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as using historical records to plan for the “100-year flood” or the “100-year drought”—will need to be revisited, and new ways of thinking about preparing and adapting to change will need to emerge. Conventional practices may even heighten risks by encouraging us to continue planting vulnerable crop varieties, harvesting threatened resources at unsustainable levels, or building homes and communities in areas at growing risk from fires, floods, or rising sea levels.

Projections of future climate depend strongly on current and future human actions. The magnitude of future climate change and the severity of its impacts are strongly dependent on how human societies produce and use energy, manage natural resources, and take other actions to respond to climate risks and vulnerabilities in the decades ahead. Not only is it impossible to anticipate all of the actions that humans might take, but the consequences of these actions for both the climate system and related human and environmental systems are subject to a large number of uncertainties and unknowns.

Climate change processes have considerable inertia and long time lags. Until GHG emissions are brought below the rate of their removal from the atmosphere, atmospheric concentrations will continue to rise. The most important GHGs remain in the atmosphere for years to centuries and continue to affect Earth’s heat balance throughout their atmospheric lifetimes. Other climate change processes also exhibit considerable inertia, which results in delays between GHG emissions and the impacts of climate change. The oceans, for example, warm much more slowly than the atmosphere in response to the buildup of heat-trapping gases. Additionally, many of the sources of GHG emissions, such as power plants and automobiles, have lifetimes of years to decades. Thus, decisions made now will shape the world for generations to come. Research has shown that individuals and organizations have trouble perceiving risks and taking action on problems with such long lead-times.

The sensitivity of the climate system is somewhat uncertain. As discussed in Chapters 2 and 6, scientists have learned a great deal about the response of the climate system to GHGs and other climate forcing agents through a combination of direct observations of recent climate change, indirect evidence of historical climate variations, and climate modeling studies. However, Earth’s climate sensitivity—which dictates how much warming would be expected if future emissions were known exactly—remains somewhat uncertain. Thus, it is possible that future temperature changes will lie below the range of current climate model projections. However, it is also possible that realized temperature changes will lie above the projected range. Additionally, climate models cannot currently simulate certain feedback processes in the Earth system, such as those related to changes in ecosystems on land or in the oceans, that could potentially

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