The Fourth National Climate Assessment (NCA4) is a U.S. federal report designed to present the current state of scientific research on the topic of climate change, together with assessments of impacts and risks under various of climate pathways, each leading to a possible climate outcome.

The NCA4 report is published in two volumes. The first volume, the "Climate Science Special Report" (CSSR), was published in 2017 and is specifically directed toward the science of climate change. The second volume, published in 2018, is directed toward impact and risk assessment, together with possible mitigation and adaptation measurements designed to minimize the disruptive potential of climate change.

The NCA4 was developed and published under the auspices of the U.S. Global Change Research Program (USGCRP), which is administratively led by the National Oceanic and Atmospheric Administration (NOAA).

The full report can be downloaded for free at the following links:
Volume 1: Climate Science Special Report

Volume 2: Impacts, Risks and Adaptation in the U.S.

The first volume of NCA4, "Climate Science Special Report", relates to the science of climate change. It is important to note that the climate of the U.S. is closely linked to the global climate. The following statements about the science of climate change apply to both the U.S. and the Earth in general.

• Average surface air temperature has increased by 1.8°F (1.0°C) over the period 1901-2016. This period is the warmest in the history of modern civilization.


• It is extremely likely that human activities, especially emissions of greenhouse gases, are the main cause for the Earth's warming. There is no scientifically convincing evidence for an alternative explanation for the observed increase in global surface temperature.


• Many other changes in global climate have been scientifically documented, including increases in surface, atmospheric and oceanic temperature, melting of glaciers, decrease in snow cover, shrinking sea ice, rising sea levels, ocean acidification, and increase in atmospheric water vapor.


• Global sea level has risen by 7-8 inches since 1900, accompanied by an increase in daily tidal flooding in many coastal cities.


• Sea level rise will continue at least through 2100, with a possible rise as much as 1-8 feet.


• Extreme weather events are increasing and will continue to increase, including heavy rainfall, extreme heatwaves, large forest fires, long-duration drought.


• The severity of the above events depends on the amount of greenhouse gases emitted globally. The current atmospheric CO2 level is 410 parts per million (ppm), a level that has not occurred since about 3 million years ago.

The full report can be downloaded for free at the following links:
Volume 1: Climate Science Special Report

Volume 2: Impacts, Risks and Adaptation in the U.S.

The second volume of the Fourth National Climate Assessment (NCA4) relates to the impacts and risks involved with climate change in the 21st century. It also discusses general measures required in dealing with climate change, emphasizing the need for mitigation and adaptation measures in coping with the potential harm to human communities, natural ecosystems, and the various Earth systems - land, sea, atmosphere, ice and biota.

Following is a list of some of the topics covered in volume 2:

• Climate change expands the risks and vulnerabilities of communities throughout the U.S., especially in regard to health, safety, quality of life, and economic growth


• Economic damage will be extensive, especially among businesses that depend on a favorable environment. These include agriculture, tourism, fisheries and other industries dependent upon natural systems.


• Impacts resulting from climate change reach across a broad range of systems - natural, built, social and more. These systems intertwine and rely on each other for successful results. Changes in the climate that damages one will likely damage the others which share a common dependency.


• Immediate action is required in the form of mitigation and adaptation to reduce risks associated with climate change. Many communities, governments and businesses are working to implement both mitigation and adaptation measures, but the scale of that effort is not yet sufficient to avoid substantial damage over the coming decades.


• The quantity and quality of water are being negatively affected by the changing climate. Droughts, floods, and contamination are damaging the U.S. water supply.


• Human health is being harmed by extreme weather events, air quality, and the transmission of disease by insects, pests, and contaminated food and water.


• The ways of life of indigenous peoples are being threatened by climate change. This extends to their livelihoods, economies, health and cultural identities.


• Ecosystems throughout the globe are being harmed by climate change, resulting in forced migration to other latitudes or, in some cases, species extinction.


• As heat, drought, floods and other climate extremes become more commonplace, agricultural environments become less and less productive.


• The U.S. infrastructure is old and deteriorating. It is further stressed by extreme weather events - heavy precipitation, coastal flooding, heat, wildfires and more. Additional deterioration must be prevented in order to avoid damage to both the U.S. national economy and essential services, but to national security as well.


• Coastal communities and ecosystems are particularly vulnerable to extreme weather events, and the effects of unmitigated climate change will have severely harmful effects on those environments if nothing is done to prevent it soon.


• Tourism and recreation depend to a large extent on nature for business success. Climate change will erode the quality of many such experiences and do considerable harm to the tourism and recreation business.

The full report can be downloaded for free at the following links:
Volume 1: Climate Science Special Report

Volume 2: Impacts, Risks and Adaptation in the U.S.