There is growing worldwide momentum to address the problem of climate change. We need to reduce the growth of greenhouse gas emissions and eventually bring those emissions significantly below current levels. In contemporary policy debates, efforts to achieve these goals are called mitigation.

Mitigation, however, will not suffice. Even with aggressive global efforts to reduce emissions, the Earth’s climate will continue to change significantly for many decades because of the magnitude of past emissions and the inertia of social and physical systems. Of course, many uncertainties remain about how best to reduce emissions and how the climate system will respond. But we can now say with confidence that rapid climate change and its impacts are at hand. As a result, we face immediate choices about how to temper its worst consequences for vulnerable populations and regions.

Alongside mitigation, then, we also need policies focused on adaptation, on making sensible adjustments in the face of unavoidable changes. Moreover, we need to coordinate adaptation with mitigation, as the success of each will depend on the other. Current efforts to reduce emissions will, in due course, determine the severity of climate change, and thus the degree of adaptation required—or even possible—in the future. At the same time, by discovering at what point climatic change becomes too severe for our capacity to adapt, we will learn more about what levels of emissions might engender serious danger.

Since the second half of the nineteenth century, global temperatures have been unambiguously on the rise. The increase in global average surface temperature, as estimated by the Intergovernmental Panel on Climate Change (IPCC), is around 0.75°C (~1.4°F). Twelve of the thirteen years just prior to 2008 rank among the thirteen warmest years on record.

The scientific evidence for a human fingerprint on this global warming is now overwhelming. Emissions of greenhouse gases, such as carbon dioxide and methane, from human activities—most crucially, the burning of fossil fuels, but also agricultural practices, deforestation, and cement production—are the primary drivers, particularly of the rapid warming since the 1970s.

The greenhouse effect and its intensification by human-induced emissions of greenhouse gases are well understood and solidly grounded in basic science. Moreover, we are already seeing the impacts of warming—such troubling changes as the melting of mountain glaciers, the Greenland ice sheet, and polar sea ice; rising and increasingly acidic seas; increasing severity of droughts, heat waves, fires, and hurricanes (the intensity and frequency of extreme events can change substantially with small changes in average conditions); and changes in the lifecycles and ranges of plants and animals.

Nevertheless, the future course of climate change is deeply uncertain. This uncertainty has two central sources: what we do (globally) to reduce emissions of greenhouse gases and how the natural climate system responds to the future course of greenhouse-gas concentrations in the atmosphere. The IPCC Fourth Assessment Report (AR4) includes climate model projections based upon six “storylines,” possible future worlds with different assumptions about future population growth, levels of economic development, and potential technological advancement, as well as the rate at which such advancements are actually deployed around the globe.

In one scenario, the IPCC assumes heavy reliance on fossil fuels and significantly increasing emissions during the current century, and projects further global average surface warming of 2.4-6.4°C by the year 2100. In a second scenario, emissions grow more slowly, peak around 2050, and then fall, with expected additional warming of 1.1-2.9°C by the year 2100. The difference between the temperature ranges for the first and second scenarios reflects the influence of different trajectories for greenhouse-gas emissions on projected climate change. The ranges within each scenario also reflect uncertainties about how the climate system will respond to increasing greenhouse-gas concentrations: how much will temperatures increase in the event of a given increase in concentrations (so-called climate sensitivity)? And how will the carbon cycle and the uptake of carbon dioxide by the ocean and by terrestrial ecosystems be altered by changing temperature and atmospheric greenhouse-gas concentrations?

These different projections for warming imply very different climate change risks, affecting other climate variables (for example, precipitation patterns) as well as the likelihood of severe impacts. Warming at the high end of the range could have widespread catastrophic consequences and few benefits, save perhaps the emergence of shipping routes across the ice-free Arctic Ocean, or oil exploration in that sensitive region. A temperature change of 5–7°C on a globally averaged basis is about the difference between an ice age and an interglacial period, though this time it would occur in a century rather than, as has always been the case historically, over millennia. Alarmingly, actual emissions of the past half-dozen years exceed even the highest considered by the IPCC scenarios, which were crafted in 2000. This suggests that large increases in greenhouse-gas concentrations are in store in the next several decades unless rapid mitigating action is taken. Warming at the low end would be less damaging, but it would likely still be significant for some communities, economic sectors, and natural ecosystems. Indeed, some systems have already shown worrisome responses to the warming over the past century.

What sorts of impacts will these temperature changes lead to?

