This Climate Science Special Report is Volume 1 of the fourth National Climate Assessment published by the U.S. Global Change Research Program — a presidential initiative under Ronald Reagan, signed into law by George H. Bush. The previous three National Climate Assessments were published in 2000, 2009 and 2014. The list of agencies participating is:
- United States Department of Agriculture
- National Oceanic and Atmospheric Administration
- United States Department of Commerce
- National Institute of Standards and Technology
- United States Department of Defense
- United States Department of Energy
- National Institutes of Health
- United States Department of Health and Human Services
- United States Department of State
- United States Department of Transportation
- United States Geological Survey
- United States Department of the Interior
- Environmental Protection Agency
- National Aeronautics and Space Administration
- National Science Foundation
- Smithsonian Institution
Volume II of NCA4–Climate Change Impacts, Risks, and Adaptation in the United States, is available for public comment and is being reviewed by the National Academies of Sciences, Engineering and Medicine for publication in December 2018
The following are excerpts from the Executive Summary of the Climate Science Special Report, [comments in square brackets are from Climate Change Now]:
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Global and U.S. Temperatures Continue to Rise
New observations and new research have increased our understanding of past, current, and future climate change since the Third U.S. National Climate Assessment (NCA3) was published in May 2014. This Climate Science Special Report (CSSR) is designed to capture that new information and build on the existing body of science. Since NCA3, stronger evidence has emerged for continuing, rapid, human-caused warming of the global atmosphere and ocean. This report concludes that “it is extremely likely that human influence has been the dominant cause of the observed warming since the mid-20th century.”
Recent data add to the weight of evidence for rapid global-scale warming, the dominance of human causes, and
the expected continuation of increasing temperatures, including more record-setting extremes. The global, long-term, and unambiguous warming trend has continued during recent years. Since the last National Climate Assessment was published, 2014 became the warmest year on record globally; 2015 surpassed 2014 by a wide margin; and 2016 surpassed 2015. Sixteen of the warmest years on record for the globe occurred in the last 17 years.
Global annual average temperature has increased by more than 1.2°F (0.65°C) for the period 1986–2016; and 1.8°F (1.0°C) from 1901–2016. Average temperatures in recent decades over much of the world have risen faster during this time period than at any time in the past 1,700 years or more.
[Global temperature increase 1880 to 2016, National Climate Assessment 2014.]
Many lines of evidence demonstrate that it is extremely likely that human influence has been the dominant cause of the observed warming. The likely human contribution is 92%–123% of the observed 1951–2010 change. [In other words, it’s likely that more than all of observed warming has been caused by humans — see here.] The global influence of natural variability, however, is limited to a small fraction of observed climate trends over decades.
[Natural climate change has actually masked a small amount of warming, reducing the global temperature more than what it should be relative to the amount of global warming pollutants emitted. National Climate Assessment 2017.]
With significant reductions in the emissions of greenhouse gases, the global annually averaged temperature rise could be limited to 3.6°F (2°C) or less. Without major reductions in these emissions, the increase in annual average global temperatures relative to preindustrial times could reach 9°F (5°C) or more by the end of this century. If greenhouse gas concentrations were stabilized at their current level, existing concentrations would commit the world to at least an additional 1.1°F (0.6°C) of warming over this century [double what has already occurred.]
Annual average temperature over the contiguous United States is projected to rise. Increases of about 2.5°F (1.4°C) are projected for the period 2021–2050 in all RCP scenarios, implying recent record-setting years may be “common” in the next few decades. Much larger rises are predicted later in the century.
In the United States, the urban heat island effect results in daytime temperatures 0.9°–7.2°F (0.5°–4.0°C) higher and nighttime temperatures 1.8°– 4.5°F (1.0°–2.5°C) higher in urban areas than in rural areas, with larger temperature differences in humid regions. [The urban heat island effect is factored out of average temperatures reported elsewhere in this report–see here.]
Temperature and Precipitation Extremes Are Becoming More Common
There have been marked changes in temperature extremes across the contiguous United States. The number of high temperature records set in the past two decades far exceeds the number of low temperature records.
[In a stable climate, record cold temperatures would be about equal to record high temperatures. National Climate Assessment 2017.]
