Now it seems that the ‘global dimming’ debate has been given an injection of that ‘uncertainty’ with which we are so familiar from the wider climate change discussion as presented by the mainstream media.
“The amount of sunlight reaching the Earth's surface is increasing, two new studies in Science magazine suggest. Using different methods, they find that solar radiation at the surface has risen for at least the last decade. Previous work had found the opposite trend, leading to a popular theory known as "global dimming". “<br> 2) Or CBC www.cbc.ca/story/science/national/2005/05/06/global-dimming050506.html “WASHINGTON - The Earth has been getting brighter since 1990, reversing a trend called global dimming, scientists reported on Friday in the journal Science.”
3) Or ‘Earth Times’<br>http://www.earthtimes.org/articles/show/2710.html
“NEW YORK: Two recent climatologic studies have nearly established that unlike the popular belief that there is global dimming, what is actually happening is a marked increase in the amount of sunlight falling on the Earth's surface.”<br> What can one say. The BBC provides the conclusion for us: “One thing which shows absolutely no sign at all of dimming is the heat of scientific debate between climate change scientists.”
The latest number of ‘Mother Jones” (“As the World Burns”
“In its giving report, ExxonMobil says it supports public policy groups that are “dedicated to researching free market solutions to policy problems.” What the company doesn’t say is that beyond merely challenging the Kyoto Protocol or the McCain-Lieberman Climate Stewardship Act on economic grounds, many of these groups explicitly dispute the science of climate change. Generally eschewing peer-reviewed journals, these groups make their challenges in far less stringent arenas, such as the media and public forums. Pressed on this point, spokeswoman Lauren Kerr says that “ExxonMobil has been quite transparent and vocal regarding the fact that we, as do multiple organizations and respected institutions and researchers, believe that the scientific evidence on greenhouse gas emissions remains inconclusive and that studies must continue.” She also hastens to point out that ExxonMobil generously supports university research programs—for example, the company plans to donate $100 million to Stanford University’s Global Climate and Energy Project. It even funds the hallowed National Academy of Sciences.”<br> Here is a question. Does this sudden burst of ‘uncertainty’ have anything to do with some such recent example of ‘generous support’
Last Edit: May 10, 2005 0:04:31 GMT -5 by Wayne Hall
At his ‘Real Climate’ website Gavin Schmidt writes the following:
“It just so happens that most of the posts on this site have tried to counteract arguments from those who would sow fake "uncertainty" in the climate debate. But lest our readers feel that we are unjustifiably certain about our knowledge, let us look at a recent example of the opposite tendency: too much certainty.
A recent BBC Horizon documentary (transcript) raised the issue of 'global dimming' and argued that this 'killer' phenomena's newly-recognised existence would lead to huge re-assessments of future global warming. As part of the hyperbole, the process of global dimming was linked very clearly to the famines in Ethiopia in the 1980s and the implication was left that worse was to come.”<br> The Horizon documentary confidently asserts that:
“Global dimming is a killer. It may have been behind the worst climatic disaster of recent times, responsible for famine and death on a biblical scale. And Global Dimming is poised to strike again.”
It is horribly premature to declare 'global dimming' the cause of this event.
The suggested 'doubling' of the rate of warming in the future compared to even the most extreme scenario developed by IPCC is highly exaggerated. Supposed consequences such as the drying up of the Amazon Basin, melting of Greenland, and a North African climate regime coming to the UK, are simply extrapolations built upon these exaggerations. Whether these conclusions are actually a fair summary of what the scientists quoted in the program wanted to say is unknown. However, while these extreme notions might make good television, they do a dis-service to the science.”<br> David Sington’s position:
1. I am the producer of the BBC Horizon Global Dimming, so I'd like to respond to Gavin Schmidt's article, and also some of his further comments.
Firstly, I want to refute the notion that Peter Cox, or any other scientist taking part in this or in any other of the films I have made, was "mugged" with trick questions and made to seem to say things he does not believe. …. Dr Schmidt's suggestion is, not to put too fine a point on it, a serious libel (tantamount to accusing a scientist of falsifying his or her data).
