-- The effects of climate change are already having an impact on the U.S., after average temperatures have risen dramatically over the last four decades, according to a draft of a government report that was obtained and published by The New York Times on Monday.
The "U.S. Global Change Research Program Climate Science Special Report," compiled by a group of scientists from 13 federal agencies, found with high confidence that it was "extremely likely that more than half of the global mean temperature increase since 1951 was caused by human influence on climate."
"Many lines of evidence demonstrate that human activities, especially emissions of greenhouse gases, are primarily responsible for observed climate changes in the industrial era," the report stated. "There are no alternative explanations, and no natural cycles are found in the observational record that can explain the observed changes in climate.”
The report is part of the National Climate Assessment, which has been congressionally mandated to be conducted at least every four years since 1990. A National Academies of Science committee reviewed the study and said it was "timely, accurate and well-written." The report contradicts claims by President Donald Trump and some members of his administration, who have disputed the connection between greenhouse gas emissions and climate change.
The report's authors also described a link between climate change and severe weather events.
"A change in the frequency, duration and/or magnitude of extreme weather events is one of the most important consequences of a warming climate," the authors wrote, pointing to an increase in heavy precipitation, extreme heat events and tropical storms.
The report states with "medium confidence" that human activities "have contributed to the observed increasing trend in North Atlantic hurricane activity since the 1970s."
On Tuesday, The New York Times released a newer draft of the report, one that uses different language to describe the strength of evidence connecting human activity to climate change.
A draft of the report from February describes the difference between weather and climate and describes warming trends over the last 150 years, saying that "many lines of evidence demonstrate that human activities, especially emissions of greenhouse (heat-trapping) gases, are primarily responsible for recent observed climate changes."
But a draft from June marked as the final draft uses stronger language, saying that new evidence shows it is "extremely likely" that human influence has been the main cause of 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 newer draft's introduction gives more detail about extreme weather events related to climate. It says that the total cost of those events in the U.S. from 1980 to 2016 exceeded $1.1 trillion — a figure not included in the earlier report.
Trump has called climate change "a hoax" and rejected evidence that human activity has contributed to rising temperatures. During an interview with ABC News' "This Week" in June, his EPA Administrator Scott Pruitt declined to confirm whether Trump still believes climate change is a hoax.
The study must be approved by the Trump administration before it may be officially released, according to the Times, which reported that some scientists were fearful that the report will be changed or suppressed by the administration.
On Tuesday the White House commented on the report.
"It's very disappointing yet entirely predictable to learn The New York Times would write off a draft report without first verifying its contents with the White House or any of the federal agencies directly involved with climate and environmental policy," a White House official told ABC News. "As others have pointed out — and The New York Times should have noticed — drafts of this report have been published and made widely available online months ago during the public comment period. The White House will withhold comment on any draft report before its scheduled release date."
Separately, Liz Bowman, an EPA spokesperson, told ABC News, "This is a draft report that has been in the public domain since January. We continue to discuss the best path forward for an honest, open dialogue in regard to climate science."
Representatives for the Global Change Research Program and the National Academy of Sciences did not respond to ABC News' requests for comment.