Climate Feedback

Predictions of climate

Posted by Oliver Morton on behalf of Kevin E. Trenberth

I have often seen references to predictions of future climate by the Intergovernmental Panel on Climate Change (IPCC), presumably through the IPCC assessments (the various chapters in the recently completedWorking Group I Fourth Assessment report ican be accessed through this listing). In fact, since the last report it is also often stated that the science is settled or done and now is the time for action.

In fact there are no predictions by IPCC at all. And there never have been. The IPCC instead proffers “what if” projections of future climate that correspond to certain emissions scenarios. There are a number of assumptions that go into these emissions scenarios. They are intended to cover a range of possible self consistent “story lines” that then provide decision makers with information about which paths might be more desirable. But they do not consider many things like the recovery of the ozone layer, for instance, or observed trends in forcing agents. There is no estimate, even probabilistically, as to the likelihood of any emissions scenario and no best guess.

Even if there were, the projections are based on model results that provide differences of the future climate relative to that today. None of the models used by IPCC are initialized to the observed state and none of the climate states in the models correspond even remotely to the current observed climate. In particular, the state of the oceans, sea ice, and soil moisture has no relationship to the observed state at any recent time in any of the IPCC models. There is neither an El Niño sequence nor any Pacific Decadal Oscillation that replicates the recent past; yet these are critical modes of variability that affect Pacific rim countries and beyond. The Atlantic Multidecadal Oscillation, that may depend on the thermohaline circulation and thus ocean currents in the Atlantic, is not set up to match today’s state, but it is a critical component of the Atlantic hurricanes and it undoubtedly affects forecasts for the next decade from Brazil to Europe. Moreover, the starting climate state in several of the models may depart significantly from the real climate owing to model errors. I postulate that regional climate change is impossible to deal with properly unless the models are initialized.

The current projection method works to the extent it does because it utilizes differences from one time to another and the main model bias and systematic errors are thereby subtracted out. This assumes linearity. It works for global forced variations, but it can not work for many aspects of climate, especially those related to the water cycle. For instance, if the current state is one of drought then it is unlikely to get drier, but unrealistic model states and model biases can easily violate such constraints and project drier conditions. Of course one can initialize a climate model, but a biased model will immediately drift back to the model climate and the predicted trends will then be wrong. Therefore the problem of overcoming this shortcoming, and facing up to initializing climate models means not only obtaining sufficient reliable observations of all aspects of the climate system, but also overcoming model biases. So this is a major challenge.

The IPCC report makes it clear that there is a substantial future commitment to further climate change even if we could stabilize atmospheric concentrations of greenhouse gases. And the commitment is even greater given that the best we can realistically hope for in the near term is to perhaps stabilize emissions, which means increases in concentrations of long-lived greenhouse gases indefinitely into the future. Thus future climate change is guaranteed.

So if the science is settled, then what are we planning for and adapting to? A consensus has emerged that “warming of the climate system is unequivocal” to quote the 2007 IPCC Fourth Assessment Working Group I Summary for Policy Makers (pdf) and the science is convincing that humans are the cause. Hence mitigation of the problem: stopping or slowing greenhouse gas emissions into the atmosphere is essential. The science is clear in this respect.

However, the science is not done because we do not have reliable or regional predictions of climate. But we need them. Indeed it is an imperative! So the science is just beginning. Beginning, that is, to face up to the challenge of building a climate information system that tracks the current climate and the agents of change, that initializes models and makes predictions, and that provides useful climate information on many time scales regionally and tailored to many sectoral needs.

We will adapt to climate change. The question is whether it will be planned or not? How disruptive and how much loss of life will there be because we did not adequately plan for the climate changes that are already occurring?

Kevin Trenberth

Climate Analysis Section, NCAR


  1. Report this comment

    Eli Rabett said:

    This is what I call the elephant in the climate problem. GHG forcing is so large that when you look at model predicitions you get a reasonable answer. It is also the reason why carping at the models wrt GHG forcing is silly, but important for the sort of other things you talk about.

