Time Scales of Climate Response

Abstract

A coupled atmosphere?ocean general circulation model (AOGCM) is integrated to a near-equilibrium state with the normal, half-normal, and twice-normal amounts of carbon dioxide in the atmosphere. Most of the ocean below the surface layers achieves 70% of the total response almost twice as fast when the changes in radiative forcing are cooling as compared to the case when they are warming the climate system. In the cooling case, the time to achieve 70% of the equilibrium response in the midoceanic depths is about 500?1000 yr. In the warming case, this response time is 1300?1700 yr. In the Pacific Ocean and the bottom half of the Atlantic Ocean basins, the response is similar to the global response in that the cooling case results in a shorter response time scale. In the upper half of the Atlantic basin, the cooling response time scales are somewhat longer than in the warming case due to changes in the oceanic thermohaline circulation. In the oceanic surface mixed layer and atmosphere, the response time scale is closely coupled. In the Southern Hemisphere, the near-surface response time is slightly faster in the cooling case. However in the Northern Hemisphere, the near-surface response times are faster in the warming case by more than 500 yr at times during the integrations. In the Northern Hemisphere, both the cooling and warming cases have much shorter response time scales than found in the Southern Hemisphere. Oceanic mixing of heat is the key in determining these time scales. It is shown that the model's simulation of present-day radiocarbon and chlorofluorocarbon (CFC) distributions compares favorably to the observations indicating that the quantitative time scales may be realistic.