Michael Schlesinger, from the University of Illinois, discussed the major uncertainties that affect estimates of climate sensitivity obtained by simulating the record of observed hemispheric-mean near-surface air temperatures using a simple climate-ocean model.

One of the largest uncertainties is in radiative forcing. With a simple energy balance climate/upwelling diffusion-ocean model, he and Natalia Andronova carried out a series of simulations using various combinations of radiative forcing agents (including long-lived greenhouse gases [GHGs], tropospheric ozone, volcanoes, solar irradiance changes, and anthropogenic sulfate aerosols), and determined which values for sensitivity (Seq) and sulfate radiative forcing in a year (1990) gave the best reproduction of the observed temperature record. They found that the sensitivity value obtained from the model is highly dependent upon which forcings are included (Schlesinger and Ramankutty, 1992; Andronova and Schlesinger, 2000, 2001).

The climate sensitivity needed to reproduce the observed changes in near-surface temperature from the middle of the nineteenth century to the present is inversely proportional to the magnitude of the radiative forcing at the top of the atmosphere (or tropopause). Thus, if the radiative forcing is increased—for example, by including putative changes in the solar irradiance—the climate sensitivity needed to reproduce the observed changes in near-surface temperature decreases by about 50 percent (see Schlesinger and Ramankutty, 1992; Andronova and Schlesinger, 2001). However, we do not know whether the solar irradiance changed as has been constructed from indirect evidence. This uncertainty in the radiative forcing, not only by the sun but also by volcanoes and anthropogenic aerosols, contributes significant uncertainty in the inferred climate sensitivity.

As the model simulation proceeds through the decades of the last century, the researchers determine what sensitivity value is needed in order to reproduce the observational record, given the radiative forcing that occurred up to that time. If the sulfate radiative forcing is known, one can home in on the true sensitivity value by learning over time. In addition, if they can estimate (rather than prescribe) sulfate radiative forcing, then they can continue this learning process into the future. Thus, the uncertainty in climate sensitivity due to climate noise can be reduced by learning over time, that is, by performing future estimations using longer observational records.

These studies utilized two different temperature records (Folland et al., 2001; Jones and Moberg, 2003) that differed in their southern hemisphere values, and thus in the interhemispheric temperature difference. As a result, the estimated sulfate radiative forcing, and thus the estimated sensitivity value, differed significantly depending upon which data record was used. Thus, if the radiative forcing by aerosols cannot be learned exogenously, but only endogenously from the observed temperature changes, then the uncertainty in southern hemisphere temperature changes must be reduced. With better aerosol radiative forcing observations, one could prescribe the forcing in the model, rather than the current approach of estimating forcing from the interhemispheric temperature difference. As a result, one could use global mean temperature values as a constraint, and thus diminish the uncertainties related to climatic noise.


Leggett: Another problem is that the model considers only sulfate; the uncertainties related to other types of aerosols have not even been accounted for.

Penner: Note that the interhemispheric temperature differences would be amplified if the radiative forcing associated with biomass aerosols were included in the analysis. Neglecting this influence means that the uncertainty in southern hemisphere temperature changes is even more important.

Schlesinger: The effect of other aerosol types was indirectly considered in one model scenario in which there was no aerosol forcing. This case represented the possible cancellation of the negative radiative forcing from sulfate aerosol by the (putative) positive radiative forcing from carbonaceous aerosol.

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