The future radiative forcing of the Earth's climate system Bill Collins, NCAR

Recent climate change has been very likely caused human-induced changes in the chemical composition of the atmosphere. The Intergovernmental Panel on Climate Change (IPCC) has concluded that these changes have caused a positive radiative forcing of the climate system. Radiative forcing is an "externally imposed perturbation in the radiative energy budget of the Earth’s climate system" (IPCC, 2001). Both natural and anthropogenic (human-induced) factors can introduce radiative forcing of the climate. The anthropogenic factors include increased levels of long-lived greenhouse gases (LLGHGs), ozone, aerosols, and disturbances of the land surface. This presentation focuses on the future radiative forcing of the Earth's climate system and challenges in accurately predicting the evolution of this forcing. The challenges include uncertainties regarding interactions between climate change and efforts to mitigate it; uncertainties in the physics of the atmosphere; and incomplete understanding of feedbacks between climate change and biogeochemical cycles. The next generation of Earth system models (ESMs) developed with support from the SciDAC program will facilitate exploration of these questions regarding radiative forcing and climate change. Quantitative evaluation and validation of climate predictions, however, will require a new class of ESMs that facilitate multi-scale refinements in spatial, temporal, and process resolution. The presentation concludes with the prospects for developing a new class of comprehensive models for the Earth and its natural and human environments.