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human and environmental systems. For example, estuaries and coral reefs are being affected by warming ocean temperatures, ocean acidification, sea level rise, and changes in runoff from precipitation, and these climate-driven impacts interact with other ongoing threats such as pollution, invasive species, coastal development, and overfishing. The impacts of these multiple stresses and interacting environmental changes on food production, water management, energy production, and other critical human activities are associated with important risks in terms of meeting human needs. The prevalence of multiple stresses and the interconnected nature of many climate-related processes also raise significant scientific and management challenges.

Vulnerabilities to climate change vary across regions, societies, and groups. Climate change will unfold in different ways across the United States and across the globe, and different sectors, populations, and regions will be differentially exposed and sensitive to the impacts of these changes. Different groups will also differ in their ability to cope with and adapt to environmental changes. In general, research suggests that the impacts of climate change will more harshly affect poorer nations and communities. Actions taken to limit future climate change and adapt to its impacts also have the potential to cause differential benefits or harm. For example, different communities or regions may experience differential exposure to the unintended side effects of alternative energy production strategies. However, our understanding and ability to predict vulnerability, adaptive capacity, and the side effects of different response strategies are less well established than our understanding of the basic causes and mechanisms of climate change.

Individually and collectively, the complexities described in the paragraphs above make it challenging to analyze the risks posed by climate change. Nonetheless, as described in Chapter 2 and discussed in detail in Part II of the report, the scientific community has high confidence in projections of a number of future climate changes and impacts. For example, (1) water availability will decrease in many areas that are already drought-prone and in areas where freshwater systems are fed by glaciers and snowpack; (2) a higher fraction of rainfall will fall in the form of heavy precipitation events, increasing the risk of flooding; (3) people and ecosystems in coastal zones will be exposed to higher storm surges, saltwater intrusion, and other risks as sea levels rise; and (4) coral reefs will experience widespread bleaching and mortality as a result of increasing temperatures, rising sea levels, and ocean acidification. There is less certainty in other projections, such as the combined impact of CO2 increases, temperature increases, precipitation changes, and other climate and climate-related changes on agricultural crops and natural ecosystems in different regions of the world, although negative impacts are expected to increase with higher temperatures. Projections of the future—in any sphere—always entail some amount of uncertainty. Nevertheless, as described in the

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