So we do have a new US – for the present. In the jargon of Trumpology, the president has tossed a sop to his climate-change denying chief strategist, Steve Bannon, and disappointed his green daughter, Ivanka. Or at least he is withdrawing the US from the Paris climate accord in three years (Bannon) but “renegotiating” it (Ivanka).

The world will not suddenly sink beneath the waves. The US’s impressive record in cleaning up its emissions since 2007 will not go into reverse. Federal subsidies for renewables may diminish, but California and New York, and other states and cities, have said they will not reverse green energy regulations. The switch away from fossil fuels will continue, if only because that is where the market is going.

What has changed is something less tangible. Trump’s contempt for internationalism and his explicit “America first” language shows him in full isolationist mode. This is the raw nationalism we saw in the early George W Bush years, and periodically back in American history, notably before the two world wars. It is one with Trump’s Mexican wall, immigration control, trade protection and anti-Nato Russophilia. He is the authentic voice of world-go-to-hell.

The perils here are obvious. The US’s leadership in the 20th century was crucial in “policing democracy”. Its power, its money, perhaps above all, its example, have overseen a clear reduction in world poverty and even conflict over the past half century. But we are now in the 21st, and this leadership has become tarnished. America’s military interventions in the Muslim world since 2001 have been obsessional and counter-productive, the agents of anarchy. Neither a bellicose Bush nor a milder Barack Obama was able to correct this. The old maxim – hold close to nurse America for fear of something worse – no longer seems the no-brainer it once did.

Trump is the authentic voice of world-go-to-hell

Just as the US itself is fracturing in response to Trump’s emissions decision, so Nato is fracturing in response to his plain hostility. Relations with Russia are unfreezing, intervention in the Middle East is growing meaningless, China is flexing its global muscles. Were this part of some coherent Trump landscape, it might become a template for a new, multi-polar world. In truth it is emotional and shambolic.

But even madmen have their uses. It is never sensible for sovereign states to rely for too long on other states, accountable to other electorates, to rule the world on their behalf. Treaties and alliances grow tired and need review – look at the EU. The Paris accord, a rare example of global collective action, remains in place. Others must now step forward to lead where the US will not, perhaps even China. It is as well to be reminded that no great power is wholly reliable. Burdens of responsibility must sometimes be carried by others.