-- Human activity is the "dominant cause" of the warmest global temperatures in modern history and has also resulted in changes including rising sea levels, more frequent heat waves and the increasing concentration of carbon dioxide in the atmosphere, according to the federal Climate Science Special Report, an annual assessment of the physical impacts of climate change.
The report, which is the result of collaboration across a number of federal government agencies and was assented to by the White House, appears to refute much of what President Donald Trump and his administration have said about the subject, notably that the impact of humans on climate change was uncertain.
The study notes that "thousands of studies conducted by researchers around the world have documented changes," including the warming climate and extreme occurrences, and that its findings "are based on a large body of scientific, peer-reviewed research, as well as a number of other publicly available sources, including well-established and carefully evaluated observational and modeling datasets."
On the changes recorded in Earth's climate, the report concludes, "There is no convincing alternative explanation supported by the extent of the observational evidence" outside of human activity.
Trump has shown himself to be skeptical of the impact of humans on the climate, ranging back to the years prior to his presidency. He has commented dismissively upon the term "global warming," calling it "an expensive hoax," and once termed climate change "b-------," as he instead called for "clean, beautiful and healthy air."
Members of the administration have offered unclear answers when asked about Trump's position on climate change.
U.S. Ambassador to the United Nations Nikki Haley told CBS News in June that the president "believes the climate is changing and he does know that pollutants are a part of that equation," after Environmental Protection Agency Administrator Scott Pruitt said days earlier that he and Trump had not discussed the specific issue.
Pruitt himself has downplayed human impact and carbon dioxide emissions on the observed changes to the planet's climate.
"I think that measuring with precision human activity on the climate is something very challenging to do and there's tremendous disagreement about the degree of impact," Pruitt said in a CNBC interview in March in response to a question about carbon dioxide's influence on the climate. "So no, I would not agree that [carbon dioxide emissions are a] primary contributor to the global warming that we see."
In addition to rising global temperatures and lowering sea levels, the Climate Science Special Report concludes that "daily tidal flooding is accelerating" in more than 25 U.S. cities, "Heavy rainfall is increasing in intensity and frequency, "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," and "earlier spring melt and reduced snowpack are already affecting water resources" in the western U.S.