05 Oct 2010: Analysis

On Climate Models, the Case
For Living with Uncertainty

As climate science advances, predictions about the extent of future warming and its effects are likely to become less — not more — precise. That may make it more difficult to convince the public of the reality of climate change, but it hardly diminishes the urgency of taking action.

by fred pearce

I think I can predict right now the headlines that will follow publication of the next report from the Intergovernmental Panel on Climate Change (IPCC), due in 2013. “Climate scientists back off predicting rate of warming: ‘The more we know the less we can be sure of,’ says UN panel.”

That is almost bound to be the drift if two-time IPCC lead author Kevin Trenberth and others are right about what is happening to the new generation of climate models. And with public trust in climate science on the slide after the various scandals of the past year over e-mails and a mistaken forecast of Himalayan ice loss, it hardly seems likely scientists will be treated kindly.

It may not matter much who is in charge at the IPCC by then: Whether or not current chairman Rajendra Pachauri keeps his job, the reception will be rough. And if climate negotiators have still failed to do a deal to replace the Kyoto Protocol, which lapses at the end of 2012, the fallout will not be pretty, either diplomatically or climatically.

Clearly, concerns about how climate scientists handle complex issues of scientific uncertainty are set to escalate. They were highlighted in a report about IPCC procedures published in late August in response to growing criticism about IPCC errors. The report highlighted distortions and
The latest climate modeling runs are trying to deal with a range of factors not dealt with in the past.
exaggerations in IPCC reports, many of which involved not correctly representing uncertainty about specific predictions.

But efforts to rectify the problems in the next IPCC climate-science assessment (AR5) are likely to further shake public confidence in the reliability of IPCC climate forecasts.

Last January, Trenberth, head of climate analysis at the National Center for Atmospheric Research in Boulder, Colo., published a little-noticed commentary in Nature online. Headlined “More Knowledge, Less Certainty,” it warned that “the uncertainty in AR5’s predictions and projections will be much greater than in previous IPCC reports.” He added that “this could present a major problem for public understanding of climate change.” He can say that again.

This plays out most obviously in the critical estimate of how much warming is likely between 1990, the baseline year for most IPCC work, and 2100. The current AR4 report says it will be between 1.8 and 4.0 degrees Celsius (3 to 7 degrees F). But the betting is now that the range offered next time will be wider, especially at the top end.

The public has a simple view about scientific uncertainty. It can accept that science doesn’t have all the answers, and that scientists try to encapsulate those uncertainties with devices like error bars and estimates of statistical significance. What even the wisest heads will have trouble with, though, is the notion that greater understanding results in wider errors bars than before.

Trenberth explained in his Nature commentary why a widening is all but certain. “While our knowledge of certain factors [responsible for climate change] does increase,” he wrote, “so does our understanding of factors we previously did not account for or even recognize.” The trouble is this sounds dangerously like what Donald Rumsfeld, in the midst of the chaos of the Iraq War, famously called “unknown unknowns.” I would guess that the IPCC will have even less luck than he did in explaining what it means by this.

The latest climate modeling runs are trying to come to grips with a range of factors ignored or only sketchily dealt with in the past. The most troubling is the role of clouds. Clouds have always been recognized as a ticking timebomb in climate models, because nobody can work out whether warming will change them in a way that amplifies or moderates warming — still less how much. And their influence could be very large. “Clouds remain one of the largest uncertainties in the climate system’s response to temperature changes,” says Bruce Wielicki, a scientist at NASA’s Langley Research Center who is investigating the impact of clouds on the Earth’s energy budget.

An added problem in understanding clouds is the role of aerosols from industrial smogs, which dramatically influence the radiation properties of clouds. “Aerosols are a mess,” says Thomas Charlock, a senior scientist at
Despite much handwringing, the IPCC has never worked out how to make sense of uncertainty.
the Langley Research Center and co-investigator in a NASA project known as Clouds and the Earth’s Radiant Energy System (CERES). “We don’t know how much is out there. We just can’t estimate their influence with calculations alone.”

Trenberth noted in Nature, “Because different groups are using relatively new techniques for incorporating aerosol effects into the models, the spread of results will probably be much larger than before.”

A second problem for forecasting is the potential for warming to either enhance or destabilize existing natural sinks of carbon dioxide and methane in soils, forests, permafrost, and beneath the ocean. Again these could slow warming through negative feedbacks or — more likely, according to recent assessments — speed up warming, perhaps rather suddenly as the planetary system crosses critical thresholds.