Heavy precipitation events in most parts of the United States have increased in both intensity and frequency since 1901. There are important regional differences in trends, with the largest increases occurring in the northeastern United States
[The 5-year storm is just one of a number of different storms tracked. The 99th percentile storm is the 100-year storm. The rarest storms are predicted to increase much faster than less rare. Example: the 100-year storm is predicted to increase in occurrence faster than the 5-year storm and 1,000-year storm faster than the 100-years. The challenge is in measurement. If a 1,000 year storm happens 50 times more often, it only happens once in 20-years. Hurricane Harvey was a 25,000 year storm.}
Recent droughts and associated heat waves have reached record intensity in some regions of the United States. Northern Hemisphere spring snow cover extent, North America maximum snow depth, snow water equivalent in the western United States, and extreme snowfall years in the southern and western United States have all declined, while extreme snowfall years in parts of the northern United States have increased. There has been a trend toward earlier snowmelt and a decrease in snowstorm frequency on the southern margins of climatologically snowy areas. Winter storm
tracks have shifted northward since 1950s. Potential linkages between the frequency and intensity of severe winter storms in the United States and accelerated warming in the Arctic have been postulated. Tornado activity in the United States has become more variable, particularly over the 2000s, with a decrease in the number of days per year with tornadoes and an increase in the number of tornadoes on these days. Confidence in past trends for hail and severe thunderstorm winds is low.
Extreme temperatures in the contiguous United States are projected to increase even more than average temperatures (very high confidence). Projections indicate large declines in snowpack in the western United States and shifts to more precipitation falling as rain than snow in the cold season in many parts of the central and eastern United States. Substantial reductions in western U.S. winter and spring snowpack are projected as the climate warms.
Little evidence is found for a human influence on observed precipitation deficits, but much evidence is found for a human
influence on surface soil moisture deficits due to increased evapotranspiration caused by higher temperatures. The incidence of large forest fires in the western United States and Alaska has increased since the early 1980s and is projected to further increase in those regions as the climate warms, with profound changes to certain ecosystems. Both physics and numerical modeling simulations generally indicate an increase in tropical cyclone intensity in a warmer world. [ Hurricane intensity and frequency have both increased in the North Atlantic, while globally the increase is not yet statistically valid.]
Oceans Are Rising, Warming, and Becoming More Acidic
The world’s oceans have absorbed about 93% of the excess heat caused by greenhouse gas warming since the mid-20th century, making them warmer and altering global and regional climate feedbacks. Ocean heat content has increased at all depths since the 1960s and surface waters have warmed by about 1.3° F (0.7° C) per century globally since 1900 to 2016.
Global mean sea level (GMSL) has risen by about 7–8 inches (about 16–21 cm) since 1900, with about 3 of those inches (about 7 cm) occurring since 1993. Human-caused climate change has made a substantial contribution to GMSL rise since 1900, contributing to a rate of rise that is greater than during any preceding century in at least 2,800 years.
Emerging science regarding Antarctic ice sheet stability suggests that, for higher scenarios, a GMSL rise exceeding 8 feet (2.4 m) by 2100 is physically possible.
As sea levels have risen, the number of tidal floods each year that cause minor impacts (also called “nuisance floods”) have increased 5- to 10-fold since the 1960s in several U.S. coastal cities. Rates of increase are accelerating in over 25 Atlantic and Gulf Coast cities). Tidal flooding will continue increasing in depth, frequency, and extent this century. [When nuisance flooding increases by 25 times, or with 14 inches of sea level rise along much of the East and Gulf coasts, resource abandonment will begin. At this point 187 U.S. cities will be in the midst of abandonment as per NOAA and the Union of Concerned Scientists.]
When nuisance flooding reaches a level 25 times greater than normal for the 20th century, NOAA says coastal resource abandonment will begin.
The world’s oceans are currently absorbing more than a quarter of the CO2 emitted to the atmosphere annually from human activities, making them more acidic, with potential detrimental impacts to marine ecosystems. The rate of acidification is unparalleled in at least the past 66 million years.
Climate Change in Alaska and across the Arctic Continues to Outpace Global Climate
Change
Annual average near-surface air temperatures across Alaska and the Arctic have increased over the last 50 years at a rate more than twice as fast as the global average temperature. Rising Alaskan permafrost temperatures are causing permafrost to thaw and become more discontinuous; this process releases additional carbon dioxide and methane resulting in additional warming.
It is virtually certain that Alaska glaciers have lost mass over the last 50 years, with each year since 1984. Over the satellite record, average ice mass loss from Greenland was −269 Gt per year between April 2002 and April 2016, accelerating in recent years. Arctic Sea ice loss since the early 1980s has decreased in extent between 3.5% and 4.1% per decade; has become thinner by between 4.3 and 7.5 feet, and is melting at least 15 more days each year. September sea ice extent has decreased between 10.7% and 15.9% per decade.
