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Explaining climate change science & rebutting global warming misinformation

Scientific skepticism is healthy. Scientists should always challenge themselves to improve their understanding. Yet this isn't what happens with climate change denial. Skeptics vigorously criticise any evidence that supports man-made global warming and yet embrace any argument, op-ed, blog or study that purports to refute global warming. This website gets skeptical about global warming skepticism. Do their arguments have any scientific basis? What does the peer reviewed scientific literature say?


What happens if we overshoot the two degree target for limiting global warming?

Posted on 18 December 2014 by Guest Author

This is a re-post from Carbon Brief by Roz Pidcock

Two degrees is the internationally-agreed target for limiting global warming, and has a long history in climate policy circles. Ambition that we can still achieve it is running high as climate negotiators gather in Lima to lay the groundwork for a potential global deal in 2015.

But against this optimistic backdrop, greenhouse gas emissions have continued to rise. With each passing year the scale of the task looms ever larger.  There are very real questions about whether or not the world will be able to stay below the two degree limit.

So what happens if we fail to meet the two degree target? What would it mean to resign ourselves to a post-two degree world? And if not two degrees, then what?

As temperatures rise, so do the risks

Two degrees above pre-industrial temperature has been agreed by countries as an appropriate threshold beyond which climate change risks become unacceptably high.

Global temperature has risen 0.85 degrees Celsius since 1880, according to the latest report from the Intergovernmental Panel on Climate Change (IPCC).

We could be due another couple of tenths on top of that as past emissions take a decade or so to reach their full effect warming. Together with current and expected emissions we're essentially already committed to about one degree of warming, scientists estimate.

AR5_SPMTemp Since 1850

Observed global mean temperature from 1850 to 2012, relative to the 1961-1990 average. Coloured lines represent three different datasets. Top panel shows yearly averages, bottom shows decadal averages. Source: IPCC 5th Assessment Report



2014 SkS Weekly News Roundup #51A

Posted on 17 December 2014 by John Hartz

3.6 degrees of uncertainty

After two weeks of grinding meetings in Lima, Peru, the world’s climate negotiators emerged this weekend with a deal. They settled on preliminary language, to be finalized a year from now in Paris, meant to help keep the long-term warming of the planet below 3.6 degrees Fahrenheit.

That upper boundary was first settled on four years ago at another round of talks in Cancun, Mexico. On the centigrade scale, it equals two degrees above the global average temperature at the beginning of the Industrial Revolution — the “2C target.”

But where did that target come from in the first place? And even if we manage to stay below it, will it really protect the planet from serious harm?

3.6 Degrees of Uncertainty by Justin Gillis, New York Times, Dec 15, 2014



2014 will be the hottest year on record

Posted on 17 December 2014 by John Abraham

For those of us fixated on whether 2014 will be the hottest year on record, the results are in. At least, we know enough that we can make the call. According the global data from NOAA, 2014 will be the hottest year ever recorded.

I can make this pronouncement even before the end of the year because each month, I collect daily global average temperatures. So far, December is running about 0.5°C above the average. The climate and weather models predict that the next week will be about 0.75°C above average. This means, December will come in around 0.6°C above average. Are these daily values accurate? Well the last two months they have been within 0.05°C of the final official results.

What does this all mean? Well, when I combine December with the year-to-date as officially reported, I predict the annual temperature anomaly will be 0.674°C. This beats the prior record by 0.024°C. That is a big margin in terms of global temperatures.

For those of us who are not fixated on whether any individual year is a record but are more concerned with trends, this year is still important. Particularly because according to those who deny the basic physics and our understanding of climate change, this year wasn’t supposed to be particularly warm.

For those who thought that climate change was “natural” and driven by ocean currents, this has been a tough year. For instance, using NOAA standards, this year didn’t even have an El Niño. NOAA defines an El Niño as 5 continuous/overlapping 3-month time periods wherein a particular region in the Pacific has temperatures elevated more than 0.5oC.

