At the time, most grown-ups wrote him off as a harmless crank - but they certainly have to take him seriously now.
Trump has promised that within 100 days of taking office he will "cancel" the Paris climate agreement of last December and "stop all payments of US tax dollars to UN global warming programmes".
He will also rescind the executive actions Obama has taken to limit US emissions of carbon dioxide, especially in the field of electricity. (In effect, this would have closed down almost all coal-fired power stations in the United States.)
In practice, Trump can't cancel the Paris Agreement, which has been signed by 195 countries. He can pull the US out of the treaty (as George W Bush, another climate change denier, pulled the US out of the Kyoto Protocol on Climate Change in 2001), but he can't stop other countries from carrying on with the agreed cuts in emissions - which they may well do, because they understand how dangerous the situation is.
He certainly can cancel all of President Obama's executive orders and encourage Americans to burn all the fossil fuels they want. Indeed, he has already appointed Myron Ebell, a professional climate-change denier, to be the new head of the Environmental Protection Agency. Ebel's mission is to gut it, and he will. But even Trump cannot save the American coal industry, because it has simply become cheaper to burn natural gas.
The net effect of a Trump presidency will certainly be to slow the rate at which American greenhouse gas emissions decline, but simple economics dictates that they will not actually rise, and might even fall a bit. Renewable energy is getting cheaper than fossil fuels in many areas, and even Trump would find it hard to increase the large hidden subsidies to oil and coal any further.
So how hard will the American defection hit the Paris Agreement, whose target is to stop the average global temperature from reaching 2C higher than the pre-industrial level? Will it cause everybody else to walk away from it too, because the US is no longer doing its share? And even if they do carry on, what does that do to their hopes of staying below 2 degrees?
The United States is the second-largest emitter of greenhouse gases (after China), accounting for about 16 per cent of global emissions. Its commitment under the Paris deal was to cut that amount by just over a quarter in the next 10 years, so what is actually at stake here is around 4 per cent of total global emissions in 2025 if the US just lets it rip. It could be considerably less in practice.
That is not a make-or-break amount, particularly given that all the pledges of cuts made in Paris last December did not get us down to the never-exceed plus-2-degree target. They got us a lot closer to it, but we would still be heading for around plus 2.7C if everybody kept all their promises. Without American co-operation we are probably heading for plus 3C, but in either case there was still a lot to do.
The unwritten assumption at Paris was that everybody would be back in a few years with bigger commitments to emission cuts, and so we would eventually stagger across the finish line just in time. It was always a dangerous assumption, but the other major players might simply refuse to go any further if the US is not doing its share. Especially China, which is responsible for 26 per cent of global emissions.
On the other hand, China is terrified of the predicted local impacts of climate change, and has installed more solar and wind power than any other country. It already gets 20 per cent of its power from renewables, and is aiming much higher. The Chinese will resent the Trump Administration's refusal to carry its share of the burden, but it will not cut off its nose to spite its face.
The world has grown wearily familiar with this aspect of American exceptionalism, and the effort to avoid a climate disaster will stumble on elsewhere even while Trump reigns in Washington.
