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Climate Change and Global Warming Michael E. Mann Department of Environmental Sciences University of Virginia Waxter Environmental Forum Sweet Briar College March 14, 2002 `There is new and stronger evidence that most of the warming observed over the last 50 years is attributable to human activity' Intergovernmental Panel on Climate Change (United Nations), Third Assessment Report, 2001 THE EMPIRICAL RECORD Surface Temperature Changes Climatic Research Unit (‘CRU’), University of East Anglia Surface Temperature Changes Shaded: 20th century Boxes: since mid 19th century Surface Temperature Changes Climate Change 2001: The Scientific Basis, Houghton, J.T., et al. (eds.), Cambridge Univ. Press, Cambridge, 2001 1998 Global Temperature Pattern EL NINO/SOUTHERN OSCILLATION (“ENSO”) Substantial interannual climate variability associated with ENSO, but decadal variability is also evident as well. The recent decadal trend towards El Nino conditions could be natural or anthropogenic. Multivariate ENSO Index (“MEI”) NORTH ATLANTIC OSCILLATION Negative Phase Positive Phase NORTH ATLANTIC OSCILLATION This NAO trend could be Anthropogenic “PACIFIC DECADAL OSCILLATION” Atlantic Multidecadal Oscillation There is evidence of multidecadal natural variability in the North Atlantic ocean circulation CO2 Related? Greenhouse Gases and Warming TREE RINGS CORALS ICE CORES VARVED LAKE SEDIMENTS HISTORICAL DOCUMENTS RECONSTRUCTED GLOBAL TEMPERATURE PATTERNS European Winter Cooling During the Little Ice Age “Little Ice Age” European Winter Cooling During the Little Ice Age Empirical LIA winter cooling in Europe associated with an NAO trend due to solar irradiance changes, interacting w/ stratospheric atmospheric dynamics and chemistry NASA/GISS Model Shindell, D.T., Schmidt, G.A., Mann, M.E., Rind, D., Waple, A., Solar forcing of regional climate change during the Maunder Minimum, Science, 294, 21492152, 2001. CLIMATE MODELS The climate represents a coupled system consisting of an atmosphere, hydrosphere, biosphere, and cryosphere General Circulation Models take into account the full threedimensional structure of the atmosphere and ocean GCMs do a fairly good job of describing the seasonal cycle in surface temperature This alone doesn’t guarantee that they should do a good job in describing climate change! January Temp (observations) January Temp (model) Modeled Internal Natural Variability Observations INFLUENCE OF EXTERNAL FACTORS The climate is governed by external factors, including the intensity of solar output and volcanic aerosols and greenhouse gas concentrations Solar Variations The climate is governed by external factors, including the intensity of solar output and volcanic aerosols and greenhouse gas concentrations Volcanoes The climate is governed by external factors, including the intensity of solar output and volcanic aerosols and greenhouse gas concentrations The climate is governed by external factors, including the intensity of solar output and volcanic aerosols and greenhouse gas concentrations GREENHOUSE EFFECT? ENHANCED GREENHOUSE EFFECT? CO2 Related? Greenhouse Gases and Warming SIMULATIONS OF CLIMATE CHANGE CLIMATE FORCINGS Volcanism 1000 1200 1400 1600 1800 Solar Natural 2000 Simulated Annual Global Mean Surface Temperatures Climate Change 2001: The Scientific Basis, Houghton, J.T., et al. (eds.), Cambridge Univ. Press, Cambridge, 2001 Forced Model simulations CLIMATE FORCINGS Anthropogenic Greenhouse Gases Industrial Aerosols Simulated Annual Global Mean Surface Temperatures Climate Change 2001: The Scientific Basis, Houghton, J.T., et al. (eds.), Cambridge Univ. Press, Cambridge, 2001 Forced Model simulations Simulated Annual Global Mean Surface Temperatures Climate Change 2001: The Scientific Basis, Houghton, J.T., et al. (eds.), Cambridge Univ. Press, Cambridge, 2001 Forced Model simulations Simulated Annual Global Mean Surface Temperatures Climate Change 2001: The Scientific Basis, Houghton, J.T., et al. (eds.), Cambridge Univ. Press, Cambridge, 2001 Forced Model simulations Energy Balance Model (“EBM”) simulation Science Future Surface Temperatures Trends? Climate Change 2001: The Scientific Basis, Houghton, J.T., et al. (eds.), Cambridge Univ. Press, Cambridge, 2001 CONCLUSIONS •Recent global surface temperatures are unprecedented this century, and likely at least the past millennium •It is difficult to explain the recent surface warming in terms of natural climate variability •Recent surface warming is largely consistent with simulations of the effects of anthropogenic influence on climate •Uncertainties remain regarding the precise sensitivity of the climate to forcing, and the regional details of expected climate changes