Interestingly, we are currently close to an El Niño, and if current patterns continue for a few weeks, an official El Niño will be announced. But it hasn’t been yet, and if we do get an El Niño, it will affect next year more than this year. How could the hottest year have occurred then, when the cards are not stacked in its favor? The obvious and correct answer is, because of continued emission of greenhouse gases.



Two degrees: Will we avoid dangerous climate change?

Posted on 16 December 2014 by Guest Author

This is a re-post from Carbon Brief by Simon Evans

Limiting warming to no more than two degrees has become the internationally accepted target for climate policy, as we saw in  the first blog of our series of pieces looking at the two degrees limit.

Scientists think the risks of climate change increase as temperatures rise. Two degrees isn't a 'safe' level of climate change, but nor is it a red line with only chaos beyond, as we'll see in part three.

It is a readily understood and useful marker of how we're doing at limiting dangerous climate change that has helped focus minds on the scale of the challenge. It's also what the world's governments have committed to achieving.

So will we manage to limit warming to two degrees above pre-industrial? We take a look at what it will take to stay below two degrees, how things are going so far, how experts say we should proceed and what we'd need to do if we wanted to follow their advice.

1. What we would need to do to stay below two degrees

The world has already warmed by 0.85 degrees celsius above the pre-industrial average and if emissions stay high we're on course for more like three to five degrees by 2100, according to the latest Intergovernmental Panel on Climate Change (IPCC) reports.

Broadly speaking, however, scientists say it is still theoretically possible to limit warming to two degrees as long as we stick within a fixed carbon budget. This is the total amount we can emit from the beginning of the industrial revolution until the day we stop adding carbon to the atmosphere.

So how big is the budget? It is likely that we'll stay below two degrees as long as we emit no more than about 2,900 billion tonnes of carbon dioxide, the IPCC says. 'Likely' here means a 66 per cent chance.



2014 SkS News Bulletin #6: LIMA COP20 / CMP10

Posted on 15 December 2014 by John Hartz

This News Bulletin is a compilation of articles about the just concluded meeting of the 20th session of the Conference of the Parties to the United Nations Framework Convention on Climate Change and of and the 10th session of the Conference of Parties to the Kyoto Protocol. The meeting was held in Lima, Peru begining on Dec 1 and ending on Dec 14 with the adoption of a report dubbed the Lima Call for Climate Action.

A climate accord based on global peer pressure

Shortly before 2 a.m. on Sunday, after more than 36 straight hours of negotiations, top officials from nearly 200 nations agreed to the first deal committing every country in the world to reducing the fossil fuel emissions that cause global warming.

In its structure, the deal represents a breakthrough in the two-decade effort to forge a significant global pact to fight climate change. The Lima Accord, as it is known, is the first time that all nations — rich and poor — have agreed to cut back on the burning oil, gas and coal.

But the driving force behind the new deal was not the threat of sanctions or other legal consequences. It was global peer pressure. And over the coming months, it will start to become evident whether the scrutiny of the rest of the world is enough to pressure world leaders to push through new global warming laws from New Delhi to Moscow or if, as a political force, international reproach is impotent.

A climate accord based on global peer pressure by Coral Davenport, New York Times, Dec 14, 2014



Two degrees: The history of climate change’s ‘speed limit’

Posted on 15 December 2014 by Guest Author

This is a re-post from Carbon Brief by Mat Hope & Rosamund Pearce

Limiting warming to no more than two degrees has become the de facto target for global climate policy. But there are serious questions about whether policymakers can keep temperature rise below the limit, and what happens if they don't.

As climate negotiators meet in Lima to discuss a new global climate deal that could limit warming to two degrees or less, we look at each of the issues in turn.

Here, we take a look at where the two degree target came from, and how it has ended up guiding international climate policy.


Woven into the fabric of climate policy

Perhaps surprisingly, the idea that temperature could be used to guide society's response to climate change was first proposed by an economist.

