This is the century in which humanitarian and environmental disasters converge. Climate breakdown has driven many millions from their homes, and is likely to evict hundreds of millions more. The famine harrowing Madagascar at the moment is the first to have been named by the UN as likely to have been caused by the climate emergency. It will not be the last. Great cities find themselves dangerously short of water as aquifers are drained. Air pollution kills 10 million a year. Synthetic chemicals in soil, air and water impose untold effects on both ecologies and people. But it also works the other way round. Humanitarian catastrophes or, to be more precise, governments’ cruel and irrational responses to them, are triggering ecological disaster. Nowhere is this more obvious than in the construction of border walls. At the moment, with the help of 140 British military engineers, Poland is starting to build a steel wall 5.5 metres high, along 180km of its border with Belarus. The assistance from British troops will help secure a new arms deal between the UK and Poland worth around £3bn. The wall is described as a “security” measure, but it’s securing Europe not from a threat but against the desperate needs of some of the most vulnerable people on Earth: particularly refugees from Syria, Iraq and Afghanistan escaping persecution, torture and mass killing. They have been cruelly exploited by the government of Belarus, which has used them as political weapons. Now, in the depths of winter, they are trapped at the border, freezing and starving, with nowhere to go. When the Berlin Wall fell, we were promised that this marked the beginning of a new era of freedom. Instead, far more walls have risen than fallen. Since 1990, Europe has built border walls six times longer than the barrier in Berlin. Worldwide, the number of fenced borders has risen from 15 to 70 since the end of the cold war: there are now 47,000 kilometres of hard frontier. For those trapped at these borders, the cruelties of capitalism are scarcely distinguishable from the cruelties of communism. The humanitarian effects of these walls are well documented. But their ecological impacts are also devastating. Roads and farmland isolate wildlife, but nothing cuts off some species as effectively as border walls. Just as we understand better than ever before the importance of ecological connectivity, we are carving up and separating habitats at unprecedented speed. We now know that, even in large reserves, wildlife species can decline towards extinction if they cannot disperse and mix with populations from elsewhere. Their genetic diversity narrows, reducing their breeding success and making them more susceptible to disease. Barriers prevent them from moving as conditions change. Conditions are now changing very quickly, as a result of climate breakdown. A trapped population, in many cases, is a doomed one. The new wall between Poland and Belarus will, among other grim impacts, slice the Białowieża Forest, the largest ancient woodland in lowland Europe, in two. Already, a temporary barrier of coiled razor wire has been strung through the middle of the forest, blocking the movement of its celebrated populations of bison, wolves, boar, lynx, deer, moose and other wildlife, and preventing bears, which have just started to return, from recolonising the woods. Yet, despite the best efforts of scientists like Dr Katarzyna Nowak of the Białowieża Geobotanical Station, the massive ecological consequences are widely ignored. There has been no environmental impact assessment of Poland’s wall, in breach of both the EU Habitats Directive and international treaties. Similar disasters are happening around the world. The border fence erected between Slovenia and Croatia in 2015 could cause the gradual extinction of the lynx in the Dinaric Mountains. The carcasses of deer that died horribly after becoming snagged on its cruel barbs have been found along its length. The barrier between India and Pakistan has caused a collapse in the population of Kashmir markhor (a rare and remarkable corkscrew-horned wild goat). The longest border fences in the world divide China, Mongolia and Russia from each other. They have isolated remnant populations of wild asses, Mongolian gazelles and other endangered species of the steppes. Trump’s wall, dividing the United States from Mexico, is a threat to several rare mammal species, as well as the pygmy owl, which flies too low to cross the barrier. In the boom-and-bust ecology of the desert, populations survive by recolonising areas after they have been driven out by drought. The wall, in many cases, will make this impossible. There is a strain of rightwing environmentalism, going back at least 100 years, that equates immigration with pollution. Madison Grant was one of the founders of the US conservation movement, who helped establish its network of national parks. He was also the author of a book called The Passing of the Great Race, published in 1916, which Adolf Hitler described as “my bible”. Grant believed that in conserving the ecosystems of North America, he was protecting the domain of the Nordic “master race”, that was being “overtaken” in the US by “worthless race types”. As secretary of the Zoological Society, he helped ensure that a kidnapped Congolese man, Ota Benga, was caged with the apes on display at the Bronx Zoo. In 2018, the Fox News host Tucker Carlson proclaimed: “I actually hate litter, which is one of the reasons I’m so against illegal immigration.” The European far right has suddenly switched from denying the environmental crisis to using it as an argument for excluding immigrants. It claims that people from elsewhere don’t share “our” environmental ethics. This slur is easily repudiated: surveys have long shown stronger environmental concerns among the public in poorer nations. Not only do these attitudes conflict with all that is best about environmentalism – its empathy and consideration towards all people and all non-human life – but the policies of separation and containment they promote are ecologically disastrous. Though border walls cause a great deal of death and suffering, and are only partially effective at their stated purpose of excluding people, they are wholly effective in excluding many other species. It’s not as if anyone who cares about people needs more arguments against the vicious policies that separate us from each other. But there are more arguments, and they are powerful. Border walls are accelerating the extinction crisis and making ecosystems inviable. Just as humanity knows no boundaries, nor does wildlife. There is no conflict between caring about the planet and caring about its people. In fact, you can’t have one without the other. George Monbiot is a Guardian columnist