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Climate modelling

Back to the future

30 Apr 2008 by Gavin

Translations: (Español) (English)

A few weeks ago I was at a meeting in Cambridge that discussed how (or whether) paleo-climate information can reduce the known uncertainties in future climate simulations.

The uncertainties in the impacts of rising greenhouse gases on multiple systems are significant: the potential impact on ENSO or the overturning circulation in the North Atlantic, probable feedbacks on atmospheric composition (CO2, CH4, N2O, aerosols), the predictability of decadal climate change, global climate sensitivity itself, and perhaps most importantly, what will happen to ice sheets and regional rainfall in a warming climate.

The reason why paleo-climate information may be key in these cases is because all of these climate components have changed in the past. If we can understand why and how those changes occurred then, that might inform our projections of changes in the future. Unfortunately, the simplest use of the record – just going back to a point that had similar conditions to what we expect for the future – doesn’t work very well because there are no good analogs for the perturbations we are making. The world has never before seen such a rapid rise in greenhouse gases with the present-day configuration of the continents and with large amounts of polar ice. So more sophisticated approaches must be developed and this meeting was devoted to examining them.

[Read more…] about Back to the future

Filed Under: Climate modelling, Climate Science, Paleoclimate

Butterflies, tornadoes and climate modelling

23 Apr 2008 by group

Ed Lorenz hiking Many of you will have seen the obituaries (MIT, NYT) for Ed Lorenz, who died a short time ago. Lorenz is most famous scientifically for discovering the exquisite sensitivity to initial conditions (i.e. chaos) in a simple model of fluid convection, which serves as an archetype for the weather prediction problem. He is most famous outside science for the ‘The Butterfly Effect’ described in his 1972 paper “Predictability: Does the Flap of a Butterfly’s Wings in Brazil Set Off a Tornado in Texas?”. Lorenz’s contributions to both atmospheric science and the mathematics of dynamical systems were wide ranging and seminal. He also directly touched the lives of many of us here at RealClimate, and both his wisdom, and quiet personal charm will be sorely missed.

[Read more…] about Butterflies, tornadoes and climate modelling

Filed Under: Climate modelling, Climate Science

The global cooling mole

7 Mar 2008 by group

By John Fleck and William Connolley

To veterans of the Climate Wars, the old 1970s global cooling canard – “How can we believe climate scientists about global warming today when back in the 1970s they told us an ice age was imminent?” – must seem like a never-ending game of Whack-a-mole. One of us (WMC) has devoted years to whacking down the mole (see here, here and here, for example), while the other of us (JF) sees the mole pop up anew in his in box every time he quotes contemporary scientific views regarding climate change in his newspaper stories.

Una traducción está disponible aqui
Tłumaczenie na polski dostępne jest tutaj.

[Read more…] about The global cooling mole

Filed Under: Climate modelling, Climate Science, Communicating Climate, Reporting on climate, skeptics

The IPCC model simulation archive

4 Feb 2008 by Gavin

In the lead up to the 4th Assessment Report, all the main climate modelling groups (17 of them at last count) made a series of coordinated simulations for the 20th Century and various scenarios for the future. All of this output is publicly available in the PCMDI IPCC AR4 archive (now officially called the CMIP3 archive, in recognition of the two previous, though less comprehensive, collections). We’ve mentioned this archive before in passing, but we’ve never really discussed what it is, how it came to be, how it is being used and how it is (or should be) radically transforming the comparisons of model output and observational data.
[Read more…] about The IPCC model simulation archive

Filed Under: Climate modelling, Climate Science, IPCC

Uncertainty, noise and the art of model-data comparison

11 Jan 2008 by Gavin

Translations: (Español) (Chinese (simplified)) (English)

Gavin Schmidt and Stefan Rahmstorf

John Tierney and Roger Pielke Jr. have recently discussed attempts to validate (or falsify) IPCC projections of global temperature change over the period 2000-2007. Others have attempted to show that last year’s numbers imply that ‘Global Warming has stopped’ or that it is ‘taking a break’ (Uli Kulke, Die Welt)). However, as most of our readers will realise, these comparisons are flawed since they basically compare long term climate change to short term weather variability.

This becomes immediately clear when looking at the following graph:

[Read more…] about Uncertainty, noise and the art of model-data comparison

Filed Under: Climate modelling, Climate Science, Instrumental Record

New rule for high profile papers

4 Jan 2008 by Gavin

New rule: When declaring that climate models are misleading in a high profile paper, maybe looking at some model output first would be a good idea.
[Read more…] about New rule for high profile papers

Filed Under: Arctic and Antarctic, Climate modelling, Climate Science

Tropical tropospheric trends

12 Dec 2007 by group

Once more unto the breach, dear friends, once more!