In the 1970s, Yale professor William Nordhaus alluded to the danger of passing a threshold of two degrees in a pair of now famous papers, suggesting that warming of more than two degrees would push the climate beyond the limits humans were familiar with:

"According to most sources the range of variation between between distinct climatic regimes is on the order of ±5°C, and at present time the global climate is at the high end of this range. If there were global temperatures more than 2° of 3° above the current average temperature, this would take the climate outside of the range of observations which have been made over the last several hundred thousand years."

Screenshot 2014-12-03 10.59.46.png
Global mean temperature change projection from Nordhaus' 1977 paper.



2014 SkS Weekly Digest #50

Posted on 14 December 2014 by John Hartz

SkS Highlights

Tamino's Guest blog post, Is Earth’s temperature about to soar?, drew the most comments of the articles posted on SkS during the past week. California just had its worst drought in over 1200 years, as temperatures and risks rise by Dana attracted the second highest number. 

El Niño Watch

Japan’s weather bureau said on Wednesday that an El Niño weather pattern, which can trigger drought in some parts of the world while causing flooding in others, had emerged during the summer for the first time in five years and was likely to continue into winter.

That marks the first declaration by a major meteorological bureau of the much-feared El Niño phenomenon, which had been widely expected to emerge this year.

First El Niño in five years declared by Japan's weather bureau, Reuters/The Guardian, Dec 10, 2104  

Toon of the Week

 2014 Toon 50

h/t to I Heart Climate Scientists



2014 SkS Weekly News Roundup #50B

Posted on 13 December 2014 by John Hartz

Building climate equity and international consensus from the ground up

Climate change poses a number of substantial equity challenges. The greatest threat from climate change is often faced by those who are the least responsible for creating the problem, frequently those already made vulnerable by poverty. Meanwhile, there are significant questions about who should combat climate change and how it should be done, from reducing emissions to making people more resilient to the impact of an altered climate.

Since the inception of the UN Framework Convention on Climate Change (UNFCCC) more than two decades ago, issues of equity have been a major sticking point in negotiations. These concerns about equity must be addressed in order to build the necessary global consensus for a strong, ambitious and durable international climate agreement in 2015.

But what is an equitable way of taking action in the context of growing emissions and climate impacts, from water scarcity and depressed agricultural yields to severe weather events? And how can we reduce emissions and build climate resilience while taking into account varying human development needs? 

Building Climate Equity and International Consensus from the Ground Up by David Waskow and Eliza Northrop, World Resources Institute, Dec 9, 2014



Skeptical Science at the 2014 AGU Fall Meeting

Posted on 12 December 2014 by dana1981

A number of Skeptical Science contributors will be presenting at the 2014 AGU Fall Meeting.  If you'll be there, come check us out.

Time Presenter Type Location


Sunday, 14 December8:00 a.m. – 12:30 p.m. John Cook Workshop InterContinental Hotel, 888 Howard St, San Francisco

Communicating Climate Science Workshop

Deadline to apply is Fri., Oct. 31

Tuesday, 16 December 2014; 8:00 AM  Andy Skuce  Poster   

Abstract ID and Title: 7106: Emissions of Water and Carbon Dioxide from Fossil-Fuel Combustion Contribute Directly to Ocean Mass and Volume Increases

Final Paper Number: G21B-0444

Session Number and Title: G21B: Variability and Projection of Regional and Global Mean Sea Level Change I Posters

 Tuesday, 16 December 2014; 2:28 PM - 2:40 PM Dana Nuccitelli  Oral  Marriott Marquis; Salon 13-15

Abstract ID: 2476

Abstract Title: How to maximize science communication efficacy by combining old and new media

Final Paper Number: PA23B-04

Session Number and Title: PA23B: We Need to Talk: Learning from Climate Experts Who Have Succeeded in the Media

Wednesday, 17 December 2014; 8:00 AM - 10:00 AM  Peter Jacobs Oral  Moscone South; 102 

Abstract ID: 11019

Abstract Title: It Ain't (Just) the Heat, It's the Humanity: Increasing Public Understanding of Scientific Consensus and Its Role in Climate Literacy

Final Paper Number: ED31H-07

Presentation Length9:30 AM - 9:45 AM

Session Number and Title: ED31H: Climate Literacy: Culture of Science AND Broader Impacts Done Well I 