The IPCC AR4 lists many potential impacts of climate change for specific regions and highlights “key vulnerabilities,” based on seven criteria (magnitude of impacts, timing of impacts, persistence and reversibility of impacts, potential for adaptation, distributional aspects of impacts and vulnerabilities, likelihood of impacts and vulnerabilities and confidence in those estimates, and the importance of the vulnerable system[s]). Possible impacts include the loss of glaciers and the advent of other factors producing rising sea levels, which could inundate low-lying coastal areas and small island nations around the world; escalating infectious disease transmission and other risks to human health; increases in the severity of extreme events such as heat waves, storms, floods, and droughts; large drops in farming productivity, especially in hotter areas; the loss of cultural diversity; and an escalating rate of species extinction. Many of these problems are already emerging in some areas.

As glaciers melt, for example, sea levels rise and water becomes scarcer in regions that depend heavily on glacier-derived water for their dry-season water supply. In South America, shrinking glaciers could put a large fraction of the population west of the Andes at risk. According to one study, glacier-covered areas in Peru have experienced a 25 percent reduction in the past three decades. The authors note that “at current rates some of the glaciers may disappear in a few decades, if not sooner,” and warn of the irreplacable loss of fossil water.

China, India, and other parts of Asia are even more vulnerable. The ice mass in the region’s mountainous area is the third largest on Earth following Arctic-Greenland and Antarctica, and as its glaciers vanish in the coming decades, diminishing water supplies will affect vast populations. The Chinese Academy of Sciences has announced that the glaciers of the Tibetan plateau are vanishing so fast they will shrink by half every decade. Researchers estimate that enough water permanently melts from them each year to fill the entire Yellow River.

Furthermore, climate change is already having significant impact on human health, particularly that of vulnerable populations and those with little capacity to adapt. For example, the increased frequency and intensity of heat waves put small children and the elderly at risk. These populations are at greatest peril where air conditioning is unavailable or unaffordable, but devastating events such as the 2003 European heat wave, now linked to the premature deaths of at least 35,000 people, illustrate that large vulnerabilities exist even in developed countries.

Increases in the frequency and intensity of floods, hurricanes, fires, and other extreme events are also troubling. They injure human health both directly and indirectly, as when smoke from wildfires damages air quality, exacerbating respiratory illnesses in downwind areas.

In some regions—particularly the Arctic, where surface air temperatures have warmed at approximately twice the global rate—climate patterns are threatening entire ways of life. Traditional hunting groups and coastal indigenous communities have had to alter their social, cultural, economic, and political lives. The island village of Shishmaref, off the coast of northern Alaska, has been inhabited for centuries. Its 600 current residents are facing the real possibility of evacuation. Rising temperatures are melting sea ice (which allows higher storm surges to reach the shore) and thawing permafrost along the coast, increasing shoreline erosion and undermining homes and water systems. The absence of sea ice in the fall makes traveling to the mainland to hunt moose and caribou more difficult. Inuit hunters in Canada’s Nunavut Territory report thinning sea ice, declining numbers of ringed seals, and new insect and bird species in their region. In the western Canadian Arctic, Inuvialuit are observing more thunderstorms and lightning—formerly very rare in this region. Norwegian Sámi reindeer herders report that prevailing winds they rely on for navigation have become more variable, forcing them to change their traditional travel routes. Unpredictable weather, snow, and ice conditions make travel hazardous, endangering lives. The precise links of these local changes in weather patterns to climate change are difficult to establish, but they are illustrative of the broader risks of extreme events.

With regard to biodiversity, important biological events for many plants and animals now occur earlier in the spring as a result of climate changes. And many species are moving poleward, up mountain slopes, or both. Scientists have found that coastlines and mountaintops are especially prone to irreversible losses. If these “geoboundaries” become inhospitable to their present occupants, extinction will be much more likely for many of the species that dwell there. Depending on the severity of its impacts and the rates of response among individual species, climate changes could dismantle existing plant and animal communities. This is worrisome not only for the loss of biodiversity, but because of the potential disruption of ecosystem “goods”—seafood, fodder, fuel wood, timber, pharmaceutical products—and “services”—air and water purification, flood control, pollination, waste detoxification and decomposition, climate moderation, and soil-fertility regeneration.

Finally, the IPCC suggested that climate change could trigger “surprises.” These are fast, nonlinear climate responses, thought to occur when environmental thresholds are crossed. Some of these surprises could actually be anticipated, but others may be truly unexpected. “Imaginable surprises,” include the collapse of the large-scale North Atlantic ocean circulation—which could cause significant and potentially rapid cooling in parts of the North Atlantic region—and the deglaciation of Greenland or the West Antarctic ice sheets, which would, over many centuries, cause a considerable rise in sea level, threatening many coastal cities and low-lying coastal areas such as river deltas. There is also the possibility of true surprises, unimaginable because of the enormous complexities of the climate system and the interrelationships between oceanic, atmospheric, terrestrial, and other systems.

