The physical and mental health of rural Alaskans is increasingly challenged by unpredictable weather and other environmental changes. The Alaska Native Tribal Health Consortium's Center for Climate and Health is using novel adaptation strategies to reduce climate-related risks including difficulty in harvesting local foods and more hazardous travel conditions. A state working group generated management options to promote recovery and reduce threats to coral reefs. Damages from the hurricanes have been compounded by the slow recovery of energy, communications, and transportation systems, impacting all social and economic sectors.
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Climate shapes where and how we live and the environment around us. Natural ecosystems, agricultural systems, water resources, and the benefits they provide to society are adapted to past climate conditions and their natural range of variability.
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A water manager may use past or current streamflow records to design a dam, a city could issue permits for coastal development based on current flood maps, and an electric utility or a farmer may invest in equipment suited to the current climate, all with the expectation that their investments and management practices will meet future needs. However, the assumption that current and future climate conditions will resemble the recent past is no longer valid Ch.
Observations collected around the world provide significant, clear, and compelling evidence that global average temperature is much higher, and is rising more rapidly, than anything modern civilization has experienced, with widespread and growing impacts Figure 1. The warming trend observed over the past century can only be explained by the effects that human activities, especially emissions of greenhouse gases, have had on the climate Ch.
Climate change is transforming where and how we live and presents growing challenges to human health and quality of life, the economy, and the natural systems that support us. Risks posed by climate variability and change vary by region and sector and by the vulnerability of people experiencing impacts.
Social, economic, and geographic factors shape the exposure of people and communities to climate-related impacts and their capacity to respond. Risks are often highest for those that are already vulnerable, including low-income communities, some communities of color, children, and the elderly Ch. Climate change threatens to exacerbate existing social and economic inequalities that result in higher exposure and sensitivity to extreme weather and climate-related events and other changes Ch. Marginalized populations may also be affected disproportionately by actions to address the underlying causes and impacts of climate change, if they are not implemented under policies that consider existing inequalities Ch.
It documents vulnerabilities, risks, and impacts associated with natural climate variability and human-caused climate change across the United States and provides examples of response actions underway in many communities. These impacts are projected to intensify—but how much they intensify will depend on actions taken to reduce global greenhouse gas emissions and to adapt to the risks from climate change now and in the coming decades Ch.
High temperature extremes and heavy precipitation events are increasing. Glaciers and snow cover are shrinking, and sea ice is retreating. Seas are warming, rising, and becoming more acidic, and marine species are moving to new locations toward cooler waters. Flooding is becoming more frequent along the U.
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Growing seasons are lengthening, and wildfires are increasing. These and many other changes are clear signs of a warming world Figure 1. Click on a topic or on the thumbnails below the image to see a relevant indicator. Scientists have understood the fundamental physics of climate change for almost years. Since the late 19th century, however, humans have released an increasing amount of greenhouse gases into the atmosphere through burning fossil fuels and, to a lesser extent, deforestation and land-use change. However, the unambiguous long-term warming trend in global average temperature over the last century cannot be explained by natural factors alone.
Greenhouse gas emissions from human activities are the only factors that can account for the observed warming over the last century; there are no credible alternative human or natural explanations supported by the observational evidence. Without human activities, the influence of natural factors alone would actually have had a slight cooling effect on global climate over the last 50 years Ch. Humans are adding carbon dioxide to the atmosphere at a rate far greater than it is removed by natural processes, creating a long-lived reservoir of the gas in the atmosphere and oceans that is driving the climate to a warmer and warmer state.
Some of the other greenhouse gases released by human activities, such as methane, are removed from the atmosphere by natural processes more quickly than carbon dioxide; as a result, efforts to cut emissions of these gases could help reduce the rate of global temperature increases over the next few decades.
However, longer-term changes in climate will largely be determined by emissions and atmospheric concentrations of carbon dioxide and other longer-lived greenhouse gases Ch. Climate models representing our understanding of historical and current climate conditions are often used to project how our world will change under future conditions see Ch. Instead, projections are typically used to capture long-term changes, such as how the climate system will respond to changes in greenhouse gas levels over this century.
Scientists test climate models by comparing them to current observations and historical changes. Confidence in these models is based, in part, on how well they reproduce these observed changes. Climate models have proven remarkably accurate in simulating the climate change we have experienced to date, particularly in the past 60 years or so when we have greater confidence in observations see CSSR, Ch.
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The observed signals of a changing climate continue to become stronger and clearer over time, giving scientists increased confidence in their findings even since the Third National Climate Assessment was released in Today, the largest uncertainty in projecting future climate conditions is the level of greenhouse gas emissions going forward. Future global greenhouse gas emissions levels and resulting impacts depend on economic, political, and demographic factors that can be difficult to predict with confidence far into the future.
