To the Editor
— In their Commentary “Quantifying climate change—too rosy a picture?”, Schwartz et al.1 suggest that the estimated uncertainties associated with future warming presented in the Intergovernmental Panel on Climate Change Fourth Assessment Report (AR4)2 do not correspond to the AR4 assessment of uncertainties associated with radiative forcing (that is, a measure of the influence that a factor has in altering the Earth's balance of energy and thereby causing warming or cooling of surface temperatures). In doing so, they criticize estimates of climate sensitivity (that is, the increase in global mean surface temperature expected from a doubling of atmospheric CO2 levels) and estimates of future warming. 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.
AR4 Fig. SPM-4 illustrates that no model simulation has reproduced observed warming by natural forcing alone for any continent (note that the data for Antarctica were insufficient to make an assessment for the AR4). Atmosphere/Ocean Global Climate Models (AOGCMs), however, are able to reproduce the observed warming on global and continental scales when taking historical natural and anthropogenic forcing into account.
It is clear that the twentieth century climate simulations in AR4 Fig. SPM-4 do not span the full range of uncertainty in radiative forcing or climate sensitivity. As Schwartz et al. point out, and as is well known, models with differing sensitivities and forcings can produce similarly skilful twentieth century simulations. However, the AR4 assessment did not derive estimates of climate sensitivity, of future warming, or of past warming attributable to greenhouse gases directly from these simulations2. Instead, uncertainty ranges for climate system properties such as climate sensitivity3,4 were derived from many sources including constraints from observed climate change and from studies that systematically varied uncertain model parameters and forcing4, the latter often covering or extending beyond the uncertainty range from the AR4 assessment (ref. 5).
Many estimates separate the observed temperature change caused by greenhouse gases from that caused by aerosols using, for example, temporal changes in aerosols4 and difference in warming between the Southern and Northern Hemisphere, which is partly a response to different amounts of aerosols in each region. In contrast to the suggestion of Schwartz et al. global climate model estimates are just one of many sources of information on climate sensitivity that are accounted for in the AR4 “likely 2–4.5 °C” range3. Indeed, the AR4 assessed the climate sensitivity to have up to a 33% chance of being outside the range of the global climate models.
In the AR4 assessment, 'likely' ranges for future changes in global mean temperature also result from the consideration of multiple sources of uncertainty. This is reflected by the grey ranges in AR4 Fig. SPM-5 (shown here as Fig. 2), which indicate the likely warming in 2100 depending on which emissions scenario is followed. These ranges are substantially wider than those of individual global climate models2,3 depicted by the coloured bands. Nevertheless, despite wide uncertainty ranges in the rate of warming in response to forcing (transient climate response) and in the total amount of forcing, some combinations of transient climate response and forcing are inconsistent with the observed record because they would yield too strong or too weak overall warming, or the wrong time-evolution or spatial pattern of warming. This has been used to narrow the range of uncertainty in future warming presented in the AR4.
A further problem in the analysis of Schwartz et al. is that they assume the uncertainties associated with forcing and with temperature response at year 2000 are linearly related (Schwartz et al. Figure 2). Such a simple relationship does not exist because temperature today is not in equilibrium with current forcing, but instead is influenced by historical forcing. Furthermore, ocean heat uptake is likely to be different for each model. Additionally, Schwartz et al. ignore that the twentieth century model results in their Fig. 2 are compared with a 1901–1950 baseline specific to each model. The model range for the year 2000 is therefore smaller than if the models had been compared with the baseline year of 1750, which is used for estimates of radiative forcing.
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.
References
Schwartz, S.E., Charlson, R.J., & Rhode, H. Nat. Rep. Clim. Change 1, 23–24 (2007).
Solomon, S. et al. (eds) Climate Change 2007: The Physical Science Basis (Cambridge Univ. Press for the Intergovernmental Panel on Climate Change, Cambridge, 2007).
Meehl, G. A. et al. in Climate Change 2007: The Physical Science Basis (eds Solomon, S. et al.) Ch. 10 (Cambridge Univ. Press for the Intergovernmental Panel on Climate Change, Cambridge, 2007).
Hegerl, G. C. et al. in Climate Change 2007: The Physical Science Basis (eds Solomon, S. et al.) Ch. 9 (Cambridge Univ. Press for the Intergovernmental Panel on Climate Change, Cambridge, 2007).
Forster, P. et al. in Climate Change 2007: The Physical Science Basis (eds Solomon, S. et al.) Ch. 2 (Cambridge Univ. Press for the Intergovernmental Panel on Climate Change, Cambridge, 2007).
Author information
Authors and Affiliations
Corresponding author
Rights and permissions
About this article
Cite this article
Forster, P., Hegerl, G., Knutti, R. et al. Assessing uncertainty in climate simulations. Nature Clim Change 1, 63 (2007). https://doi.org/10.1038/climate.2007.46a
Issue Date:
DOI: https://doi.org/10.1038/climate.2007.46a
This article is cited by
-
Assessing uncertainty in climate simulations - authors' response
Nature Climate Change (2007)