Simulation of Past Climates: Studies using NCAR Climate Models and Computers

While climate models are being developed and improved to address questions of present and future climates, they are also being used to explain the causes of past climates and climate change. The use of climate models to study and understand the mechanisms of past climate changes began at NCAR, and elsewhere, in the 1970s, and continues in expanding fashion today. Warren Washington has played a key role in NCAR's work in this fascinating area through his leadership in recognizing the importance and relevance of studies of past climate, through his direct involvement in this research, through his encouragement of NCAR scientists and visiting scientists in pursuing this work, and through his support of university scientists in using NCAR climate models and computers. This paper reviews some of these studies. In the 1970s, the NCAR general circulation model was used in initial studies of ice age climates, of the warm climates of the Cretaceous with changed positions of the continents, of the response of climate to changes in the height of mountains and plateaus, and of the response of climate to changes in greenhouse gas concentrations. Warren Washington was a coauthor in all of these studies and thereby helped launch NCAR's involvement in this area of interdisciplinary research involving geology and ecology as well as climatology. In the 1980s, and especially with the development of the NCAR Community Climate Model, version 0 (CCM0), the study of the causes of climate change expanded to include other climate forcing mechanisms ( including the role of orbital changes). In the 1980s, and continuing into the 2000s, improved versions of the CCM, and other climate models, have incorporated more interactive components - soil moisture, sea ice, mixed-layer ocean, dynamic ocean, dynamic vegetation. These multi-component climate models are being used to greatly extend the early studies of past climates by simulating both the initial response of climate to changes in external forcing and the subsequent internal adjustments and feedbacks within the atmosphere-ocean-cryosphere-biosphere system. Some of the achievements of the past 25 years will be illustrated by comparing results from early studies and recent studies.