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Atmospheric Sciences & Global Change
Research Highlights

July 2005

Calculations for Rising Global Temperature Vary Significantly Based on Greenhouse Gas Emission Models

Critics of the Intergovernmental Panel on Climate Change's Special Report (published in 2000) on Emission Scenarios claim that the use of market exchange rates (MER) rather than purchasing power parity (PPP) to measure the Gross Domestic Product—the primary indicator of economic activity used in the analysis of national economies—has led to a signifi cant upward bias in projections of greenhouse gas emissions, and hence unrealistically high future temperature forecasts and economic impacts.

Pacific Northwest National Laboratory's Jae Edmonds, an international expert in climate change, joined with Alan Manne, Professor Emeritus at Stanford University, and Richard Richels, Director of Global Climate Change research at EPRI, to explore the degree to which the use of PPP rather than MER could affect global mean surface temperature estimates in the absence of policies to limit emissions of greenhouse gases.

Employing an integrated assessment model designed to examine a variety of issues in the climate debate they found that under some conditions using labor productivity growth assumptions based on PPP lead to lower global mean surface temperatures than when MER was used. Differences between a PPP-based and a MERbased scenario were explored using two alternative reference cases. In both instances differences between a PPP-based and MER-based scenario were larger at the end of the 21st Century than earlier, and depended on several factors, such as the reference scenario level of labor productivity. The authors concluded that using the wrong variable could systematically bias results under certain conditions, but that the scale of the potential error was smaller than uncertainties associated with core determinants of potential future climate change such as, for example, the base rate of growth in labor productivity and the climate sensitivity.

This work was begun in late 2003 and completed in the summer of 2004.

Shifting from MER to PPP reduced global mean surface temperature in the year 2100 from 5.0 to 5.6 percent despite the fact that corresponding reductions in 2100 annual emissions ranged from seven to 15 percent. The smaller effect on temperature refl ects inertia in the global atmosphere-climate system stemming from such factors as the cumulative nature of carbon emissions, ocean thermal lag, and the logarithmic relationship between CO2 concentrations and radiative forcing.

While signifi cant, changes in climate resulting from a shift from MER to PPP were smaller than changes associated with alternative reference economic growth assumptions. Changes in global CO2 emissions and concentrations, and global mean surface temperature that resulted from alternative economic growth assumptions were respectively 35 percent, 15 percent, and 10 percent in 2100.

The authors showed that under some circumstances using PPP could yield significantly different results than using MER. Thus, care should be taken when choosing measures of economic activity, particularly when scenarios are defined in terms of some arbitrary end state of economic welfare. Another key observation was that other factors, such as the reference scenario rate of economic growth, particularly in developing nations, future energy technologies, and climate sensitivity, remain larger sources of overall uncertainty in determining long-term climate change.

The Intergovernmental Panel on Climate Change is a body of climate change experts that regularly conduct and publish assessments on the main areas of concern for influencing climate change. Edmonds has served as the lead author on several of theses assessments. The findings by Edmonds, Manne and Richels have been accepted for publication in Climatic Change.

About Jae Edmonds

Dr. Jae Edmonds heads an international global change research program with active collaborations in more than a dozen institutions and countries around the world, and he is the principal investigator for the Global Energy Technology Strategy Program, an international public-private collaboration addressing climate change. His research is focused on the integrated assessment of climate change and the examination of interactions between energy, technology, policy and the environment. He has served as the lead author for all three major assessments of the Intergovernmental Panel on Climate Change, frequently delivers testimony before Congress, frequently briefs members of the Executive Branch, such as the Department of Energy, is widely published, and has authored Global Energy Assessing the Future, and A Primer on Greenhouse Gases.

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