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Global Warming in a Nutshell

Posted on 15 April 2012 by LarryM

Occasionally it's good to step back from the details of global warming science and offer non-technical visitors a "Global Warming 101" perspective, sort of like The Big Picture, but starting from the very beginning and touching on many aspects of this broad topic. This article was revised and re-posted from Larry's website. The figures supplement the main text with key data, but they are mostly independent and reading the figures is not necessary for understanding the text, and vice versa.

The Greenhouse Effect

The Earth is a giant rock, hurtling through space in its orbit around the sun. It would be a frozen lifeless rock like the moon if not for the thin layer of atmosphere that traps solar energy and insulates the Earth's surface, like a transparent blanket. The way the atmosphere traps solar energy is called (somewhat inaccurately) the Greenhouse Effect, because the effect is similar to a greenhouse or a closed car heating up in the sun. Sunlight comes in through a transparent window and is absorbed by whatever it hits, heating up the interior. Some of that heat is trapped inside, partly because glass is less transparent to heat than it is to light, and the temperature increases. In the atmosphere, sunlight is absorbed by the Earth's surface or rooftops or whatever, and that energy is radiated as heat (infrared energy) back toward space. Most of that heat doesn't make it to space, because it gets absorbed by certain gases in the atmosphere, mainly water vapor, carbon dioxide, and methane. Normally this is a good thing, because without the heat trapped in the atmosphere by "greenhouse gases", our planet would be frozen. But it turns out that too much of a good thing is a bad thing. If extra carbon dioxide that is not part of the natural carbon cycle is added to the atmosphere, then extra heat is trapped that would otherwise escape to space, and the atmosphere gets warmer. So in a nutshell, Global Warming is an increase in the Earth's overall average temperature caused by adding extra carbon dioxide and other greenhouse gases to the atmosphere that absorb and trap heat.

Average Global Temperature 1880-2009

Instrumental Temperature Record

Figure 1: Average global temperature during the period 1880-2009, compiled from various data sources by NASA. Zero in this figure corresponds to the average global temperature for the period 1961-1990. The red curve is a 5-year average that smooths the year-to-year variability. All measurements are imperfect for a variety of reasons, including accuracy limitations of the instrumentation, and sampling uncertainty from incomplete data coverage. The uncertainty analysis for this dataset gives an estimated increase in the average global temperature over the 20th century of 0.57±0.17°C, where ±0.17°C is the uncertainty in the estimate at the 95% confidence level.

Figure created by Robert Rohde from published data for Global Warming Art, where references for the data sources and analysis methods are given.

Carbon Dioxide

Carbon dioxide (or CO2) is the familiar gas that bubbles out of carbonated beverages, and in its solid form it's called dry ice. Carbon dioxide is also a waste product of animal metabolism, after oxygen from the air is combined with carbon-containing compounds in food to produce the energy we need to live. In a beautiful carbon cycle, plants take in carbon dioxide and solar energy to live, and they "exhale" the oxygen that we need to live. For most of human existence, the amount of CO2 in the atmosphere has been stable at about 280 parts per million (ppm), meaning that out of every million molecules in the air, 280 of them are carbon dioxide. It's a pretty small fraction of the atmosphere (0.028%), but it's the right amount of carbon dioxide to absorb just enough heat so that the Earth has the overall average temperature that we and everything else have gotten used to.

Then came the Industrial Revolution, when we learned to make the great amounts of electricity and heat needed to build modern civilization by burning "fossil fuels", or coal, oil, and natural gas (methane). Fossil fuels are the remains of plants and animals that died millions of years ago and sank to the bottom of stagnant water that lacked the oxygen needed to decompose them, so they became buried under layer after layer of sediment and compressed into coal or oil. Today we mine that coal and drill for oil and then burn them in power plants and cars. Fossil fuels are mostly made of carbon, and burning them turns them into carbon dioxide that goes up the smoke stack or out the tailpipe and into the atmosphere. This is the extra carbon dioxide, beyond that nice amount we used to have when the carbon cycle was in balance, that is causing global warming. Carbon dioxide doesn't just go away, it builds up in the atmosphere and traps more heat and makes the planet warmer. The amount of carbon dioxide in the atmosphere in 2010 was 390 ppm (Fig. 2), which is up 39% from pre-industrial levels, and it's increasing at a rate of 1.9 ppm/year as more and more humans burn more and more fossil fuels. It's astonishing that humans can have such a large effect on the whole planet's atmosphere, but it's a fact that they do.

Direct CO2 Measurements 1958-2009

Mauna Loa Carbon Dioxide plot

Figure 2: Concentration of atmospheric carbon dioxide measured at Mauna Loa, Hawaii from 1958 to 2009. The red curve shows monthly measurements of CO2 concentration in parts per million (ppm), and the blue curve is the annual average. The seasonal cycle is caused by the respiration (breathing) of all Earth's plants, where there is more plant activity in the northern-hemisphere summer to take in CO2 and reduce its concentration in the atmosphere. The 280 ppm CO2 concentration prior to the industrial revolution had increased to 315 ppm by 1958, and by 2010 it was 390 ppm and increasing, mostly in response to increased burning of fossil fuels and deforestation. Deforestation is linked with global warming because fewer plants on Earth means less CO2 is removed, so its concentration in the atmosphere remains higher. This graph is known as the Keeling curve, after Charles David Keeling who maintained this excellent record over the years. This is the first significant evidence of rapidly increasing CO2 levels in the atmosphere, and Keeling is often credited with first bringing to the world's attention the effect that human activity is having on the Earth's atmosphere and climate.

Figure created by Robert Rohde from published data for Global Warming Art.

Global Warming is about Climate, not Weather

Global warming is NOT about the daily weather, and there's no clear connection between global warming and any single hurricane or snow storm or drought. That's not the right way to think about it. Instead, adding energy to the whole Earth System leads to such things as more frequent severe weather events that on average are stronger and more damaging. That is, it's a statistical thing that has to do with averages and long-term trends, rather than one's own experience with the daily weather. There's a great deal of day-to-day and even year-to-year variability in the weather, and the 0.6°C (or 1.1°F) increase in the average global temperature over decades shown in Fig. 1 is a statistical trend that an individual can't really detect. However, scientific research brings to bear lots of measurements and data analysis and computer modeling and debate and discussion among thousands of experts who spend their careers studying this in great detail. That's what makes them experts, and why they deserve to be listened to with an open mind.

Global warming IS about an overall increase in the amount of energy in the whole Earth System caused by an increase in heat-trapping greenhouse gases. The experts are only talking about a few degrees of average temperature increase, which doesn't sound like much, but consider this example. Imagine a glass of water and ice cubes in a refrigerator whose temperature is set right at the freezing point of water, 0°C or 32°F. The mixture of ice and water will remain pretty much as it is, but if the temperature is raised by even 1 degree, the ice cubes will start to melt, and at 2 degrees they will melt faster. Everything was in balance at the old temperature, but at the slightly warmer temperature you eventually end up with all water and no ice, much like what is happening right now to Earth's Arctic sea ice (Fig. 3) and mountain glaciers (Appendix, Fig. A1).

