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Confidence in climate forecasts

Posted on 4 August 2010 by Kevin Judd

Guest post by Kevin Judd

Climate scientists are telling us that the earth's average temperature is going to rise 2 to 3 degrees over the next 50 to 100 years. How do they make this prediction? And why are they confident their prediction will be correct? Climate scientists make this prediction using a climate model. So what is a climate model?

Perhaps you have seen, or even had a ride on, one of those model stream-trains. These are miniature working replicas of real steam-engines. Climate models are the same; they are a working replica of the earth, only instead being made of rock and water and other materials, they are made from mathematical equations.

These mathematical models are the basis of science and technology. There are models for how microwave ovens work; models for car engines and power stations; models for jet-aircraft. Models allow scientists and engineers to build things that have never been built before, by testing how they will work before they are made. Models were used to build the rockets that took astronauts to the moon and back safely. Models allows scientists to predict complex things like the weather.

These models make correct predictions because they are based on general scientific principles, often referred as "Laws", like the law of gravity. General scientific principles are important because they connect phenomena that are not obviously connected. For example, the principles of microwave ovens are related to the greenhouse effect. The principles of car engines and power stations are related to how the earth will warm up. The principles of aircraft are related to winds, storms, and ocean currents.

This interconnectedness gives scientific models great power. If the general principles of climate models were wrong, scientists would have known long ago: microwave ovens wouldn't work, aircraft wouldn't fly, weather couldn't be forecast.

Based on general principles alone climate scientists have every reason to be confident in their predictions, because the principles have been well tested. Furthermore, the climate models of twenty years ago accurately forecast temperature rises over the last 20 years. This successful prediction further validates the general principles, and gives us confidence in climate models. Add to this that the climate models of today are much better than 20 years ago.

Of course there will always be some uncertainty about how the details of climate change will play out, but there is no doubt on the basic story that the earth's average temperature is going to rise 2 to 3 degrees over the next 50 to 100 years. Anyone who says otherwise, either does not understand how science works, or is being deliberately misleading.

In my next segment I'll consider what the consequences of this warming will be.

NOTE: this post is also being "climatecast" by Kevin Judd on RTR-FM 92.1 around 11.30 AM WAST today. You can listen to a streaming broadcast of RTR-FM online via

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Comments 101 to 115 out of 115:

