Over the past few decades, the amount of carbon dioxide in the atmosphere has increased steadily. Global temperatures have risen higher and higher.
The long-term effects of that increase are likely to include higher sea levels (as ice caps melt and water expands), more severe precipitation events and extended droughts.
Over the past decade, the subject has been partisan. Over the past few years, views have shifted.
Recent polling from Gallup shows that concern about climate change has surged, with Americans accepting that human activity is causing the shift and that the long-term effects of climate change have already begun.
This week, Gallup reported that half the country now identifies as "concerned believers" - people who accept climate science and are worried about addressing it. Trump is not among that group.
2. What does Trump plan to do?
There are three main components.
Firstly, Trump hopes to dismantle a rule advanced by the Obama Administration that would reduce carbon dioxide emissions from coal-burning power plants.
The Obama Administration also had a rule affecting newly constructed power plants, limiting allowable carbon dioxide levels, but that didn't affect older plants.
What's more, many older plants still in use were made exempt from key provisions of the 1970 Clean Air Act, under the assumption they would eventually be replaced.
In many cases, they weren't. Secondly, Trump wants to promote fossil fuel extraction, in an effort to make American energy independent and to fulfill a campaign pledge to bolster the coal industry.
Trump will remove a moratorium on coal leasing by the government, allowing federal land to be used for coal mining. Thirdly, Trump wants to strip consideration of the long-term effects of climate change from routine government decision-making.
3. Can Trump save the coal industry?
Trump can probably tack a few years onto the lifespan of the coal industry, but he almost certainly can't revive it as a viable economic engine for communities where it was once prominent.
Between 1985 and 2000, the number of people working in the coal industry in the United States fell from 178,000 to 71,000.
There are a lot of reasons for that decline, including the increased use of automation and that extracting coal from the same mine necessarily becomes harder over time, as easily accessible seams are tapped out.
From 2000 to 2012 or so, the industry saw a brief resurgence. But then it collapsed, falling from about 90,000 employees to 50,000 or so. The blame for that recent collapse has been placed largely on Obama's regulatory actions.
Charles Murray, a fervently conservative coal mining executive, told the Guardian this week that he considered Obama's regulatory actions to be "fraudulent" and the Obama Administration's effort to protect waterways from mining an attempt to "destroy our nation's underground coalmines and put our nation's coalminers out of work".
A 2012 study estimated that a fifth of streams in southern West Virginia had been polluted by coal mining; Trump overrode Obama's protection rule.
When he spoke to the President at the White House last month, Murray said he told Trump to "temper his expectations" about restoring coal jobs.
"He can't bring them back," Murray said. He told the Guardian "we have an energy cost problem". There's another big factor in coal's recent decline.
4. Will Trump help the United States become energy independent?
Over the course of his presidential campaign, Trump repeatedly decried the inability of fossil fuel companies to drill on government property. The impression he gave was of a fossil-fuel industry hampered by government to the point of ineffectiveness.
In fact, the fossil fuel industry thrived under Obama for one simple reason: Improvement in horizontal drilling techniques, which led to the fracking boom and surges in production of oil and natural gas from 2005 on.
The boom in new natural gas production meant that prices fell dramatically. While extracting natural gas releases the potent greenhouse gas methane into the atmosphere, it burns much more cleanly than coal.
That and the cheap price led a number of power producers to move from burning coal to burning gas in their facilities. In 2015, a third of American energy was produced with coal and a third was produced with natural gas.
Twenty years prior, more than half of the electricity production in the US came from burning coal and only 13 per cent from gas.
Another American energy growth industry is wind and solar energy.
As of the end of last year, America produces more energy from wind than from the use of hydroelectric dams - and wind, of course, doesn't produce any greenhouse gas emissions at all. Wind receives some subsidies from the government, which Trump has criticised in the past.
The American energy mix is evolving quickly. The fracking boom meant that net petroleum imports to the US in 2015 made up about a quarter of all consumption, the smallest percentage in nearly half a century.
Complete energy independence is unlikely for a number of reasons, but lower fuel prices, increased American production and shifting energy systems make "energy independence" look somewhat different than it used to.
5. Will Trump's actions make climate change worse?
The US has consistently been the biggest per capita producer of greenhouse gases. The US is not the biggest emitter of them overall - China is.
For years, this was a point of argument against the US taking action to address climate change: It's a global problem and China pollutes more than we do, so why should we limit ourselves economically when China doesn't? In 2014, Obama announced a landmark agreement with China to do exactly that.
Obama also committed the US to a more sweeping climate agreement, the 2015 Paris Accord - an agreement from which Trump once said he would withdraw.
Globally, the trend is clear: movement away from fossil-fuel-burning power plants - particularly coal-burning ones - and towards cleaner and renewable sources. That's also the trend in the US, for reasons beyond government regulatory actions.
So, while Trump can unwind Obama's climate legacy to some extent, the economic and political forces that have spurred those underlying shifts are largely beyond his control.
His efforts will likely mean that America's greenhouse gas emissions - which were lower for much of Obama's Administration largely because of the economic slowdown - may tick upward.
But the good news for that majority of the country worried about the threat of climate change is that Trump mostly can't touch that broader global trend.