Arctic Sea Ice Extent 1979-2005

Arctic Sea Ice Decline

Figure 3: During the summer, Arctic sea ice melts at the edges and the polar ice cap shrinks, then during the winter there is freezing and growth at the edges. This figure compares the summer Arctic sea ice minimum in 1979 (red) with the sea ice minimum in 2005. Since 1979, more than 20% of the Polar Ice Cap has melted in response to warmer air and ocean temperatures. This melting does not contribute to a rise in sea level because it is ocean ice that was already contributing to the sea level before it melted. However, disappearing sea ice is causing the decline of the polar bears because they rely on having sea ice within swimming distance for hunting. Disappearing sea ice is also causing the loss of coastal villages to erosion that didn't previously occur because there was sea ice rather than open ocean on the coast. Land-based ice, including the Antarctic and Greenland ice sheets and mountain glaciers, does contribute to sea level rise when it melts because it is new water added to the oceans.

Source: NASA and the Natural Resources Defense Council (NRDC). Satellite data of Arctic sea ice minimums from 1979 and 2005 are from NASA and the National Snow and Ice Data Center.

Impacts of Global Warming

What happens when the planet gets warmer? More extreme weather, disappearing Arctic sea ice, and receding glaciers have consequences, such as less habitable coastal areas, decline of the polar bears, and disappearing fresh water supplies for billions of people. The current rate of sea level rise is 3.3 mm/year (Fig. 4), which is cause for concern in low-lying or hurricane-prone coastal areas like Bangladesh or certain disappearing Pacific islands or the U.S. Gulf coast. This becomes a national security and military concern when there are millions of "climate refugees" with nowhere to go. They will do what they must to survive, as we all would, and they'll go to China, Australia, the U.S., and elsewhere, and they probably won't be welcomed with open arms. Other consequences of global warming include extended droughts and encroaching deserts, increasing wildfires and insect infestations, and changing rainfall and agricultural patterns. The oceans and marine life are doubly affected by global warming: first by an increase in temperature, which intensifies hurricanes and melts sea ice, and second by increased acidity caused by dissolved CO2 (Fig. A2). And these aren't even the worst-case scenarios.

Global Mean Sea Level

Figure 4: This record of the global mean sea level (GMSL) was derived from tide gauge measurements taken since 1870 (dark blue line). The light blue region is the range of uncertainty in the measurements. The tide gauge data are consistent with very accurate satellite measurements of GMSL since 1993 (red line). Sea level has increased by about 240 mm (24 cm, or 9.4 inches) since 1870. Since 1993, sea level has increased by almost 60 mm (6 cm, or 2.4 inches) at a relatively constant rate of 3.2 mm/year, which is over 50% faster than the average rate of sea level rise over the 20th century. The sea level increases when new water is added to the oceans from the melting of ice on land, and sea level decreases when evaporated water from the oceans is retained on land by the growth of ice sheets and glaciers (as during a glacial period). Sea level is also affected by the temperature of the water through "thermal expansion", where warmer water takes up more volume than cooler water, so the oceans expand in response to an increase in temperature. A warmer atmosphere from global warming leads to a warmer ocean and increase in sea level, after a time lag for the heat transfer and mixing to deeper levels to occur.

Figure prepared from published data by the Sea Level group at CSIRO (the Australian national scientific research organization).

Timescales, Positive Feedbacks, and Tipping Points

These three factors make confronting global warming both more difficult and more urgent. Natural processes that permanently remove CO2 from the atmosphere take place on a timescale of decades or longer, so we are already committed to a certain amount of additional warming even if humanity stopped burning fossil fuels today. When a system such as Earth's climate system is perturbed from a stable state, "positive feedback" processes can cause the system to move even farther from its previous state, while "negative feedback" processes tend to restore a perturbed system to its previous state. There are several positive feedbacks in the Earth's climate system that could lead to a "tipping point", which is a threshold condition that, once reached, leads to an irreversible change no matter what we do, because once it's done it's done.

One example of a positive feedback in the climate system concerns the decreasing Arctic ice cap shown in Fig. 3 (20% smaller since 1979). Ice is a very reflective surface with an albedo of 0.5-0.7, meaning that 50-70% of sunlight is reflected back to space and doesn't contribute (much) to global warming. Ocean is one of the least reflective surfaces with an average albedo of only 0.08 (only 8% reflected, 92% absorbed), so most of the solar energy hitting the ocean participates in global warming. As the size of the Arctic ice cap decreases from melting, a less reflective surface (the ocean) replaces a more reflective surface (the ice), so more solar energy is absorbed and the rate of global warming increases, leading to faster ice cap melting, leading to faster warming, etc., until all the ice is gone.

A second positive feedback in the climate system concerns the thawing of permafrost soil in northern latitudes. Vast stretches of permafrost contain vast quantities of CO2 and methane locked up in the frozen soil, which is released into the atmosphere when the soil melts, which traps more heat and increases the rate of global warming, which melts more and deeper permafrost, which further increases the rate of global warming, etc. These and other positive feedbacks could lead to a tipping point, where dramatic changes in the Earth System happen quickly and are permanent and irreversible no matter what we do about CO2 emissions.

Energy and Climate Policy

Climate science is enormously complex (see Fig. A4), yet sufficient progress has been made that we know the broad outlines of what is needed to avoid the worst consequences of global warming. We need to make major changes in the way we get and use energy, and we need to do it soon, or the world will change in ways we won't like and our children will like even less.

Having a specific goal can sometimes motivate progress, and some climate experts have suggested that we should aim to stabilize the atmospheric CO2 concentration at no more than 350 ppm. Recall that the pre-industrial CO2 level was 280 ppm, and we're currently at 390 ppm and increasing at a rate of 1.9 ppm/year, so we're already above the desired level and rapidly heading in the wrong direction. The target of 350 ppm originated with a paper by eminent NASA climate scientist Dr. James Hansen, which involved an analysis of past climate conditions and their associated CO2 concentrations. A commonly cited target for policy considerations is to limit global warming to 2°C (3.6°F), which gives us a chance to avoid positive feedbacks such as widespread melting of permafrost that could lead to a tipping point. The 2°C limit roughly corresponds to an 80% reduction in CO2 emissions by the year 2050, if substantial reductions begin immediately. Meeting such targets requires transformational energy policy that establishes clear requirements and a mechanism to meet them, and in one way or another puts a price on the emission of carbon to drive reductions. Right now there is zero cost for contributing to global warming, so there's no incentive to stop, and taxpayers will continue to pay for the effects of global warming.