Wednesday, 17 December 2014: 10:20 AM - 12:20 PM John Cook Oral MS, 102

MS, 102
Applying Agnotology-Based Learning in a Mooc to Counter Climate Misconceptions

Wednesday, 17 December 2014; 1:40 PM John Mashey Poster  Moscone South; Poster Hall

Abstract ID and Title: 2428: The Machinery Of Climate Anti-Science, Its Efforts Against Education, Top To Bottom

Final Paper Number: ED33B-3513

Session Number and Title: ED33B: Climate Literacy: Overcoming Barriers—Research Outcomes and Best Practices for Supporting Education and Informed Decision Making II Posters

Wednesday, 17 December 2014; 5:30 PM - 5:45 PM Julian Brimelow Oral Moscone West; 3010 Abstract Title: Hydroclimatological Aspects of the Extreme 2011 Assiniboine River Basin Flood

Final Paper Number: H34C-07

Session Number and Title: H34C: Global Floods: Forecasting, Monitoring, Risk Assessment, and Socioeconomic Response II
Thursday, 18 December 2014: 01:40 PM - 03:40 PM John Cook Oral MW, 3004

Scientists Are from Mars, Laypeople Are from Venus: An Evidence-Based Approach to Consensus Messaging

Thursday, 18 December 2014; 3:00 PM - 3:20 PM Kevin Cowtan Oral   Moscone West; 3003

Abstract ID: 3409

Abstract Title: Biases in the instrumental temperature record: the policy and communications context

Final Paper Number: GC43G-05

Session Number and Title: GC43G: Quantifying Uncertainty in Climate, Earth System, Integrated Assessment, and Impact Models and Observations II



Global warming continues despite continuous denial

Posted on 11 December 2014 by John Abraham

Human emissions of greenhouse gases cause the Earth to warm. We’ve known that for decades – actually for over 100 years. But how do we measure warming? How fast is the planet heating? Turns out, this is conceptually easy to answer, even though it is difficult to implement the required measurements.

In order to measure how fast the planet is heating, we can measure the difference in incoming and outgoing energy at the top of the atmosphere (just like keeping track of a bank account by comparing deposits to withdrawals). Another way to measure a warming planet is to simply to measure how much energy is stored within the planet’s climate (like watching the balance of a bank account). Both methods should give the same answer. If you have more deposits than withdrawals to a bank account, you will see your balance increase.

Fortunately both of these methods, when used by climate scientists, tell the same story. The Earth is out of balance; we are gaining 0.5 – 1 Watts per square meter of area.

Okay, so what are the impacts of this heat? What should we experience? The answers to these questions were addressed in a recent paper (Abraham, Fasullo and Laden) published by the National Center for Science Education. This paper, which is geared toward an informed but not expert audience, focuses on the current energy imbalance, the recent trends in atmospheric temperatures, and the issue of the so-called “pause” of global warming. As a spoiler, we show that the “pause” is not really a thing.

First, there has been no pause or even slowdown in the warming of the planet. We provide updated information from NOAA which clearly shows a continued heating of the world’s oceans – the reservoir where most heat ends up. We compare energy contained in different layers and discuss the transfer of energy from the surface regions to the lower regions. When you look at the oceans, the so-called pause is simply a redistribution, a burying of heat to deeper waters.

Ocean heating data from the National Oceanic and Atmospheric Administration. Ocean heating data from the National Oceanic and Atmospheric Administration.



Is Earth’s temperature about to soar?

Posted on 10 December 2014 by Guest Author

This is a re-post from tamino's Open Mind, showing that there's no statistical evidence the short-term slowing of global surface warming is a change in the long-term warming trend

A recent blog post on RealClimate by Stefan Rahmstorf shows that when it comes to recent claims of a “pause” or “hiatus,” or even a slowdown in global surface temperature, there just isn’t any reliable evidence to back up those claims.