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    m phelps said:

    I must take issue with the statement that “the science is convincing that humans are the cause. Hence mitigation of the problem: stopping or slowing greenhouse gas emissions into the atmosphere is essential”. First we need to know what the (net) costs of climate change are. Then whether mitigation rather than adaptation (or more accurately what mix of the two)is required does not depend on whether climate change is human caused or not. What matters is the cost of adaptation compared to the cost of mitigation. It is perfectly possible to imagine that climate change is man-made but the costs of mitigation are higher than the costs of adaptation.

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    Tom Saksa said:

    It is difficult for me to imagine that even a very large reduction in greenhouse gas emissions could not be made without resulting in economic disaster. At one time I thought that it would be difficult to cut emissions by more than a small percentage without making huge sacrifices. But when I tried it myself I found that it wasn’t. Many of us who live in industrialized countries waste so much energy without even thinking that it that I found it was actually relatively easy to reduce my personal GHG emissions by over 50%, and still live quite well. And I was not even starting from what most people would consider a very high consumption level.

    It also surprises me that many of my friends who have very little background in anything even closely related to climate research feel quite justified in parroting objections to the conclusions reached by thousands of the most experienced scientists in the field. Or that some of the people I know who live only a few blocks from their workplace would not consider for a moment going to work without their car even for just a few days a month.

    So many of the arguments I hear either that global warming does not exist, or how economically suicidal it would be to address, are starting to sound a lot like the defeatist arguments I usually hear from teenagers.

    Teenager, “I need money for stuff I want to do over the summer ”.

    Dad “The best way I know to get money is to get a job.”

    Teenager, “I can’t get a job. There are no jobs out there”

    Dad “Sure there are, there are hundreds of them in the paper. You just haven’t looked”

    Teenager, “I can’t get a job without experience”

    Dad “Lot’s of these jobs say no experience is needed.”

    Teenager, “I can’t get a job until you buy me a car.”

    Dad “Many of these jobs are within walking distance, or are near the bus route. And you also have a bike, or you could car pool with you mom if you found a job downtown”

    Teenager, “I already told you I can’t get a job without experience”


    It is not that we do not have enough facts. It is not that we could not make really significant reductions if we wanted to. The big problem is that most people have no real will to make any changes. Perhaps we should expect the next IPCC document to include a statement that reads “There is at least a 95% chance that the human race was not really worth saving anyway”.

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    Eduardo Zorita said:

    This is in my opinion a very interesting, and at the same time somewhat troubling, comment by one of the leading climate scientist. Climate models are believed to incorporate our best knowledge of the physics and chemistry of the present climate. When used to simulate the effects of increasing concentrations of atmospheric greenhouse trace gases, they represent a theory of anthropogenic greenhouse climate change, i.e. a theory that explains the observed increase in surface temperatures in the last decades, along with other related phenomena, like sea-level rise, stratospheric cooling, trends in seasonality amplitude and changes in daily temperature range. As all scientific theories, it should offer the possibility to be falsified, and successfully passing these tests, strengthening itself in the process.

    The troubling aspect of Trenberth’s comment is that if the IPCC climate simulations do not represent real predictions for the future, how can the theory be falsified? The most obvious alternative , as with other theories in geosciences where experimenting is almost impossible, is to identify not-yet-observed phenomena predicted by the theory, which should be finally observed. Unfortunately, – although others may correct me here- I am not aware of one clear example of this. Along the past years and decades, the theory of anthropogenic greenhouse climate change has explained ’ a posteriori’ an impressive observational body, to an extent that other competing theories have not attained. To some extent, however, these explanations rely on ad-hoc factors – for instance the effect of aerosols to explain the observed lack of warming between roughly 1940 and 1975. The effects ad-hoc factors usually contain large uncertainties, so that a ‘devils advocate’ could argue that these factors have been tuned to reproduce part of the observations. Other example is the European heat-wave in summer 2003. Before this date, a dearth of papers were devoted to summer heat-waves – after this date, the mechanisms leading to such heat-waves appeared crystal-clear in simulations with climate models.