Energy Choices, and Who to Trust

Energy choices made today will determine the climate in coming decades. Historically it has been difficult for human beings to get together to confront a major problem until some catastrophe occurs, which does not bode well for addressing global warming because we must act far in advance of any catastrophe due to the long timescale for removal of CO2 from the atmosphere. THE SOLUTIONS ARE CLEAR: We need to use energy more efficiently, as about half of it is just wasted by inefficient cars, appliances, buildings, electrical grid, etc., and we need to transition to clean and renewable (non-fossil fuel) sources of energy, which we already know how to do.

However, the forces opposing change are formidable and well funded, and frankly, very good at deceiving a poorly-informed public about a complex and long-term issue. Coal was great for powering the industrial revolution and electrifying civilization, but now we know there are long-term global consequences, and now we know how to make electricity in smarter ways than using fire to boil water to turn a generator. Oil has plenty of problems in addition to being a major contributor to global warming, including volatile prices, environmental damage such as the Gulf oil disaster or tar sands mining, and the loss of life, political destabilization, and enormous expense to taxpayers of protecting oil supplies. Even relatively "clean" natural gas (methane) is not really very clean unless much more is done to prevent leakage of this potent greenhouse gas into the atmosphere.

Confronting global warming will require a combination of strong legislation crafted by informed and courageous legislators, personal actions to reduce one's own energy use and to support renewable energy, and teaching others about the reality of global warming and what's at stake. One thing that any individual can do is to elevate the importance of strong clean energy and climate legislation when making voting decisions.

For me, this issue is way above politics, it's about the future of my daughter and my species. Regarding who to believe about global warming, consider this simple analogy. If I want to know how to fix my plumbing, I will call a plumber, not a climate scientist. But if I want to know if all the hoopla about global warming is real and should we do something about it, it seems like common sense to ask a climate scientist, not the Coal Lobby, or the American Petroleum Institute, or some (but not all) politicians.


Links to Additional Information

NASA evidence page Observed climate change evidence from NASA.
Center for Climate and Energy Solutions Observed climate change impacts from C2ES (formerly the Pew Center on Global Climate Change).
National Snow and Ice Data Center Repeat photography showing the retreat of glaciers worldwide, and other information about changes happening in Earth's frozen regions.
Global Warming Art An excellent source of information and downloadable graphics that document the scientific data and analysis of global warming research.
The Risk Management Perspective Above I suggest that it makes more sense to trust the vast majority of climate scientists (that is, experts) than to trust representatives of the fossil fuel industies (that is, vested interests). This popular You Tube video presents another angle on how to think about global warming where you don't have to trust anyone, by putting it as a question of "risk management", which is how big and complicated questions are often approached in business, engineering, and elsewhere.
Climate and Clean Energy Advocacy One way that an individual can stand for reason-based decision-making in government is to join and donate to reputable advocacy organizations, to counter the very well-funded deception and denial campaign of the fossil fuel lobbies. My favorite such organization based on their long record of results is the Natural Resources Defense Council (NRDC). I also like the emphasis on scientific integrity and opposition to political interference in federally-funded research at the Union of Concerned Scientists. For decades I've been a fan of Amory Lovins' "soft energy path" and his "think and do tank" called the Rocky Mountain Institute.

Appendix -- Additional Figures

Glacier Mass Balance 1980-2008

Glacier Mass Balance

Figure A1: The mass of glaciers, which is the total amount of water they contain, is a combination of the area of the glacier and its average thickness. The World Glacier Monitoring Service oversees annual reporting on the "mass balance" of over 90 glaciers worldwide. Mass balance is the net gain or loss of mass in units of "mm w.e." (millimeters of water equivalent), which is the average thickness of ice that was gained or lost if that amount was melted. The figure shows the average mass balance for all reported glaciers (black) and for a subset of 30 reference glaciers (red) for 1980-2008. Over this time period the glaciers on average lost an amount of mass equivalent to a thickness of 12,000 mm (12 meters, or 39 feet) of water, and the loss rate (slope) has been increasing since the mid 1990s. This water ends up in the oceans and contributes to a rising sea level, although the contribution is small compared to that of the Greenland and Antarctic ice sheets. A particularly distressing aspect of disappearing mountain glaciers is that they are the source of water for billions of people in various parts of the world.

The loss of ice is uneven across a glacier, and the lower portion of glaciers has been disappearing, which is known as "glacial retreat". This is captured visually with repeat glacier photography, such as these examples at the National Snow and Ice Data Center and at Global Warming Art. The Extreme Ice Survey has documented the changes and the fascinating behavior of glaciers around the world with time-lapse video and photography. The PBS/Nova Extreme Ice documentary is very interesting and can be downloaded or purchased on DVD, and James Balog's TED talk is a good overview of the project and its conclusions.

Figure is from published data at the World Glacier Monitoring Service.

Dissolved CO2 and Ocean Acidity (pH)

Ocean Acidification

Figure A2: This figure shows the correlation between rising levels of carbon dioxide (CO2) in the atmosphere at Mauna Loa (red) with rising levels of CO2 dissolved in the ocean at nearby Station Aloha (blue), and the consequent increase in acidity of the ocean that is seen as a decrease in ocean pH (green, right scale). Global warming deals a double blow to the oceans because not only does the ocean warm by absorbing part of the atmospheric temperature increase, it also absorbs about one-third of the increase in CO2, which reacts with water to produce carbonic acid. So, the oceans help to decrease atmospheric CO2 and slow global warming, but with major negative consequences for the chemistry and biology of the oceans.

Many creatures in the ocean require a fairly narrow range of temperature and/or pH to survive. A warming ocean causes creatures that can move, like jellyfish, to move northward to cooler waters, while those that can't move, like coral, die of heat stress. Acidification interferes with the formation of shells made of calcium carbonate, because decreasing pH means decreasing availability of carbonate ions. Some of those shelled creatures are microscopic and are at the base of the food chain, so their demise affects all ocean life as well as we humans who depend on it. The acidity of the oceans has already increased by 30% (0.1 pH units) from pre-industrial times, and it will continue to increase as long as humans continue burning fossil fuels and adding more CO2 to the atmosphere. For more information about ocean acidification, see this report from the Center for Climate and Energy Solutions.

Source: Ocean Acidification by R.A. Feely, page 60 in Levinson, D.H. and J.H. Lawrimore, 2008: State of the Climate in 2007. Bull. Amer. Meteor. Soc., 89, S1-S179. Also: Feely, R.A., S.C. Doney, and S.R. Cooley, 2009: Ocean acidification: Present conditions and future changes in a high-CO2 world. Oceanography 22(4):36-47.

