Session V7: Current Issues in Climate Change

Overview of Climate Change 2007: IPCC WG1 Key Findings

The Intergovernmental Panel on Climate Change (IPCC) was jointly established by the World Meteorological Organization and the United Nations Environment Programme (UNEP) in 1988. The purpose of IPCC is to assess available information on the science of climate change and to provide policy-relevant but not policy-prescriptive assessments of interest to policymakers, scientists, and the public. IPCC will release its fourth comprehensive Working Group 1 (WG1) assessment report on the state of understanding of the physical science basis of climate change in early 2007. This talk will summarize the key scientific findings of that report, including the understanding of observations of changes in the atmosphere, ocean, and ice, forcing agents such as carbon dioxide and aerosol, feedbacks in the climate system, paleoclimatic observations and theory, and projections of future changes in coming decades and centuries.   

The uncertain hockey stick: a statistical perspective on the reconstruction of past temperatures

A reconstruction of past temperatures based on proxies is inherently a statistical process and a deliberate statistical model for the reconstruction can also provide companion measures of uncertainty. This view is often missed in the heat of debating the merits of different analyses and interpretations of paleoclimate data. Although statistical error is acknowledged to be just one component of the total uncertainty in a reconstruction, it can provide a valuable yardstick for comparing different reconstructions or drawing inferences about features. In this talk we suggest a framework where the reconstruction is expressed as a conditional distribution of the temperatures given the proxies. Random draws from this distribution provide an ensemble of reconstructions where the spread among ensemble members is a valid statistical measure of uncertainty. This approach is illustrated for Northern Hemisphere temperatures and the multi-proxy data used by Mann, Bradley and Hughes (1999). Here we explore the scope of the statistical assumptions needed to carry through a rigorous analysis and use Monte Carlo sampling to determine the uncertainty in maxima or other complicated statistics in the reconstructed series. The principles behind this simple example for the Northern Hemisphere can be extended to regional reconstructions, incorporation of additional types proxies and the use of statistics from numerical models.   

Counting the Clouds

Cloud processes are very important for the global circulation of the atmosphere. It is now possible, though very expensive, to simulate the global circulation of the atmosphere using a model with resolution fine enough to explicitly represent the larger individual clouds. An impressive preliminary calculation of this type has already been performed by Japanese scientists, using the Earth Simulator. Within the next few years, such global cloud-resolving models (GCRMs) will be applied to weather prediction, and later they will be used in climate-change simulations. A ``multi-scale modeling framework'' can be used as a bridge between current low-resolution climate models and future GCRMs.   

Simulating Earth's Past and Future Greenhouse Climates

Earth's climate has experienced dramatic changes through geologic time. Specific intervals in the geologic record indicate climates that were much warmer than the past few thousand years. These warm climates, or greenhouse climate, are associated with times of elevated levels of carbon dioxide relative to current levels. For a number of these greenhouse climates major perturbations to ocean circulation occurred, which had significant impacts on marine life. Our ability to simulate these past climates with global climate models provide tests for models that are used to project Earth's future climate. I will present simulations of greenhouse climates of the past using a comprehensive global climate model, the Community Climate System Model. I will then present simulations of Earth's potential climate for the end of the 21$^{st}$ century. I will show how our understanding of simulations of past greenhouse climates can provide information on where Earth's climate is heading for the end of this century and beyond. I will also indicate how Earth's greenhouse effect has evolved over Earth's history and how these past changes provide a context for future changes in Earth's greenhouse effect.