Some old-timers will remember a series of ‘bombshell’ papers back in 2004 which were going to “knock the stuffing out” of the consensus position on climate change science (see here for example). Needless to say, nothing of the sort happened. The issue in two of those papers was whether satellite and radiosonde data were globally consistent with model simulations over the same time. Those papers claimed that they weren’t, but they did so based on a great deal of over-confidence in observational data accuracy (see here or here for how that turned out) and an insufficient appreciation of the statistics of trends over short time periods.

Well, the same authors (Douglass, Pearson and Singer, now joined by Christy) are back with a new (but necessarily more constrained) claim, but with the same over-confidence in observational accuracy and a similar lack of appreciation of short term statistics.
[Read more…] about Tropical tropospheric trends

Filed Under: Climate modelling, Climate Science, Greenhouse gases, Instrumental Record

A phenomenological sequel

27 Nov 2007 by rasmus

Grading

Does climate sensitivity depend on the cause of the change?
Can a response to a forcing wait and then bounce up after a period of inertness?
Does the existence of an 11-year time-scale prove the existence of solar forcing?
Why does the amplitude of the secular response drop when a long-term trend is added?
[Read more…] about A phenomenological sequel

Filed Under: Climate modelling, Climate Science, Paleoclimate, Sun-earth connections

The certainty of uncertainty

26 Oct 2007 by group

A paper on climate sensitivity today in Science will no doubt see a great deal of press in the next few weeks. In “Why is climate sensitivity so unpredictable?”, Gerard Roe and Marcia Baker explore the origin of the range of climate sensitivities typically cited in the literature. In particular they seek to explain the characteristic shape of the distribution of estimated climate sensitivities. This distribution includes a long tail towards values much higher than the standard 2-4.5 degrees C change in temperature (for a doubling of CO2) commonly referred to.

In essence, what Roe and Baker show is that this characteristic shape arises from the non-linear relationship between the strength of climate feedbacks (f) and the resulting temperature response (deltaT), which is proportional to 1/(1-f). They show that this places a strong constraint on our ability to determine a specific “true” value of climate sensitivity, S. These results could well be taken to suggest that climate sensitivity is so uncertain as to be effectively unknowable. This would be quite wrong.

[Read more…] about The certainty of uncertainty

Filed Under: Climate modelling, Climate Science

Regional Climate Projections

27 Aug 2007 by rasmus

Regional Climate Projections in the IPCC AR4

glasses How does anthropogenic global warming (AGW) affect me? The answer to this question will perhaps be one of the most relevant concerns in the future, and is discussed in chapter 11 of the IPCC assessment report 4 (AR4) working group 1 (WG1) (the chapter also has some supplementary material). The problem of obtaining regional information from GCMs is not trivial, and has been discussed in a previous post here at RC and the IPCC third assessment report (TAR) also provided a good background on this topic.

The climate projections presented in the IPCC AR4 are from the latest set of coordinated GCM simulations, archived at the Program for Climate Model Diagnosis and Intercomparison (PCMDI). This is the most important new information that AR4 contains concerning the future projections. These climate model simulations (the multi-model data set, or just ‘MMD’) are often referred to as the AR4 simulations, but they are now officially being referred to as CMIP3.

One of the most challenging and uncertain aspects of present-day climate research is associated with the prediction of a regional response to a global forcing. Although the science of regional climate projections has progressed significantly since last IPCC report, slight displacement in circulation characteristics, systematic errors in energy/moisture transport, coarse representation of ocean currents/processes, crude parameterisation of sub-grid- and land surface processes, and overly simplified topography used in present-day climate models, make accurate and detailed analysis difficult.

I think that the authors of chapter 11 over-all have done a very thorough job, although there are a few points which I believe could be improved. Chapter 11 of the IPCC AR4 working group I (WGI) divides the world into different continents or types of regions (e.g. ‘Small islands’ and ‘Polar regions’), and then discusses these separately. It provides a nice overview of the key climate characteristics for each region. Each section also provides a short round up of the evaluations of the performance of the climate models, discussing their weaknesses in terms of reproducing regional and local climate characteristics.

[Read more…] about Regional Climate Projections

Filed Under: Climate modelling, Climate Science, IPCC

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