Arctic sea ice loss is expected to continue through the 21st century, very likely resulting in nearly sea ice-free late summers by the 2040s. It is very likely that human activities have contributed to observed arctic surface temperature
warming, sea ice loss, glacier mass loss, and northern hemisphere snow extent decline.
Arctic Sea Ice, National Climate Assessment 2017.
Limiting Globally Averaged Warming to 2°C (3.6°F) Will Require Major Reductions in
Emissions
The observed increase in global carbon emissions over the past 15–20 years has been consistent with higher scenarios. In 2014 and 2015, emission growth rates slowed as economic growth became less carbon-intensive. Even if this lowing trend continues, however, it is not yet at a rate that would limit the increase in the global average temperature to well below 3.6°F (2°C) above preindustrial levels.
Global mean atmospheric carbon dioxide (CO2) concentration has now passed 400 ppm, a level that last occurred about 3 million years ago, when global average temperature and sea level were significantly higher than today. Continued growth in CO2 emissions over this century and beyond would lead to an atmospheric concentration not experienced in tens of millions of years. The present-day emissions rate of nearly 10 GtC per year [gigatons of carbon, or about 40 gigatons of CO2,] suggests that there is no climate analog for this century any time in at least the last 50 million years.
Stabilizing global mean temperature to less than 3.6°F (2°C) above preindustrial levels requires substantial reductions in net global CO2 emissions prior to 2040, and likely requires net emissions to become zero or possibly negative later in the century. Achieving global greenhouse gas emissions reductions before 2030 consistent with targets and actions announced by governments in the lead up to the 2015 Paris climate conference would hold open the possibility of meeting the long-term temperature goal of limiting global warming to 3.6°F (2°C) above preindustrial levels, whereas there would be virtually no chance if net global emissions followed a pathway well above those implied by country announcements. Actions in the announcements are, by themselves, insufficient to meet a 3.6°F (2°C) goal; the likelihood of achieving that depends strongly on the magnitude of global emissions reductions after 2030.
[Very important: as this report makes abundantly clear, impacts increase non linearly with warming. The statements above concerning the 2 degrees C warming threshold to dangerous climate change are currently under scrutiny and will almost certainly be revised to at least 1.5 C with the upcoming Intergovernmental Panel on Climate Change Special Report on 1.5 C Warming due out in 2018.]There is a Significant Possibility for Unanticipated Changes
[This is extremely important.]Humanity’s effect on the Earth system, through the large-scale combustion of fossil fuels and widespread deforestation and the resulting release of carbon dioxide (CO2) into the atmosphere, as well as through emissions of other greenhouse gases is unprecedented. There is significant potential for humanity’s effect on the planet to result in unanticipated surprises and a broad consensus that the further and faster the Earth system is pushed towards warming, the greater the risk of such surprises.
There are at least two types of potential surprises: compound events, where multiple extreme climate events occur simultaneously or sequentially (creating greater overall impact), and critical threshold or tipping point events, where some threshold is crossed in the climate system (that leads to large impacts). The probability of such surprises—some of which may be abrupt and/or irreversible—as well as other more predictable but difficult-to-manage impacts, increases as the influence of human activities on the climate system increases.
Positive feedbacks (self-reinforcing cycles) within the climate system have the potential to accelerate human-induced climate change and even shift the Earth’s climate system, in part or in whole, into new states that are very different from those experienced in the recent past (for example, ones with greatly diminished ice sheets or different large-scale patterns of atmosphere or ocean circulation). Some feedbacks and potential state shifts can be modeled and quantified; others can be modeled or identified but not quantified; and some are probably still unknown.
The physical and socioeconomic impacts of compound extreme events (such as simultaneous heat and drought, wildfires associated with hot and dry conditions, or flooding associated with high precipitation on top of snow or waterlogged ground) can be greater than the sum of the parts (very high confidence). Few analyses consider the spatial or temporal correlation between extreme events.
While climate models incorporate important climate processes that can be well quantified, they do not include all of the processes that can contribute to feedbacks, compound extreme events, and abrupt and/or irreversible changes. For this reason, future changes outside the range projected by climate models cannot be ruled out (very high confidence). Moreover, the systematic tendency of climate models to underestimate temperature change during warm paleoclimates suggests that climate models are more likely to underestimate than to overestimate the amount of long-term future change.