2014 SkS Weekly News Roundup #50A

Posted on 9 December 2014 by John Hartz

2014 surprisingly rough on coral reefs, and El Niño looms in 2015

An outbreak of coral bleaching—the loss of corals’ food-producing algae—in the Pacific and the Caribbean occurred this past summer, most likely tied to a brewing El Niño. The reefs of the Florida Keys observed their worst bleaching impacts since 1997-1999, when a major El Niño was quickly followed by a major La Niña. The surprising intensity of bleaching across multiple ocean basins in 2014 has scientists wondering what to expect in 2015, when El Niño is forecasted to finally develop.

2014 surprisingly rough on coral reefs, and El Niño looms in 2015, NOAA, Dec 5, 2014



How the world's economic growth is actually un-economic

Posted on 9 December 2014 by Guest Author

By Robert Costanza, Australian National University

The focus of the recently concluded G20 summit was economic growth. The final communiqué begins:

“Raising global growth to deliver better living standards and quality jobs for people across the world is our highest priority."

The word “growth” is mentioned 29 times in the three-page document.

Climate is mentioned only in article 19, out of 21. While the parties pledge to “support strong and effective action to address climate change”, this is clarified to mean support for “economic growth, and certainty for business and investment”.



California just had its worst drought in over 1200 years, as temperatures and risks rise

Posted on 8 December 2014 by dana1981

A new paper published in the journal Geophysical Research Letters by Griffin & Anchukaitis concludes that the 2012–2014 drought in California was its most intense in at least 1,200 years.

The study used drought reconstructions from tree-ring cores, from the North American Drought Atlas (NADA) and from cores Griffin & Anchukaitis collected from blue oak trees in southern and central California. Blue oak tree ring widths are particularly sensitive to moisture changes. According to Griffin,

California’s old blue oaks are as close to nature’s rain gauges as we get

Pencil-like tree-ring cores are collected non-destructively using a Swedish increment borer. May 2014, image by Daniel Griffin. Pencil-like tree-ring cores are collected non-destructively using a Swedish increment borer. May 2014, image by Daniel Griffin.

The study compared today’s drought conditions in California to those reconstructed over the past 1,200 years using the Palmer Drought Severity Index (PDSI), an estimate of available soil moisture. The data showed that California is experiencing its most intense drought in over a millennium,



2014 SkS Weekly Digest #49

Posted on 7 December 2014 by John Hartz

SkS Highlights

Dana's Volcanoes may be responsible for most of the global surface warming slowdown attracted the highest number of comments of the articles posted on SkS during the past week. Tied for second place were Drought and Deforestation in Brazil by Alexandre Lacerda and Even climate change experts and activists might be in denial by Steffen Böhm and Aanka Batta.

El Niño Watch

The first Thursday of every month is when we do the CPC/IRI ENSO status update, when NOAA officially answers the question “Are we there yet?” This month, the answer is...close, but no cigar.

Sea surface temperatures (SSTs) in the Niño3.4 region of the Pacific were quite warm during November, with the most recent weekly Niño3.4 index value at +1.0°C above average. The threshold for El Niño conditions is +0.5° above average for one month, and most of the climate models are forecasting that SSTs will stay above average for at least a few more months. Then why haven’t we changed from an “El Niño Watch” (favorable for development of El Niño conditions) to an “El Niño Advisory” (El Niño conditions are present)?

December's ENSO Update: Close, but no cigar. by Emily Becker, NOAA, Dec 4. 2014 

Toon of the Week

 2014 Toon 49

h/t to I Heart Climate Scientists



2014 SkS Weekly News Roundup #49B

Posted on 6 December 2014 by John Hartz

Canada's climate inaction leaves it 'increasingly isolated'

Canada is looking "increasingly isolated" as former climate policy laggards such as the U.S. and China take action to tackle climate change, policy experts say.

Earlier this month, the U.S. and China announced an agreement tosignificantly cut and cap their greenhouse gas emissions.

Not only does that give a big boost to the global momentum to tackle climate change, but it cranks up the international pressure on Canada ahead of the annual United Nations Climate Change Conference (COP 20), which opens today in Lima, Peru.