What about the substance of the programme, and Dr Schmidt's criticisms of it? Here, it is important to make a distinction between criticisms of the science itself, and criticisms of the way it is presented.
The film explores what might be expected to happen if a) nothing is done to curb GHG emissions and b) the climate sensitivity is in the higher range Peter Cox and other leading scientists now believe possible. Of course, we are not saying that this is what will happen (to quote the film "this is not a prediction - it is a warning") - but in assessing policy options (which is what in a democracy we are calling upon our fellow-citizens to do) a proper appreciation of the worst case is vital.
Can I just finish by saying that the Horizon film was seen by 3.5 million viewers (representing about 7% of the adult population of the UK) and that copies were requested by the Prime Minister's office. The issues it discussed are being actively debated in Britain.
David Sington DOX Productions Ltd
[— 6 Feb 2005 @ 11:49 am
Caroline Campbell’s position:
I found that story and made that programme. It has been an utterly absorbed year in the making and I was preparing to leave it behind and move on. Having just sat down to surf the internet for feedback, I now sit here deeply motivated by your comments. Thank you. I will do my best to find a way to take awareness of this issue ‘beyond the Horizon’…
There are a few points to explain. Global Dimming has not hit the media more because few people knew about it. The programme is as I write kicking up quite a storm behind closed doors. Horizon has been thrown into this slightly unusual situation of having broken a major climate story. Sure, Farquhar, Stanhill and Ramanathan have been waving their results in the air for decades but these results appeared to contradict a lot of what climatologists already believed … without enough peer-reviewed data no self-respecting climatologist would translate this gathering evidence into ‘what it meant for the climate’… until Peter Cox came along that is.
He wrote his paper only in the last few months of us making this programme. In it, he predicts we are looking at a 9.3 degree rise over the next century if we eliminate Global Dimming but do nothing about Global Warming. It will be very interesting to see how this paper is taken by the scientific community.
I do not wish to mislead you. Horizon does have a specific structure to its programme-making and we were advised to lean on the disaster-scenario BUT nothing presented in the programme was not the honest belief of the scientists we spoke to.
If you want my honest opinion. I think that an awful lot more work needs to be done to truly understand this one. The data these scientists have is strongly suggestive but it is not a full global picture. We’d need more long-term records from more countries: records that agree though measured using different methods before we can be absolutely certain of the true extent the world has been dimming. Then we need to understand which particles do the heat-absorbing and which ones do the reflecting and which do most cloud-seeding to understand the effects of cutting these down.
However, there is a problem. Time.
The climate predictions are only going to be as good as the data we already have and it is not perfect. To wait 30 years for a whole new set could be suicidal.
A huge frustration of many of the scientists in this programme is that an increasing amount of money is made available to the guys who sit at their computers crunching numbers and generating predictions (with the windows closed) whilst the guys outside who are watching the skies, taking the measurements etc are struggling. Governments, Media etc. all want fast answers. We have to remember that fast answers are not always available. With Global Dimming, we are forced to take a leap of faith. We may not have every scrap of evidence in place, but if we ignore it, it could be suicidal.
Motivating people to act is difficult. I think it does have to come from political leaders because they can make swift decisions and the mounting environmental crisis needs that. I wish I could now run off a list of things YOU can do, choices YOU can make. Now. But the real issue is our reliance on fossil fuels, living a more energy-efficient lifestyle is a great start but it is political will that is needed. I believe, in the time given, nuclear power is the answer driving a baseline electricity-based economy. Hybrid and electric cars.
As for aeroplanes… I don’t know. There is simply too much gained from using them to stop. I must go and do some research into alternatives to the jet engine!