  1. Don't forget that as the earth warms (leading to higher and more common temperature records), there will also be more precipitation : As Earth gets warmer, large regions will experience heavier rain and snowfall as weather becomes generally more intense, according to a new study. LiveScience I wonder how many so-called skeptics are continuing to ignore reality in Pakistan, India, Afghanistan or China ? Maybe they'll be trying to convince themselves that it was just the same about 30 years ago, or whenever.
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  2. #92 Pete Ridley. "Perhaps the real culprit causing those global weather events that are being misinterpreted as human-made global climate change is El Ni˜no and not our use of fossil fuels." Have you not heard the expression? Climate trains the boxer, weather throws the punches.
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  3. Pete Ridley #92 Or somewhere between adelady's simple explanation, and the more complex one in the moderator's response: ENSO is a process which moves heat around the planet surface. So if heat is used in one place it will by necessity not be used elsewhere (law of conservation of energy). So this means that ENSO can not explain the rising temperatures, as there has to be an energy input in order for this to happen. This could come from a few places, including the sun, volcanic activity (transfer of deep heat to surface) and greenhouse gasses. Unfortunately for your argument, it's pretty well established that greenhouse gasses (largely CO2 from fossil fuels) are the only reasonable explanation we have left.
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  4. 2010 seems to be doing a splendid job of smashing previous record high temperatures in Moscow.
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  5. Argus at 02:58 AM on 7 August, 2010 I am assuming that you use modeling in your profession at some level, as I do. Models are almost universal tools, but we know they are simplifications, and to use them successfully we must understand and accept their limitations. Where climate models have fallen down in the past they have (at least to my knowledge) done so because they have not included factors which turn out to be more significant than previously thought (such as water vapour or cloud feedback, in many older models) or have not been calculated to high enough resolution (for instance to resolve eddy effects in Ocean studies). The models constantly improve, but by adding in more factors, increasing complexity and by using improved processing power to increase resolution, not by some sort of fudge factor “fitting”. As has been pointed out, many of the IPCC models -in general- underestimated the reality of evolving sea level rise or Arctic ice loss. However another extremely important point is that models are the only way of running climate experiments and control experiments in parallel. We do not have that luxury in reality. Your point about media bias cuts both ways, but this is in many cases not objective science (either way).
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  6. Pete Ridley at 06:59 AM on 8 August, 2010 Technically, I can agree on the semantic distinction between projection and prediction, von Storch makes this point clearly. We are dealing with possibilities and probabilities derived from models which are as representative or reductionist as any set of equations. Each model generates a projection with an ever expanding uncertainty envelope based on estimates of uncertainty associated with the input parameters. Different models, differing sophistication, and different weightings produce different projections. The range of projections offer us “scenarios”, we can infer worst case and best case from the range of possibilities, and consider adjusting policy to avoid the worst case. The graphics I have seen of IPCC model results always show a range of projections. Simply put, they are the best indicators we currently have of what might happen. They are certainly more objective than wishful thinking. However, before discussing modeling further, we must address your understanding of the current status of the input data and current theory, and your sweeping generalizations which appear to disregard or willfully misinterpret this data. How you can be so pedantic about the term “prediction” and yet lose your critical objectivity when it comes to some of the sources and anecdotes you cite and the statements you make? “Perhaps the real culprit causing those global weather events that are being misinterpreted as human-made global climate change is El Ni˜no and not our use of fossil fuels”?? As a non-expert in any climate related field, you seem to have formed a certain fixed viewpoint and now search around to find evidence to support it which you seem to “enjoy”. This is the opposite of an objective scientific evidence based approach. Von Storch discusses uncertainty, but he also discusses current AGW theory being consistent with the overall weight of empirical evidence, and this is really the crux of the matter, - gaining confidence in the theories and models based on data, or rejecting (or certainly losing confidence in) alternative hypotheses based on this same data. As more evidence accumulates, and models become increasingly refined, uncertainties in measurements, trends, and models are reduced. This has lead to increasingly high confidence (rather than absolute certainty) in the mainstream view. On this von Storch appears to agree, as do a majority of climate scientists. There are a minority of notable exceptions of course, as we do not expect views to be monolithic on any matter involving human “rationality”. Please take time to read the up to date NOAA State of the Climate report. It is written in clear terms by the some of the leading scientific experts in their respective fields. Look at the author list. It’s a great read, enjoy.
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  7. JMurphy at 06:42 AM, if you go to the site indicated below as a starting point, you will be able to go back through the archives to the 2007 forecasts and follow how the predictions evolved. Also go to the email discussion link within the page that provides some background as to how the implications of coming up with a strongly negative forecast were being closely considered given most other agencies were providing more rosy outlooks, at a time when the rural sector was particularly wanting a rosy picture painted. The print media gave quite a bit coverage to the matter when they picked up on the story the following year.
