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All IPCC definitions taken from Climate Change 2007: The Physical Science Basis. Working Group I Contribution to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change, Annex I, Glossary, pp. 941-954. Cambridge University Press.

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It's a natural cycle

What the science says...

A natural cycle requires a forcing, and no known forcing exists that fits the fingerprints of observed warming - except anthropogenic greenhouse gases.

Climate Myth...

It's a natural cycle
"Global warming (i.e, the warming since 1977) is over. The minute increase of anthropogenic CO2 in the atmosphere (0.008%) was not the cause of the warming—it was a continuation of natural cycles that occurred over the past 500 years." (Don Easterbrook)

"What if global warming is just a natural cycle?" This argument is, perhaps, one of the most common raised by the average person, rather than someone who makes a career out of denying climate change. Cyclical variations in climate are well-known to the public; we all studied the ice ages in school. However, climate isn't inherently cyclical.

A common misunderstanding of the climate system characterizes it like a pendulum. The planet will warm up to "cancel out" a previous period of cooling, spurred by some internal equilibrium. This view of the climate is incorrect. Internal variability will move energy between the ocean and the atmosphere, causing short-term warming and cooling of the surface in events such as El Nino and La Nina, and longer-term changes when similar cycles operate on decadal scales. However, internal forces do not cause climate change. Appreciable changes in climate are the result of changes in the energy balance of the Earth, which requires "external" forcings, such as changes in solar output, albedo, and atmospheric greenhouse gases. These forcings can be cyclical, as they are in the ice ages, but they can come in different shapes entirely.

For this reason, "it's just a natural cycle" is a bit of a cop-out argument. The Earth doesn't warm up because it feels like it. It warms up because something forces it to. Scientists keep track of natural forcings, but the observed warming of the planet over the second half of the 20th century can only be explained by adding in anthropogenic radiative forcings, namely increases in greenhouse gases such as carbon dioxide. 

Of course, it's always possible that some natural cycle exists, unknown to scientists and their instruments, that is currently causing the planet to warm. There's always a chance that we could be totally wrong. This omnipresent fact of science is called irreducible uncertainty, because it can never be entirely eliminated. However, it's very unlikely that such a cycle exists.

Firstly, the hypothetical natural cycle would have to explain the observed "fingerprints" of greenhouse gas-induced warming. Even if, for the sake of argument, we were to discount the direct measurements showing an increased greenhouse effect, other lines of evidence point to anthropogenic causes. For example, the troposphere (the lowest part of the atmosphere) is warming, but the levels above, from the stratosphere up, are cooling, as less radiation is escaping out to space. This rules out cycles related to the Sun, as solar influences would warm the entire atmosphere in a uniform fashion. The only explanation that makes sense is greenhouse gases.

What about an internal cycle, perhaps from volcanoes or the ocean, that releases massive amounts of greenhouse gases? This wouldn't make sense either, not only because scientists keep track of volcanic and oceanic emissions of CO2 and know that they are small compared to anthropogenic emissions, but also because CO2 from fossil fuels has its own fingerprints. Its isotopic signature is depleted in the carbon-13 isotope, which explains why the atmospheric ratio of carbon-12 to carbon-13 has been going up as anthropogenic carbon dioxide goes up. Additionally, atmospheric oxygen (O2) is decreasing at the same rate that CO2 is increasing, because oxygen is consumed when fossil fuels combust.

A natural cycle that fits all these fingerprints is nearly unfathomable. However, that's not all the cycle would have to explain. It would also have to tell us why anthropogenic greenhouse gases are not having an effect. Either a century of basic physics and chemistry studying the radiative properties of greenhouse gases would have to be proven wrong, or the natural cycle would have to be unbelievably complex to prevent such dramatic anthropogenic emissions from warming the planet.

It is indeed possible that multidecadal climate variabilityespecially cycles originating in the Atlantic, could be contributing to recent warming, particularly in the Arctic. However, the amplitude of the cycles simply can't explain the observed temperature change. Internal variability has always been superimposed on top of global surface temperature trends, but the magnitude - as well as the fingerprints - of current warming clearly indicates that anthropogenic greenhouse gases are the dominant factor.

Despite all these lines of evidence, many known climatic cycles are often trumpeted to be the real cause, on the Internet and in the media. Many of these cycles have been debunked on Skeptical Science, and all of them either aren't in the warming phases, don't fit the fingerprints, or both.

For example, we are warming far too fast to be coming out of the last ice age, and the Milankovitch cycles that drive glaciation show that we should be, in fact, very slowly going into a new ice age (but anthropogenic warming is virtually certain to offset that influence).

The "1500-year cycle" that S. Fred Singer attributes warming to is, in fact, a change in distribution of thermal energy between the poles, not a net increase in global temperature, which is what we observe now.

