4th National Climate Assessment, Volume II: Impacts, Risks, and Adaptation in the United States

On November 23, 2018, the U.S. Global Change Research Program released Volume II of the Fourth National Climate Assessment (NCA4) entitled Impacts, Risks and Adaptation in the United States. NCA4 includes sixteen chapters focusing on national topics and specific sectors, nine chapters focusing on different regions of the country, and two chapters focusing on both mitigation (reducing greenhouse gas emissions) and adaptation responses to climate change. NCA4 concludes that: “ [o]bservations 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. 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.”

Observed Change

NCA4 documents observed changes in the climate that are already being experienced in communities across the country:

  • Global average temperatures have risen 1.8°F (1.0°C) since 1901, predominantly due to human activity.
  • Sea-levels have risen on average 9 inches along the U.S. coast since the early 20th century.
  • Sea-level rise has already increased the frequency of high-tide flooding in some coastal communities by 5 to 10 times since 1960.
  • Sixteen of the last seventeen years have been the warmest years on record.
  • More frequent and intense extreme weather events are already affecting communities across the country and emissions have already contributed to an observed increase in the frequency of hurricane activity in the Atlantic ocean since 1970. The 2017 hurricane season alone caused more than $250 billion in damages and more than 250 deaths.
  • Rising air and water temperatures, changing precipitation patterns, and reduced snowpack are affecting flooding, water supply, and water quality.
  • Costly wildfires are occurring with greater frequency; in 2015, over 10 million acres of forest burned across the U.S., with over $1 billion in losses in California alone.
  • Climate change has doubled the acres burned by wildfire in the West from 1984 to 2015.
  • Higher temperatures and increasing wildfires are affecting air quality and increasing health risks.
  • Climate change is already affecting biodiversity and ecosystems, and diminishing the services provided by these natural resources.

Future Scenarios

In assessing future climate impacts, NCA4 uses the higher emissions scenario with more warming (RCP8.5) and the lower scenario with less warming assuming a greater reduction in global emissions by the end of the century (RCP4.5). NCA4 also includes an extreme scenario for future sea-level rise of upwards of 8 feet that accounts for rapid melt of Greenland and Antarctic ice sheets. When discussing risks and impacts, the assessment includes the “confidence” of the findings based upon the scientific evidence, and the “likelihood” or probability of occurrence. NCA4 includes the following projections for future climate change effects and impacts in the U.S.

  • Without significant reductions in emissions, global average temperatures could rise by up to 9°F or more by the end of the century; an average of 2.3°F of warming is anticipated by the middle of the century (relative to 1986-2015).
  • Certain regions of the country could see an additional 60 days of temperatures in above 100°F (38°C) by 2050.
  • Global average sea-levels are very likely to rise by 1 to 4 feet by the end of the century, and 8 feet of rise is physically possible under a extreme scenario factoring in rapid ice sheet melt.
  • In coastal communities, $1 trillion of coastal real estate is threatened by rising sea-levels and more extreme storms.
  • Of Department of Defense military sites assessed, 10 percent are vulnerable to extreme temperatures and 6 percent of sites are vulnerable to increasing flood risks.
  • Economic losses in some sectors could exceed $100 billion per year and extreme temperatures could cost $160 billion in lost wages annually by the end of the century.
  • Crop yields and are expected to decline in many areas of the country causing economic losses to the agricultural sector.
  • The incidence of West Nile virus is anticipated to double by 2050 costing the U.S. $1 billion annually.

The national topical chapters focus on potential climate impacts to water, energy, land-use and land cover, forests, ecosystems, coasts, oceans and marine resources, agriculture and rural communities, built environment and urban systems, transportation, air quality, human health, tribes, international interests, and cross-sector interactions and complex systems.

Regional chapters focus on observed changes and potential future impacts in specific regions of the U.S. including the Northeast, Southeast, U.S. Caribbean, Midwest, Northeast and Southeast Great Plains, the Pacific Northwest, Southwest, Alaska, Hawaii and the Pacific Islands. These chapters also provide case study examples of efforts being taken in these regions to adapt to future climate risks. One of the key findings from the Adaptation chapter is that: "(p)roactive adaptation initiatives - including changes to policies, business operations, capital investments, and other steps - yield benefits in excess of their costs in the near term, as well as over the long term. Evaluating adaptation strategies involves consideration of equity, justice, cultural heritage, the environment, health, and national security."

Response chapters discuss mitigation and adaptation efforts across the U.S. including a discussion of benefits, tradeoffs, best practices.

NCA4  also includes new economic analysis to quantify the financial costs of climate change to the specific regions and economic sectors and the financial damages that could be avoided through efforts to mitigate climate change.

An Overview chapter provides a high-level summary of findings of observed impacts from climate change, an update to the best available science on projected impacts from future climate change, a synthesis of potential impacts to specific sectors of the economy and regions, and highlights of what actions are being taken to prepare for the impacts of climate change and to reduce emissions.

The federal government is required to produce a national climate assessment every four years by the Global Change Research Act of 1990. The U.S. Global Change Research Program (USGCRP) produced NCA4 with input from 13 federal agencies and other experts. The climate science that informed USGCRP was released in November, 2017 as Volume I of the 4th National Climate Assessment, Climate Science Special Report.

 

Publication Date: November 23, 2018

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