(CNN) For four years, President Donald Trump has careened from one crisis to the next, many of his own making.
Still, through the Mueller investigation, two impeachments, the deadliest pandemic in a century, and even a failed and dangerous attempt to overturn his own election defeat, Trump and his administration remained steadfast in at least one quest: to weaken many of the country's bedrock climate and environmental guardrails.
Considered in the course of humanity -- or the 4.5-billion-year history of this planet -- a single presidential term is barely a blink of an eye.
But in just four years, Trump has cemented a legacy -- particularly on climate change -- that will be felt by generations to come.
"It's pretty much been an unequivocal disaster," said Christine Todd Whitman, the former Republican governor of New Jersey who was EPA administrator under President George W. Bush. "To just roll back [regulations] whole cloth because they came from a previous administration has made no sense, and really what's happening is that they're putting the health of Americans and the health of our environment in jeopardy.
"The mission [of the EPA] is to protect human health and the environment — pretty simple and pretty straightforward," Todd Whitman added. "It seems to me they've totally ignored the mission."
Much of the environmental protections that Trump dismantled can be rebuilt by the incoming Biden administration, experts say.
However, doing so will take time. And in the case of global warming, the hour has grown late to stop the worst effects.
The most lasting part of Trump's climate legacy -- and one that cannot be undone -- may be the time the administration wasted in the face of a worsening climate crisis, some scientists and experts say.
"I'm kind of hopeful that many of the worst and most damaging climate policies are capable of being reversed," said Kim Cobb, a professor and climate scientist at Georgia Tech's School of Earth and Atmospheric Sciences. "But the lost years in terms of progress on emissions reductions we can't ever take back, and that is something that will have a finite impact on coming climate change impacts."
Trump's deregulatory crusade began with his first EPA administrator, Scott Pruitt, a former Oklahoma attorney general who had sued the agency a dozen times over environmental protections before being tabbed to lead it.
After he resigned in the face of multiple ethics scandals, Trump picked a former coal lobbyist, Andrew Wheeler, to take his place.
Pruitt and Wheeler -- in parallel with the Interior Department and the Department of Energy -- have worked to complete dozens of industry rollbacks, gutting regulations on everything from greenhouse gas emissions from power plants to showerhead water efficiency.
Along the way, fossil fuel interests have applauded Trump's moves, even if he didn't revive America's coal industry as he had promised.
"They took industry's wish lists and translated them into agency orders or regulations," said Michael Gerrard, the director of the Sabin Center for Climate Change Law at Columbia Law School. "They were only partially successful because they were so sloppy in following the necessary procedures that they were often slapped down by the courts."
In an interview this week with the Washington Post, Wheeler defended his legacy, saying that on his watch the EPA has "proven that you can reduce pollution and have cost-effective regulations."
But critics see a willful and costly ignorance of science, one that has colored the administration's response to the world's biggest crises, from the Covid-19 pandemic to climate change.
"I think their [policies] are dangerous. That's the bottom line," Cobb said. "I think it's even worse than anti-science."
The Trump administration's pullbacks on climate regulations came at a time when the science has never been clearer on the urgent need for the planet's biggest polluters to make big cuts to their greenhouse gas emissions.
Preliminary estimates by the private data analytics firm the Rhodium Group show US emissions did plummet by 10% last year, the largest drop since World War II. But experts attribute most of the reduction to the pandemic, which kept many Americans out of their cars and off planes, and expect emissions to rebound as the effects of Covid-19 wane.
In 2019, the UN warned that to hold global warming below the 1.5 degree Celsius threshold, global emissions would need to fall by 7.6% each year from 2020 to 2030.
Trump's moves also coincided with a seemingly unending rash of extreme weather events, which brought the destruction fueled by global warming to the doorsteps of millions of Americans.
In Trump's first year in office in 2017, he announced that he planned to pull the US out of the Paris Agreement on climate. That same year, Hurricanes Maria, Harvey and Irma left behind a wake of death and destruction from Puerto Rico to Texas, a year that saw all disasters in the US cause a record $321 billion in damages.
In 2020, a record-breaking 22 separate billion-dollar weather and climate disasters -- including unprecedented wildfires in the American West -- caused a total of $95 billion in damages across the country.
Many of those disasters bore the fingerprints of climate change.
During Trump's presidency, global average temperatures have also continued to climb. The last six years have been the hottest six years ever recorded, with 2020 tying 2016 as the hottest year.
Atmospheric carbon dioxide concentrations also climbed to a new high in 2020, reaching levels unseen in millions of years.
And because of how long greenhouse gases stay in the atmosphere, the Trump presidency could influence global warming for years to come.
When humans burn fossil fuels, it sends heat-trapping carbon dioxide into the air, where it accumulates in the atmosphere like a blanket, and can stay to heat the Earth for hundreds of years.
Some studies have tried to quantify how much of a contribution Trump's regulatory rollbacks will make over time to accelerating global warming.