CO2 and Temperature from Ice Cores - 800,000 years

CO2-Temperature Long-Term Record

Figure A3: This is the long-term (800,000 year) record of atmospheric CO2 concentration (right scale) and reconstructed temperature (left scale) from analysis of air bubbles trapped in Antarctic ice cores acquired by EPICA (European Project for Ice Coring in Antarctica). The temperature scale is the difference from the average temperature over the last 100 years, so it is degrees difference from modern times. Over the last few hundred thousand years there has been a sequence of alternating glacial ages (mini ice ages) and "inter-glacial warm periods" (we've been in one for about 11,000 years). The data show that a glacial age corresponds to a global temperature that is lower than today's by 7-8°C (12-14°F).

The good correlation between CO2 concentration and temperature strongly supports the link that is predicted from basic global warming theory. On the timescale of this figure, the last few thousand years are squashed up against the right axis, where the CO2 concentration has been 270-280 ppm. The arrow shows the current CO2 level of 390 ppm that has occurred in only the last century, much faster and higher than anytime in the last 800,000 years. The atmosphere has begun to respond to the increased CO2 level with a temperature increase of about 0.6°C in the last century, and the figure gives no reason to doubt that warming will continue in response to the unprecedented increase in CO2. We are now conducting a grand experiment to see what happens to the planet when we increase the temperature, and since this is our one and only planet, some would say that it's not a very smart experiment.

Figure created by Leland McInnes from published EPICA data.

Some Physics Behind the Greenhouse Effect and Global Warming

Atmospheric Transmission

Figure A4: The Earth System and Global Warming are enormously complex, and their study involves many areas of scientific research. One area concerns the details of what happens to solar radiation (light) as it passes through the atmosphere and is either absorbed in the atmosphere or by the surface or is reflected back to space. A related concern is what happens to the heat (infrared radiation) that is emitted by the surface of the Earth and is either absorbed by greenhouse gases and clouds in the atmosphere or it escapes to space. This information-packed figure nicely summarizes what happens to sunlight (red) and heat (blue) as they pass through and interact with the atmosphere, but understanding it may require some background.

Sunlight is not one single thing, but rather it is composed of a "spectrum of colors", where "color" is the response of the human eye to different wavelengths or energies of light particles ("photons"). Our eyes are only sensitive to a small portion of the entire "electromagnetic spectrum", namely the wavelength region called "visible" in the top panel. Heat, or "infrared" radiation in the figure, has a longer wavelength (or lower energy) than visible light, and "ultraviolet light" (UV) has a shorter wavelength (or higher energy) than visible light. So, light and heat (and microwaves and radio waves and x-rays and gamma rays) are all different wavelengths or energies of the same thing (electromagnetic radiation), but each wavelength interacts differently with gas molecules in the atmosphere, or with the Earth's surface, or clouds, or human beings.

The red curve in the top panel is the spectrum of electromagnetic radiation emitted by the sun, which peaks in the visible portion of the spectrum but also contains both infrared and UV radiation. The filled-in red area is the spectrum of sunlight that actually reaches the ground after traveling through the atmosphere, where the missing radiation is either absorbed by one of the specific molecules shown in the lower panels, or it is scattered into a different direction by molecules in the air ("Rayleigh scattering", which is why the sky is blue). The middle panel shows the total percentage of each wavelength of radiation that is absorbed or scattered by the atmosphere, so wherever there is a "strong absorption band" for one of the molecules in the lower panels, that radiation will not reach the surface and contribute to the red area in the top panel. The top and middle panels are two complementary ways of showing the same thing, namely how much solar radiation of a given wavelength reaches the surface. After sunlight reaches the surface it is either absorbed by the surface or reflected back toward space for a second trip through the atmosphere. The figure shows that much of the UV radiation emitted by the sun is either Rayleigh scattered or is absorbed by oxygen and especially ozone. Thankfully we now have regulations against emitting chemicals that destroy the ozone layer in the upper atmosphere, which protects us from the most harmful "extreme UV" radiation from the sun. The other important molecule for absorption of solar radiation is water vapor, which absorbs a small amount of the visible light and a medium amount of the infrared light emitted by the sun. The chemical properties of a molecule determine which specific wavelengths of electromagnetic radiation it will absorb.

The other half of the Earth's radiation balance concerns the fate of infrared or thermal radiation (heat) that is emitted by the Earth's surface, atmosphere, and clouds. All objects emit a spectrum of infrared radiation whose peak is at a wavelength that depends on its temperature. The blue curve is the spectrum of infrared radiation emitted by the Earth as a whole as seen from space, at an effective temperature of about 250 K (-23°C or -9°F). This is colder than the Earth's average surface temperature of about 287 K (14°C or 57°F) because some of the outgoing infrared radiation is absorbed by greenhouse gases or cloud particles and is re-emitted from colder levels in the atmosphere (as nicely explained in the article Greenhouse Effect Basics: Warm Earth, Cold Atmosphere). The figure shows that 70-85% of the outgoing infrared radiation is absorbed by molecules in the atmosphere, and water vapor is the dominant greenhouse gas because it absorbs a large fraction of the infrared radiation. Carbon dioxide is the next most important greenhouse gas, followed by methane.

The average temperature of the Earth is the result of a balance between energy that is added to the Earth System by the Sun, and infrared energy that is emitted by the Earth System and escapes to space. Global warming is about increasing the concentration of greenhouse gases in the atmosphere, which absorb more of the infrared radiation that would otherwise escape to space. This adds energy to the Earth System, which increases its temperature, which causes the Earth to emit more infrared radiation at the higher temperature, until a new balance with the energy input from the sun is established.

Disclaimer: This is a super highly oversimplified description of one aspect of the complex climate puzzle, and this description doesn't do justice to even this one aspect of the current understanding of climate processes that are represented in modern climate models. For an excellent description of energy flows in the Earth System, see Trenberth on Tracking Earth’s energy: A key to climate variability and change.

Figure created by Robert Rohde, a physicist and mathematician who documents climate science as a hobby at Global Warming Art.

Postscript (17 April 2012): A few small changes to the original post have been made in response to the comments, as described in Comments #10 and #35.

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Comments

Comments 1 to 41:

  1. Nice summary. Let's also not forget the issue of Energy Balance, and the role the oceans play in taking up, not just excess carbon dioxide but heat as well. Looking at ocean heat content over the past 40 years shows an equally consistent (if not more so) rise, but is even more impressive when one considers that it represents many times more energy than the atmosphere. The atmosphere can be subject to much more natural variation or noise over short periods, whereas the oceans are buffered from this to some extent, and over the past 40 years have shown an amazing 23 x 10^2 Joules of storage down to about 2000m.
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    • What happens when the planet gets warmer?

    Life is easier. Longer growing seasons, more rain, more arable land and increased productivity due to increased CO2. Less ice and snow, lower heating bills, less winter wear & tear on infrastructures ...