The next models will be working hard to take these factors into better account. Whether they go as far as some preliminary runs published in 2005, which suggested potential warming of 10 degrees C (18 degrees F) or more is not clear. Of course, uncertainty is to be expected, given the range of potential feedbacks that have to be taken into account. But it is going to be hard to explain why, when you put more and better information into climate models, they do not home in on a more precise answer.

Yet it will be more honest, says Leonard Smith, a mathematician and statistician at the University of Oxford, England, who warns about the “naive realism” of past climate modeling. In the past, he says, models have been “over-interpreted and misinterpreted. We need to drop the pretense that they are nearly perfect. They are getting better. But as we change our predictions, how do we maintain the credibility of the science?”

The only logical conclusion for a confused and increasingly wary public may be that if the error bars were wrong before, they cannot be trusted now. If they do not in some way encapsulate the “unknowns,” what purpose do they have?

Despite much handwringing, the IPCC has never worked out how to make sense of uncertainty. Take the progress of those errors bars in assessing warming between 1990 and 2100.

The panel’s first assessment, published back in 1990, predicted a warming of 3 degrees C by 2100, with no error bars. The second assessment, in 1995, suggested a warming of between 1 and 3.5 degrees C. The third, in 2001, widened the bars to project a warming of 1.4 to 5.8 degrees C. The fourth assessment in 2007 contracted them again, from 1.8 to 4.0 degrees C. I don’t think the public will be so understanding if they are widened again, but that now seems likely.

Trenberth is nobody’s idea of someone anxious to rock the IPCC boat. He is an IPCC insider, having been lead author on key chapters in both 2001 and 2007, and recently appointed as a review editor for AR5. Back in 2005 he made waves by directly linking Hurricane Katrina to global warming. But in the past couple of years he has taken a growing interest in highlighting uncertainties in the climate science.

Late last year, bloggers investigating the “climategate” emails highlighted a message he sent to colleagues in which he said it was a “travesty” that
Trenberth questioned if the IPCC wouldn’t be better off getting out of the prediction business.
scientists could not explain cool years like 2008. His point, made earlier in the journal Current Opinion in Environmental Stability, was that “it is not a sufficient explanation to say that a cool year is due to natural variability.” Such explanations, he said, “do not provide the physical mechanisms involved.” He wanted scientists to do better.

In his Nature commentary, Trenberth wondered aloud whether the IPCC wouldn’t be better off getting out of the prediction business. “Performing cutting edge science in public could easily lead to misinterpretation,” he wrote. But the lesson of climategate is that efforts to keep such discussion away from the public have a habit of backfiring spectacularly.

All scientific assessments have to grapple with how to present uncertainties. Inevitably they make compromises between the desire to convey complexity and the need to impart clear and understandable messages to a wider public. But the IPCC is caught on a particular dilemma because its founding purpose, in the late 1980s, was to reach consensus on climate science and report back to the world in a form that would allow momentous decisions to be taken. So the IPCC has always been under pressure to try to find consensus even where none exists. And critics argue that that has sometimes compromised its assessments of uncertainty.

The last assessment was replete with terms like “extremely likely” and “high confidence.” Critics charged that they often lacked credibility. And last August’s blue-chip review of the IPCC’s performance, by the InterAcademy Council, seemed to side with the critics.

The council’s chairman, Harold Shapiro of Princeton, said existing IPCC guidelines on presenting uncertainty “have not been consistently followed.” In particular, its analysis of the likely impacts of climate change “contains many statements that were assigned high confidence but for which there is little evidence.” The predictions were not plucked from the air. But the charge against the IPCC is that its authors did not
We need to get used to greater uncertainty in imagining exactly how climate change will play out.
always correctly portray the uncertainty surrounding the predictions or present alternative scenarios.

The most notorious failure was the claim that the Himalayan glaciers could all have melted by 2035. This was an egregious error resulting from cut-and-pasting a non-peer reviewed claim from a report by a non-governmental organization. So was a claim that 55 percent of the Netherlands lies below sea level. But other errors were failures to articulate uncertainties. The study highlighted a claim that even a mild loss of rainfall over the Amazon could destroy 40 percent of the rainforest, though only one modeling study has predicted this.

Another headline claim in the report, in a chapter on Africa, was that “projected reductions in [crop] yield in some countries could be as much as 50 percent by 2020.” The only source was an 11-page paper by a Moroccan named Ali Agoumi that covered only three of Africa’s 53 countries (Morocco, Tunisia, and Algeria) and had not gone through peer review. It simply asserted that “studies on the future of vital agriculture in the region have shown... deficient yields from rain-based agriculture of up to 50 percent during the 2000-2020 period.” No studies were named. And even Agoumi did not claim the changes were necessarily caused by climate change. In fact, harvests in North Africa already differ by 50 percent or more from one year to the next, depending on rainfall. In other words, Agoumi’s paper said nothing at all about how climate change might or might not change farm yields across Africa. None of this was conveyed by the report.

