The first group focuses on climate change science. In the opening chapter of this section, Jerry Mahlman (Senior Research Fellow at the National Center for Atmospheric Research) describes what he terms the "global warming dilemma.'' According to Mahlman, the scientific community has reached an effective consensus that immediate and quite aggressive steps would be required to avoid climatic changes that are large in comparison with those observed in the Earth's geological record. Stabilizing atmospheric concentrations of carbon dioxide, for example, would require permanent emissions reductions of roughly 60-80%. Moreover, the long lags in the Earth's response to changes in the composition of the atmosphere suggests that even this stringent scenario would be insufficient to prevent moderate temperature increases in the coming decades. Based on his reading of the scientific literature, Mahlman concludes that deferring action until climate change has broadly recognized deleterious effects would most likely "lock in'' quite profound environmental impacts with effects lasting for centuries and even xi millennia. In terms of mechanisms, this argument appeals to the view that today's greenhouse gas emissions might use up the Earth's assimilative capacity, thus increasing the length of time that greenhouse gases remain in the atmosphere. On top of this, Mahlman notes that most scientific studies have emphasized time scales of one century or less in evaluating climate impacts. But impacts such as sea-level rise, which would be strongly affected by the melting and breakup of glacial formations such as the West Antarctic Ice Sheet, occur over much longer time horizons with a high degree of irre-versibility. This makes climate change an issue of intergenerational fairness that pits present society's willingness to bear significant economic costs against the goal of protecting future generations from environmental harms that are hypothetical and yet potentially catastrophic.

The chapter by William Schlesinger (Dean of the Nicholas School of the Environment at Duke University) describes the basic science of the carbon cycle and the pathways through which human activities affect global climate. In particular, Schlesinger examines the potential for employing land-use and land-use change as techniques for removing carbon dioxide from the atmosphere, thus partially mitigating the need to reduce fossil consumption as a means of stabilizing the Earth's climate. This topic is of major importance given the emphasis placed on carbon sequestration in international negotiations. In particular, the United States has long held that emissions credits should be conferred on countries that undertake actions to store carbon dioxide in managed ecosystems. At present, fossil fuel consumption emits roughly 6 billion tons of carbon to the atmosphere on an annual basis. Schlesinger carefully traces the physical and biological processes through which carbon dioxide is transferred to the oceans, biomass, and terrestrial and marine sediments. While it is well recognized that deforestation in tropical nations is a net source of carbon dioxide emissions, the magnitude of this effect is largely offset by the regrowth of forests on abandoned farmland in North America and Eurasia. This observation has fostered hope that reforestation could substantially reduce the pace of climate change. Schlesinger, however, notes that current forecasts suggest that carbon dioxide emissions will rise to 15 billion tons per year by 2050, while even optimistic studies suggest that forest regrowth could reduce the future accumulation of carbon dioxide in the atmosphere by no more than 5-10%. While Schlesinger acknowledges the potential for technologies that capture carbon dioxide at the point of fuel consumption with subsequent geological storage, he notes that substantial uncertainties surround both the cost-effectiveness and the environmental impacts of this approach. Accordingly, he concludes that substantial and immediate reductions in fossil fuel consumption would be required if decision-makers aimed to reduce the rate of climate change with reasonable confidence.

In the concluding chapter of this section, John Weatherly (Cold Regions Research and Environmental Laboratory) explores the notion that the Arctic may be the "canary in the coal mine'' of the climate change debate. As is well known, general circulation models suggest that the temperature increases caused by greenhouse gas emissions would likely be three times higher at the poles than for the world as a whole. This prediction is matched by the observation that temperatures have already increased significantly in many polar environments and that the thickness of sea ice in the Arctic Ocean has thinned by roughly one-third since the 1960s. As Weatherly notes, changes in sea ice have been accompanied by ecological changes such as a transition from tundra vegetation to woody shrubs in northern Alaska. Perhaps most dramatically, the collapse of the Larsen B ice shelf in Antarctica released 720 billion tons of ice in March of 2002. As Weatherly notes, data from the Earth's geological record suggest that rapid climate change has in the past disrupted the Gulf Stream ocean current that is crucial in maintaining Europe's temperate climate. Accordingly, some scientists speculate that greenhouse gas emissions might ironically give rise to a human-induced ''deep freeze'' in Europe even as (and indeed because) the world as a whole grew warmer. In discussing the current state of scientific opinion on this issue, Weatherly concludes that a major disruption of ocean circulations patterns is unlikely but, if it occurred, would have devastating impacts.

Taken together, Mahlman, Schlesinger, and Weatherly provide a well-rounded picture of what we know about global warming and what we need to learn.

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