One estimate published last year by the Rhodium Group found that the administration's moves to weaken greenhouse gas regulations could add the equivalent of 1.8 gigatons of CO2 to the atmosphere by 2035, equaling nearly one-third of all US emissions in 2019.
However, there was a bit of good news hidden in the massive Covid-19 relief package that passed last December and which Trump signed.
Buried in the $900 billion stimulus package were some significant climate legislation which calls for phasing out the use of hydrofluorocarbons -- a class of super heat-trapping gases used in refrigerators and air conditioners -- and an extension of a carbon capture technology tax credit for industry.
Over time, analysts say those line items could go a long way toward negating the emissions impact of Trump's rollbacks.
"We estimate that this is one of the single biggest climate actions the U.S. has taken in at least in a decade," said Kate Larsen, the director of the Rhodium Group's international energy and climate research team. "When you look at the remaining policy rollbacks that Trump implemented and that will remain standing, these are largely equivalent to making up the damage that the Trump administration has done in terms of regulatory rollbacks."
Still, the long-term impact of Trump's other major climate moves is harder to quantify.
Chief among those is the US' exit from the Paris Agreement on climate, which was completed last November.
Leaving Paris marked the second time the US has bailed on an international climate agreement after it led the negotiations. The first exit was from the Kyoto Protocol, a previous climate pact which the US signed during the Clinton administration, only to drop out during George W. Bush's presidency.
President-elect Joe Biden has pledged to rejoin the Paris Agreement on Day One of his presidency, but experts say repairing the damage to the country's international standing that was done by Trump abandoning the accord will not be easy.
"In one sense, it's easy for President Biden to announce on the first day he's in office that the US will rejoin," said Jody Freeman, a Harvard law professor who served as counselor for energy and climate change in the White House under President Barack Obama. "The hard part is to put together an ambitious, credible pledge for what the US is prepared to do to meet their Paris Agreement commitments."
Trump's interior department also cleared the way for new fossil fuel extraction on federal lands, including in Alaska's Arctic National Wildlife Refuge (ANWR), one of the country's largest remaining pristine wildlife areas.
Though there was ultimately little interest in the rights to drill in ANWR from bidders, experts say it could be difficult for the Biden administration and environmental groups to challenge those leases.
"Once the lease has been sold, it creates a property right," Gerrard said. "There will be litigation about whether the leases were validly issued, but if any of them are upheld in court, it becomes more difficult to revoke them."
Throughout the campaign and the transition period, Biden has made it clear that he intends to try and make a complete 180-degree turn on federal climate policy.
He appointed former Secretary of State John Kerry to a new Cabinet-level position as special climate envoy, where he will have a seat on the National Security Council, and has tapped other Obama administration alums to join his climate team.
During the presidential campaign, he unveiled a $2 trillion climate plan that calls for the US to reach 100% clean electricity generation by 2035, and make huge investments in green infrastructure, from expanding wind and solar to power generation to building a nationwide electric car-charging network.
Experts say there is plenty of opportunity for his administration to push through parts of his plan to reduce US greenhouse gas emissions and to inspire action internationally.
"Trump took a wrecking ball to the nation's environmental regulations," Gerrard said. "Fortunately, it's not Humpty Dumpty, and most of it can be put back together again."
But the need to reverse moves finalized by his predecessor and the political realities of a razor-thin Democratic Senate majority could hamper Biden's more ambitious climate proposals.
Some of Trump's rollbacks can be undone with the stroke of a pen or overturned by a simple majority in both houses of Congress through the Congressional Review Act, Gerrard said.
Others like reinstating or tightening the Obama-era standards on car and truck emissions can be accomplished through EPA rulemaking, but the process takes time -- anywhere from a few months to a year, Gerrard said.
Youth-led climate action organizations like the Sunrise Movement have indicated that they intend to hold the new administration accountable for delivering on Biden's climate promises.
And a newly-minted Democratic Senate majority will open the door to some legislative opportunities that wouldn't have been possible in a chamber controlled by Senate Majority Leader Mitch McConnell.
Still, climate policy experts say that the narrow Senate majority will force Biden to find measures that can garner bipartisan support -- like spending on green infrastructure -- in the vein of the climate legislation that passed in the end-of-year Covid-19 stimulus package.
"When the Senate is sort of evenly split, the type of progress we're going to make on climate is not going to look like comprehensive climate legislation," Larsen said. "It's going to be these targeted wins on things that can largely get bipartisan support."
But whatever progress Biden's administration is able to make on halting climate change over the next four years, some say the lost time of the last four years will still loom large.
"The primary effect is that the new administration will be occupied for its first couple of years with reversing all the damage rather than continuing to make progress," said Ted Lamm, a senior research fellow at the Center for Law, Energy and the Environment at the University of California-Berkeley. "Particularly in the case of climate change, where we are facing a ticking clock, that lost time is potentially disastrous and harmful."