Archive for the ‘models’ Category

Climate models and climate "sensitivity" for dummies (me too); a recent bibliography

October 14th, 2008 No comments

Inspired by Bob Murphy’s Mises post on “Economic and Climate Models“, I decided to put together a few links to (i) further information on global climate models, (ii) criticism of them (and responses), and (iii) the state of the science on climate “sensitivity” (that is, the range of temperature increases that are expected to eventually result from a doubling of atmospheric levels of CO2).

1. General information on climate models

Spencer R. Weart, “Simple Question, Simple Answer … Not”, Forum on Physics & Society, October 2008, American Institute of Physics

Discovery of Global Warming site at the American Institute of Physics, created by Spencer Weart with support from the National Science Foundation and the Alfred P. Sloan Foundation 

Modelling the climate, by

A model approach to climate change (Feb 1, 2007,, by Adam Scaife and Chris Folland of the Hadley Centre for Climate Prediction and Research, Met Office, UK, and John Mitchell, chief scientist at the Met Office, UK.

The physics of climate modeling (Physics Today, Jan 2007), by Gavin Schmidt, research scientist at the NASA Goddard Institute for Space Studies.

The IPCC: (regional models)


Principles of Planetary Climate, detailed online book on the physics and modelling of climate by Raymond T. Pierrehumbert (U. Chi.)

The Global Warming Debate, 8. Climate Models


2. Shortcomings of Models, Environmental Information Coalition (EIC), National Council for Science and the Environment.

“A climate of alarm”, Feb 1, 2007, (interview of Richard Lindzen) (Lindzen PowerPoint) (Lindzen – Stephan Rahmstorf dialogue)

“The Sloppy Science of Global Warming”, Roy W. Spencer, Mar. 20, 2008,

Roy Spencer interview,


Lay criticism:, Warren Meyer, who describes himself as “a small business owner and author of Coyote Blog” with “an engineering degree from Princeton and an MBA from the Harvard Business School.”


Responses to critics:

The following posts at Skeptical Science by John Cook, an ex-physicist (majored in solar physics at the University of Queensland): (review of Lindzen’s “iris” theory)

Brian J. Soden, Darren L. Jackson, V. Ramaswamy, M. D. Schwarzkopf, Xianglei Huang, The Radiative Signature of Upper Tropospheric Moistening

“Climate models predict that the concentration of water vapor in the upper troposphere could double by the end of the century as a result of increases in greenhouse gases. Such moistening plays a key role in amplifying the rate at which the climate warms in response to anthropogenic activities, but has been difficult to detect because of deficiencies in conventional observing systems. We use satellite measurements to highlight a distinct radiative signature of upper tropospheric moistening over the period 1982 to 2004. The observed moistening is accurately captured by climate model simulations and lends further credence to model projections of future global warming.”

3. The state of the science on climate “sensitivity”

“Sensitivity” refers to the expected equilibrium temperature increase expected to result from a doubling of atmospheric levels of CO2.  The sensitivity reflects not simply the direct warming effects of increased CO2 levels, but the short- and long-term of feedbacks, which are generally expected to be positive, such as increases in atmospheric water vapor resulting from higher temperatures, reduced surface reflectivity (albedo) as ice melts and darker ocean and land surfaces absorb greater solar radiation, and releases of methane (a much more potent GHG than CO2) from permafrosts (and possibly also ocean floor deposits).  Equilibrium effects are expected to be felt over hundreds and thousands of years, so we are already committed to future climate change – of an imperfectly known degree – as a result of the current 1/3 increase of atmospheric CO2 over preindustrial levels.  Plus, atmospheric levels continue to increase, and at increasing increments, as fossil fuel use grows worldwide.

Climate models have been used to estimate a fast-feedback (changes of water vapor, clouds, climate-driven aerosols, sea ice and snow cover) CO2 sensitivity of 3 ±1.5°C., but models cannot define climate sensitivity more precisely, as it is unknowable whether models realistically incorporate all feedback processes.  The model results reported by the IPCC do not include long-term sensitivity (including the effects of changes to long-lived atmospheric gases, ice sheet area, land area and vegetation cover).  Further, as a recent paper (Dana L. Royer, Robert A. Berner & Jeffrey Park) notes, “Most estimates of climate sensitivity are based on records of climate change over the past few decades to thousands of years, when carbon dioxide concentrations and global temperatures were similar to or lower than today, so such calculations tend to underestimate the magnitude of large climate-change events and may not be applicable to climate change under warmer conditions in the future.”

Studies of the Earth’s history provide empirical inferences of both fast feedback climate sensitivity and  long-term sensitivity.  Climate sensitivity has been estimated based on the 20th Century data, the constraints from responses to volcanic eruptions, and the last glacial maximum (LGM); these data have been combined on a Bayesian basis by one group to conclude that fast-feedback sensitivity is very unlikely (< 5% probability) to exceed 4.5°C, but some evidence is inconsistent with this range.  Jim Hansen has argued for a long-term sensitivity of up to 6°C.

A recent paper concludes, based on a comparison of estimations of carbon dioxide concentrations over the past 420 million years with a proxy record, that (1) “a climate sensitivity greater than 1.5 °C has probably been a robust feature of the Earth’s climate system over the past 420 million years, regardless of temporal scaling”, (2) “deep-time geological records exclude the possibility of weak climate sensitivities … the amount of warming for every doubling of carbon dioxide must be at least 1.5 °C.”, and (3) although high climate sensitivities cannot be entirely excluded, their best fit for the past 420 million years was about 2.8 °C per doubling.

Further, scientists have shown that, given the existence of net positive feedbacks, that there is an irreducible uncertainty about the climate effects of GHG increases, but that this uncertainty is not symmetric – rather, physics-based models and empirical evidence force the conclusion that climate change has a long tail on the “bad” side and a very short tail on the “good” side.   Gerard H. Roe and Marcia B. Baker recent article in Science “Why Is Climate Sensitivity So Unpredictable?” concludes:

“we show that the shape of these probability distributions is an inevitable and general consequence of the nature of the climate system. Further, we show that the breadth of the distribution and, in particular, the probability of large temperature increases are relatively insensitive to decreases in uncertainties associated with the underlying climate processes.”

“… it is evident that the climate system is operating in a regime in which small uncertainties in feedbacks are highly amplified in the resulting climate sensitivity. We are constrained by the inevitable: the more likely a large warming is for a given forcing (i.e., the greater the positive feedbacks), the greater the uncertainty will be in the magnitude of that warming.” (italics added)

A quick explanation of the Roe and Baker paper is laid out by John Cook here:  Does model uncertainty exaggerate global warming projections?


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