Climate Change and Variability: Lessons from Earth System History

The challenge to develop a robust understanding of the coupled earth system and then to develop a predictive capability for natural variability and future climate change is limited, among other things, by the length of the instrumented record and by our ability to demonstrate the capability of climate models to simulate conditions very different from the present day. The importance of Earth system history to global change research stems from unique capabilities to: (1) assess the temporal and spatial characteristics of ecosystems and natural climate variability, (2) define the nature of Earth sensitivity to a large number of forcing factors, including changes in ocean circulation and in greenhouse gases, (3) examine the integrated climatic, chemical and biologic response of the Earth system to a variety of spatial and temporal perturbations, (4) validate the predictions of numerical models for conditions very different from the present day, and (5) assess the rates of change associated with the evolution of the Earth and its components. Earth system history provides a great diversity of examples yielding a remarkable opportunity to develop insights into a broad range of issues and problems associated with the evolution of our planet. Three examples provide a focus for discussion. First, a careful analysis of climate proxies over the last millennia demonstrates the distinctive character of climate changes over the last half of the 20th century. Second, Earth system history provides a series of known forcing factors or perturbations that can be utilized to examine potential changes in climate. For example, known variations in freshwater fluxes in polar latitudes due to modest ice sheet melt provide a remarkable opportunity to examine the sensitivity of the ocean circulation to potential future perturbations. Third, High resolution simulation of European climates during Oxygen Isotope Stage 3 (approximately 30,000 to 50,000 years ago at the time of transition from Neanderthalers to modern humans) permits careful model-data comparison, and therefore a test of the sensitivity of climate models to external forcing.