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  8. More words to puzzle over: Belarus records its hottest temperature in history for the second day in a row The Russian heat wave has also affected the neighboring nations of Ukraine and Belarus. All three nations have recorded their hottest temperatures in history over the past few weeks. Belarus, on the western border of Russia, recorded its hottest temperature in history on Saturday, August 7, when the mercury hit 38.9°C (102°F) in Gomel. This broke the all-time record for extreme heat set just one day before, the 38.7°C (101.7°F) recorded in Gorky. Prior to 2010, the hottest temperature ever recorded in Belarus was the 38.0°C (100.4°F) in Vasiliyevichy on Aug. 20, 1946. As I described in detail in Saturday's post, Belarus' new all-time extreme heat record gives the year 2010 the most national extreme heat records for a single year--seventeen. These nations comprise 19% of the total land area of Earth. This is the largest area of Earth's surface to experience all-time record high temperatures in any single year in the historical record. Looking back at the past decade, which was the hottest decade in the historical record, seventy-five countries set extreme hottest temperature records (33% of all countries.) For comparison, fifteen countries set extreme coldest temperature records over the past ten years (6% of all countries). Earth has now seen four consecutive months with its warmest temperature on record, and the first half of 2010 was the warmest such 6-month period in the planet's history. It is not a surprise that many all-time extreme heat records are being shattered when the planet as a whole is so warm. Global warming "loads the dice" to favor extreme heat events unprecedented in recorded history. Dr. Jeff Masters' Wundergound Blog My eyebrows are up.
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  9. JMurphy at 06:42 AM, in response to your comment about the June model forecasts all being reasonably well aligned. Given that a La Nina pattern is now well established and is being predicted to be stronger and remain in place for over a year, then one would expect most credible models to be aligned. Where the models do differ is generally when patterns are changing with different models throwing up conflicting forecasts, and this is where the reputation of models is generally formed. A further refinement is considering how far out from such changes each of the models can begin forming reliable forecasts. Most will be able to forecast an event when it is imminent or already established, as BOM does, but being able to reliability predict a year ahead is beyond most, but not all, at present.
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  10. michael sweet at 10:22 AM, whilst it is a common cliche that climate is not weather, it is impossible to quantify climate without first understanding, and then quantifying the existing pattern of all the weather processes. Thus the GCM's that do understand, combine and quantify all weather processes provide the basis from which climate models are then able to articulate how the weather will (or should have) manifest(ed) itself at any point in time, both looking forward, and back. The main focus of the AGW theory is the role IR plays in the retention of heat energy. However IR only plays a small role in the movement of heat energy, about 25%(?), convection and conduction move and disperse the vast majority of heat energy, and of course these are weather processes, with clouds being an integral and dominating part of those processes. This then leads straight back to the inability of climate models in being able to successfully and adequately quantify such a dominating process. It is not enough to just focus on those warming processes that work to retain heat energy, those cooling processes that disperse and remove accumulated heat must also be quantified. Whilst the forecast of the next El-Nino/ La-Nina event may fall under the realm of weather forecasts, it is the frequency of such events and how they alternate that is relevant to the climate. A quick glance at the Quinn reconstruction of past El-Nino events over the last 500 years show clearly that such events are not regularly spaced but go through periods where they are more frequent and occur one upon another, and then times when they were infrequent. Given it is acknowledged that they are a result of redistributing heat energy then the obvious question is what caused such irregular patterns, patterns that seem to indicate as much, if not more regular occurrences in the past than at present. As an example, the extended drought that was responsible for the American dust bowl era, should it be weather forecasting, or climate forecasting, that can be used to forecast when such conditions might return? It is thought that another such pattern may be in the process of forming now. Which branch of the science does the responsibility for making predictions that may follow such longer term patterns, weather or climate modeling?
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  11. JohnD, clouds introduce uncertainty to GCMs in terms of radiative effects, not atmospheric heat transport. GCM functionality w/regard to redistribution of atmospheric heat appears quite good, not particularly controversial.
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  12. JohnD "It is not enough to just focus on those warming processes that work to retain heat energy, those cooling processes that disperse and remove accumulated heat must also be quantified." Nothing 'removes' accumulated heat from the system except radiative transfer out to space I would have thought. The energy within the system concentrates in different places at different times. But the redistribution of heat between various parts of the land, ocean and atmosphere is exactly that. Redistribution. The more energetic the system, the more energetic the redistribution. Heat is only 'removed' from the system at large at the top of the atmosphere. The fact that the atmosphere and oceans within the system are over-excited and rocking and rolling all over the surface doesn't matter.
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  13. Doug @ 108 - great graphic over at Jeff Masters' blog, very interesting to see how it compared with the 2003 European heat wave. I have one eyebrow raised.
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  14. Johnd: Around 1900 Arrhenius calculated a climate sensitivity (by hand) of 4.5 C/W2 without knowledge of the weather that you claim is required for that calculation. His number is still within the range of accepted values. The models of the 1970's and 1980's predicted, with skill, current warming, arctic amplification, sea ice loss, Arctic/Antarctic anamolies, etc. These models did not have the weather knowledge you claim is required. The forecast of weather, both short term of weeks and long term of a few months or years is simply different from the long term forecast of weather out decades. Weather is chaotic while climate is not. I was going to refer you to the "weather is not climate" page but I see you are a frequent visitor already. Perhaps it is worth another read.
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  15. For some reason my earlier post of this comment was removed so I’ve modified it slightly. I can’t see any violation of the comments policy.
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    Moderator Response: Putting duplicate comments into multiple threads leads to incoherent discussion.

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