    I think this state of affairs is not satisfactory. My question to those interested and with more knowledge than me would be: beyond explanatory power, has the theory made testable predictions of unobserved phenomena?

    Eduardo Zorita

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    Rejean Gagnon said:

    Re: Tom Saska’s Comment

    Your comment is quite correct in saying many people do not reduce their environmental footprint when they easily could, probably due to convenience and laziness, whatever. But at the heart of the matter, you are saying that people who disagree with AGW hysteria are content to waste energy and pollute the earth. This is simply not true. I bike and run to work myself – I do as much as possible at my home to reduce our power requirements (changing bulbs, getting rid of old appliances and replacing with more efficient models, turning everything off, using a programmable thermostat, etc…). Yet I have read much and do not believe in what I have seen on CO2 as a culprit in “global warming”. This is not even to say that I do not agree that there very likely is some warming taking place!

    In short – you’re placing everyone who does not believe in AGW (CO2-based) into the “enemy camp” (You are either for or against us mentality).

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    Mark Bahner said:

    Eduardo Zorita writes, “This is in my opinion a very interesting, and at the same time somewhat troubling, comment by one of the leading climate scientist.”

    I think Dr. Trenberth’s opinion, while being unquestionably correct (“There are no predictions by the IPCC at all”) is far more than “somewhat troubling.”

    In my opinion, the fact that “there are no predictions by the IPCC at all” constitutes scientific fraud. Specifically, as I have written on my blog:

    “The IPCC Third Assessment Report’s (TAR’s) projections for methane atmospheric concentrations, carbon dioxide emissions and atmospheric concentrations, and resultant temperature increases constitute the greatest fraud in the history of environmental science.”

    The IPCC TAR (and AR4) “projections” constitute scientific fraud

    Further, I would be happy to engage in a scientific debate with anyone who thinks otherwise.

    I would be happy to debate this matter here on Nature’s Climate Feedback blog (where presumably the writers and editors of Nature think otherwise), on Scientific American’s blog (where the writers and editors clearly think otherwise), at the Real Climate blog (where the authors presumably think otherwise), at the Prometheus blog (if they’re interested, and are not afraid of the consequences of the public seeing that the scientific emperor has no clothes), at my blog, or anywhere else.

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    hunter said:

    E. Zorita hits the nail on the head. Ther AGW movement cannot be falsified. The comment this thread is based on seems to have two very different parts: The part that shows that climate projections used by the IPCC are worthless and a huge backdown from the predictive part of the IPCC. The second is a rather truncated defense of the tenets of AGW. So it is unsettled, except for its main claims. This makes no xense.

    The IPCC is a document of science except it does not predict anything except when it does and is edited by committee. The science is settled except the projections are worthless and the projections do not take into account the important factors of weather. The problem of climate change is properly described by the IPCC except it is much worse.

    At the end of the day AGW is a black box where no matter what goes into it, good projections or bad, hot or cold weather, inaccurate weather station observations, etc., the answer is always the same.

    It raises reasonable questions as to the nature of the black box.

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    Kevn said:

    An argument with that many caveats does not require action. I think this is the main problem. People want to fill a box full with conjecture, close the box and call it fact. The models simply do not work. Any implication that they do is either ignorance of the nature of science or out right fraud. The problem with the whole field of weather prediction is that no one is ever responsible for a wrong answer. What kind of organization do you develop when no one is ever responsible or called to answer for failures in the product they produce and allowed to doctor the statistics on which they grade themselves?

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    Fred056 said:

    Some of the above respondents seem to be overlooking an even bigger elephant. Climate change is not a theory. It is an established fact (like dinosaurs are, or what we know about the early history of Homo genus, are facts). If we can successfully model an “ideal earth”, or one with no anthropic tinkering, it should behave, over tens of thousands of years, like the real thing has been doing over its past few tens of thousands (with periods of interrupted glaciation). Then presumably you can “play” with this model, maybe try to “tweak” it so it behaves more like today’s world (which now has a bit more CO2 and other stuff in its atmosphere). Of course the model will only ever be an ideal (and I personally have no idea how “close” the scientists are to one). Modelling any climate is a demanding (especially of computers) task, and weather is a complex and chaotic system. The Earth not a bald sphere.