      More extreme weather,

    I see conflicting reports on whether or not this is happening so far*

      disappearing Arctic sea ice,

    So what?

      and receding glaciers have consequences, such as less habitable coastal areas,

    Ice makes a coastline more habitable?

      extinction of the polar bears,

    I just saw a report that their numbers are up, maybe an all time high.

      and disappearing fresh water supplies for billions of people.

    If a glacier disappears, it will still rain and snow in the watershed where the glaciers was and the rivers in such watersheds will still flow.

      The current rate of sea level rise is 3.3 mm/year (Fig. 4), which is cause for concern in low-lying or hurricane-prone coastal areas like Bangladesh or certain disappearing Pacific islands or the U.S. Gulf coast.

    Why has an acceleration of sea level rise not been observed during the altimeter era?

      This becomes a national security and military concern when there are millions of "climate refugees" with nowhere to go.

    The scary stuff requires more than a a great deal more than a 3.3 mm/yr rate in sea level rise.

      They will do what they must to survive, as we all would, and they'll go to China, Australia, the U.S., and elsewhere, and they probably won't be welcomed with open arms.

    Hasn't happened yet.

      Other consequences of global warming include extended droughts and encroaching deserts,
      The IPCC's AR4 Report tells us that in a "future warmer climate ... Globally averaged mean water vapour, evaporation and precipitation are projected to increase."
      ipcc.ch/publications_and_data/ar4/wg1/en/ch10

    That's not a recipe for more droughts and encroaching deserts.

      The oceans and marine life are doubly affected by global warming: first by an increase in temperature, which intensifies hurricanes

    *It's not the increase in temperature that intensifies storms, it's the increase in the difference between the air masses involved in the storms. And as the IPCC tells us in a warming world:

      "Almost everywhere, daily minimum temperatures are projected to increase faster than daily maximum temperatures, leading to a decrease in diurnal temperature range. Decreases in frost days are projected to occur almost everywhere in the middle and high latitudes, with a comparable increase in growing season length."

      ipcc.ch/publications_and_data/ar4/wg1/en/ch10

    In other words, the warming will be mainly at night, in winter and in the Arctic; day time, summer time and in the tropics, not so much, meaning that the difference in temperatures between air masses will be less. Which does not mean that the storms will be more extreme.

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    Moderator Response: TC: Due to the very general nature of the OP, there is a risk that discussion will become confused with too many subtopics intermingled so that the progress of discussion on any particular topic will be difficult for our readers to follow. To avoid that situation, we ask that all commenters discuss particular topics on threads more directly related to that particular topic, as Eric (skeptic) has in fact done. In particular we ask that responses to Steve Cases' post pick a particular point and discuss it on the more directly related thread; and ask that Steve Case do the same to those responses that have already been made. Thank you.
  2. Steve Case#2: "Life is easier."

    Now that you've posted your unsubstantiated opinion, how about a word from science?

    Here's a good one from 2009:

    The study notes that decreases in rainfall that last not just for a few decades but over centuries are expected to have a range of impacts that differ by region. Such regional impacts include decreasing human water supplies, increased fire frequency, ecosystem change and expanded deserts. Dry-season wheat and maize agriculture in regions of rain-fed farming, such as Africa, would also be affected.

    Some of that good news is already upon us.
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  3. Steve Case said... "extinction of the polar bears,
    I just saw a report that their numbers are up, maybe an all time high."

    You didn't spend much time researching this one I take it.

    http://mediamatters.org/blog/201204090004

    There were so many things that were wrong with the shoot-from-the-hip response coming out of denierville on this one.
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  4. Steve: You need to read the actual reports, not take the pieces you’re looking for and repeat them out of context…. Of course it might also help to actually pay attention to what’s going on in the world.
    and receding glaciers have consequences, such as less habitable coastal areas,
    Ice makes a coastline more habitable?
    No, Steve, when glaciers melt, the water that used to be stored on the continents as glacial ice contributes to sea level rise.
    If a glacier disappears, it will still rain and snow in the watershed where the glaciers was and the rivers in such watersheds will still flow. No, Steve, actually they might not….
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  5. @ R. Gates #1:
    Yes, the oceans soak up more of the heat retained in the Earth's climate system by global warming than the atmosphere does, which contributes to melting of the Arctic ice cap, and effects on marine life as mentioned in Fig. A2, and also to intensification of hurricanes (which nicely ties into the next comment).

    @Steve #2:
    I'm afraid you've misstated how hurricanes form. The primary driver of hurricane formation is energy from warm sea surface temperatures, with overlying rotating air. As ocean temperatures increase, more energy is available, and hurricanes are expected to intensify (on average, of course, as with all things related to global warming). The jury is still out on the expected relationship between hurricane frequency and global warming, but the intensity of hurricanes is strongly correlated with sea surface temperature, and the "power dissipation index" (a measure of hurricane intensity) is observed to be increasing. Please see What is the link between hurricanes and global warming?, and also an entertaining NASA article called Recipe for a hurricane.

    Regarding the expectation that global warming will lead to extended drought and desertification is some regions, part of the answer lies in the very link that you provided from the IPCC report. The key phrase in your quote is "globally averaged". Overall, in a warmer world there will be more water vapor in the atmosphere and more precipitation in many areas such as the tropics, but not everywhere, and the other part of the quote in the report is the expectation of less precipitation in the subtropics.

    If you know of a reliable report that polar bear numbers are at an all time high, please provide that reference.

    Regarding life being easier and heating bills lower in a warmer world, well, I suppose that all depends on who and where you are. For others, life will be harder and cooling bills will be higher.
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  6. Impact section is very slightly marred by absolutist "extinction" of polar bears. It's a good piece overall and I am not a "concern troll", I will elaborate in the thread where I made my very first posts at this site on this thread
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  7. I prefer to expand the scope to describe it as geological impacts of our anthropogenic era - and to atmosphere and lithosphere.

    Changes would include: global warming - climate change leading to ocean acidification, sea level rise, ice cap melt leading to isostatic rebounds & earthquakes.

    Humans have been amazingly powerful.
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  8. @LarryM:

    The contents of your “Links to Additional Information” box should be updated to reflect the fact that the Pew Center on Climate Change was transformed into the Center for Climate and Energy Solutions (C2ES) in Nov, 2011.
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  9. @John Hartz:
    Thanks, the name and link have been updated.

    @Eric (skeptic):
    That's a fair criticism, as some polar bear populations may survive. I changed the word "extinction" to "decline".
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  10. Just thought it's worth mentioning, that there is up-to-date information for all of these subjects available.

    Best summary sheet that I've found for up-to-date records of the temperature data set providers is here:-
    http://junksciencearchive.com/MSU_Temps/Warming_Look.html

    The temperature graphs it plots also show the CO2 levels from the Mauna Loa observatory in Hawaii.