BEATE LIEPERT’S POSITION
Dear David and readers, Again, I am one of the interviewed scientists and one of the leading experts on "global dimming" and its climate consequences and I initiated the meeting which started the entire debate (Liepert et. al. 2004). Gavin Schmidt is my colleague at NASA GISS and we have been discussing my research on "global dimming" for many years now. He has been very supportive and a great colleague. Graham Farquhar and Michael Roderick (the other interviewed scientists) are frequent guests at our institutions. Furthermore during the research process for the documentary I repeatedly raised my concerns about linking the indirect effect and the Sahel drought without mentioning the study by Giannini et al (Science, 2004). Her study is regarded as the most convincing explanation. She provides good evidence that the Indian Ocean sea surface temperature was the driving factor behind the Sahel drought. My colleagues Rotstayn and Giannini correspond with each other. I respect and know them both quite well. Science might seem pretty boring to film makers in this respect. But that's what it is.
Last Edit: May 10, 2005 0:56:46 GMT -5 by Wayne Hall
The DOX program was not scripted in the way that I would have done. But I guess that you'd have to say that if I scripted it, only my mother would have watched it. Best wishes Graham
Professor Graham Farquhar, FAA, FRS Head, Environmental Biology Group Research School of Biological Sciences & Chair, Forum of the Institute of Advanced Studies Australian National University
From David Travis
I was clearly in the dark about the intent of your first e-mail but after reading through your reply and the various e-mail excerpts you included at the bottom I think I'm getting closer to understanding the debate that is going on here. Let me give you some summarystatements below as general eplies/responses to some of the commentary and then, as time permits, I'd be happy to try and clarify or rebut any specifics you or the others might want to get into.
1. First of all, I believe the Horizons show on global dimming was definitely over-produced and over-dramatized. However, I don't think this is necessarily a bad thing. Without such effects much of the younger audience would likely have lost interest half-way through and the sort of discussions that are going on now would probably not be happening. I have heard from an amazing amount of young people (high school and college students) since that show aired. They aren't necessarily "scared" but instead just want to learn more about global dimming. In my opinion, these sorts of shows have two primary responsibilities: (1) Educate the public about latest scientific research on important topics and (2) impact on them the importance of how these findings could potentially affect their lives (or their children's) some day. The latter is crucial in order to have any sort of potential impact on policy makers by getting their attention. I do believe, however, that a fine line is balanced because if it goes too far in the sensationalist direction the risk is to lose credibility. As a scientist, I did find myself feeling uncomfortable in spots where the statements seemed a bit too bold without sufficient evidence to back them up (even one of my own!).
************************************************ Dr. David Travis Professor and Chair Department of Geography and Geology University of Wisconsin-Whitewater ************************************************ ________________________________
Extracts from a dialogue:
The BBC "Global Dimming" documentary was undoubtedly a danger-mongering programme. I have seen it now, and I don't think that is an inaccurate characterization. But I also think it true that the scientists were saying what they said on the basis of a conviction that they were not exaggerating anything about the seriousness of the situation.
Danger-mongering and sensationalism, by the Pentagon, the BBC, the makers of "The Day After Tomorrow" has been pressed into service (i) as a substitute for giving the whole picture in relation to climate change, including the whole 'mitigation' dimension. Whatever the reasons for the suppression of this latter dimension (avoidance of private litigation, 'national security' classification, fear of 'mass panic') the effect of omitting it is to take the edge off the genuine urgency of the climate change issue.
Dangermongering also serves as a false solution to the dilemma posed by the fact that there are public issues where peer-reviewed scientific consensus is not allowed to prevail over views that are kept in the public arena for political reasons by powerful interest groups and defended ideologically on grounds of 'freedom of speech'.
Because 'climate change sceptics' cannot be silenced, climate change non-sceptics resort to sensationalism and hysteria.
THIS IN TURN GIVES ANTHROPOGENIC CLIMATE CHANGE SCEPTICS THE OPPORTUNITY TO REGAIN THE ‘MORAL HIGH GROUND’.
I am writing to you in response to your recent broadcast on "global dimming", produced for BBC's Horizon Series.
