WASHINGTON — Last month, deep in a 500-page environmental impact statement, the Trump administration made a startling assumption: On its current course, the planet will warm a disastrous 7 degrees by the end of this century.
A rise of 7 degrees Fahrenheit, or about 4 degrees Celsius, compared with preindustrial levels would be catastrophic, according to scientists. Many coral reefs would dissolve in increasingly acidic oceans. Parts of Manhattan and Miami would be underwater without costly coastal defenses. Extreme heat waves would routinely smother large parts of the globe.
But the administration did not offer this dire forecast as part of an argument to combat climate change. Just the opposite: The analysis assumes the planet’s fate is already sealed.
The draft statement, issued by the National Highway Traffic Safety Administration (NHTSA), was written to justify President Trump’s decision to freeze federal fuel efficiency standards for cars and light trucks built after 2020. While the proposal would increase greenhouse gas emissions, the impact statement says, that policy would add just a very small drop to a very big, hot bucket.
‘‘The amazing thing they’re saying is human activities are going to lead to this rise of carbon dioxide that is disastrous for the environment and society. And then they’re saying they’re not going to do anything about it,’’ said Michael MacCracken, who served as a senior scientist at the US Global Change Research Program from 1993 to 2002.
The document projects that global temperature will rise by nearly 3.5 degrees Celsius above the average temperature between 1986 and 2005 regardless of whether Obama-era tailpipe standards take effect or are frozen for six years, as the Trump administration has proposed. The global average temperature rose more than 0.5 degrees Celsius between 1880, the start of industrialization, and 1986, so the analysis assumes a roughly 4 degree Celsius or 7 degree Fahrenheit increase from preindustrial levels.
The world would have to make deep cuts in carbon emissions to avoid this drastic warming, the analysis states. And that ‘‘would require substantial increases in technology innovation and adoption compared to today’s levels and would require the economy and the vehicle fleet to move away from the use of fossil fuels, which is not currently technologically feasible or economically feasible.’’
The White House did not respond to requests for comment.
World leaders have pledged to keep the world from warming more than 2 degrees Celsius compared with preindustrial levels, and agreed to try to keep the temperature rise to 1.5 degrees Celsius. But the current greenhouse gas cuts pledged under the 2015 Paris climate agreement are not steep enough to meet either goal. Scientists predict a 4 degree Celsius rise by the century’s end if countries take no meaningful actions to curb their carbon output.
Trump has vowed to exit the Paris accord and called climate change a hoax. In the past two months, the White House has pushed to dismantle nearly half a dozen major rules aimed at reducing greenhouse gases, deregulatory moves intended to save companies hundreds of millions of dollars.
If enacted, the administration’s proposals would give new life to aging coal plants; allow oil and gas operations to release more methane into the atmosphere; and prevent new curbs on greenhouse gases used in refrigerators and air-conditioning units. The vehicle rule alone would put 8 billion additional tons of carbon dioxide in the atmosphere this century, more than a year’s worth of total US emissions, according to the government’s own analysis.
Administration estimates acknowledge that the policies would release far more greenhouse gas emissions from America’s energy and transportation sectors than otherwise would have been allowed.
David Pettit, a senior attorney at the Natural Resources Defense Council who testified against Trump’s freeze of fuel efficiency standards this week in Fresno, Calif., said his organization is prepared to use the administration’s own numbers to challenge their regulatory rollbacks. He noted that the NHTSA document projects that if the world takes no action to curb emissions, current atmospheric concentrations of carbon dioxide would rise from 410 parts per million to 789 ppm by 2100.
‘‘I was shocked when I saw it,’’ Pettit said in a phone interview. ‘‘These are their numbers. They aren’t our numbers.’’