    Arctic sea ice extent (as well as Antarctic sea ice extent) can be found here:-
    http://nsidc.org/arcticseaicenews/

    The problems with determining ice volume are discussed in a RealClimate.org article here:-
    link

    Sea level information is collated here:-
    http://sealevel.colorado.edu
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    Moderator Response: [RH] Added hot link to fix broke page formatting.
  11. I would like to see a link to this article a prominent feature of the home page. If that is not possible, perhaps it could be linked to from the 'Newcomers Start Here' section.

    Unfortunately, if it were to form such a feature, then it would be necessary to either delete the comments, or preferably take the second comment’s string of unsubstantiated opinion and rebut each point raised. I would imagine anyone who would find the article a significant source of information on the topic may well hold the same opinions (it is after all what the mainstream media would have us believe) and thus the rebuttals would be of added value to them.

    Who knows, a certain member of the British aristocracy might learn something, seeing as said peer seems to hold many of the same misguided views on the topic and is even proud to parade his ignorance to all who will listen (and run away when challenged with the facts of the matter).
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  12. Excellent introductory to the topic, which I will glady point people to when they ask for information.

    For the sake of people reading these comments and wondering about the so-called debate on climate change, note that Steve Case @ 2 made a number of claims, but did not stick around to discuss them in a real debate. This type of activity is known as a 'drive by' and is a common tactic by those whose purpose is to spread confusion and misinformation. His list of points is known as a Gish Gallop, where there are so many things wrong that it is difficult to know where to start in addressing them.

    To be clear: there is no debate, amongst scientists who are actively researching and publishing peer-reviewed papers in the relevant fields, that anthropological global warming (AGW) is real, is happening now and is dangerous.
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    Moderator Response: TC: To be fair to Steve Case, he is still actively discussing issues arising from one of his claims on a more appropriate thread as requested by the moderators. Therefore his actions cannot be characterized as a 'drive by'. As the moderator who explicitly made the request, I would like to thank Steve Case for his cooperation.
  13. The real danger of Natural Gas is that it will be a glut and hence cheap. This will force down the price of all fossil fuel and cause us to use far more energy than otherwise. Instead of replacing coal and hence giving us a small respite from our carbon emissions, the use of Natural gas will simply mean we have further to fall when the gas runs out as it certainly will. Worse, it will trash the initiative to switch to renewable energy.
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  14. I was thinking it might be useful to put together a list of links where most, if not all, of Steve Case's talking points have already been hashed out. But, the list of good refutations is long, and perhaps people are more convinced by things they find for themselves than what they've been pointed toward. Simple searches on just this site alone yield multiple pages per topic, nevermind if you go off-site.

    Steve's arguments sound simple, and simple sounds good; to refute them you have to get more technical, but if your target audience doesn't even know what a Hadley cell is, how do you convince them about changes in rain bands and expanding deserts? In the end it comes down to an appeal to authority, unless you happen to actually be an authority, a publishing researcher in the area.

    So, I can only ask that any newcomers verify the information in the article independently, and ask themselves questions like (on just one topic): Ice sheet mass loss is measurably accelerating; where will this water end up? Why does Steve think that the sea level rise will not accelerate? Looking at the geologic record, has the sea level transition between one glacial state and another ever been slow and steady over the whole transition? What makes Steve think it will be this time? Is the habitable land area potentially gained from underneath ice masses more or less that the habitable land lost from a coinciding rise in sea level? Look at pictures of land exposed by receding ice; does it look very habitable to you?

    OK, here is as good a start link for that one topic as any:

    Climate and Sea Level: An Emerging Hockey Stick
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  15. Coral islands may well be destroyed by climate change but probably not because of sea level rise.
    http://mtkass.blogspot.com/2011/09/by-by-coral-atolls.html
    It is important to get the reason right because if you don't and your argument is discredited, it is hard to convince anyone that your new argument is correct.
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    Moderator Response: [DB] Fixed link.
  16. WRT: "...which is colder than the average surface temperature because some of the infrared emission comes from clouds at colder levels in the atmosphere."

    Mmm, while it is true that the tops of clouds are generally colder than the areas without clouds, I don't think it is necessary to invoke clouds. It might be more accurate to say that the percentage of longwave not absorbed by the GHGs above any point of emission, and therefore actually escaping to space, increases as the altitude increases. Absorption is a function of density, and density drops rapidly with altitude. Near the surface, very few of the photons which could be absorbed by a GHG make it to space without interacting with a GHG. As the density drops, a higher percentage of the photons emitted reach space (and GHGs both absorb and emit). Temperature drops with altitude, up to the tropopause. The mean emission altitude is around 6km; this is below the mean tropopause height (which varies). So, an instrument measuring outbound photons sees a distribution of photons that represents a temperature that is less that what the surface actually is. More of any GHG effectively raises the distribution of outbound photons a little higher in the atmosphere. Using an environmental lapse rate of 6.5 K/km, and not counting albedo changes, you only need to raise the mean altitude of emission by about 300m in order to cause a 2 K warming at the surface.

    OK, at this point I think that all I've done is demonstrate that more accurate is sometimes less clear. My longer description is still a simplification, and I suspect I've left some readers behind.

    Or, maybe I've demonstrated that when things are simplified, you can have more than one seemingly different explanation for the same effect, that might appear to be, but are not really at odds with each other. I suspect that some bloggers take advantage of this kind of inconsistency-that-isn't scenario in order to imply doubt where really none exists.
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  17. Chris G, you may appreciate this attempt to not leave people behind.
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  18. I guess I'm representative of the sort of people this post is intended to inform. I started to inform myself a couple of years ago, and came up with the following hypersimple outline of the scientific argument:


    1. Earth’s human population has grown, very rapidly in the last 80 years. from about two billion, to around seven billion in 2011. It is predicted to exceed nine billion by 2050.

    2. Human activity – manufacture, agriculture, trade, travel - has increased. Not only are there more of us, but per capita we are doing more.

    Together, (1) and (2) are what we call ‘growth’

    3. Growth takes energy.

    4. The energy has been supplied by the heat from burning carbon: wood, then coal , then oil and gas.

    5. Burning (oxidising) carbon produces carbon dioxide.

    6. Carbon dioxide in the atmosphere causes the planet to warm.

    It seems to me that each of the above points, elaborated, is as near to incontrovertable as you can get.

    So the issue now is how to get the message out, and how to stoke action on the basis of the science. To that end I have tried to develop a website for use in a small-group, net connected educational project. See

    www.theclimateargument.com (Sorry: dunno how to make the link active)

    The central information source for the project is a paper 'convergence' which is on the website.It covers much the same ground and uses many of the same figures as Larry's Of course, beyond all this is a question for politics and political economy.....What is to be Done?