Faced with these grave risks, and great uncertainty, what should we do? While we cannot know the precise temperature increase and consequences likely to result from future emissions, we do know a few things with confidence. We know that cutting back emissions will reduce the level of temperature increase that would otherwise occur and thus reduce climate change risks: that is why mitigation is so important. But we also know that some further climate change will occur no matter how quickly we are able to reduce emissions. We know, too, that emissions are increasing rapidly, and are, as decribed above, at higher levels than assumed in the most dire IPCC scenario. The combination of historical and currently increasing emissions levels has locked in further warming for many decades. Therefore, we need both mitigation and adaptation.

How can we get the right mix?

Mitigation and adaptation have often been presented as tradeoffs, as if efforts to adapt to climate change will attenuate efforts to reduce greenhouse-gas levels, and vice versa. The assumption seems to be that each would divert attention and resources from the other. But there is growing recognition that the two policies must be complementary and concurrent.

Mitigation can keep warming on a lower trajectory, and delays in mitigation will lock in further warming, making adaptation that much harder. Adaptation, in contrast, is a response to warming, not a means of slowing it. It is necessary as a response to the impacts associated even with lower-emissions scenarios. Because of the decades of inertia in both climatic and economic systems, the benefits of mitigation take time to materialize, so adaptation is essential in responding to near-term climate changes. Failures to adapt could be disastrous.

Mitigation and adaptation also yield fundamentally different benefits and have correspondingly different policy implications. Mitigation provides long-term, global gain. A central challenge of mitigation policy, then, is to balance global factors: responsibility for historical emissions, growth in current emissions, and capacity to reduce emissions vary widely among nations. The benefits of adaptation strategies, in contrast, are both more immediate and more focused on specific regions and sectors. Given the wide range of climate change impacts on different regions, groups, and sectors, the need for adaptation varies widely as well.

The IPCC described two types of adaptation: autonomous and planned. Autonomous adaptation is not guided by policy; it is a reactive response prompted by the initial impacts of climate change. Consider a physiological example of autonomous adaptation. People who now live in warmer areas have acclimatized to those conditions, becoming less vulnerable to temperatures that would cause significant heat-related illnesses for people living in more temperate areas. Even so, there are limits to such adaptation, particularly if warmer temperatures spur increased use of air conditioning, and therefore less acclimatization.

Planned adaptation can also be reactive. For example, after the 2003 European heat wave, European countries instituted more coordinated plans to deal with periods of extreme heat. Buying additional water rights to offset the impacts of a drying climate or purchasing crop insurance where available are also reactive responses.

Reactive adaptations to climate change will almost certainly not be fast or easy. Some have argued that farmers adapt to changes in markets, technology, and climatic conditions when necessary. But a large number of factors are bound to complicate such adaptations: farmers may resist unfamiliar practices, have difficulties with new technologies, or face unexpected pest outbreaks. Moreover, the high degree of natural variability of weather may mask clear identification of emerging climatic trends. Adaptations to slowly evolving trends embedded in a noisy background with a great deal of variability are likely to be delayed by decades as farmers attempt to sort out true climate change from random fluctuations. Imagine a sequence of weather anomalies: say, a series of very dry years, which are precisely the opposite of what a long-term climatic trend toward wetter conditions would lead one to anticipate. Such a sequence could easily be mistaken for a new climatic regime and actually lead to maladaptive practices, such as investing in additional water storage that becomes unnecessary, instead of investing in better flood protection.

Another kind of planned adaptation—anticipatory or proactive—has greater policy potential. Anticipatory adaptation might include improving or expanding irrigation for agriculture, engineering crop varieties that are better able to cope with changing climate conditions, building sea walls to protect coastal infrastructure, and constructing reservoirs or implementing water recycling strategies to improve water management. Such actions may be similar in substance to reactive adaptation, but they anticipate future changes rather than responding to past shifts.

One region, among many, that could benefit greatly from adaptive initiatives is California. With its Mediterranean climate of wet winters and dry summers, California relies heavily on melting snow pack stored in the Sierras for agricultural and urban water supply. Warming is expected to reduce the snow pack considerably as more precipitation falls as rain instead of snow and the snow pack melts earlier in the year. Proactive adaptations are now under consideration, and they might include regulatory and political actions: connecting protected lands to create wildlife migration corridors, setting up networks to disseminate climate information and potential adaptive actions, and creating insurance mechanisms or support funds for disadvantaged and vulnerable groups that might not have the capacity to adapt on their own. Some of these actions would also foster sustainable development.

