Climate model tests of the anthropogenic influence on greenhouse-induced climate change: the role of early human agriculture, industrialization, and vegetation feedbacks

We test the early anthropogenic hypothesis that greenhouse-gas emissions produced by early agricultural activities in recent millennia kept the climate warmer than its natural level and offset an incipient glaciation. We use versions of the NCAR's Community Climate System Model to investigate the natural climate that might exist today if CO 2 and CH 4 concentrations had fallen to their average levels reached during previous interglaciations (while ignoring the effects of aerosol changes). The model is run in a coupled atmosphere–slab ocean configuration with fixed land cover in one experiment and interactive vegetation changes in the other. With lowered greenhouse-gas concentrations, global-mean temperature falls by 2.75 K under fixed land cover and by 3.0 K with vegetation feedbacks included. Of the total global cooling with fixed land cover, 38% (62%) is attributable to early agricultural activities (industrialization), while early agriculture accounts for approximately half of the expanded permanent snow cover area. The large-scale cooling is amplified in polar regions, where mean-annual temperatures fall by up to 9 K and sea-ice area more than doubles in the Southern Hemisphere. In high latitudes, boreal forest is replaced by tundra and tundra by polar desert, causing a higher surface albedo that amplifies regional Arctic cooling by up to 2 K annually and 5 K during spring. The greenhouse cooling in our simulations triggers widespread glacial inception in the Northern Hemisphere, where permanent snow cover expands by 80% (206%) without (with) vegetation feedbacks. The regional pattern of incipient glaciation is strongly influenced by both vegetation feedbacks and atmospheric circulation changes. While these results are fully consistent with the early anthropogenic hypothesis, further simulations with a dynamical ocean, higher spatial resolution, and other models are still needed to investigate the origin of human influence on global climate.