In general, the InterAcademy Council’s report noted a tendency to “emphasise the negative impacts of climate change,” many of which were “not supported sufficiently in the literature, not put into perspective, or not

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expressed clearly.” Efforts to eliminate these failings will necessarily widen the error bars on a range of predictions in the next assessment.

We are all — authors and readers of IPCC reports alike — going to have to get used to greater caution in IPCC reports and greater uncertainty in imagining exactly how climate change will play out. This is probably healthy. It is certainly more honest. But it in no way undermines the case that we are already observing ample evidence that the world is on the threshold of profound and potentially catastrophic warming. And it in no way undermines the urgent need to do something to halt the forces behind the warming.

Some argue that scientific uncertainty should make us refrain from action to slow climate change. The more rational response, given the scale of what we could face, is the precise opposite.

POSTED ON 05 Oct 2010 IN Biodiversity Climate Climate Policy & Politics Science & Technology Australia 


Your article listed many of the flaws that skeptics have identified in the IPCC reports. In most cases, the authors and manager of the reports appeared to be aware these were present and published anyway because they wanted to influence the public and government to take action.

Now you are concerned that publishing accurate and honest information will reduce credibility, and you continue the mantra that action is needed now. To meaningfully reduce carbon dioxide emissions, the entire coal, oil, and natural gas fired electrical generating capacity and transportation capacity of the world must shut down and replaced with something not yet available or even identified. Billions of people will be adversely affected, but we should do it anyway because there is a chance that sea levels might rise and the globe might warm by 1 C (or more) in the next century.

Credibility would be enhanced by more honesty and better judgment. Credibility would be enhanced by funding and publishing articles exploring alternative explanations for climate change such as natural variability. Credibility would be enhanced if the people promoting the CAGW side would stop attacking the intelligence and morals of anyone who points out the very flaws you mention in this article. Credibility was lost because it was not credible to assert that the 'science is settled' when in fact it is not.

Credibility will be difficult to recover as long as the same people who caused it to be lost are in charge of publishing the IPCC and similar reports.

Posted by AGW sceptic 99 on 05 Oct 2010

The article confuses matters when it should not. The error bars on warming published by IPCC to date were evaluated using the climate models then used. Now in the next assessment report climate models will be used that contain more processes (such as interactive ecosystem
models) that were neglected before due to poor knowledge of how they work. More processes in the model means more feedbacks, both negative and positive and, potentially, a wider range of outcomes. Since error bars are always relative to the prediction system used there is no confusion to be had in a more complete system having larger error bars. I actually think that can be communicated to the public effectively. I've managed to get this point across in a number of settings and think I succeeded.

Posted by Richard Seager on 05 Oct 2010

Pearce's emphasis on the models is peculiar given the state of the science. Knowledge of paleoclimate has proceeded in leaps and bounds, most pertinently regarding the interglacials and Pliocene and Miocene warm periods that provide a wealth of information about the climate conditions we seem determined to experience in the near future. As Jim Hansen says, we are informed by paleoclimate, modern observations and only then by the models. Furthermore, the paleoclimatic view isn't much affected by problems modeling clouds and aerosols.

We know, for example that the last time CO2 was at approximately current levels, in the mid-Pliocene, that global temperature was 2-3 degrees C higher than current and sea level was about 25 meters higher. While that information is only somewhat helpful in determining the Charney sensitivity for doubled CO2 (the model benchmark), it's quite useful on its face.

Re Trenberth's "travesty" comment, Pearce neglects to mention that it referred to the lack of sufficient observing systems, in particular satellites, which has hardly been a secret. So much for the whiff of scandal.

Posted by Steve Bloom on 07 Oct 2010

I used to have a lot of time for Fred Pearce but it seems in recent times he has become a "closet" denier, or maybe he always has been.

The extreme cautions and conservative reportings of the IPCC, and other organisations engaged in AGW research, is the result of people such as Fred failing to stand up and demand or take action against the numerous denier organisations who consistantly spread disinformation. What on earth else do you expect from the people who work for and contribute to the IPCC, when one knows they are going to be attacked by unchallenged unscrupulous people they will always tend on the side of safety. The sad end result is "the truth, the whole truth and nothing but the truth" gets masked.