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Like previous climate assessments, NCA4 relies on a suite of possible scenarios to evaluate the implications of different climate outcomes and associated impacts throughout the 21st century. RCPs drive climate model projections for temperature, precipitation, sea level, and other variables under futures that have either lower or higher greenhouse gas emissions. Each RCP leads to a different level of projected global temperature change; higher numbers indicate greater projected temperature change and associated impacts.
The higher scenario RCP8. Current trends in annual greenhouse gas emissions, globally, are consistent with RCP8. In some cases, throughout this report, a very low scenario RCP2. Each RCP could be consistent with a range of underlying socioeconomic conditions or policy choices.
The effects of different future greenhouse gas emissions levels on global climate become most evident around , when temperature Figure 1. With substantial and sustained reductions in greenhouse gas emissions e. Early greenhouse gas emissions mitigation can reduce climate impacts in the nearer term such as reducing the loss of arctic sea ice and the effects on species that use it and in the longer term by avoiding critical thresholds such as marine ice sheet instability and the resulting consequences for global sea level and coastal development; Ch.
Annual average temperatures in the United States are projected to continue to increase in the coming decades. Regardless of future scenario, additional increases in temperatures across the contiguous United States of at least 2. As a result, recent record-setting hot years are expected to become common in the near future. By late this century, increases of 2.
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Alaska has warmed twice as fast as the global average since the midth century; this trend is expected to continue Ch. High temperature extremes, heavy precipitation events, high tide flooding events along the U. These and other changes are expected to increasingly impact water resources, air quality, human health, agriculture, natural ecosystems, energy and transportation infrastructure, and many other natural and human systems that support communities across the country. The severity of these projected impacts, and the risks they present to society, is greater under futures with higher greenhouse gas emissions, especially if limited or no adaptation occurs Ch.
Some climate-related impacts, such as increasing health risks from extreme heat, are common to many regions of the United States Ch. Others represent more localized risks, such as infrastructure damage caused by thawing of permafrost long-frozen ground in Alaska or threats to coral reef ecosystems from warmer and more acidic seas in the U. Caribbean, KM 2 ; Ch. Many places are subject to more than one climate-related impact, such as extreme rainfall combined with coastal flooding, or drought coupled with extreme heat, wildfire, and flooding. The compounding effects of these impacts result in increased risks to people, infrastructure, and interconnected economic sectors Ch.
Impacts affecting interconnected systems can cascade across sectors and regions, creating complex risks and management challenges. For example, changes in the frequency, intensity, extent, and duration of wildfires can result in a higher instance of landslides that disrupt transportation systems and the flow of goods and services within or across regions Box 1. Many observed impacts reveal vulnerabilities in these interconnected systems that are expected to be exacerbated as climate-related risks intensify. Under a higher scenario RCP8. Without more significant global greenhouse gas mitigation and regional adaptation efforts, climate change is expected to cause substantial losses to infrastructure and property and impede the rate of economic growth over this century Ch.
Regional economies and industries that depend on natural resources and favorable climate conditions, such as agriculture, tourism, and fisheries, are increasingly vulnerable to impacts driven by climate change Ch. Reliable and affordable energy supplies, which underpin virtually every sector of the economy, are increasingly at risk from climate change and weather extremes Ch. The impacts of climate change beyond our borders are expected to increasingly affect our trade and economy, including import and export prices and U.
Some aspects of our economy may see slight improvements in a modestly warmer world. However, the continued warming that is projected to occur without significant reductions in global greenhouse gas emissions is expected to cause substantial net damage to the U. The potential for losses in some sectors could reach hundreds of billions of dollars per year by the end of this century Ch.
Existing water, transportation, and energy infrastructure already face challenges from heavy rainfall, inland and coastal flooding, landslides, drought, wildfire, heat waves, and other weather and climate events Figures 1. Many extreme weather and climate-related events are expected to become more frequent and more intense in a warmer world, creating greater risks of infrastructure disruption and failure that can cascade across economic sectors Ch. For example, more frequent and severe heat waves and other extreme events in many parts of the United States are expected to increase stresses on the energy system, amplifying the risk of more frequent and longer-lasting power outages and fuel shortages that could affect other critical sectors and systems, such as access to medical care Ch.
Current infrastructure is typically designed for historical climate conditions Ch.
Infrastructure age and deterioration make failure or interrupted service from extreme weather even more likely Ch. Climate change is expected to increase the costs of maintaining, repairing, and replacing infrastructure, with differences across regions Ch. Recent extreme events demonstrate the vulnerabilities of interconnected economic sectors to increasing risks from climate change see Box 1.