Human activity is the primary cause of rising global temperatures in recent decades, and there is no convincing evidence to the contrary, according to a report issued Friday by 13 U.S. agencies. The assessment, approved by the White House, contradicts public statements by President Donald Trump and several top members of his administration.
The findings came in the Fourth National Climate Assessment, an authoritative review of climate science. The assessment finds that the period from 1901-2016 was the warmest in modern civilization and warns that temperatures and sea levels will rise much more if no action is taken.
"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," the report finds.
"For the warming over the last century, there is no convincing alternative explanation supported by the extent of the observational evidence."
Trump has called climate change a "hoax." Environmental Protection Agency Administrator Scott Pruitt and Energy Secretary Rick Perry both told CNBC earlier this year that carbon dioxide emissions from human activity are not the primary driver of climate change.
The Trump administration has rolled back a series of Obama-era rules and initiatives to reduce greenhouse gas emissions and mitigate the effects of climate change. Trump announced in June that he will pull the United States out of the Paris Agreement, a global effort to prevent world temperatures from rising 2 degrees Celsius above preindustrial levels.
Temperatures in the United States have risen by 1.8 degrees Fahrenheit, or 1 degree Celsius, since 1901, the report finds. The authors forecast that between 2021 and 2050 U.S. temperatures could rise by 2.5 degrees Fahrenheit above 1975-2005 levels in all "plausible future scenarios."
"The magnitude of climate change beyond the next few decades will depend primarily on the amount of greenhouse gases (especially carbon dioxide) emitted globally," the authors wrote.
Annual average temperatures could increase by 9 degrees Fahrenheit above preindustrial levels by the end of the century if humans do not reduce emissions, they warn.
"In addition to warming, many other aspects of global climate are changing, primarily in response to human activities," the report finds. Those include "changes in surface, atmospheric, and oceanic temperatures; melting glaciers; diminishing snow cover; shrinking sea ice; rising sea levels; ocean acidification; and increasing atmospheric water vapor," according to thousands of studies.
Global average sea levels have risen by 7 to 8 inches since 1900, with about half of the rise occurring since 1993, according to the assessment. The authors say a further 8-inch rise by 2100 cannot be ruled out, and the U.S. East and Gulf coasts in particular will be impacted.
Changes in extreme weather events are putting humans, infrastructure, agriculture, water supplies and ecosystems at risk.
The White House told CNBC that the Trump administration supports "rigorous scientific analysis and debate," and will continue to "support technology, innovation and the development of modern and efficient infrastructure" to reduce emissions and mitigate climate-related risks.
"The climate has changed and is always changing. As the Climate Science Special Report states, the magnitude of future climate change depends significantly on 'remaining uncertainty in the sensitivity of Earth's climate to [greenhouse gas] emissions,'" said White House principal deputy press secretary Raj Shah, in a statement.