I just love the line by AGW sceptic 99 when he/she says, "Credibility would be enhanced by more honesty", when did the denier industry show honesty? For a picture of the latest dishonesty of the denier industry just have a look at http://www.desmogblog.com/koch-funded-attacks-prop-23-include-manufacturing-science

Leaving all the scientific and anti-scientific reporting aside I know a few indisputable facts:-
1/ Our planet has been heating up for the past 150 years and it is continueing to heat.
2/ Carbon dioxide in the atmosphere has been increasing and it is a proven greenhouse gas.
3/ The excess carbon dioxide in the atmosphere is as a result of human activities.
4/ The worlds glaciers and ice sheets are melting, (based on where we are in the Milankovich cycle these should be increasing not diminishing)
5/ The world's oceans are becoming more acidic, caused by the extra carbon dioxide being absorbed from the atmosphere.
6/ Putting aside any debate on cloud cover or water vapour I know from the history of 135 million yeara ago when atmospheric carbon dioxide levels were in excess of 1,000 ppm the global temperatures where about 10 degrees warmer than today, and, this planet was ice free and ocean levels were more than 300 feet higher than today
7/ As Steve Bloom points out in his comment, with Carbon Dioxide levels only a little higher than today (400 ppm versus the 390 ppm we are currently at) the temperatures in the mid-Pliocene were 2 - 3degrees warmer than now, also with less ice and ocean levels more than 80 feet higher than now.
8/ If we fail to act to stop the increases in carbon dioxide emissions, indeed if we don't do enough to actually reduce their levels, we are entering a world completely uncharted in the history of homo sapiens sapiens and it is this uncertainty we should be afraid of, not the statistical variation of a model.

Posted by Patrickdj on 07 Oct 2010

A lot of the science behind CO2-induced global warming is actually based on climate models.
The "projections" for the 21st century are almost entirely based on them.

And yet, there is nowhere near enough discussion of the real state of climate modelling - and whether the models that we have today are a good and useful representation of the Earth's climate.

Climate modelling is a relatively youthful art - no more than 30 years old. It is a huge and complex challenge. The Earth's climate is an enormous, non-linear system, involving many independent factors, some of which we barely understand. The modelling of turbulent fluid dynamics is a serious challenge in other scientific disciplines. The climate adds to that the behaviour of water - in all its phases - plus a huge layer of convective heat transfer.

Meanwhile, we barely have global measurements of any of the significant aspects of the climate system. Temperature, humidity, wind speed, rainfall, clouds, aerosols, surface albedo, motions of the water in the oceans...

How is it possible to believe that any of the models so far created are in any way a true representation of the real climate system? Have the present models been validated adequately? Do they accurately represent the behaviour of all the many factors in the climate system? Can they model the exchanges that take place between the oceans and the atmosphere and account for the quasi-periodic behaviour that appears to take place in the real world?

We would do well to pause and reflect with some humility here. Some problems in science are hard. Some are extremely hard. And then there is the problem of modelling the Earth's climate system. To imagine that we have got even a fraction of the way there yet is to fool ourselves.

Meanwhile, we are building policies for ourselves and our grandchildren on these foundations of sand...

There is an excellent discussion of the state of climate modelling and its associated uncertainties over at Judith Curry's blog:


Posted by Mike Edwards on 08 Oct 2010

If only Judy knew much abut climate modeling, Mike. Your first sentence demonstrates that you sure don't. Just keep repeating it, though; someone might believe it.

Posted by Steve Bloom on 08 Oct 2010

There's as much uncertainty about whether humans will obliterate their slim chances for a future through the deliberate or accidental use of nuclear and biological weapons. But that doesn't seem to stop us stupid monkeys from making more of them.

Thus, the IPCC should assume that in our unbridled hubris we will indeed dig up all known fossil reserves and burn them until we reach an EROEI of 1:1 and then give up.

That should easily set off the frozen methane bomb in the ice-free polar regions, which will raise sea levels at least 100 metres, providing lots of new jobs for refugees in the construction industry. This will temporarily relieve the uncertainty of cornucopian economists.

Posted by Dave Inkstall on 10 Oct 2010

"Prediction" and "projection" must not be used interchangeably. Part of the problem appears to be that (ignorant?) skeptics seem to visualise climate scientists being actively engaged in chrystal balling, rather than using sophisticated (computer-based) models to project possible outcomes and develop appropriate scenarios.


Posted by Alexander on 12 Oct 2010

There are three areas often overlooked, ignored, or shunted aside in discussions about climate change that shouldn't be, points even strong skeptics can agree with.

