The ACPI Climate Change Simulations

Abstract. The Parallel Climate Model (PCM) has been used in the Accelerated Climate Prediction Initiative (ACPI) Program to simulate the global climate response to projected CO2, sulfate, and other greenhouse gas forcing under a business-as-usual emissions scenario during the 21st century. In these runs, the oceans were initialized to 1995 conditions by a group from the Scripps Institution of Oceanography and other institutions. An ensemble of three model runs was then carried out to the year 2099 using the projected forcing. Atmospheric data from these runs were saved at 6-hourly intervals (hourly for certain critical fields) to support the ACPI objective of accurately modeling hydrological cycles over the western U.S. It is shown that the initialization to 1995 conditions partly removes the un-forced oceanic temperature and salinity drifts that occurred in the standard 20th century integration. The ACPI runs show a global surface temperature increase of 3–8 ◦C over northern high-latitudes by the end of the 21st century, and 1–2 ◦C over the oceans. This is generally within ±0.1 ◦C of model runs without the 1995 ocean initialization. The exception is in the Antarctic circumpolar ocean where surface air temperature is cooler in the ACPI run; however the ensemble scatter is large in this region. Although the difference in climate at the end of the 21st century is minimal between the ACPI runs and traditionally spun up runs, it might be larger for CGCMs with higher climate sensitivity or larger ocean drifts. Our results suggest that the effect of small errors in the oceans (such as those associated with climate drifts) on CGCM-simulated climate changes for the next 50–100 years may be negligible. 1