    S.H.
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    Moderator Response: TC: Added html link. The comments policy contains a number of hints for using html code, including basic commands for links, pictures and emphasis (italics, bold, underlining).
  19. Hello,
    Can some please make it clear (with some numbers)
    why the earth would be a frozen ice ball if our atmosphere had zero CO2 content.
    Thanks!
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    Moderator Response:

    [DB] In addition to the help already given below, you will find the numbers you seek here:

    Atmospheric CO2: Principal Control Knob Governing Earth’s Temperature

    "Without the radiative forcing supplied by CO2 and the other noncondensing greenhouse gases, the terrestrial greenhouse would collapse, plunging the global climate into an icebound Earth state."

  20. davpaf, the map below shows the approximate extent of ice during the last glacial maximum.



    Note that the ice sheet extending over North America on this map was more than a mile thick. That ice coverage was due to atmospheric CO2 levels dropping from ~280 ppm to ~180 ppm. Dropping the CO2 level to 0 ppm would cause the temperature to drop by about three times as much as it did during the last glacial maximum.

    You'd need a complete climate model to get a tightly constrained estimate of the temperature drop and total ice coverage. My guesstimate would be around -18 C and ice extending to the tropics of Capricorn and Cancer.
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  21. CBDunkerson @21, actually dropping CO2 levels to 0 ppmv would global mean temperature from about 14 degrees C to about - 20 degrees C. That is, the drop would be around 6 or more times the difference between the LGM and preindustrial temperatures:

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  22. Thanks for the info. Have any papers been published on modelling such an atmospheric situation?

    The -18C you are referring supposedly originate from the Solar Radiation and the Earth's Energy Balance? It would be great if someone could explain to me why such a representation is valid in describing the global mean surface temperature of the planet.
    Thanks
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    Moderator Response: [DB] The Lacis paper I linked for you in the earlier response to you is published.
  23. Yes thank you for the link to this paper. Interesting and valid results, but all based on their statement on page 1:

    "The difference between the nominal global
    mean surface temperature (TS = 288 K) and the
    global mean effective temperature (TE = 255 K)
    is a common measure of the terrestrial greenhouse
    effect (GT = TS – TE = 33 K). Assuming
    global energy balance, TE is also the Planck radiation
    equivalent of the 240 W/m2 of global
    mean solar radiation absorbed by Earth."

    This still leaves me pretty uncertain why it is valid to assume that TS should be equal TE.

    Thanks
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  24. Tom @18,
    I missed that. Good one; I think these two articles should be linked to each other.

    Looking at the comments, apparently there Steve Case believes that CO2 has an effect, and here that albedo changes and other feedbacks/forcings do not. Wondering how he would explain how Milankovitch cycles work without mentioning ice albedo. Maybe we should put him and Lindzen in a room together.

    davpaf @20,
    It might help to point out the observation that in the past, when the atmosphere has dropped below a certain CO2 level, an icehouse state ensued. (And it took very high levels of CO2 to flip it out of that state.)
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  25. Neoproterozoic `snowball Earth'
    simulations with a coupled
    climate/ice-sheet model


    There are several items in the References section that look look like good starting points for geologic evidence.
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  26. davpaf - "This still leaves me pretty uncertain why it is valid to assume that TS should be equal TE."

    Because if there were no absorbing greenhouse gases (in this thought experiment) emissions to space would be directly from the surface, hence TE = TS. Greenhouse gases shift emission up in the atmosphere to colder regions, hence the present difference between surface and emission temperatures.
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  27. KR – Wouldn’t emission to space be from the complete earth surface/atmosphere system, since they both have a temperature. How much each of these parts contributes to this total emission is of course debatable.

    So I am still unclear why one can assume that the total emission temperature of these 2 parts (surface emission through its temperature) (atmosphere emitting through its temperature) should be equal to the global mean surface temperature.

    Are there any papers on this specific issue?

    Thanks folks
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    Moderator Response:

    [DB] "How much each of these parts contributes to this total emission is of course debatable"

    Debatable? Not really. This is an area well-studied and well-understood. See this SkS post by Dr. Kevin Trenberth and this attribution study by Schmidt et al, 2010:

    Attribution of the present-day total greenhouse effect

  28. davpaf - "Wouldn’t emission to space be from the complete earth surface/atmosphere system, since they both have a temperature."

    Yes. But in the Gedankenexperiment of no greenhouse gases, you're left with essentially oxygen, nitrogen, and some argon - none of which absorb/radiate significantly in the IR spectra. in the absence of GHG's only the Earth's surface will radiate - at about 0.98 to 0.99 emissivity in IR. See figure A4 above regarding gas spectra.

    "How much each of these parts contributes to this total emission is of course debatable."

    No, it is not. Not after 150 years or so of spectroscopy.
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  29. Tom & KR, note that the original hypothetical was just the removal of all carbon dioxide from the atmosphere... not all greenhouse gases.

    Of course, it would be impossible to remove all CO2 without also removing all methane (which breaks down into CO2)... and the temperature drop from decreased CO2 would perforce cause a significant reduction in atmospheric H2O...

    However, I ignored all of that in my response... which was then still just a wild guess. The ~33/34 C values you are citing would be removal of the entire greenhouse effect... though I believe even then it isn't accounting for additional cooling which would result from the albedo shift.

    Basically, it is a physically impossible hypothetical and thus the answer depends in large part on what assumptions we make to 'gloss over' the inherent contradiction of CO2 levels somehow changing to zero independently of all interconnected factors. It would be cold exactly how cold is impossible to say without understanding the magical process which allows the atmospheric CO2 to drop to zero.
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  30. I am not in any way in doubt about the radiative characteristics of the different gases!

    Here what I understand so far:
    As you have pointed out in the figure of this article, various gases have different characteristics of emitting radiation. According to the figure the major players seems to be Water vapour & CO2.
    So if in our thought experiment an atmosphere without CO2 is present, there are still gasses available which will provide radiation to space, hence the atmosphere will be radiating to space. So the two components from before, atmosphere + surface, both radiating are still present, leaving me with my original question why it is valid to disregard the atmospheres part.

    Thanks for the link to the paper admin.
    As I understand it is mostly on the effectiveness of the individual greenhouse gases, and how individual and combined effects are important. But still the initial sentence of the paper is the basic assumption, which is unclear:

    “The global mean greenhouse effect can be defined as the difference between the planetary blackbody emitting temperature (in balance with the absorbed solar irradiance)
    and the global mean surface temperature. The actual mean surface temperature is larger (by around 33°C, assuming a constant planetary albedo) due to the absorption and emission of long‐wave (LW) radiation in the atmosphere by a number of different “greenhouse” substances.”

    I have also checked the reference of this quote in this paper
    (Charney, J. (1979), Carbon Dioxide and Climate: A Scientific Assessment,Natl. Acad. Press, Washington, D. C.)

    but don´t seem to find its origin.
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  31. CBDunkerson, davpaf - My apologies, I was thinking of the more extreme thought experiment with no greenhouse gases, not just missing CO2 (and methane).