First, anyone with even rudimentary familiarity with scientific projections at the layman level understands that projections aren't writ in stone and have to account for a range of possibilities when we're talking about something as confoundingly complex as climate and changes in it.

Second, and directly related to the first, we need to look at these projections in terms of probability theory, keeping in mind the gaps in our knowledge. Some projected possible scenarios are less likely than other ones are. And there indeed are some strong arguments for at least pausing -- at length -- to plunge headlong in attempting to stave off or mitigate the possible effects in worst-case scenarios, such as preparing for a 10-degree (C) temperature increase in just a century. At the other end of the spectrum, there's little suggesting we can simply the best-case scenario will be very likely to be the one to play out of no or inconsequential climate change. The logical goal, therefore, is to look in between, and fine-tuning as best we can as our knowledge grows and methodology improves. Based on what we know (or think we know) so far, maybe it's best for us to anticipate a temperature rise of, say, 2-4 degrees Celsius. Yes, that range is itself open to discussion, and might shift as we understand better -- it's almost certainly a moving target.

Thirdly -- forget climate change entirely and dismiss it as pure fiction. There are still enormous benefits to be had by reducing pollution, developing alternative energy sources, and the like. What's there *not* to like about cleaner air, soil, and water? What's wrong with moving ahead in preparation for the point at which we'll simply run out of fossil fuels?

In conversations with skeptical friends in recent years, I've found that if I approach matters based on these three points, even the most skeptical among them has agreed, at least to some extent, with all three, especially the third one. A fair number of my skeptical friends work in the oil industry, in the field, both onshore and offshore, yet even they tend to agree.

In short, there is common ground to be found, but skeptics and believers both need to stop demonizing each other and seek that common ground. That doesn't mean people can't or won't have spirited debates about the details of achieving any agreed-on plans -- they will. But that's good -- it get more heads gathered around the campfire, so to speak.

Posted by Mekhong Kurt on 02 Nov 2010

When Trenberth said "it is not sufficient to say that a cool year is due to natural variability" because "he wanted scientists to do better", it was because he thinks that "doing better" is, for the first time, something that is possible to do.

There has been a reduction in uncertainty. A flood of new data is coming in, such as the ARGO system that is sampling heat in the oceans down to 2000m worldwide, or the observations of sea level that allow millimeter precision.

Trenberth wants to banish the words "natural variability" from explanations scientists offer, in the same way that "here be dragons", as used by map makers of old, is no longer put on maps.

More than 90% of the heat accumulating in the planetary system is going into the oceans, yet until now most have been riveted on fluctuations in the average planetary surface temperature chart, even though the primary causes of those fluctuations are changes in the circulation of heat in the oceans.

Trenberth wrote in "The ocean is warming, isn't it?", 20 May 2010 Nature, that detailed study of heat in the oceans derived from sea level and ARGO data is in its "infancy". He predicted that "ocean heat content is likely to become a key indicator of climate change".

This is a major advance and will prove to be a major reduction in uncertainty yet I'm hearing some, like Pearce, tell us watch out, more uncertainty ahead.

I find that Trenberth, and many other experts writing about climate could express themselves more precisely. Eg. In "More knowledge, less certainty", Trenberth starts out asserting that the IPCC has never done "predictions" before. Yet only two paragraphs later he writes "the uncertainty in AR5's climate predictions and projections will be much greater than in previous IPCC reports". You can't have greater uncertainty about something you've never done before.

Hansen provides an excellent discussion in his 2008 Bjerknes Lecture about what his "principal" source for his understanding of climate change is: "from increasingly detailed knowledge of how the Earth responded in the past to changes of boundary conditions, including atmospheric composition", i.e. paleoclimate. His second most important source is present day observations. Third is models. "Climate models, used with understanding of their limitations, are useful, especially for extrapolating into the future, but they are certainly number three on the list".

As for "critics", we don't have to worry. If a doctor advised us to start exercising and eat less because our extreme obsesity had us on the road to a heart attack, the "skeptics" would be poring over any studies available for anything they could distort, while making up as many reasons as possible to persuade us to ignore the doctor's advice.

Posted by David Lewis on 04 Nov 2010

Comments have been closed on this feature.
Fred Pearce is a freelance author and journalist based in the UK. He is environment consultant for New Scientist magazine and author of numerous books, including When The Rivers Run Dry and With Speed and Violence. His latest book is The Coming Population Crash and Our Planet’s Surprising Future. In earlier articles for Yale Environment 360, Pearce has written about global climate talks in the wake of Copenhagen and the fallout from the “Climategate” incident.



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