Sensitivity experiments with general circulation models show that uplift of plateau and mountain regions in Southern Asia and the American west during the late Cenozoic was an important factor in the evolution of Northern Hemisphere climate. The climatic trends simulated in the uplift experiments agree in direction with most trends observed in the geological record, including the tendencies toward greater regional differentiation of climate, and particularly the fragmentation into wetter and drier climatic patterns at middle latitudes. These climatic trends result from (1) increased orographic diversion of the mid-latitude westerlies, and (2) increased summer heating and winter cooling over the plateaus, which enhances seasonally reversing (monsoonal) changes in wind directions.

Most previous hypotheses addressing the physical impact of orography on climate have focused on mountain ranges and have stressed relatively local responses such as upslope precipitation maxima, cooling of mountain crests due to lapse-rate effects on rising terrain, and lee-side rainshadow effects. In contrast, our results emphasise the importance of large-scale plateau orography. By redirecting the basic directions of wind flow both at surface and upper-tropospheric levels, these rising plateaux cause far-reaching climatic changes that extend across the continents as well as over the oceans.