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    Vernon said:

    I often wonder how anyone can claim that climate change is due to CO2 when Hansen himself says that is not.

    If Hansen is right in his 2000 paper “Global warming in the twenty-first century: An alternative scenario” then how does anyone prove there is CO2 based warming by observing the climate?

    To quote from Hansen in the report:

    “Our estimates of global climate forcings indicate that it is the non-CO2 GHGs that have caused most observed global warming.”

    Further, Hansen went on to say:

    “Fossil fuel use is the main source of both CO2 and aerosols, with land conversion and biomass burning also contributing to both forcings. Although fossil fuels contribute to growth of some of the other GHGs, it follows that the net global climate forcing due to processes that produced CO2 in the past century probably is much less than 1.4 W/m2. ”

    Which reads as burning fossil fuels produces CO2 and aerosols which cancel either other out. That leaves only the other GHGs as the source of 20th century warming.

    The IPCC assumes a 4 W/m2 forcing but as the Hansen found “Most climate simulations, as summarized by the IPCC, do not include all of the negative forcings; indeed, if they did, and other forcings were unchanged, little global warming would be obtained." In this study Hansen predicted that “Global warming at a rate 0.15 +/- 0.05 degrees C per decade will occur over the next several decades.” This works out to being 1.5 degrees +/- .5 C.

    The IPCC agrees that there are problems with the instrumented and proxy readings for the 20th century and it can be seen ">”> here

    In their large-scale reconstructions based on tree ring density data, Briffa et al. (2001) specifically excluded the post-1960 data in their calibration against instrumental records, to avoid biasing the estimation of the earlier reconstructions (hence they are not shown in Figure 6.10), implicitly assuming that the ‘divergence’ was a uniquely recent phenomenon, as has also been argued by Cook et al. (2004a). Others, however, argue for a breakdown in the assumed linear tree growth response to continued warming, invoking a possible threshold exceedance beyond which moisture stress now limits further growth (D’Arrigo et al., 2004). If true, this would imply a similar limit on the potential to reconstruct possible warm periods in earlier times at such sites. At this time there is no consensus on these issues (for further references see NRC, 2006) and the possibility of investigating them further is restricted by the lack of recent tree ring data at most of the sites from which tree ring data discussed in this chapter were acquired.

    GISSTEMP has the following global temps for 2001-2006 (J-D): 57, 69, 67, 60, 76, and 65. Anyone want to point out where the warming is hidden in there? Which could also be why the big news was warming was going to start again after 2009. Do not get me wrong, I believe that we have had warming since the LIA. But I do not see accelerated warming that the instrumented direct readings are indicating. The proxies are not showing this accelerated warming.

    So either the proxy readings are right and there is something wrong with the way we are doing the direct instrumented readings, or the proxy readings are wrong and we just lost the basis to say anything out of the ordinary is happening now.

    Warming in the Antarctic is mainly limited to the Antarctic Peninsula and even then manly in the portion that is outside the Antarctic Circle as can be clearly seen here. The interior of Antarctic is clearly cooling. This is not consistent with any of the GCMs. If the CO2 theory is correct with the sensitivity that is expected by the proponents, then warming should be happening at both poles.

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    Macnerdzcare said:

    the worst thing about climate change is that the thirld world countries are the ones suffering first but it is the first world countries who started the climate change.

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    gary said:

    “the worst thing about climate change is that the thirld world countries are the ones suffering first but it is the first world countries who started the climate change.”

    Quite possibly the stupidest quote that I have ever read regarding climate change. Let us imagine for a second that the worst possible scenarios that Al Gore and other alarmists predict… er… project: Famine, disease, displacement of families, thousands dying from malaria… Guess what? Its all been happening for quite some time, it wasn’t caused by global warming…er… climate change and nobody’s doing anything about it. Tens of thousands dying from malaria Mr. Gore? There are already millions dying from it! We are currently robbing starving countries of arable land so that we can grow ethanol to save the earth.

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