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.
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; https://www.nytimes.com/interactive/2017/08/07/climate/document-Draft-of-the-Climate-Science-Special-Report.html.
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 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; https://www.nytimes.com/interactive/2017/08/07/climate/document-Draft-of-the-Climate-Science-Special-Report.html.
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.
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).
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.
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.