There is strong evidence of marked changes in temperature extremes across the contiguous United States. Extremely cold days have become warmer since the early 1900s, and extremely warm days have become warmer since the early 1960s. In recent decades, extreme cold waves have become less common while extreme heat waves have become more common. Heavy precipitation events in most parts of the United States have increased in both intensity and frequency since 1901. That is the conclusion of a report prepared by scientists from NOAA, NASA, the Department of Energy, the U.S. Global Change Research Program, U.S. national laboratories, universities, and the private sector, and peer-reviewed by an expert panel at the National Academy of Sciences. Furthermore, the report found that “it is extremely likely that human influence has been the dominant cause of the observed warming since the mid-20th century. For the warming over the last century, there is no convincing alternative explanation supported by the extent of the observational evidence.”
The National Climate Assessment summarizes the impacts of climate change on the United States, now and in the future. The last report NCA3 was released in 2014. In preparation for the Fourth National Climate Assessment (NCA4), the U.S. Global Change Research Program (USGCRP) has produced a special Climate Science Special Report (CSSR) which is a detailed analysis the state of the science of how climate change is affecting weather and climate across the United States. The Science Steering Committee had representatives from NOAA, NASA, and the Department of Energy (DOE), the U.S. Global Change Research Program and three Coordinating Lead Authors. 30 Lead Authors, all scientists from Federal agencies, national laboratories, universities, and the private sector were selected by the steering committee. An early draft was released for public comment 15 December 2016 – 3 February 2017. A draft was peer-reviewed by an expert panel at the National Academy of Sciences in April 2017. The final draft of the CSSR is currently under review by the Trump Administration. This draft was published on the website of The New York Times August 7, 2017.
Temperature and precipitation extremes
The average annual temperature over the contiguous United States has increased by 1.8°F for the period 1901-2016 (Very high confidence). Surface and satellite data are consistent in their depiction of rapid warming since 1979 (High confidence). Longer-term climate records over past centuries and millenia indicate that average temperatures in recent decades over much of the world have been much higher, and have risen faster during this time period, than at any time in the past 1,700 years or more, the time period for which the global distribution of surface temperatures can be reconstructed (High confidence)
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 (Very high confidence). Extremely cold days have become warmer since the early 1900s, and extremely warm days have become warmer since the early 1960s. In recent decades, extreme cold waves have become less common while extreme heat waves have become more common. (Very high confidence).
Heavy precipitation events in most parts of the United States have increased in both intensity and frequency since 1901 (High confidence).
Since 1980, the cost of extreme events for the United States has exceeded $1.1 trillion.
Rising sea level and tidal flooding
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 (Very high confidence).
As sea levels have risen, the number of tidal floods each year that cause minor impacts called “nuisance floods” have increased 5- to 10-fold since the 1960s in several coastal cities (Very high confidence).
Arctic and Alaska warming
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 (Very high confidence)
Since the early 1980s annual average arctic sea ice 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 per year. September sea ice extent has decreased between 10.7% and 15.9% per decade (Very high confidence)
Losses of Arctic sea ice and Greenland Ice Sheet mass are accelerating, and Alaskan mountain glaciers continue to steadily melt (Very high confidence). Alaskan coastal sea ice loss rates exceed the Arctic average (Very high confidence).
Rising Alaskan temperatures are causing permafrost to thaw releasing carbon dioxide and methane from the decomposition of previously frozen organic matter, adding to the global greenhouse gases that are driving climate change. (High confidence)
While some observational studies suggest a linkage between enhanced arctic warming (arctic amplification) and mid-latitude Northern Hemisphere weather, specifically for an increase in highly amplified jet stream patterns in winter over the United States, other studies show mixed results. Therefore, a definitive understanding of the effects of arctic amplification on mid-latitude winter weather remains elusive.
Global mean atmospheric carbon dioxide (CO2) concentration has now passed 400 ppm, a level that last occurred about three million years ago, when global average temperature and sea level were significantly higher than today (High confidence). It is estimated that three million years ago global mean temperatures were 3.6° to 6.3°F (2° to 3.5°C) higher and sea levels were 66 ± 33 feet (20 ± 10 meters) higher than today.
The world’s oceans are currently absorbing about a quarter of the carbon dioxide emitted to the atmosphere annually from human activities making them more acidic (Very high confidence). The world’s oceans have absorbed 93% of the excess heat caused by greenhouse gas warming since the mid-20th century making them warmer (Very high confidence). Over the last half century, major oxygen losses have occurred in inland seas, estuaries, and in the coastal and open ocean. (High confidence)
Due to the complex interactions of the processes that govern terrestrial biogeochemical cycling, terrestrial ecosystem responses to increasing CO2 levels remains one of the largest uncertainties in long-term climate feedbacks and therefore in predicting longer-term climate change.
Human emissions of carbon dioxide (CO2), methane (CH4 ), and other greenhouse gases now overwhelm the influence of natural drivers on the external forcing of the Earth’s climate. The report concludes that “Many lines of evidence demonstrate that it is extremely likely that human influence has been the dominant cause of the observed warming since the mid-20th century. Over the last century there are no convincing alternative explanations supported by the extent of the observational evidence. Solar output changes and internal natural variability can only contribute marginally to the observed changes in climate over the last century, and we find no convincing evidence for natural cycles in the observational record that could explain the observed changes in climate. (Very high confidence)”