However, even if policies to facilitate adaptation are instituted, maladaptation can still occur. Such actions can be more damaging to developing countries and marginalized groups who have limited financial and other resources than not adapting at all. For them, even one round of successful adaptations will be taxing, let alone multiple rounds when the early measures prove misguided. A generally effective near-term strategy will need, then, both to identify and address immediate threats, and to strengthen the ability to cope with climate variability, while also anticipating that we may face more intense and/or more frequent extreme events than we have seen historically. Such an approach would build resilience and a capacity to make new adaptations while new information regarding the trajectory of future climate change becomes available.

But the near term cannot be our only focus. There are certain areas that require long-term planning to avoid severe impacts, planning that may take the form of investments in long-lived infrastructure in coastal zones or habitat protection for threatened or endangered species. In these cases, paying attention to the full range of climate projections over the next century is important. Furthermore, good policy coordination—explicit consideration of the interactions of adaptation policies across sectors and regions, as well as the interactions between adaptation and mitigation—is crucial.  For example, certain adaptations, such as recharging groundwater to increase water supply, may be energy-intensive. If the energy is generated from fossil fuels, the result may be to increase greenhouse-gas emissions.

Anticipatory adaptation is an investment. Most studies about its potential assume that countries and groups can afford such investment. Unfortunately, this is not universally true, especially for countries where development is a priority. Several funds have therefore been established to foster adaptation in developing countries, the best-known being the so-called Marrakech Funds (established at the seventh Conference of the Parties in Marrakech, Morocco in 2001) and the Global Environmental Facility’s (GEF) Climate Change Operational Programme, funded by world governments. These funds are a promising development, but guidelines for determining which adaptation projects deserve funding are lacking. The GEF requires such projects to show “global environmental benefits” and the Marrakech Funds try to assure funding of adaptation to long-term climate change rather than to short-term climate variability. Yet adaptation projects are hard to assess on these grounds because they are local (and therefore bring local, rather than global, benefits) and will likely improve an area’s ability to adapt to climate change and climate variability even if they focus immediately on the short-term. Moreover, the funds available are probably much less than the needed amount. Most developed countries have long underinvested in sustainable development, with or without the prospect of climate change. Likewise, adaptations to help the most vulnerable—often those who contributed the least to the accumulation of greenhouse gases in the atmosphere—are significantly under-funded.

Indeed, planned and autonomous adaptation both have their limits. Sensitivity to changing climate conditions may be higher than currently estimated. Without significant mitigation of greenhouse-gas emissions, warming and the intensity of climate impacts may exceed the coping capacity of these measures.

A third form of adaptation, more drastic than autonomous and planned, is called geoengineering. Schemes to modify environment systems themselves or to control climate have been proposed for more than fifty years as efforts to, say, increase temperatures in high latitudes, increase precipitation, decrease sea ice, create irrigation opportunities, or offset potential climate change. Various proposals would inject iron into the oceans or sea-salt aerosol in the marine boundary layer or spread dust in the stratosphere to reflect an amount of solar energy equivalent to the amount of heat trapped by increased greenhouse gases.

In a similar vein, a variety of ecoengineering strategies have been proposed to use managed relocation or colonization as a way to prevent extinction of species that are unable to adapt independently to climate change.  But relocating species to new areas may have unintended consequences for the ecosystem into which the species are introduced. It will be a difficult ethical dilemma to assess whether moving a species threatened with extinction at the top of a warming mountain to another taller mountain, thereby turning it into an invasive species at its new home, is justified.

These approaches—geoengineering and ecoengineering—attempt to offset the effects of one manipulation of the Earth system (climate change) with another large-scale manipulation of physical or biological systems. Recent geoengineering schemes have been championed as a cheaper method of counteracting climate change than conventional emissions-reduction strategies, an intervention that responds to foot-dragging by governments, or a unilateral alternative to global coordination.

But while proponents—including industries negatively affected by climate-control policies—argue for geoengineering on grounds of cost-effectiveness or as a last resort, critics express skepticism that any geoengineering scheme would reliably work as planned or that the long-term international political stability and cooperation needed to maintain such schemes is possible. Moreover, geoengineering risks transnational conflicts; such activities may produce—or be perceived to produce—damaging climatic events, and thus provoke political conflict. These controversial schemes are desperate measures: understandable, but not what the current situation demands. In reducing risks, nothing can substitute for the hard work of aggressive mitigation combined with anticipatory adaptation.

