A major U.S. government report on Friday affirmed that global warming is real and "extremely likely" caused by human activities, a stark contrast with President Donald Trump's position on climate change.
The 477-page report, part of a congressionally mandated review conducted every four years known as the National Climate Assessment, showed that global annually averaged surface air temperature has increased by about 1.0 degree Celsius over the last 115 years.
"This period is now the warmest in the history of modern civilization," said the assessment, officially titled the Climate Science Special Report.
"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," it said. "These trends are expected to continue over climate timescales."
Based on extensive evidence, the report concluded that it is "extremely likely" -- meaning a 95 to 100 percent probability -- 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," it noted.
The report also warned that 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 9 degrees Fahrenheit (5 Celsius) or more by the end of this century," it said.
"With significant reductions in emissions, the increase in annual average global temperature could be limited to 3.6 degrees Fahrenheit (2 Celsius) or less."
The two-degree target is the cornerstone of the 196-nation Paris Agreement, inked in 2015. Trump has announced in June that the U.S. is withdrawing from the agreement.
Trump once called climate change a "hoax" and his administration considered efforts to fight global warming just to "be a waste of your money," a position met with widespread criticism both at home and abroad.
Just one month ago, the Trump administration took a formal step to repeal the Clean Power Plan, former President Barack Obama's centerpiece regulation that would limit greenhouse gas emissions from existing power plants.
Rachel Licker, senior climate scientist at the non-profit Union of Concerned Scientists, said in a statement that "the assessment is like a doctor's report that evaluates a patient's vital signs and uses that information to diagnose a medical condition."
"Unlike a physician, the climate assessment stops short of offering up a specific prescription or treatment plan," she said.
"Instead, the American public must hold legislators and policy-makers accountable for taking action commensurate with the problem."
Former Vice President Al Gore, a prominent environmental activist, also warned on Twitter that "America's leading scientists have confirmed what Mother Nature has been making plenty clear: the climate crisis is real and urgent."
"If the Administration won't listen & if the federal gov't won't act, states, cities, businesses, investors, & citizens will," Al Gore added.