I recently read some commentary by a person on your staff, Caroline Campbell. This comment in particular interests me:
"I do not wish to mislead you. Horizon does have a specific structure to its programme-making and we were advised to lean on the disaster-scenario BUT nothing presented in the programme was not the honest belief of the scientists we spoke to."
I am curious exactly who advised the filmmakers to "lean on the disaster-scenario"?
Is this a standard practice in BBC documentary film work?
I would appreciate any answers you could provide for my questions.
DANGERMONGERING AS A SUBSTITUTE FOR FULL EXPOSURE OF ALL ASPECTS OF REALITY, WHATEVER LEGAL AND POLITICAL PROBLEMS THIS MAY LEAD TO, IS A RECIPE FOR CONTINUING POLITICAL DEFEAT OF ECOLOGISTS.
THE BBC HORIZON AND ABC FOUR CORNERS ON-LINE DEBATES FOLLOWING SCREENING OF DAVID SINGTON’S ‘GLOBAL DIMMING’ DOCUMENTARY IN BRITAIN AND AUSTRALIA WERE NAIVE AND UNINFORMED, EASILY MISLED BY INPUTS LIKE THE ABOVE CONTRIBUTION FROM A FULL-TIME ANTHROPOGENIC CLIMATE CHANGE DEBUNKER. ANY AUDIENCE OF ‘CHEMTRAIL ACTIVISTS’ WOULD HAVE YIELDED AN INFINITELY HIGHER LEVEL OF DEBATE, PROTECTION FROM MISLEADERS AND COMMITMENT TO DEALING WITH CLIMATE CHANGE.
Last Edit: May 10, 2005 1:00:15 GMT -5 by Wayne Hall
I am a member of the Athens Ecological Movement at present doing Greek subtitling for David Sington's BBC Horizon documentary on Global Dimming.
You are one of the Australian scientists appearing in the documentary. You are quoted as saying: "What our model is suggesting is that these droughts in the Sahel in the 1970s and the 1980s may have been caused by pollution from Europe and North America affecting the properties of the clouds and cooling the oceans of the northern hemisphere."
We are aware that there has been some controversy among scientists over the connection between the droughts in the Sahel and global dimming. We are not qualified to express any opinion one way or another on the subject.
We would however be interested to hear your opinion on whether a more comprehensive picture of the probable causes of 'global dimming' including the factor of climate mitigation (whatever other problems such discussion may perhaps give rise to) might perhaps be preferable to the methodology adopted in David Sington's documentary. For a start, the 'scare-mongering' approach gives anthropogenic climate change sceptics a chance to seize the moral high ground. A chance they grab with both hands.
We seem to have reached some level of agreement with David Travis, another scientist who appeared in the documentary, on this subject, and we would be interested to what extent you agree with the comments he makes below.
I would like to make it clear that we are not 'out to get' David Sington, or anyone else on the side of the debate wishing to draw attention to the problem of climate change. What we want to do is more effectively combat the 'sceptics'.
We would be very interested to hear your comments. Specifically on whether you agree that a different approach to this subject should be adopted by the media, an approach targeting the best-informed, and not the worst-informed, members of the general public.
Reply from Leon Rotstayn:
The study on which the comments are based can be seen at
I share your concern about the way some of the climate sceptics misrepresent the facts, but I also agree that some of the words in the Global Dimming Documentary were alarmist. It screened in Australia a few weeks ago, with some changes to the voice over to make it a little less alarmist. It seems to have had a strong impact on many people who saw it, and I have mixed feelings about whether it is justified to be slightly "alarmist" in order to get a strong message across. After all, if I had written the documentary, complete with caveats and qualifications, it would have put most of the viewers to sleep! On the other hand, as a professional scientist, I feel that it is important to be as accurate as possible.
The cited study shows that one can make a reasonable hypothesis for a connection between pollution and Sahelian drought, as summarized in the comment that you have quoted below. I am very aware that this is only the first step in establishing such a link. We hope to address this question further in more detailed simulations that are currently under way. Other modelling groups will probably also look at it.
