CLIMATE CHANGE OVER THE LAST MILLENNIUM: THE ROLE OF NATURAL VARIABILITY

Abstract:

A major challenge is to understand and communicate the causes of observed trends in climate, particularly at local and regional scales. Such trends are often interpreted in the context of rising anthropogenic emissions of greenhouse gases; however, internally-generated (unforced or natural) variability is also a major contributor to regional climate change over periods of several decades and longer. Paleoclimate records provide important insights into the causes of past climate change, whether forced (e.g., in response to volcanic activity) or unforced. In this study, we emphasize the latter by utilizing a large ensemble of simulations with the Community Earth System Model over the Last Millennium. Ensemble members extend from 850 to 2006 using reconstructions for the transient evolution of solar intensity, volcanic emissions, greenhouse gases, aerosols, land use conditions, and orbital parameters, together and individually, in addition to long 850 and 1850 control simulations. These simulations provide an unprecedented opportunity to quantify the role of natural climate variability in producing significant climate events over the last millennium, such as prolonged regional droughts or pluvials, as well as how past forced changes in climate were enhanced or lessened by natural variability. In other words, we use the Last Millennium ensemble to characterize not only the forced climate response, but also the accompanying uncertainty due to internal variability. By examining the range of natural variability under past conditions, we also gain insights into the role of natural variability in future climate change on regional scales.