Nature correspondence on uncertain climate model assessments

September 6, 2007 at 2:55 pm | Posted in Climate change, Correspondence, Letters to Editors, Nature, Uncertainty | 2 Comments

Original Commentary dated 27 June 2007

The latest report from the Intergovernmental Panel on Climate Change assesses the skill of climate models by their ability to reproduce warming over the twentieth century, but in doing so may give a false sense of their predictive capability.

Cooling, or negative forcing, occurs primarily through direct effects such as scattering of light in cloud-free air and indirect effects such as enhanced reflection of light by clouds. These aerosol forcings are much less certain than the greenhouse-gas forcings, and hence the total forcing is likewise quite uncertain. The new report estimates total anthropogenic forcing to be 0.6 to 2.4 W m-2 (5–95% confidence range). This factor of four range greatly limits the ability to evaluate the skill of climate models in reproducing past temperature changes and to infer climate sensitivity from observed change because a given temperature increase might result from a large forcing and low climate sensitivity or alternatively from a small forcing and high climate sensitivity.”

wrote Stephen E. Schwartz1, Robert J. Charlson2 & Henning Rodhe3 in Quantifying climate change — too rosy a picture?


To the Editor (of Nature, posted today)

“…However, they have misinterpreted the use of AR4 Fig. SPM-4 (shown here as Fig. 1) and, as such, their criticisms are misplaced. In the following, we summarize key aspects of the AR4 uncertainty analyses that were neglected by Schwartz et al.

We conclude that the skill of climate models in reproducing warming over the twentieth century shown in AR4 does not imply that the IPCC has under-estimated uncertainty in future warming. Uncertainty ranges are not based on the ability of the multi-model ensemble to reproduce twentieth century global mean warming. Instead, the AR4 assessment of future warming provides important new probabilistic information that incorporates multiple sources of uncertainty, including observational data.”

wrote Piers Forster1, Gabriele Hegerl2, Reto Knutti3, V. Ramaswamy4, Susan Solomon5, Thomas F. Stocker6, Peter Stott7 & Francis Zwiers8 in Assessing uncertainty in climate simulations


Authors’ response (posted today)

“In their reply to our Commentary1, Forster et al.2 mainly criticize us for points which we did not make regarding climate sensitivity and future global warming and fail to come to grips with our central point, namely that in assessing the skill of climate models by their ability to reproduce warming over the twentieth century, the latest report from the IPCC3 may give a false sense of their predictive capability.”

wrote S. E. Schwartz1, R. J. Charlson2 & H. Rodhe3 in Assessing uncertainty in climate simulations – authors’ response

That original Commentary and its reinforcing Authors’ response echoes points a certain professor made in London, which is what drew my attention to this exchange of letters.

(Here’s the Nature correspondence in PDF format: one is the original ‘Commentary’ and the other is ‘To the Editor’ combined with ‘Authors’ response’.)

If there is more uncertainty in climate change than current research indicates, we should be even more aggressive in tackling emissions—immediately! Strangely, the “uncertainty is too great” camp often prefers to argue that we should do nothing to jeopardize economic growth, favoring further study instead. In so doing, they put zero value on our natural world. (Could it be they just don’t care?)

Still, I am not convinced that the IPCC reports present “too rosy a picture” in terms of scientists’ ability to quantify climate change. (That is just another ‘talking point’, i.e. what the doubters want us to think.) If anything, recent indicators in the natural world tend to show that the IPCC reports under-estimate the rate and extent of change, and that is what we should concentrate efforts on, rather than the uncertainties.

It’s time to look at solutions (that is, what we can do now—and do as much as we can) instead of dwelling on unknowns (that is, what we do not know for sure—hurdles ensuring we do as little as possible).



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  1. Oh dear, I hate it when you can’t tell who is right without having to bother read and understand their words.

    In the spirit of that, let me point out that SS has been badly wrong just recently on a very similar topic: see

  2. Thanks!

    Read? Understand words? Not to mention study graphs and (edits to) notes accompanying graphs. For instance, the first thing I noticed when I printed off the PDF yesterday was that SS had added that green bar from six-year-old TAR to emphasise uncertainty (mixing representations), removed the IPCC copyright notice and changed the explanatory text that accompanied the graph he took from AR4 Figure SPM.2.
    He ADDED and deleted from that IPCC text to make his point about uncertainty thus:

    Figure SPM.2. Global average radiative forcing (RF) estimates and ^UNCERTAINTY^ ranges in 2005 ^RELATIVE TO THE PREINDUSTRIAL CLIMATE^ for anthropogenic carbon dioxide (CO2), methane (CH4), nitrous oxide (N2O) and other important agents and mechanisms, together with the typical geographical extent (spatial scale) of the forcing and the assessed level of scientific understanding (LOSU). The net ^TOTAL^ anthropogenic radiative forcing and its range ^ASSOCIATED UNCERTAINTY (5-95% CONFIDENCE INTERVAL^ are also shown^1^. These require summing asymmetric uncertainty estimates from the component terms, and cannot be obtained by simple addition. Additional forcing factors not included here are considered to have a very low LOSU. Volcanic aerosols contribute an additional natural forcing but are not included in this figure due to their episodic nature. The range for linear contrails does not include other possible effects of aviation on cloudiness. {2.9, Figure 2.20} ^ADDED TO THE FIGURE (GREEN BAR AT BOTTOM AND ASSOCIATED UNCERTAINTY RANGE) IS THE ESTIMATE FROM THE 2001 IPCC REPORT 2 OF THE TOTAL FORCING PROJECTED FOR 2100, WHERE THE UNCERTAINTY DENOTES THE RANGE OF ESTIMATES FOR DIFFERENT EMISSION SCENARIOS^

    In my layman’s take on this, Commentary status (which I guess is not peer-reviewed, even if printed in Nature) still does not warrant unprofessional edits to original IPCC figures, does it?
    Research into the effects aerosols have on climate should have an explicit goal to reduce the uncertainty associated with negative radiative forcings. Otherwise, the study of aerosols may be prolonged forever … since they have local effects and are linked with local meteorological events, this kind of research could continue ad infinitum without necessarily contributing to the understanding of RF components, methinks 😉
    Instead, his website hints at doubts about AGW and suggests further concerns: such as the extent to which aerosol forcings are balancing ghg forcings, and what may happen should emissions of aerosols (emitted in the process of procuring energy from fossil fuels) be curtailed.

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