Assessment of Twentieth-century regional surface temperature trends using the GFDL CM2 coupled models

Historical climate simulations of the period 1861-2000 using two new GFDL global climate models (CM2.0 and CM2.1) are compared with observed surface temperatures. "All-forcing" runs include the effects of changes in well-mixed greenhouse gases, ozone, sulfates, black and organic carbon, volcanic aerosols, solar flux, and land-cover. Indirect effects of tropospheric aerosols on clouds and precipitation processes are not included. Ensembles of size 3 (CM2.0) and 5 (CM2.1) with all forcings are analyzed, along with smaller ensembles of natural-only and anthropogenic-only forcing, and multi-century control runs with no external forcing.

Observed warming trends on the global scale and in many regions are simulated fairly realistically in the all-forcing and anthropogenic-only forcing runs. In the all-forcing runs, the ensemble mean simulated warming over 1949-2000 is significantly smaller than observed over Northern Asia, Canada, and much of the Indian Ocean, but greater than observed over parts of the tropical North Atlantic, tropical central Pacific, and South America. Differences in Arctic Oscillation behavior between model and observation contribute substantially to the Northern Asia temperature discrepancies. In the all-forcing and natural-only runs, a temporary global cooling in the models during the 1880s not evident in the observed temperature records is volcanically forced. El Niño interactions complicate comparisons of observed and simulated temperature records for the El Chichón and Pinatubo eruptions during the early 1980s and early 1990s.

The simulations support previous findings that 20th century global warming has resulted from a combination of natural and anthropogenic forcing, with anthropogenic forcing being the dominant cause of the pronounced late-20th century warming. The regional results provide evidence for an emergent anthropogenic warming signal over many, if not most, regions of the globe. The warming signal has emerged rather monotonically in the Indian Ocean/Western Pacific warm pool during the past half century. The tropical and subtropical North Atlantic and the tropical eastern Pacific are examples of regions where the anthropogenic warming signal now appears to be emerging from a background of more substantial multi-decadal variability.