Below are the highlights of the draft U.S. Global Change Research Program Climate Science Report, known as the 4th National Climate Assessment
Full report available here: https://science2017.globalchange.gov/downloads/CSSR2017_FullReport.pdf
The climate of the United States is strongly connected to the changing global climate. The statements
below highlight past, current and projected climate changes for the United States and the globe.
Global annually averaged surface air temperature has increased by about 1.8Â°F (1.0Â°C) over the last 115
years (1901â€“2016). This period is now the warmest in the history of modern civilization. The last few
years have also seen record-breaking, climate-related weather extremes, and the last three years have
been the warmest years on record for the globe. These trends are expected to continue over climate
This assessment concludes, based on extensive evidence, that it is extremely likely that human activities,
especially emissions of greenhouse gases, are 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.
In addition to warming, many other aspects of global climate are changing, primarily in response to human
activities. Thousands of studies conducted by researchers around the world have documented
changes in surface, atmospheric and oceanic temperatures; melting glaciers; diminishing
snow cover; shrinking sea ice; rising sea levels; ocean acidification; and increasing atmospheric
For example, global average sea level has risen by about 7â€“8 inches since 1900, with almost half
(about 3 inches) of that rise occurring since 1993. Human-caused climate change has made a substantial
contribution to this rise since 1900, contributing to a rate of rise that is greater than during any
preceding century in at least 2,800 years. Global sea level rise has already affected the United States;
the incidence of daily tidal flooding is accelerating in more than 25 Atlantic and Gulf Coast cities.
Global average sea levels are expected to continue to riseâ€”by at least several inches in the next
15 years and by 1â€“4 feet by 2100. A rise of as much as 8 feet by 2100 cannot be ruled out. Sea level
rise will be higher than the global average on the East and Gulf Coasts of the United States.
Changes in the characteristics of extreme events are particularly important for human safety, infrastructure,
agriculture, water quality and quantity, and natural ecosystems. Heavy rainfall is increasing in
intensity and frequency across the United States and globally and is expected to continue to increase.
The largest observed changes in the United States have occurred in the Northeast.
Heatwaves have become more frequent in the United States since the 1960s, while extreme cold
temperatures and cold waves are less frequent. Recent record-setting hot years are projected to become
common in the near future for the United States, as annual average temperatures continue to
rise. Annual average temperature over the contiguous United States has increased by 1.8Â°F (1.0Â°C) for
the period 1901â€“2016; over the next few decades (2021â€“2050), annual average temperatures are
expected to rise by about 2.5Â°F for the United States, relative to the recent past (average from
1976â€“2005), under all plausible future climate scenarios.
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 changes, with
profound changes to regional ecosystems.
Annual 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. Under higher scenarios,
and assuming no change to current water resources management, chronic, long-duration hydrological
drought is increasingly possible before the end of this century.
The magnitude of climate change beyond the next few decades will depend primarily on the
amount of greenhouse gases (especially carbon dioxide) emitted globally. Without major reductions
in emissions, the increase in annual average global temperature relative to preindustrial times
could reach 9Â°F (5Â°C) or more by the end of this century. With significant reductions in emissions, the
increase in annual average global temperature could be limited to 3.6Â°F (2Â°C) or less.
The global atmospheric carbon dioxide (CO2
) concentration has now passed 400 parts per million
(ppm), a level that last occurred about 3 million years ago, when both 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 to hundreds
of millions of years. There is broad consensus that the further and the faster the Earth system is pushed
towards warming, the greater the risk of unanticipated changes and impacts, some of which are potentially
large and irreversible.
The observed increase in carbon emissions over the past 15â€“20 years has been consistent with higher
emissions pathways. In 2014 and 2015, emission growth rates slowed as economic growth became
less carbon-intensive. Even if this slowing trend continues, however, it is not yet at a rate that would
limit global average temperature change to well below 3.6Â°F (2Â°C) above preindustrial levels.