It was also the year the world's second largest polluter, the United States, turned its back on the 196-nation Paris climate deal meant to limit global warming to under two degrees Celsius (1.5 degrees Fahrenheit) over pre-industrial levels.
President Donald Trump, who has dismissed global warming as a Chinese hoax, vowed to quit the 2015 Paris accord and tapped fossil fuel allies to key environmental posts.
His administration also dropped climate change from the list of national security threats, announced plans to auction off vast swaths of the Gulf of Mexico to oil and gas drilling, and signed a proposal to eliminate the Clean Power Plan, aimed at limiting the release of polluting greenhouse gases.
"Together, we are going to start a new energy revolution -- one that celebrates American production on American soil," Trump said in June.
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In October, Trump signed a proclamation to make America a net energy exporter by 2026, reviving the coal industry and seeking to access the estimated $50 trillion in untapped shale, oil, and natural gas reserves, particularly on federal lands.
While the fossil fuel industry has applauded the moves, scientists have expressed alarm.
"The Trump administration, in less than a year, has done more to undermine climate policy than even the worst previous administration on climate (i.e. George W Bush) had done over the course of two full terms," said Michael Mann, a climatologist and geophysicist at Pennsylvania State University, in an email to AFP.
"They must be stopped," he added, because their actions "pose an existential threat to us and our children and grandchildren."
The more fossil fuels we burn, the hotter the planet becomes due to the accumulation of heat-trapping greenhouse gases.
The world is currently on track for its third warmest year in modern times.
Experts say global warming can make certain events, like floods, drought and hurricanes, more frequent and sometimes worse.
Among the fiercest storms seen this year were severe monsoon rains in Bangladesh, India and Nepal that killed more than 1,200 people and affected 40 million people, destroying homes, livestock and crops, according to the UN Office for the Coordination of Humanitarian Affairs.
Meanwhile, an unusually active hurricane season roiled the Atlantic, Gulf of Mexico and Caribbean with 17 big storms, the most since 2005.
Major, deadly hurricanes included Harvey, which flooded Texas with 50 inches (125 centimeters) of rain in some places. The massively powerful Irma devastated the Caribbean and Florida, while Maria flattened much of Puerto Rico.
California Governor Jerry Brown, whose state is currently battling its third largest wildfire since 1932, spoke this month of how such devastation could be the "new normal."
That phrase was echoed in the 2017 Arctic Report Card, an international peer-reviewed report on the fragile Arctic, which is warming at twice the pace of the rest of the world.
"The Arctic environmental system has reached a 'new normal,'" that will raise sea levels, alter weather patterns and unleash more extreme weather across the globe, it said.
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