The Trump administration has offered oil companies a chunk of the American west and the Gulf of Mexico that’s four times the size of California – an expansive drilling plan that threatens to entrench the industry at the expense of other outdoor jobs, while locking in enough emissions to undermine global climate policy.

Energy companies have leased 9.9m acres from the unprecedented 461m acres put up for rent by the Trump administration, according to a new analysis from the Wilderness Society.

The fossil fuels extracted from those leases could equal half a year of emissions from China, the world’s top carbon polluter.

Trump’s Democratic opponents vow to close public lands to new drilling. But they probably can’t stop the extraction that Trump has already started.

Despite a glut in US oil supply, the federal government has proposed leasing places like the Slickrock Bike Trail in Moab, Utah, where visitors pedal through petrified sand dunes and ancient seabeds.

Locals have warned the potential air and water pollution aren’t worth damaging this national treasure.

“If we really get to the point that we need to burn the Picassos to heat the house for an hour, we could still do that, but there’s no reason to lease these parcels now when they have a higher and better use,” said Ashley Korenblat, CEO of Western Spirit Cycling and managing director of the not-for-profit group Public Land Solutions.

The leases are being signed as world scientists stress that pollution from oil, gas and coal needs to decline rapidly to avert catastrophe. Conservationists say the developed land will never again be wild and experts have repeatedly shown the sales aren’t even earning a fair return for taxpayers, costing the federal government and states billions of dollars.

“We’re in an era now where fundamental questions need to be raised about whether there should be more leasing or not. Millions of acres are already under lease that are not being developed,” said David Hayes, the former deputy secretary of the interior department under President Barack Obama. “So why is the administration going so hard and fast over putting additional acreage up?”

Just as the Obama administration halted coal leasing on federal lands, Hayes argues the federal government should consider a moratorium for oil and gas. At the least, Trump officials should not allow drilling on frontier and sensitive areas, said Hayes, who is now the director of the State Energy and Environmental Impact Center at New York University School of Law.

Nearly all the Democrats running for president would ban fossil fuel extraction on federal land.

Chase Huntley, energy and climate change director with the Wilderness Society, said the “tremendous area” the administration has offered to private companies and its disregard for the environment and climate “suggests the administration’s real interest here: which is advancing their agenda of energy dominance regardless of who it hurts”.

The interior department and its Bureau of Land Management, which handles onshore leasing, did not respond to requests for comment. Neither did the trade group for the US oil and gas industry, the American Petroleum Institute.

Much of the drilling on Trump-leased areas won’t happen now, while global supply is high and oil and gas prices are so low that the acres wouldn’t turn a large enough profit, but the Wilderness Society report shows that as a portion of the leases are acted upon they will be responsible for substantial pollution.

On the low end, the leases could result in emissions equal to the annual output of Brazil.

Pete Erickson, a senior scientist with the Stockholm Environment Institute, reviewed the analysis and called the low-end estimates “conservative”.

Earth is already 1.1C hotter than it was before humans began to burn fossil fuels for industry.

To keep the planet from climbing to 1.5C hotter, oil emissions would need to drop from 13 gigatons of carbon dioxide per year to less than 8 gigatons by 2030, according to an analysis by the climate change news organization Carbon Brief. Gas emissions would need to decline from 8 gigatons to fewer than 5 gigatons.

In a report ordered by the Obama administration, the US Geological Survey in 2018 found that public lands account for 24% of the country’s emissions and vegetation on the same lands absorbs 15% of the nation’s carbon dioxide pollution.

But the Trump administration has largely refused to consider what its actions on public lands will mean for the climate crisis.

Last month the White House proposed changes to how it applies a bedrock environment law – the 50-year-old National Environmental Policy Act. Under the revisions, the government would no longer need to count climate impacts when approving infrastructure such as pipelines or leasing land for oil and gas drilling.

The administration has also made revisions to rules that protected the environment but proved inconvenient for the oil and gas industry.

“If there’s anything in the way of oil and gas leasing, it’s going to be pushed aside,” Hayes said, noting the reinterpretation of the Migratory Bird Act, changes to an agreement for protecting sage grouse, the shrinking of national monuments in Utah and the opening of sensitive portions of the Arctic National wildlife refuge and National Petroleum reserve to development.

Independent analyses have found the US is not earning a fair amount from oil and gas leasing either.

The Congressional Budget Office in 2016 concluded the federal government could increase its share of income from onshore leases by up to $1.2bn over 10 years. That figure would be about twice as large if it counted the money that would go to states. Audits by the Government Accountability Office have drawn similar conclusions that the government could demand more money.

If the government had raised royalty rates years ago, it could have earned up to $12bn more for taxpayers between 2009 and 2019, according to the non-partisan fiscal policy group Taxpayers for Common Sense. States would have received about half that income.

“We have policies that govern the way we lease our federal lands for oil and gas development that are a century old, and they just haven’t kept pace. And we have others that haven’t been looked at in decades. This has led to really a tremendous giveaway [to the oil and gas industry],” said Autumn Hanna, vice-president of the non-partisan fiscal policy group Taxpayers for Common Sense.

In addition to expanding leasing opportunities, the Trump administration has opened lands that were once protected from industry. Last week the interior department finalized plans that allow mining, drilling and other development on lands recently removed from Bears Ears and Grand Staircase-Escalante national monuments in Utah.

The landscapes there feature “classic red rock canyons”, “forested mesas”, and “fantastical geologic features”, ranging from standing rock pillars to streams that flash-flood several times a year, according to Steve Bloch, the conservation director at the Southern Utah Wilderness Alliance. The region has a dense concentration of prehistoric sites, where researchers are able to study the cultures of indigenous tribes.

Bloch said that doesn’t matter to the federal government.

“It’s clear from the administration’s approach to public lands, to their energy dominance agenda, that they believe the highest and best use of these places is for fossil fuel development,” Bloch said.

Of the 9.9m acres leased, 2.2m acres are in Wyoming and 1.3m acres are in Alaska. Nearly 5m acres are in the Gulf of Mexico.

Adam Kolton, executive director of the Alaska Wilderness League, said “at the broadest level, what we’re witnessing is a sort of wholesale turnover of America’s Arctic to the oil and gas industry”.

Kolton said drilling on public lands was once done alongside arguments that the US needed to be energy independent, but the country is now a net exporter of oil. He believes the expansion of drilling, particularly in the Arctic, will be among Trump’s most damaging and longest-lasting environmental legacies.

“Right now there’s an attempt to not just roll back what was done under Obama in Alaska, but to turn back the clock half a century and look to an era where oil, mining and logging operations were unchecked,” Kolton said.