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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 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 irreversibility. 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.

To make the issue stark, let us begin with a few assumptions. I believe that these assumptions are probably roughly accurate, but none is certain, and I will not try to justify them here. Instead, I will simply take them for granted for the sake of argument.¹

Whether we like it or not, global warming is shaping up as one of the most important challenges of the 21st century. It is going to drive far-reaching changes in how we live and work, power our homes, schools, factories, and office buildings, get from one place to another, manufacture and transport goods, and even farm and manage forests. It touches every aspect of our economy and our lives, and to ignore it is to live in a fantasy land where nothing ever has to change – and where we never have to accept what science tells us about what is happening to our world.

One feature of Utilitarianism that provides its link to rational decision making is that the basic principle of utility demands that one maximize the good. One can disagree about what exactly the good is – perhaps it is pleasure, autonomy, beauty, or some set of items on a list mixing a variety of intrinsic goods. However, whatever the good turns out to be, we ought – morally – to maximize it. A failure to maximize the good is seen as not only a moral failure but also a rational one. So, for example, suppose that a friend of yours offers you a choice between $100 and $10. Most would hold that the rational thing to do is maximize the good and take the $100, all other things being equal. The person who took the $10 option would be considered irrational and imprudent. Maximizing or optimizing one's finances, all other things being equal, would be prudent, but the general point about maximizing carries over to the moral area. What one ought to do, morally, is maximize the good. In the moral area, this means maximizing human well-being, impartially considered. The above illustration is an artificial one, and real life introduces all sorts of complexities such as how to weigh disparate goods and how to deal with risk and uncertainty. However, the basic point that one ought to maximize the good, or do the best one can, stands.

In this chapter I claim that climate change poses important questions of global justice, both about mitigating the change that is now under way and about adapting to its consequences.¹ I argue for a mixed policy of mitigation and adaptation, and defend one particular approach to mitigation. I also claim that those of us who are rich by global standards and benefit from excess emissions have strenuous duties in our roles as citizens, consumers, producers, and so on to reduce our emissions and to finance adaptation.

Although the general theory of greenhouse warming has been understood by scientists since the end of the nineteenth century, an international regime to address the problem of climate change began to develop only in the late 1980s.¹ In the decade and a half since then, the regime has undergone a remarkable evolution. In 1992, states adopted the UN Framework Convention on Climate Change (UNFCCC), which took effect in 1994 and serves as the “constitution” for the international climate change regime.² In 1997, the UNFCCC was supplemented by the Kyoto Protocol, which requires industrialized countries to reduce their emissions of carbon dioxide and five other gases that contribute to the greenhouse effect (so-called “greenhouse gases” or GHGs for short). And the 2001 Marrakesh Accords further elaborate the Kyoto Protocol's regulatory regime, setting forth detailed rules for how the Kyoto Protocol will operate.

A variety of gases, including water vapor (H2O), carbon dioxide (CO2), methane (CH4), and nitrous oxide (N2O), add to the radiative forcing of Earth's atmosphere, meaning that they absorb certain wavelengths of infrared radiation (heat) that is leaving the Earth and thus raise the temperature of its atmosphere. Since glass has the same effect on the loss of heat from a greenhouse, these gases are known as “greenhouse” gases. It is fortunate that these gases are found in the atmosphere; without its natural greenhouse effect, Earth's temperature would be below the freezing point, and all waters on its surface would be ice. However, for the past 100 years or so, the concentrations of CO2, CH4, and N2O in the atmosphere have been rising as a result of human activities. An increase in the radiative forcing of Earth's atmosphere is destined to cause global warming, superimposed on the natural climate cycles that have characterized Earth's history.

Fairness is central to any multilateral regime, that is, any agreement between multiple nation-states to address and resolve a common problem. Climate change mitigation is among the key global environmental concerns that will require a common agenda, approach, and set of actions by the community of nations. To that end, global climate negotiations under the United Nations Framework Convention on Climate Change (UNFCCC, 1992) are centered on establishing a multilateral framework to control greenhouse gas (GHG) emissions from all nations and to help those who would be affected by climate change.