One of the most depressing ironies of our age – and there’s some pretty stiff competition out there – is that, at a time when the most serious threat facing our species is man-made climate change, a problem which requires people from all over the world to work together towards a common goal, we seem to be increasingly obsessed with demarcating and defending our own little patches of dirt. Well, make that a little patch of dirt with a moat in the case of the UK; a big patch of dirt in the case of the United States. When Donald Trump, the climate change ostrich-in-chief, first started slobbering semi-coherently about building a “big, beautiful wall” along the US-Mexico border, he shocked a lot of people, but Trump was in tune with the times: since 9/11, border walls and fences have become quite the thing. According to Canadian academic Elisabeth Vallet, the world had seven border walls at the end of the Second World War, still just 15 by the time the Berlin Wall came down in 1989, yet now we have a total of 77. Our heads of state are looking more and more like a posse of bad-tempered, shotgun-toting farmers, taking it in turns to shriek “get off my land!” And meanwhile, the planet literally burns.