Past Climate

CCR is involved in a wide variety of studies of the past behavior of the climate system and the earth system, including biosphere feedbacks. These studies have intrinsic interest for documenting the earth's environmental history, for examining the interrelationships between civilizations and climate (see civilizations/climate), and for studying the process of coupled earth system behavior at times when the system was changing abruptly, or nonlinearly, or when the magnitude of change was large. An additional result of these studies, one that bears on future climate research, is that we can evaluate the accuracy of our simulations of past climate by comparing them to observations. These comparisons provide a measure of the overall reliability of climate models, a measure of validation that is of key importance when using future climate scenarios to evaluate possible impacts of climate change on food, water and land resources.

The recent period of conventional climatological observations with thermometers and rain guages does not begin to record the full range of possible changes of climate, and therefore it is important to obtain records that can be interpreted in terms climate or other indicators of the state of the earth's environment at past times. Some of this field work is ongoing in CCR and includes paleoecological studies in the Everglades and on Cape Cod (see paleoecology lab). Other field work is part of collaborative research with marine geologists, geologists, paleoecologists, and archeologists at other institutions.

CCR research projects are studying past climates in four time periods and in relation to a variety of changes in external forcing;

The past several thousand years. During this period, climate changes are accurately dated by historical sources, tree rings, corals, annually-laminated lake sediments, annually layered ice cores, or by careful radiocarbon dating. Changes in volcanic activity, also accurately dated, are proving to be one of the important causes of climate variability during this period. Records of solar variability are also being documented. CCR is using climate models to simulate climate changes of the past several thousand years and collaborates with an international network of archeologists in comparing the simulated climate with paleoecological and cultural data (see civilizations/climate).

The past 125,000 years. During this period, the climate has changed from interglacial to glacial and then back to the present-day interglacial. Changes in earth's orbital parameters (the tilt of the earth's rotational axis, the eccentricity of the earth's orbit about the sun, and season of perihelion) are proving to be very important for explaining the fundamental timing of interglacial and glacial events in middle and polar latitudes, and of waxing and waning of summer monsoon cirulations in the northern tropics. However, the phasing and the amplititude of the climate response to orbital changes is very non-linear and involves ocean, ice sheet, and vegetation feedbacks. CCR is using climate models and earth-system models to simulate these changes, and collaborates in this work with a network of individuals at other universities and institutes (Brown, Oregon, USGS, MPI for Biogeochemistry) involved in field work, synthesis of observations, and comparison of observations and models.

The past 10,000,000 years. During this period, climate has generally cooled, leading first to the onset of glaciation of Greenland, and then to the onset of glacial-interglacial cycles in North America and Europe. Changes in the height of mountains and plateaus, produced by tectonic processes, are proving to be very important for explaining the timing of major climatic transitions through this long interval. Changes in ocean circulation, terrestrial vegetation, and, perhaps, decreases in atmospheric carbon dioxide concentration, accompanied major uplift events. Large increases in the elevation and extent of the Tibetan Plateau occurred between 9 and 6 million years ago, and caused major increases in the strength of the Asian summer and winter monsoon circulations. CCR is using earth-system models to simulate these changes, and collaborates with researchers at other universities (Brown, Virginia) and institutes (Earth Environment Institute, Chinese Academy of Sciences) involved in field work, analysis,and data/model comparisons.

The period between about 280 and 250 million years ago. This period is the most recent example of a major global warming, a period when major southern hemisphere ice sheets melted. Changes in the location of the continents, changes in ocean currents, changes in vegetation, and changes in atmospheric carbon dioxide concentration may all have played a role in this global warming event. CCR is using climate models to simulate these changes and works in close collaboration with researchers at the University of Chicago who have assembled data sets of coals, evaporites, and floral indicators for these periods.