"They make [Prime Minister Stephen] Harper look increasingly isolated," said Simon Dalby, CIGI chair in the political economy of climate change at the Balsillie School of International Affairs in Waterloo, Ont.

Canada's climate inaction leaves it 'increasingly isolated' ahead of COP 20 by Emily Chung, CBC News, Dec 1, 2014



Even climate change experts and activists might be in denial

Posted on 6 December 2014 by Guest Author

By Steffen Böhm, University of Essex and Aanka Batta, University of Essex

Another month, another important UN climate change conference. The latest is in Lima, the capital of Peru. Thousands of experts from the world of politics, business, academia and civil society – and Leonardo DiCaprio – have flown around the world to urge us all to curb our carbon emissions.

Recent meetings have failed to make significant progress. Yet, this year there are high hopes that the US-China climate deal and the New York UN Climate Summit will allow Lima to provide a stepping stone for a binding emissions agreement at next year’s meeting in Paris.

However, even if a deal can be reached – despite the urgent need for it – there is no guarantee that global greenhouse gas emissions will actually come down significantly and dangerous climate change can be averted. Psychoanalytic theory provides disturbing insights into why this may be so – and it is all to do with the split psychological make-up of those who work at the forefront of climate science, policy and activism.

Climate denial can be unconscious

For at least a century, psychoanalysis has taught us that we might be consciously thinking and saying one thing, but unconsciously doing another. In this context that means people are very consciously aware of the threats posed by climate change, even if they aren’t doing too much about it.

Not a week goes by without the media showing catastrophic images of environmental damage and social suffering seemingly caused by a changing climate. Research suggests that such threats lead us to adopt various unconscious coping and defence mechanisms.



Cutting carbon pollution is the key to curbing global warming

Posted on 5 December 2014 by John Abraham

Not all greenhouse gases are created equal. If we want to limit the temperature rise of the Earth, we really need to focus on the long-lasting greenhouse gases. They’re the ones that matter, according to a new study published in the Proceedings of the National Academy of Sciences by Joeri Rogelj and his colleagues.

Most of us already know this, but not all greenhouse gases are created equal. There are some greenhouse gases that, when emitted, only stay in the atmosphere for a short time. There are other greenhouse gases (like carbon dioxide) that stay aloft for decades to centuries. Finally, there are some that stay airborne for an intermediate duration. It has often been stated that we can “buy time” by focusing on short-lived greenhouse gases. Reducing things like black carbon or methane can give us some extra years to get our act together on carbon dioxide.

But this suggestion is challenged in the PNAS paper. There are two major issues that suggest we really need to focus on the long-lived gases. First, since short-lived gases only stay airborne for a brief period, any emissions that we make now will not impact the temperatures we can expect say in 2100. Reducing our emissions of short lived gases will affect the rate of temperature increase in the next few years, but will have very little impact on the maximum temperature that will be obtained. In fact, the present study is clear in stating,

Maximum temperature increases (peak warming) is to first order determined by the cumulative emissions of the long-lived greenhouse gases until the peak and by the annual emissions of the short-lived greenhouse gases at the time of the peak.

This doesn’t mean that reducing short-lived greenhouse gases isn’t important. In fact, the short-lived gases become more important if we have already reduced carbon dioxide. As stated in the paper,

Methane mitigation measures in the latter half of the century become important if carbon dioxide emissions have already been curbed, and warming thus peaks before 2100. Early action on methane is less important for limiting warming to below 2C.

Part of the paper’s conclusions are based on simple calculations with a climate model. But another part of the study is based on the interdependence of these greenhouse gases. The gases are interdependent because they may be cogenerated – they may be emitted from the same source.



Drought and Deforestation in Brazil

Posted on 4 December 2014 by Guest Author

This is a guest post by Alexandre Lacerda who lives in Serra Negra, a small town about 150 Km north of São Paulo city, Brazil, where he works as an attorney and entrepreneur. He has contributed many of the Portuguese translations of Skeptical Science arguments and this is his first blog post.