    However, the link Tom Curtis gave above looks to be a model of just that situation, removal of non-condensing GHG's - a cooling Earth, condensation of water vapor reducing the absolute humidity, tapering off to a temperature of about -20 C.
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  32. davpaf - "...the initial sentence of the paper is the basic assumption, which is unclear"

    Energy balance in the climate is determined by the difference between what comes in from the sun (240 W/m^2), and what gets radiated out to space (~239 W/m^2 or so at present, given the observed warming).

    Without GHG's, given the IR emissivity of the ground and water on the Earth's surface, 240 W/m^2 could be radiated with a surface temperature of about -18 C. That's as per the basic Stephan-Boltzmann equation.

    With the presence of various GHG's absorbing/emitting in the lower atmosphere, and the lapse rate of temperature fall with altitude, the top of troposphere location for those GHG's to effectively emit to space is both higher and colder than the surface - and since IR emission scales with T^4, they emit less energy than the surface or near-surface air could if directly exposed to space.

    This is often expressed as an "effective emissivity", or the proportion of a theoretic blackbody emission at surface temperatures. For our climate (as measured from space, relative to a 15 C blackbody) the effective emissivity is ~0.612, meaning that the Earth emits ~240 W/m^2 to space - rather than the ~396 W/m^2 emitted by the surface at that temperature.

    GHG's slow the effective cooling to space - and hence a higher temperature on the surface is required (higher than required without GHG's) to match incoming energy with outgoing IR.
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  33. A side note to the last few comments:

    A simple thought experiment of removing GHG's shows an expected temperature of -18C. Tom Curtis linked to a model that included some secondary effects, such as albedo changes from ice growth and cloud cover, which reaches -20C even with some water vapor still present.

    But the core of the discussion remains, whether you attempt to account for all details of the extreme thought experiment - we're a lot warmer with greenhouse gases than we would be without...
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  34. @Chris G #17 and #25:
    I made a few changes to the caption of Fig. A4 to clarify that it is not just cloud particles but also re-emission of absorbed IR radiation that occurs at lower temperature levels and contributes to the lower effective temperature of the Earth's outgoing longwave radiation as seen from space. The excellent article suggested by @Tom Curtis #18, Greenhouse Effect Basics: Warm Earth, Cold Atmosphere, has also been linked. I also added a link to the excellent article mentioned by the moderator in comment #28, about energy flows in the Earth System, called Trenberth on Tracking Earth’s energy: A key to climate variability and change.
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  35. davpaf @24, there is no reason why TS should equal TE, and unless you remove all greenhouse gasses it will not be. In removing all greenhouse gases, you need to remove not only well mixed GHG, but also water vapour and clouds.

    In the model runs by Lacis et al 2010, whose results I showed @22 above, TE is reduced by about 15 degrees C because the albedo in increased from 0.3 to about 0.42. That is compensated for by the remained greenhouse effect from water vapour and clouds so that by coincidence TS approximately equals the value of TE prior to the removal of CO2.

    Please note that in his comment @27, KR assumes zero greenhouse effect, which is a different thought experiment (see his comment @ 32).

    CBDunkerson @30, as already noted by KR, the model results I presented are for the removal of CO2 (strictly, all well mixed GHG so that it removes methane as well as CO2), not for the removal of all GHG including water vapour.

    Personally I am surprised at the results. It is known that the Earth entered a "snowball" state, with extensive continental ice at sea level at the equator, at least twice between 1 billion and 600 million years ago. The sun emitted about 10% less radiation then, which would make that easier, but the model results suggest that it is no longer possible for the Earth to enter a Snowball state, even with the complete removal of all CO2.

    A solution to the paradox may be that the model does not allow for the growth of ice sheets. That is, it only incorporates fast feedbacks. With the growth of ice sheets temperatures may fall further, flipping the Earth into a true snow ball state. Lacis et al also mention the simple ocean they use in the model as also potentially introducing a false stability to temperature.

    Regardless, I think we are in safe territory if we conclude that the removal of all well mixed GHG would reduce the Global Mean Surface Temperature by at least 30 degrees C, and probably significantly more if (as you say, magically) the Earth's atmosphere could be kept free of well mixed greenhouse gases.
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  36. Morning,
    OK, I think I also got a bit mixed up with CO2 and all the other GHG's in this thought experiment and after reading the quotes from the papers again and your comments, I am a bit clearer I think

    So would it be correct to say then, that in a situation without any GHG's, TS = TE, which would actually mean, the complete atmosphere is not active in radiating to space, only the surface.

    Wouldn't this be effectively the representation of a planet system without an atmosphere?
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  37. davpaf wrote: "Wouldn't this be effectively the representation of a planet system without an atmosphere?"

    If the atmosphere were composed entirely of gases which are 'transparent' to outgoing radiation (i.e. if all 'greenhouse' gases were removed) then it wouldn't matter... an atmosphere 100% transparent to all outgoing radiation is the same as no atmosphere at all for this purpose.

    That said, there are a lot of things in the atmosphere (e.g. dust, ozone) which block incoming radiation. Thus, if Earth had no atmosphere at all the incoming radiation and TS would be higher... at that, if we removed all GHG that'd mean no clouds, a correspondingly lower albedo, and thus higher TS (with or without an atmosphere).

    Anyway, the takeaway message is that GHG are the only reason TE does not equal TS - irrespective of any changes in the value of TS due to linked factors.
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  38. Ok, so by equating TS and TE, one assumes TS is based on an existing atmosphere (affecting the amount of incoming solar radiation, as stated by clouds, dust, ozone) and TE is based only on the earth surface radiating into space, not the atmosphere.

    Shouldn’t strictly speaking all GHG effects be also removed from the TS part, I mean in this way it seems a bit inconsistent?

    One more question, doesn’t any material with a temperature above absolute zero, radiate heat? So if say one puts a parcel of air into space, it will eventually cool down.

    I understand that GHG, have specific bands of emission/absorption characteristics for specific radiation wavelengths, which oxygen/nitrogen don’t have. But both must be radiating if they have a temperature, right?
    Thanks.
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  39. davpaf
    "doesn’t any material with a temperature above absolute zero, radiate heat?"
    A black body radiates following the Stefan-Bolzmann law. You need to multiply it by ε, the emissivity, for any real substance. If ε is zero, or in the frequency range where it is zero, so will be the radiation flux. Then the answer to your question is "more or less", depending on ε.
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  40. davpat,

    KR answered your questions in #27, and Tom Curtis answered them again in #36. Tom also has a great post at this site on the greenhouse effect. I think you should take the discussion further at that post, if you are still confused.

    I found this review article an excellent introduction to the greenhouse effect.

    http://geosci.uchicago.edu/~rtp1/papers/PhysTodayRT2011.pdf
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