Many of the scientists (i.e., those who go to conferences) have been aware of this for at least six months. To say that there was a reversal of dimming since about 1990 is not the same as saying "there is no dimming". In fact, the dimming, followed by brightening, is broadly consistent with what we know about aerosols. It is well established that in Europe, North America and Japan, emissions have been decreasing since the 1980s, due to emission controls, as well as the economic downturn in Eastern Europe. I mentioned this on p2113 of my paper, and it adds some support to the hypothesized connection with Sahelian drought, since the drought has been less severe since the 1980s.
However, the climate model results I have seen to date do not give a large enough dimming from 1950 to 1990 (or a large enough brightening thereafter). So there may be other factors causing the dimming besides aerosols - this is one aspect that was glossed over in the documentary. The possibility of other factors is also suggested by the results from Antarctica, where dimming followed by brightening has also been measured, and it is hard to blame it on aerosols down there.
Hope this helps. It is not all as "well worked out" as the documentary would suggest!
.....[Editor's note: Glancing at the headlines, it may seem as if this caption disagrees with the U.S. Department of Energy's news release, titled Earth Lightens Up.
However, a more careful read of that press release reveals that this caption is in agreement. That DOE press release refers to brightness at surface, or the amount of sunlight that penetrates all the way down through the atmosphere, whereas this (NASA) caption refers to the fraction of incoming sunlight that gets reflected.
Researchers suspect that an overall reduction in aerosol in the air is allowing more sunlight to penetrate to the surface, thus causing the brightening, which could also explain why Earth's albedo is in decline.]..... END excerpt.
Surfing through the mainstream media these days gives one the impression that a fair degree of consensus exists regarding climate change. It’s getter hotter…, Glaciers are retreating…, Sea levels are rising... - it all seems to be a pretty straightforward we’re-all-gonna-die sort of scenario. But let’s leave the fear factor aside for a moment and consider how complex issues like climate change are reported in the media, and more importantly, how governments make decisions about such complex issues.
In 1950’s science fiction movies, there was usually a scene where white-coated boffins would earnestly sit down with the President to brief him on how to defeat the flying saucer menace. This sort of direct communication channel between the laboratory and the seat of power is still thought to exist by many people. It’s generally assumed that governments are fully conversant with complex scientific issues and act accordingly, dutifully consulting with scientific experts when necessary. But this has never been the case and nowadays decision-making that involves science is informed more by politics than scientists. Climate change is a good example of how science is often either missing or completely misinterpreted in policy-making.
Recently, scientists hotly pursued by the press, informed us that our atmosphere is dimming due to the by-products of fossil fuel emissions not only producing greenhouse gasses, but particles (aerosols) that affect the properties of clouds. As a result, it was reported that less solar radiation was reaching the earth’s surface because the sun’s radiation was being reflected back into the atmosphere. A good thing apparently as this phenomenon has so far provided relief from the full effects of global warming. Unfortunately, any short-term relief that this may have provided was short lived. A new study then revealed that the dimming trend is now in reverse, and as a result, the earth will be subject to global brightening1. This trend, we are told, reveals that the cloud pollutants shielding the Earth’s surface from greater solar radiation are decreasing. Why? Well again there is little agreement on this, but there is yet more doom-and-gloom to this story. While particles previously shielding us from solar radiation are diminishing, greenhouse gasses like carbon dioxide are not. Some greenhouse gasses have an extended life span and can hang for a century or more. So in this respect, some scientists are claiming we are already committed to an unacceptable rise in global temperatures in the future. But climate change is just as much an argument about whether humans are the cause of it as it is about whether global warming is happening at all. The collective body of scientists who comprise the Intergovernmental Panel on Climate Change2 (IPCC) claim that human activity does contribute to the problem. But how much are we contributing to what might otherwise be naturally occurring changes? What can we do about the melting polar ice caps? Is the earth dimming or brightening? And if it’s dimming does that mean I can keep using fossil fuels? Or is it the other way around? Science is not supposed to be a realm of ambiguity and uncertainty. It is supposed to be the impartial final word. The truth is that the hard scientific facts that filter down to most of us are merely snapshots of an ongoing process conducted in relatively politically neutral social spaces, which are then politicized and popularized.
