The Fifth National Climate Assessment - 285

The Fifth National Climate Assessment is the US Government’s preeminent report on climate change impacts, risks, and responses. It is a congressionally mandated interagency effort that provides the scientific foundation to support informed decision-making across the United States.

The following is the official summary.  Whatever your understanding of climate change, I encourage everyone reading this to take a few minutes and review the entire assessment which can be found at, https://nca2023.globalchange.gov/

Earth’s climate is changing because humans are increasing the concentrations of greenhouse gases in the atmosphere by burning fossil fuels and through other activities. The increase in greenhouse gases is warming the planet and driving other observed climate trends, including increases in the frequency and severity of many types of extreme weather events. Future changes and impacts depend largely on the choices humans make about future greenhouse gas emissions.

Human activities are changing the climate. The evidence for warming across multiple aspects of the Earth system is incontrovertible, and the science is unequivocal that increases in atmospheric greenhouse gases are driving many observed trends and changes. There are more greenhouse gases in the atmosphere primarily because humans have burned and continue to burn fossil fuels for transportation and energy generation. Industrial processes, deforestation, and agricultural practices also increase greenhouse gases in the atmosphere. As a result of increases in the atmospheric concentrations of these heat-trapping gases, the planet is on average about 2°F (1.1°C) warmer than it was in the late 1800s. No natural processes known to science could have caused this long-term temperature trend. The only credible explanation for the observed warming is human activities.

Climate change is happening now in the United States. Including Alaska, the continental US has been warming about 60% faster than the planet as a whole since 1970. This temperature change has driven increases in the frequency and severity of some extreme events, consistent with the scientific understanding of climate change. There has always been extreme weather, which occurs even in an unchanged climate due to the natural variability of the Earth system. However, recent advances in attribution science mean that the role of climate change in some extreme events can now be quantified in real time. For example, climate change made the record-breaking Pacific Northwest heatwave of June 2021 2° to 4°F hotter, and in 2017, Hurricane Harvey’s rainfall was estimated to be about 15%–20% heavier than it would have been without human-caused warming.

Climate change is already affecting people in the United States. Extreme heat was estimated to be responsible for more than 700 deaths per year between 2004 and 2018, although some estimates put heat-related mortality closer to 1,300 deaths annually. Disasters are now coming more frequently and causing more damage. In the 1980s, the country experienced, on average, one (inflation-adjusted) billion-dollar weather disaster every four months. Now there is one, on average, every three weeks.

Disaster risk in a complex society such as the United States is never determined simply by extreme weather events. It also depends strongly on exposure (who or what lies in the path of hazards) and vulnerability (their ability to cope with hazards). Climate change interacts with existing social, political, and economic structures—increases in property values as well as increased development in hazard hotspots have also contributed to the increase in billion-dollar disasters—and exacerbates existing inequalities. Certain groups are more vulnerable to extreme events due to socioeconomic or demographic factors. Americans over 65 are several times more likely to die of heat-related cardiovascular disease than younger people, while Black Americans die from heat-related diseases at a rate twice that of the general population. The extreme rainfall brought by Hurricane Harvey increased the flooded area in the Greater Houston area by 14%, which led to 32% more homes flooded in Harris County, with a disproportionate impact on low-income Hispanic neighborhoods. The spatial distribution of climate impacts partially reflects current and past policy choices: low-income neighborhoods, including those historically affected by redlining or other discriminatory policies, can be as much as 12°F hotter during heatwaves than wealthier neighborhoods in the same city and are at a substantially higher risk of flooding.

Focus on Western Wildfires:

Climate change is leading to larger and more severe wildfires in the western United States, bringing acute and chronic impacts both near and far from the flames. Wildfires have significant public health, socioeconomic, and ecological implications for the Nation.

Climate change has other wide-ranging consequences for people’s health and well-being and the land and ocean ecosystems on which we depend. The 2021 Pacific Northwest heatwave, which resulted in more than 1,400 heat-related fatalities, also led to widespread die-offs of shellfish and other marine organisms, tree and crop damage, and other impacts on the region’s ecosystems. Western wildfires, made more severe by climate change, have destroyed towns and infrastructure and contributed to an increase in the frequency and persistence of high levels of air pollution across the US West. These extreme events occur against a changing backdrop as climate change pushes aspects of the Earth system into a “new normal.”

Long-term warming trends are associated with shifts in other aspects of the climate system. For example, both drought in the western US and heavier precipitation and increased flood risk across much of the US are linked to rising temperatures. Sea level rise threatens the coasts and makes storm surges higher. Scientists cannot rule out the possibility of still more dramatic shifts if certain tipping elements trigger rapid and irreversible changes. While immediate and aggressive reductions in greenhouse gas emissions can mitigate future warming and reduce the risk of exceeding tipping points, temperatures will continue to increase until emissions of carbon dioxide reach net zero. When or if warming stops, long-term responses to the temperature changes that have already occurred will continue to drive changes for decades. Put simply, communities across the country are built for a climate that no longer exists.