A draft of the Fourth National Climate Assessment reports that the global average temperature for the 30-year period from 1986 to 2016 rose by 1.2°F (0.7°C). It is extremely likely that activities by humans have been the principal cause of this warming. Extreme temperature and rainfall events have increased over this time, as have forest wildfires.
Arctic land-based ice has been lost to melting, and the extent and thickness of sea ice has decreased. The mean sea level has risen about 7-8 in (about 26-21 cm) since 1900. Ocean waters have taken up 93% of the excess heat of the Earth system due to global warming since the 1950s.
Global greenhouse gas emissions are projected to continue and consequently global temperatures will continue increasing and related trends will continue. Limits to the intended increase require that humanity reduce annual emissions to zero by 2100.
The draft states “Choices made today will determine the magnitude of climate change risks beyond the next few decades.”
The Fourth National Climate Assessment (NCA) is due in 2018 (See Background at the end of this post). However the U.S. Global Change Research Program (USGCRP), which oversees preparation of the NCA, has prepared a Final Draft of a Climate Science Special Report (CSSR; see Note 1 at the end of the post) that has become publicly available as a freestanding document on which the actual NCA will be based.
This post is based on the CSSR Executive Summary (ES). Confidence levels and likelihoods given here in italicsare taken directly from the ES. They are carefully defined in the CSSR. Phrases in quotes are taken verbatim from the CSSR text.
The Historical Record
The global average temperature has risen above the average for the six decades 1901-1960 by 1.2°F (0.7°C) for the recent period from 1986 to 2016 (very high confidence). The map below shows temperature increases gridded across the globe.
Color-coded global map grid of historical changes in the average temperature for the period 1986-2016 relative to the average from the six-decade reference period 1901-1960, in °F. No data are available at the poles, indicated by gray. Source: CSSR.
It is extremely likely that activities by humans have been the “dominant” cause of the warming observed since the middle of the 20th century. No alternatives, such as the cyclical changes in solar energy reaching the Earth or variations in natural planetary factors, can explain the observed climate changes (very high confidence).
Extreme climate-related weather events have increased in number and severity. Since 1980 the cost of such calamities in the U. S. is over US$1 trillion. Extreme events can impact water quality, agriculture, human health, infrastructure, and lead to disaster events. In the U. S. the number of high temperature records in the past 20 years is much higher than the number of low temperature records (very high confidence).
The occurrence of large forest wildfires has increased in the U. S. West and Alaska since the early 1980s (high confidence).
The waters of the oceans have absorbed about 93% of the heat accumulating in the Earth system due to global warming since the 1950s (very high confidence). This affects climate patterns around the world.
In the Arctic, ice sheets overlaying land have been melting for at least the last three decades; in some locations the rate of loss is accelerating (very high confidence). The rate of melting of ice sheets over Greenland has accelerated in the last few years (high confidence). As this ice melts the water flows to the ocean, resulting in a net increase of sea level.
Arctic sea ice has been imaged since satellite flights permitted. The sea ice floats on the Arctic Ocean; its area expands and contracts in freeze-thaw seasonal cycles without any net change to global sea levels. Rather, the extent responds to changes in air and sea temperatures. The least extent, i.e., the most melting, occurs typically in September. Striking images showing the loss of September sea ice from 1984 to 2016, both in thickness (color coded white as having been formed at least four years earlier) and in overall surface area, are shown in the images below:
Satellite images of Arctic sea ice extent and thickness in September, for 1984 (top) and 2016 (bottom). The color bar shows the local age of the ice in years, a proxy for its thickness, from recent (dark gray) to more than 4 years (white). Source: Adapted from CSSR.
The mean sea level has risen about 7-8 in (about 26-21 cm) since 1900 (very high confidence). This is attributed “substantially” to human-induced climate change (high confidence). The rate of sea level rise is greater than any found in the last 2,800 years (medium confidence).
Ocean waters are absorbing more than 25% of the carbon dioxide emitted into the atmosphere by burning fossil fuels. Carbon dioxide is weakly acidic when dissolved in water, increasing its acidity (very high confidence). This negatively impacts marine ecosystems in many important ways.