I live in São Paulo, the richest state in Brazil, and also the one with the largest population. I live in a small town up on the hills about 150 Km north of São Paulo city, a large city with some 20 million inhabitants if you include all its surrounding metropolitan area.

The climate here is quite pleasant: dry, mild winters in the middle of the year (remember, this is the southern hemisphere) when we get a few frosts and hot summers cooled by frequent torrential rain in December-January. Well, at least that’s how it used to be. Frosts became a rare event, when they happen at all. And summer rains failed last year, which is what this article is about.

In our small town, public water reservoirs usually get quite low during winter, and get filled up again every summer with reliable rainfall. That’s something we learned to count on almost as we count on the sun rising tomorrow. The low reservoirs were something we looked at as something normal for the season, just as you don’t usually panic when your fridge is empty: after all, it will be replenished when needed.

Then, after a pretty unremarkable December last year, with a few good showers, January came and it was sunny. Day after day, we enjoyed the summer vacation with the feeling that the fun sunny days would any time be over. But they lingered on all month, then the next month, then the month after that. It slowly dawned on us that the rainy season would be over, but the necessary rain would not come.

Reservoir Brazil Normal

Reservoir Brazil Drought

 Local reservoir in normal condition (on top) and at the present drought (above). (personal archive)



Volcanoes may be responsible for most of the global surface warming slowdown

Posted on 3 December 2014 by dana1981

A new study has found that when particulates from small volcanic eruptions are properly accounted for, volcanoes may be responsible for much of the slowdown in global surface warming over the past 15 years.

Sulfur aerosol particulates pumped into the atmosphere from volcanic eruptions cause short-term cooling by blocking sunlight. Until recently, climate scientists thought that only large volcanic eruptions had a significant impact on global temperatures. There haven’t been any big eruptions since Mount Pinatubo in 1991. However, studies published over the past few years have found that even moderate volcanic eruptions can pump significant amounts of aerosol particulates into the atmosphere.

Virtually all research into the climate influence of volcanic aerosols has used satellite measurements of particulates in the upper atmosphere (the stratosphere). These satellite measurements only monitor the volcanic aerosol at heights of 15 km and above. The new paper by David Ridley and colleagues studied the amount of volcanic aerosols in portions of the stratosphere that lie below 15 km.

To do this, the researchers combined data from satellites, ground-based instruments in the AERONET program, and from instruments on weather balloons. The study was co-authored by 17 climate scientists, including some leading experts in aerosol research.

By combining all of these measurements, the scientists found that there is also a significant amount of volcanic aerosol in portions of the stratosphere below 15 km They concluded that for recent eruptions, between 30 and 70% of the overall amount of volcanic aerosol in the stratosphere has come from below 15 km. Since the year 2000, the study estimates that volcanoes have had a cooling influence on global surface temperatures. The likely range of this volcanic cooling influence lies between 0.05 and 0.12°C.

As the authors of the paper note, this cooling influence is not taken into account in the climate model simulations incorporated into the latest IPCC report,

The climate model simulations evaluated in the IPCC fifth assessment report [Stocker et al., 2013] generally assumed zero stratospheric aerosol after about 2000, and hence neglect any cooling effect of recent volcanoes

Although the global surface temperature data have been within the range of model simulations, they’ve been towards the lower end of those model runs over the past 10–15 years.

IPCC AR5 Figure 1.4. Solid lines and squares represent measured average global surface temperature changes by NASA (blue), NOAA (yellow), and the UK Hadley Centre (green). The colored shading shows the projected range of surface warming in the IPCC First Assessment Report (FAR; yellow), Second (SAR; green), Third (TAR; blue), and Fourth (AR4; red).

IPCC AR5 Figure 1.4. Solid lines and squares represent measured average global surface temperature changes by NASA (blue), NOAA (yellow), and the UK Hadley Centre (green). The colored shading shows the projected range of surface warming in the IPCC First Assessment Report (FAR; yellow), Second (SAR; green), Third (TAR; blue), and Fourth (AR4; red).



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