The Little Ice Age following the Medieval Warm Period ended due to a slight increase in solar output (changes in both thermohaline circulation and volcanic activity also contributed), but that increase has since reversed, and global temperature and solar activity are now going in opposite directions. This also explains why the 11-year solar cycle could not be causing global warming.

ENSO (El Nino Southern Oscillation) and PDO (Pacific Decadal Oscillation) help to explain short-term variations, but have no long-term trend, warming or otherwise. Additionally, these cycles simply move thermal energy between the ocean and the atmosphere, and do not change the energy balance of the Earth.

As we can see, "it's just a natural cycle" isn't just a cop-out argument - it's something that scientists have considered, studied, and ruled out long before you and I even knew what global warming was.

Last updated on 11 January 2011 by climatesight.

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Comments 1 to 10:

  1. From what I hear from people in everyday life and what I see on conservative news outlets and skeptical blogs, "it's a natural cycle should be near the top of the list of skeptic arguments.
  2. Reply to comment from here.

    "Natural climate variability has always existed and it didnt suddenly just disappear."

    No one said that 'natural variability' has disappeared. However, there is excellent evidence that this variability is insufficient to account for the current warming. The graph below shows we've already exceeded the limits of natural variability. There is therefore ample rationale to accept that man-made global warming will far exceed anything natural.

    --- from Chapman and Davis 2010 via ourchangingclimate

    Chapman and Davis explain: This collection of curves suggests that the Northern Hemisphere was relatively warm around 1000 C.E. (but not as warm as current temperature), that the period 1500–1850 was relatively cool, and that there has been considerable warming since 1900. ... All of the emissions scenarios considered by the IPCC yield global warming in the 21st century that dwarfs warming seen in the past millennium.

    [DB] Upgraded image quality.

  3. Another illustration of why its not 'natural cycles.'

    -- Figure 4 from Dr. Thomas Karl's testimony to US Congress, July 2006
    (originally IPCC, 2001)

    As an example of how models are used to detect human influence on the climate system Figure 4 shows that without including all the observed forcing mechanisms the models cannot replicate the observed global temperature changes.
  4. Playing the pile-it-on game, here's a composited version of muoncounter's graphic:

    Courtesy John P. Reisman (his site is an excellent resource, BTW)

    The Yooper
  5. #4: Nicely done. His myths page is quite nice: even a slot for der Beckster. Surely Bill O's who-put- the-moon-there deserves one.
  6. Certainly forcing produce multiple reactions. 2 reactions that I have not seen integrated into the CO2 mix, are the absorption of CO2 due to increased acidification, which seems to be accelerating. Coral and shells dissolve more rapidly as acidification increases, which allows for increased CO2 absorption. Also CO2 release into the atmosphere, from the ocean, decreases as the thermohaline circulation slows due to continued warming. There have been precious few, if any studies done on these CO2 reducing reactions, and incorporated into the CO2/AGW puzzle. This is one of the few references I have found to either.
    Harvard Magazine - Nov 2002 - The Ocean Carbon Cycle
    Response: See the post "PMEL Carbon Program."
  7. is climate change a natural cycle. i don't want to know that it is or isn't. i want to see some evidence plaease.

    [DB] Try here.

  8. Kat77#7:

    Here is some evidence.

    And more evidence.

    Even more evidence.

    And for balance, a discussion of your natural cycles.

    Now weigh the evidence.
  9. Cross-post from this thread with my inquiry of Eric (skeptic) where it is on topic.
  10. Thanks Composer99 for pointing to this thread. There are many natural cycles that get discussed on the fringes of science, everything from pure Fourier analysis results (temperature effects with no discussion of cause) to complex orbital or lunar mechanics (causes that generally lack a plausible magnitude of effect). I don't find those very convincing. Instead I would like to put forth a combination of solar, oceanic and AGW (with or without AGC). SkS analyzes each non-anthro cause in separate articles, such as PDO and It's (not) the sun

    Those arguments are a divide and conquer approach to GW, including some oversimplification such as using TSI as the primary solar factor. In contrast, I believe GW results from a combination of some natural cycles and AGW. Here is an example of oceanic cycles from showing the NH temperature response to PDO and AMO:

    Second, an example of a weather response to solar: This is important because the weather controls some climate in general and sensitivity in particular. In this paper, the increase in solar UV is shown to decrease CAPE leading to weaker cyclones. The higher solar cycles of the 80's and 90's (including higher minimums) caused an overall increase in solar UV which caused lower CAPE, weakened tropical cyclones and thus less heat loss to space. The latter is an old and well-established fact (e.g. that increased tropical convection, regardless of cause, will result in more latent heat transfer and cooling. I would then have to show that there was a worldwide decrease in convection, particularly tropical, coinciding with the greatest warming of the 80's and 90's. I do not yet have a good data source for that claim.

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