Even with an optimal mix of mitigation and adaptation, however, the results may still be unfair. The most marginalized groups often have limited access to information, little political or economic power, and therefore limited influence over decisions: much less than people with political and economic power, who are generally less vulnerable to climate change damages. Hence, policies cater to such special interest groups as the coal industry or to the United States, China, and other powerful countries at the expense of more needy groups or nations.

To avoid overlooking already-marginalized groups when forming local, national, and international climate policy, decision-makers need to consider the effects of actions and inactions on the distribution of people’s well-being and the sustainability of other species. In a framework of distributive justice, priority should go to disadvantaged countries and groups. Side payments from the prime sources of the problem (generally, richer countries) to those who have contributed less of the problem (generally, less-developed countries) and have less ability to fix it can also be fashioned for fairness and political cooperation (for example, agriculture or clean-energy technology transfer, or assistance for population mobility). Both the direct effects of climate change and the differential impacts of climate policy can lead to inequitable results for the many populations grappling with this worldwide phenomenon.

Thus, good governance in the realm of climate policy will require protecting the planetary commons by mitigating emissions, initiating smart adaptations and improving adaptive capacities, and dealing on fair terms with those most disadvantaged by either climate impacts or impacts of climate policies.

Vulnerability assessment is essential to the progress of adaptation. It is an important tool for informing the development of climate change policies, particularly adaptation strategies. Vulnerability depends on exposure to a stress, susceptibility to that stress, and capacity to adapt or respond to it. With climate change as the source of stress, mitigation reduces vulnerability by reducing exposure, while adaptation reduces vulnerability by turning adaptive potential into adaptive capacity.

The distinction between adaptive potential and adaptive capacity is critical. We know that the vulnerability of New Orleans to a direct hit by a Category III hurricane was much higher than was widely believed prior to Katrina, though a few academics and engineers had been issuing warnings for decades. Adaptive potential was quite high—for example, levees could have been strengthened in advance—but this potential was not realized, and therefore adaptive capacity was low. In general, adaptive capacity is related to the level of development in a country. But events such as Katrina (affecting primarily poor residents), and the 2003 heat wave in Europe (affecting primarily the elderly) highlight the vulnerability of specific populations and regions, even within highly developed nations.

Assessing vulnerability to climate change is a complex matter. It requires analysis of climatic conditions and potential impacts, projections of future impacts in the context of alternative socioeconomic development paths, and an evaluation of how well different adaptation strategies will do at reducing vulnerabilities. Detailed understanding of the affected sectors, communities, and management systems; the interactions of non-climatic stressors with a changing climate; and each system’s ability to respond to changing conditions are often lacking. There is a critical need for research that couples climate projections with studies of vulnerability that focus on specific economic sectors (agriculture, services, manufacturing), specific regions, and specific groups. And these need to be generated in close communication with relevant stakeholders to increase the potential usefulness and likelihood of actual use of such knowledge in choosing adaptations.

Given the uncertainties in climate science and impact estimates, we need to reduce considerably the rate at which we add to atmospheric greenhouse gas levels. This will give us more time to understand climate risks and to develop lower-cost mitigation options while making climate surprises less likely. Greenhouse-gas abatement policies will provide incentives to invent cleaner, cheaper technologies, and developed countries should aggressively lead that effort, both because of their historical contribution to the problem and their greater capacity to help.

Simultaneously, the needs of developing countries and marginalized groups should be accommodated through coordinated adaptation and mitigation actions. Developed countries should shoulder this burden as well, as required by the United Nations Framework Convention on Climate Change. When leaders of developing countries say they will not join mitigation efforts until they catch up with OECD countries in per capita emissions, and some OECD countries assert that they won’t abandon fossil-based energy generation that props up their economic growth, we face real, potentially catastrophic environmental danger. We will need international negotiations and bargaining to help the developing world leapfrog the traditional technologies of growing economies—such as massive coal burning or dramatic increases in individual car use. Lower-emitting technologies such as electric vehicles and alternative energy sources can be implemented at much faster rates with cooperation and political will.

Slowing down pressure on the climate system and addressing marginalized- country and group needs are the main insurance policies we have against potentially dangerous, irreversible climate events and the injustices that will inevitably accompany them. Even as the world struggles to fashion fair and effective forms of mitigation, adaptation will be essential to minimize the worst consequences of climate change.