Tropical temperature, measured on an outdoor thermometer, global

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 pre-industrial 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 sealed.

The draft statement, issued by the National Highway Traffic Safety Administration, was written to justify President Donald 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 U.S. Global Change Research Program from 1993 to 2002.

The document projects 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 C. or 7-degree F. increase from pre-industrial 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 technologically feasible or economically feasible.”

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 C.

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 C. rise by the century’s end if countries take no meaningful actions to curb their carbon output.

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