The Status of Climate Modeling

The Intergovernmental Panel on Climate Change (Houghton et al., 2001) has summarized of the status of climate modeling in its 2001 assessment of the science of climate change. The scientific consensus is that climate models generally project an increase of temperature for expected increases in greenhouse gases of from 1.5°C to 4.5°C by 2100. The spatial distribution of warming shows that land generally will warm more than oceans in the transient simulations, similar to results of equilibrium results. A minimum in the warming occurs around Antarctica and in the northern North Atlantic in the transient simulations which is related to the deep oceanic mixing in those areas as discussed in Unit 1-10 (see green hatched areas in the sketch of oceanic mixing, Figure 16 . The general tendency is for all models to show a warming, principally in late fall and winter, in high northern latitudes in connection with the reduce surface area of sea ice. The diurnal range (difference between day time maximum and night time minimum) of temperature is reduced over land in most seasons. Effects of aerosols are not uniformly distributed over the globe, but favor a reduced warming in the middle latitudes of the Northern Hemisphere (close to the sources of the aerosol). All models produce an increase in mean global precipitation and an increase in the south Asian monsoon rainfall. Models show decreased intensity of circulation in the North Atlantic ocean, principally because of increased high-latitude precipitation that reduces ocean salinity and hence suppresses deep water production in the North Atlantic (see Unit 1-10).

As of this writing (Feb 2004) one of the most recent simulations, Figure 17, of the climate of the last 140 years in comparison with the observed trend is given by transient global climate model of the National Center for Atmospheric Research reported by Wigley and available in C&E News (Hilleman, 1999) . This simulation includes the warming effect of enhanced greenhouse gas concentrations, cooling effects of sulfate aerosols, and variations due to fluctuating solar radiation. Addition of aerosols to the model has suppressed the warming as was discussed in Unit 1-12. It was pointed out in that unit that an important difference between CO₂ warming and aerosol cooling is that the longer lifetime of CO₂ means continuing emissions, even at a non-increasing rate, lead to increased radiative forcing. For aerosols, with limited lifetimes, constant emission rates simply replace the aerosol particles being washed out of the atmosphere. So the aerosol radiative forcing for emissions is constant. This fact is evident in the simulation showing the individual effects of CO₂ and aerosols. The effect of fluctuations in output of the sun apparently have settled two of the recent criticisms of global climate model simulations of the 20th century, namely that their results showed substantial warming before the CO₂ levels had a chance to build up and that the cooling of the climate between 1950 and 1970 could not be simulated. Evidently, variations in output of the sun have been a significant factor in causing these two signatures of the global mean temperature record.

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