Richard Dawkins3, best known for his writings on evolutionary science, argues that we should all become more knowledgeable about science. Well I couldn’t agree more, especially since 40 percent of Americans think that early man shared the Earth with dinosaurs. The dimwitted public is thus captive to the authoritarian and agenda setting voices of the politician and the media. With Joe Public having a less than adequate understanding of science, politicians can, and do, cherry-pick scientific research to strengthen their case for climate change policy.
Academic Roger Pielke4, of the University of Colorado’s Center for Science and Technology Policy Research, takes the view that politicians are hijacking scientific discovery for their own ends, and getting away with it because of our faith in indisputable ‘scientific fact’. Pielke says that the interface between actual science, and mediated science, is the difference between the meaning and significance of findings. But forget politicians, they’re in the dark as much as we are, and scientists should be left to do what they do best. It’s true that science should be conducted in as neutral a space as is possible. And scientists probably feel at home in a vacuum anyway. However, being free from the framing and spin-doctoring of government and political advocates can only last so long, as those ideas have to emerge at some point, and once in the public realm developments in science are fair game. While many of us like to keep pace with scientific developments we often miss some of science’s subtleties and are, on occasion, guilty of glossing over the fluid and transient nature of the physical world. This is the kind of stark realization that many have no doubt had to face of late, after being confronted by the increasingly un-simple reports on climate change that may leave many bewildered. If the public are to contribute to scientific debate and eventual policy direction at all they are going to have to be properly informed. This leaves the bulk of the responsibility to journalists. There is no question that scientific consensus promoted by the mainstream media affects public attitudes on climate change, which in turn establishes what politicians can get away with.
The public needs to become more science savvy, as well as being skeptical of any scientific consensus on climate change, especially when it is quite likely that it is informed more by politics than science. In the cold light of day, reason tells me that we, the public, are at the wrong end of the science information loop. Regardless, I turn on the PC, pick up the newspapers and assume my public duty of taking a sthingyful of science to help the complexity go down.
Last night, 29th June, the BBC 'Global Dimming' documentary was shown at the Ama Lahi taverna in Athens.
Around forty people showed up, many in response to notification in one of the mass-circulation newspapers.
The following is the translation of a document that was distributed, along with Teller's 'Sunscreen for Planet Earth':
What new element is added to the discussion by the documentary we are to see tonight? It is the original idea that atmospheric pollution, specifically visible particle pollution, not invisible pollution from greenhouse gases, is something to be welcomed in the sense that it has a cooling effect, mitigating the warming caused by those greenhouse gases.
This conception is virtually unknown to the general public, though commonplace among climate scientists. Its least unknown expression is doubtless the 1997 text of Edward Teller ‘Sunscreen for Planet Earth’.
In public discussion on climate change the image that is projected is that on the one hand the Europeans and the rest of the world support the Treaty of Kyoto as a necessary measure in combating fossil-fuel dependence while on the other the Americans sabotage Kyoto because Mr. Bush and his supporters have not been persuaded of the reality of anthropogenic climate change.
I see this representation of the situation as a caricature. For a start, the Americans do not interpret the scientific data differently. This is confirmed by the content of last year’s ‘Pentagon Report’ on climate change. It is also confirmed by refutations by President Bush’s scientific advisor in the course of the current G8 negotiations of claims that “the White House does not believe in global warming”.
How can one explain the political gulf separating the USA from the rest of the world on this issue?
The answer may be provided by the climate scientists Peter Cox, who towards the end of the ‘Global Dimming’ documentary says: “If we carry on pumping out the particles it will have terrible impact on human health, I mean particles are involved in all sorts of respiratory diseases… If you fiddle with the radiative balance of the planet, you affect all sorts of circulation patterns like monsoons, which would have horrible effects on people. So it would be extremely difficult, in fact impossible, to cancel out the greenhouse effect just by carrying on pumping out particles, even if it wasn't for the fact that particles are damaging for human health.”