By the end of the 21st century if the world generates significant reductions in greenhouse gases the global average temperature increase could be limited to 3.6°F (2.0°C) or less. This would require a pathway of annual GHG emissions reaching near zero by then. In contrast, minimal constraints on the annual emissions rate could result in a rise of 5.8-11.9°F (3.2-6.6°C) (high confidence).
But realistic projections all foresee continued GHG emissions into the future. U. S. temperatures will continue to rise (very high confidence); new records for high temperatures will be frequent (virtually certain). Temperatures by the end of this century will be much higher than the present (high confidence).
Heavy precipitation events are projected to continue increasing over the 21st century (high confidence). In the western U. S., large reductions in mountain snowpack, and more precipitation falling as rain rather than snow, are projected as the climate warms (high confidence). These trends are attributed to human activity (high confidence). They will likely worsen considerably as the climate warms (very high confidence). In the absence of reductions in emission rates long-duration hydrological drought, due to decreased retention of soil moisture, becomes more likely by the end of the century (very high confidence).
The mean sea level will continue increasing, to varying extents depending on future emission rates, by at least 1 ft (30 cm; very high confidence) and as much as 4 ft (130 cm; low confidence) by 2100. If the Antarctic ice shelf is lost due to high emission rates the upper bound could be as high as 8 ft (260 cm). It is extremely likely that sea level will continue rising beyond 2100 (high confidence) as ice continues melting.
Further loss in Arctic sea ice will continue throughout the 21st century, very likely resulting in a virtually ice-free ocean by the 2040s (very high confidence).
Limiting the total global average temperature increase to 3.6°F (2.0°C), or less, from a 19th century baseline will require significant constraints on future GHG emission rates. Even though annual emission rates decreased slightly in 2014 and 2015, they are still too high to meet commitments that nations made upon entering the 2015 Paris Agreement (high confidence). Indeed, present and projected emission rates would bring the atmospheric level of GHGs to levels so high that they have not occurred for at least the last 50 million years (medium confidence).
New carbon dioxide released “today” is long-lived, persisting in the atmosphere for decades to thousands of years. Therefore it’s important to note that the relationship between total atmospheric CO2 concentration and the increase in global temperature is a linear one.
The ES states “Choices made today will determine the magnitude of climate change risks beyond the next few decades. Stabilizing global mean temperature below 3.6°F (2°C) or lower relative to preindustrial levels requires significant reductions in …CO2emissions…before 2040 and likely requires net emissions to become zero….” If humanity continues emitting GHGs at rates higher than called for here we would reach the 3.6°F limit only two decades from now, with further temperature increases later.
[Update August 17, 2017: The science journal Nature has published a news article that discusses the CSSR and the political considerations facing the U. S. administration as it weighs issuing the Fourth NCA. Climate scientists are concerned about the fate of the Report. Nature notes that the Heartland Institute, a conservative think tank that promotes skepticism about global warming, is consulting with the Environmental Protection Agency on this issue.]
This NCA is only the latest in a long series of reports detailing the reality of warming and specifying the harms that global warming and climate change cause to our planet. In particular, it attributes the cause to human activity, including the burning of fossil fuels.
We must all undertake to reduce emissions of GHGs in our personal lives, and support policies promoting reductions at the state, national and international levels.
The U. S. Global Change Research Act of 1990 mandates preparation of assessments of global change every four years to “assist the nation and the world to understand, assess, predict, and respond to human-induced and natural processes of global change”. It assesses the current state of scientific understanding of global change on the natural and human environments. Its tasks, however, do not include formulation of policies to address global warming.
Climate scientists and related specialists drawn from thirteen U. S. government departments and agencies (see Note 2 at the end of this post), as well as a large number of scientists in nongovernmental research facilities, prepared the CSSR and the NCA. They critically assessed peer-reviewed research and similar public sources, including primary datasets and recognized climate modeling frameworks.