WASHINGTON -– Climate change is no longer a distant threat, but a real and present danger in the United States, according to a government report issued Tuesday.

The report is the latest update from the U.S. Global Change Research Program, and details ways that climate change -- caused predominantly by the emission of heat-trapping greenhouse gases -- is already being felt across the country.

"Climate change, once considered an issue for a distant future, has moved firmly into the present," the report says in its introduction. The full report, at more than 800 pages, is the most comprehensive look at the effects of climate change in the U.S. to date, according to its authors. (Even the "highlights" document provided to reporters the day before the release weighed in at 137 pages). The report includes regional and sectoral breakdowns of current and anticipated impacts, which have implications for infrastructure, agriculture, human health, and access to water.

Those impacts include increased severity of heat waves and heavier downpours. On the coasts, sea level rise is already contributing to increased flooding during high tides and storms, the report notes. And in the West, conditions are getting hotter and drier, and the snowpack is melting earlier in the year, extending wildfire season.

Average U.S. temperatures have increased 1.3 degrees to 1.9 degrees Fahrenheit (depending on the part of the country) since people began keeping records in 1895, and much of that warming has come in recent decades. The report notes that the period from 2001 to 2012 was warmer than any previous decade on record, across all regions of the country.

The length of time between the last spring frost and the first fall frost also has increased across the U.S. The average time between frosts in the Southwest increased by 19 days in the years 1991 to 2012, compared with the average from 1901 to 1960.

Heat waves are already the top cause of weather-related deaths in the U.S., and that will only get worse. Extreme heat can cause more heart, lung and kidney problems, especially among the poor, sick and elderly. The number of days where temperatures top 100 degrees is predicted to increase in the future. If emissions continue to rise, temperatures on the very hottest days during the last 20 years of this century may be 10 degrees to 15 degrees hotter across most of the country, the report finds. Under a lower-emission scenario, those hottest days of the years 2081 to 2100 would still be 3 degrees to 4 degrees warmer than now.

Another impact that scientists are already seeing that they have linked to climate change is an increase in major precipitation events. In the Northeast, for example, there has been a 71 percent increase in storms that would classify as "very heavy" -– in the top 1 percent -- from 1958 to 2012.

While the outlook could be considered bleak, Radley Horton, a scientist at Columbia University Earth Institute’s Center for Climate Systems Research and the lead author for the assessment’s chapter on the Northeast, said the report "delves into much more detail about opportunities to address climate change."

"The climate hazards are looking as severe as ever, but I think there is a message contained in the report that our ability to respond is about getting going," Horton told The Huffington Post. "The question is, are we able to meet the challenges, given the growing understanding of how much the climate could change this century?"

The amount of climate change in the future, the report says, "will still largely be determined by choices society makes about emissions."

The report notes that American society and its infrastructure were built for the past climate -- not the future. It highlights examples of the kinds of changes that state and local governments can make to become more resilient. One of the main takeaways, said David Wolfe, a professor of plant and soil ecology at Cornell University and a coauthor of the chapter on the Northeast, is that "you don't want to look at the weather records of yesteryear to determine how to set up your infrastructure."

This report, said Wolfe, signals that the country is "beginning to move beyond the debate about whether climate change is real or not, and really getting down to rolling up our sleeves" and addressing it.



A 60-person advisory committee comprised of government, private and academic representatives oversaw the assessment, which took four years and involved more than 300 scientists, engineers, and technical experts.

In an appearance at the White House press briefing on Monday, White House senior counselor John Podesta said the updated assessment provides "practical, usable knowledge" for state and local decision-makers as they prepare for climate impacts and is the "most authoritative and comprehensive" to date.

The reports are supposed to be issued at least every four years under the Global Change Research Act of 1990, and are meant to analyze "the effects of global change on the natural environment, agriculture, energy production and use, land and water resources, transportation, human health and welfare, human social systems, and biological diversity." The reports are to be presented to the president and Congress.

This is the third report of its kind. The first came in 2000, during the Clinton administration. The George W. Bush administration was accused of tampering with reports from the office (at that time called the "Climate Change Science Program"), altering them to downplay the connection between greenhouse gas emissions and global warming. The last report from the program was released in June 2009, as the Obama administration was making a push for climate legislation in Congress early in its first term.