Bear in mind that in the course of the controversy over the political declaration on climate change to be issued by the G8, the Observer newspaper revealed that the US government insisted on deleting from the final communiqué the statement that ‘every year atmospheric pollution causes respiratory problems and premature deaths for millions of people.’
The real difference between the United States and the rest of the world on the subject of climate change has to do not with diagnosis but with treatment. The treatment preferred by the American government and its supporting oil companies appears much worse than the disease itself.
Of course the so-called ‘mitigation policies’ are not merely a national preference of the United States. They are the strategy of all the international community, including relevant agencies of the United Nations such as the Intergovernmental Panel on Climate Change (IPCC). It is for this reason that open critics of the strategy - and there are a number of them, particularly in Italy and the German-speaking world - do not enjoy very good relations with climate scientists and are in fact marginalised.
Nevertheless, in my opinion it is more expedient for public discussion to be conducted with heretics such as activists who reject IPCC policies on climate mitigation than it is for there to be inclusion of so-called climate change ‘sceptics’ who are funded by oil companies and oppose the IPCC for opposite reasons: because this committee of the United Nations seeks to end dependence of the international economy on fossil fuels.
Last Edit: Jun 30, 2005 1:09:08 GMT -5 by Wayne Hall
Global warming looks set to be much worse than previously forecast, according to new research. Ironically, the crucial evidence is how little warming there has been so far.
Three top climate researchers claim that the greenhouse gases already in the atmosphere should have warmed the world more than they have. The reason they have not, they say, is that the warming is being masked by sun-blocking smoke, dust and other polluting particles put into the air by human activity.
But they warn that in future this protection will lessen due to controls on pollution. Their best guess is that, as the mask is removed, temperatures will warm by at least 6°C by 2100. That is substantially above the current predictions of 1.5 to 4.5°C.
“Such an enormous increase would be comparable to the temperature change from the previous ice age to the present,” says one of the researchers, Meinrat Andreae of the Max Planck Institute for Chemistry in Mainz, Germany. “It is so far outside the range covered by our experience and scientific understanding that we cannot with any confidence predict the consequences for the Earth.”
The calculations assume a doubling of greenhouse gases such as carbon dioxide (CO2) in the atmosphere by 2100 compared to pre-industrial levels.
Andreae and his two British colleagues, Peter Cox and Chris Jones, are leading authors from the UN’s Intergovernmental Panel on Climate Change. These new findings are likely to be reflected in the IPCC’s next assessment of climate change science, scheduled for 2007.
The cooling effect of aerosols has been known for some time. But, says Andreae, past assessments have underestimated its influence. Because of this, they have also underestimated the sensitivity of the atmosphere to the warming effect of greenhouse gases.
The new modelling study finds that only high estimates of both aerosol cooling and greenhouse warming can explain the history of global temperatures over the past 50 years.
One foot on the gas
The problem for future climate is that the cooling aerosols only stay in the air for a few days, whereas the warming gases stick around for decades or centuries. So while the cooling effect is unlikely to grow much, the gases will accumulate and have an ever-bigger effect on global temperature.
The world, says Andreae, is “driving the climate with one foot on the gas and the other on the brake. When the brake comes off, it makes a hell of a difference".
The authors have added another previously unrecognised element to the temperature forecast - the effect of all this on nature and the natural carbon cycle.
Natural ecosystems are currently absorbing up to half of the CO2 that humans put into the atmosphere. Most climate models assume this will continue. But there is growing evidence that from about 2050, soils and forests will stop absorbing CO2 and start releasing it instead.
The authors calculate that this switch in the natural carbon cycle could accelerate the build-up of CO2 in the air by more than 50%, producing a total warming that “may be as high as 10°C” by 2100.