Even an immediate and dramatic cut in global greenhouse gas emissions would not fully prevent climate change impacts. The climate system responds to changes in greenhouse gas levels with a time lag, in part because of the oceans' thermal inertia. Past and present emissions have already committed the earth to at least some climate change in the 21st century. Natural ecosystems and human societies will be sensitive to both the magnitude and the rate of this change. Therefore, while controlling emissions is vital, it must be combined with efforts to minimize damage through adaptation.

The most vulnerable ecological and socio-economic systems are those with the greatest sensitivity to climate change and the least ability to adapt. Sensitivity is the degree to which a system will respond to a given change in climate; it measures, for example, how much the composition, structure, and functioning of an ecosystem will respond to a given temperature rise. Adaptability is the degree to which systems can adjust in response to, or in anticipation of, changed conditions. Vulnerability defines the extent to which climate change may damage or harm a system; this depends not only on the system's sensitivity, but on its ability to adapt.

Ecosystems that are already under stress are particularly vulnerable. Many ecosystems are sensitive to humanity's management practices and increasing demands for resources. For example, human activities may limit the potential of forest ecosystems for adapting naturally to climate change. Fragmentation of ecosystems will also complicate human efforts to assist adaptation, for example by creating migration corridors.

Social and economic systems tend to be more vulnerable in developing countries with weaker economies and institutions. In addition, people who live in arid or semi-arid lands, low-lying coastal areas, flood-prone areas, or on small islands are at particular risk. Greater population densities in many parts of the world have made some sensitive areas more vulnerable to hazards such as storms, floods, and droughts.

Adapting to climate change can be a spontaneous or planned act. Individuals, businesses, governments, and nature itself will often adapt to climate change impacts without any external help. In many cases, however, people will need to plan how to minimize the costs of negative impacts and maximize the benefits from positive impacts. Planned adaptation can be launched prior to, during, or after the onset of the actual consequences.

Six general strategies are available for adapting to climate change. Measures can be taken in advance to prevent losses, for example by building barriers against sea-level rise or reforesting degraded hillsides. It may be possible to reduce losses to a tolerable level, including by redesigning crop mixes to ensure a guaranteed minimum yield under even the worst conditions. The burden on those directly affected by climate change can be eased by spreading or sharing losses, perhaps through government disaster relief. Communities can also change a use or activity that is no longer viable, or change the location of an activity, for example by re-siting a hydro-electric power utility in a place where there is more water or relocating agricultural activities from steep hill slopes. Sometimes it may be best to restore a site, such as an historical monument newly vulnerable to flood damage.

Successful strategies will draw on ideas and advances in law, finance, economics, technology, public education, and training and research. Technological advances often create new options for managed systems such as agriculture and water supply. However, many regions of the world currently have limited access to new technologies and to information. Technology transfer is essential, as is the availability of financial resources. Cultural, educational, managerial, institutional, legal, and regulatory practices are also important to effective adaptation, at both the national and international levels. For example, the ability to incorporate climate change concerns into development plans can help ensure that new investments in infrastructure reflect likely future conditions.

Many adaptation policies would make good sense even without climate change. Present-day climatic variability, including extreme climatic events such as droughts and floods, already causes a great deal of destruction. Greater efforts to adapt to these events could help to reduce damage in the short term, regardless of any longer-term changes in climate. More generally, many policies that promote adaptation – for example by improving natural resource management or bettering social conditions – are also vital for promoting sustainable development. Despite such synergies, however, it is clear that adaptation will also involve real costs and will not prevent all of the expected damage.

Crafting adaptation strategies is complicated by uncertainty. It is still not possible to quantify with any precision the likely future impacts on any particular system at any particular location. This is because climate change projections at the regional level are uncertain, current understanding of natural and socio-economic processes is often limited, and most systems are subject to many different interacting stresses. Knowledge has increased dramatically in recent years, but research and monitoring will remain essential for gaining a better understanding of potential impacts and the adaptation strategies needed to deal with them.