School of Science Research Highlights
The Climate Conundrum: Challenging Science, Economics, Technology, and Policy
Ron Prinn, ScD '71
TEPCO Professor of Atmospheric Science, Department of Earth, Atmospheric and Planetary Sciences
I must confess that my views have evolved over the many years that I have worked in climate science. Twelve years ago, at the 1997 “Countdown to Kyoto” hearings in the U.S. House of Representatives, I testified that I was not convinced that human activity had yet made a detectable impact on the natural variations of the climate. But in the ten years that followed—amid the observations of continued overall warming, and the recent improvements in climate theory and modeling—I had good reason to change my conclusions. By the time I was invited to appear before the U.S. House Ways and Means Committee in 2007, I could confirm that human activity was indeed affecting the Earth’s climate.
That we are experiencing a change of climate is well documented: An examination of the actual temperature records since 1860 shows some ups and a few downs, but an overall warming of about 0.8°C—a seemingly small, but yet significant increase. However, the crux of the intense debates on climate change is not about whether warming has occurred; rather, it is about whether warming is the result of human activity or is simply part of the naturally occurring fluctuations in temperature observed over the Earth’s history.
Central to these debates is the influence of greenhouse gases on global warming. Any global imbalance between the energy the Earth receives as visible light from the Sun and the energy it radiates back to space as infrared radiation will drive global warming or cooling. The greenhouse effect is one driver of climatic warming, where clouds and gases absorb outgoing infrared radiation and re-emit it back toward the Earth’s surface, causing temperatures to rise. A significant portion of the greenhouse effect occurs naturally through quickly dissipating water vapor, and is countered by the cooling properties of snow, desert sand, clouds, and colorless sulfate aerosols that reflect sunlight back to space.
However, concerns that human activity is propelling global warming arise from the accumulation of long-lived greenhouse gases, notably carbon dioxide, methane, nitrous oxide, chlorofluorocarbons, and lower atmospheric ozone. The concentrations of these gases have increased substantially over the past two centuries, due in large part to human activity that can upset the long-term balance between global warming and cooling mechanisms.
When I first testified at the U.S. House of Representatives in 1997, there were not enough powerful observations, theories, and models to sort out how much of the climate change we had experienced could be attributed to a natural warming phenomenon and how much could be attributed to human activity. But since then, the Intergovernmental Panel on Climate Change (IPCC) has combined multiple models to separate the “noise” of natural variability in climate from the “signal” of human-caused changes. Model runs that include human influences and those that exclude them were compared. If human activity has a negligible effect on the climate, then models that exclude human influence should be able to simulate the patterns of climate change already observed over the past 50 to 100 years. The reverse is also true: If human activity has a significant impact on the climate, then those models that account for human influence should match observed patterns. The IPCC concluded in 2007 that the latter case was correct: There is a greater than 90 percent chance that most of the observed increase in globally averaged temperatures since the mid-20th century is due to the observed increase in anthropogenic greenhouse gas levels.
Given this human influence, integrating and understand-ing the diverse human and natural components of the problem is essential if we are to objectively address uncertainty in forecasts and inform policy development and implementation. Toward this end, over the past 15 years at MIT, we have developed the Integrated Global System Model (IGSM). To make effective forecasts of climate change, the IGSM combines models not only of natural biogeochemical cycles, climate dynamics, and natural ecosystems, but also urban air pollution and the increase in emissions from such essential human activities as energy and food production linked with economic growth.
Though the IGSM accounts for numerous variables affecting climate change, there is still some amount of uncertainty associated with climate forecasts—a model, after all, can only give us probable outcomes. Toward this end, we have used thousands of IGSM runs applying different assumptions to quantify the uncertainty of future climate change forecasts. We can provide the probability range of outcomes expected for proposed policies and account for important feedback relationships between submodels, the sensitivities of certain policy-relevant variables (such as rainfall, temperature, and ecosystem state) to the accuracy of assumptions built in the submodels, structural uncertainties present in existing models, and uncertainties in economic parameters such as labor productivity and efficiency growth.
No matter how persuasive comprehensive models like the IGSM are to the scientific community, the results of such an uncertainty study must be communicated with force and clarity to the public and policymakers to be useful. My colleagues and I have found our “greenhouse gamble” wheels to be effective in communicating the value of climate policy despite the uncertainties. The probabilities of various amounts of warming from the above IGSM runs are projected onto two exemplary wheels. One represents the range of probable results when implementing no policy, and the other reflects the results from implementing a stringent policy that keeps total long-lived atmospheric greenhouse gas levels in the year 2100 just below the equivalent of 660 parts per million (ppm) of CO2. If there are no significant efforts to curb greenhouse gas emissions, the “no policy” wheel shows about a 1-in-4 chance of greater than 6 °C warming between now and 2100. Most climate scientists regard such an amount of warming as undeniably dangerous. The “policy” wheel indicates that the odds of exceeding a 6°C warming drop to less than 1 in 400, clearly showing the value of the policy in lowering the risks. Therefore, the exact odds of various amounts of warming depicted in the two wheels are not as important as the differences between them.
To illustrate the dangers of a 6°C warming, I note that the warming of the Arctic and Antarctic regions is predicted to be about twice that of the global average warming shown on these wheels. A 12°C polar warming would undoubtedly lead to total loss of the Greenland and West Antarctic ice sheets that together contain the equivalent of 12 meters (39 feet) of sea-level rise. The last time the polar latitudes were only a few to several degrees warmer than present for an extended period (about 125,000 years ago), reductions in polar ice volume led to 4 to 6 meters of sea-level rise. Arctic tundra and frozen soils (which contain the equivalent of over 200 years of current fossil fuel carbon emissions that would be released on melting) and Arctic summer sea ice cover (an already diminishing polar cooling mechanism) are also vulnerable.
We cannot wait for perfection in either climate forecasts or impact assessments before taking action. The long-lived greenhouse gases emitted today will last from decades to centuries in the atmosphere. Added to this is the multi-decade period needed to change the global infrastructure for energy and agricultural production and utilization without serious economic impacts.