changes in atmospheric GHG concentrations.1 This is because the world’s oceans can store a large amount of heat—so it takes a long time for the climate system to warm up in response to changes in GHG concentrations2—and because impacts such as sea level rise and the melting of ice sheets can take several centuries or even millennia to be fully expressed. Some GHGs (such as methane) are removed from the atmosphere within about a decade, but CO2 persists much longer—approximately 20 percent of the CO2 emitted today will remain in the atmosphere more than a millennium from now.3 Thus, a failure to reduce GHG emissions in the near-term will “lock in” a certain amount of future climate change for decades, if not centuries, to come.

There are also significant time lags in human response systems. GHG emissions are to a large extent built into societal infrastructure (e.g., buildings, power plants, settlement and transportation patterns) and into human habits and organizational routines, few of which change quickly. Market incentives affecting capital investments leave little room for considering consequences on century or longer time scale. Nevertheless, making major reductions in GHG emissions and preparing to adapt to the effects of climate change will require transformative changes, for instance, in how the country produces and uses energy (see Box 3.1), builds buildings and transportation infrastructure, and manages water and other natural resources. It will likewise require significant changes in consumer choices, travel behavior, and other individual and household-level decisions. Overcoming the inertia of the status quo in advancing these sorts of transformations will pose challenges for government, industry, agriculture, and individual citizens alike.

An issue of particular concern is that much of the equipment and infrastructure that leads to GHG emissions (e.g., roads, vehicles, buildings, power plants) have lifetimes of decades. There are often strong economic pressures to continue use of such equipment and infrastructure, rather than retrofitting or replacing with a lower-emitting option. Making substantial emission reductions within the next few decades will require accelerating this turnover faster than projected business-as-usual rates.4

Risks, judgments about risk, and adaptation needs are highly variable across different contexts. Different regions, economic and resource sectors, and populations will experience different impacts from climate change, will vary in their ability to tolerate and adapt to such impacts, and will hence differ in their judgments about the potential risks posed by climate change. For instance, coastal communities that are vulnerable to serious disruptions could be expected to view the risks of climate change as quite serious. Actions that are taken in response to climate change will also pose differing types of risks to different regions, sectors, and populations. For instance,



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