On November 3, the first volume of the Fourth National Climate Assessment (NCA4), the Climate Science Special Report (CSSR), was released by a coordinated group of 13 federal agencies. The legally-mandated report concludes that humans are unequivocally the dominant cause of the observed global warming that has been taking place since the mid-20th century. The report states that over the last 115 years, global average temperatures have increased 1.8 degrees Fahrenheit, contributing to progressively worse record-breaking weather extremes, and that these trends are expected to continue. The report assigns a more than 95 percent likelihood that human influence has been the dominant cause of the observed warming since the mid-20th century. There are no plausible alternative explanations supported by the evidence. Fluctuations in radiation from the sun and natural variability caused by ocean-atmosphere interactions have affected only a tiny fraction of climate trends when considered over decades.

The NCA4 was developed as a two-volume set. Volume I, the Climate Science Special Report (CSSR), provides the foundational science and factual information undergirding climate change observations. Volume II, Climate Change, Risks, and Adaptation in the United States, draws upon the information developed in Volume I to assess and forecast climate change impacts on human welfare, society and the environment for ten U.S. regions. The latest draft of Volume II was also released on November 3 and will be open for public comments through January 31, 2018. It is expected to be published in final form in December 2018.

The first volume underwent six levels of review by more than 300 federal and non-federal experts, including scientists at federal agencies, national laboratories, universities, and businesses. It was also reviewed by the National Academies of Sciences, Engineering and Medicine.

The report was mandated by the Global Change Research Act of 1990, which requires that a National Climate Assessment be delivered to Congress no less than every four years.


Worldwide Environmental Impacts

The CSSR begins by stating that there is a global, long-term, unambiguous warming trend that has continued for more than the past 50 years. Since 2014, when the last National Climate Assessment was published, each year has surpassed the previous year as the warmest year on record globally. Sixteen of the warmest years on record for the globe have occurred in the last 17 years.

The burning of fossil fuels is also causing many other environmental changes, including glacier melting, diminishing snow cover, warming of the oceans, shrinking sea ice, ocean acidification, and increasing atmospheric water vapor. Global average sea level has risen 7-8 inches since 1900 (more than in any preceding century in at least 2,800 years), with almost half of that occurring since 1993. This too, is caused by human/industrial activities. Incidences of tidal flooding are increasing in almost all coastal cities, especially on the Atlantic and Gulf coasts. Global average sea levels are expected to continue to rise by at least several more inches in the next 15 years and by 1-4 feet (and possibly 8 feet) by 2100.

All of these trends are conspiring to negatively impact human safety, infrastructure, agriculture, water quality, and natural ecosystems. For example, heat waves have become more frequent in the United States, and the trend will likely continue. Over the next few decades, average annual U.S. temperatures are expected to rise another 2.5° F by 2050, and much larger rises are projected for the latter half of the century. Heavy rainfall, and the flooding it creates, is increasing in intensity and frequency globally, and is expected to continue to increase. Extreme precipitation events in most parts of the United States have increased over the past century, with the largest increases occurring in the Northeast.


Regional Trends: Varied Impacts from Alaska to the Tropics

Trends for some other types of extreme weather events such as floods, droughts and severe storms have more variable regional characteristics. The frequency and severity of “atmospheric rivers” (narrow streams of moisture that are associated with severe flooding events) on the West Coast, for example, will increase as a result of higher atmospheric water vapor associated with increasing temperature. Similarly, the incidence of severe tropical hurricanes and typhoons is projected to increase as a result of human-induced global warming.

The incidence of large forest fires in the western United States and Alaska has increased since 1980 and is projected to further increase as climate change progresses. Trends toward earlier spring melt and reduced snowpack are already affecting water resources in the western United States and these trends are expected to continue. Without changes to water resource management, chronic, long-duration droughts, which have serious economic and societal impacts, will become more common before the end of this century.

Average near-surface air temperatures across Alaska and the Arctic have increased over the last 50 years more than twice as fast as the global average temperature. Since 1980, the average amount of Arctic sea ice has decreased significantly, has become much thinner, and is melting more days per year. It is projected that the Arctic will be sea ice-free in late summers by 2040. The report gives a greater than 90 percent likelihood that human activities have contributed to Arctic surface temperature warming, sea ice loss, glacier mass loss, and snow extent decline.

The world’s oceans have absorbed 93 percent of the excess heat caused by greenhouse gas warming since the mid-20th century, making them warmer and altering climate feedbacks. The oceans are currently absorbing more than a quarter of the CO2 emitted into the atmosphere annually from human activities, making them more acidic and negatively impacting marine ecosystems.


Threatening Outlook for the Future

Global temperatures are projected to continue to increase over this century and beyond. The magnitude of climate change beyond the next few decades will depend on the amount of greenhouse (heat-trapping) gases emitted globally and on the sensitivity of the Earth’s climate to those emissions. With significant reductions in greenhouse gas emissions, the global annually averaged temperature rise could be limited to 3.6°F (2°C), which is the internationally-agreed upon limit established by the Paris Climate Agreement to avoid the worst effects of climate change. Without major reductions in these emissions, however, global average temperatures could increase by 9°F (5°C) or more by the end of this century.

While carbon emissions have slowed in recent years as economic growth has become less carbon intensive, they are not yet low enough to keep global average temperatures less than 3.6°F above preindustrial levels. Keeping the global mean temperature to less than 3.6°F above pre-industrial levels will require significant reductions in net global CO2 emissions prior to 2040, and will likely require net emissions to become zero or possibly negative later in the century. There is a significant possibility of unanticipated and difficult or impossible to manage changes in the climate system throughout the next century, as passing critical thresholds (a.k.a. tipping points) can cause a self-driven acceleration of greenhouse gases being emitted into the atmosphere, causing irreversible climate-related extreme events.

One potential tipping point has already been reached. The concentration of carbon dioxide (CO2) in the atmosphere has now passed 400 parts per million (ppm), which last occurred 3 million years ago when the average global temperature and sea level were significantly higher than today. Continued growth in CO2 emissions would lead to an atmospheric concentration last present tens to hundreds of millions of years ago. The further the Earth’s ecosystems are pushed, the greater the risk of unanticipated impacts, some of which may be irreversible.

Volume II of the NCA4, currently being reviewed by a panel of the National Academies of Science, Engineering and Medicine, as well as by the public, will delve deeper into the current and future impacts of climate change. The draft report outlines some of the impacts that have been caused by climate change since the last report (NCA3) was published in 2014, such as hurricanes, floods, wildfires, and increases in high tide flooding related to sea level rise. It makes the connection between climate change and flooding events in coastal cities, rising ocean temperatures causing disruptions to U.S. fisheries, agricultural losses from drought, infrastructure damage, and loss of life.

Even if the Trump Administration chooses not to act based on the large, carefully reviewed National Climate Assessment, the data in it is available for all policymakers, at the subnational or international levels, to use. References to the NCA4 are already being made by several other governments at the U.N. Climate Change Conference (COP23), currently taking place in Bonn, Germany, and which will conclude November 17, 2017.


Author: Richard Nunno