As migrant arrivals go, the appearance of Thor the Walrus in British waters last week was encouraging. Thousands flocked to greet the huge mammal as he meandered up England’s east coast after his arrival in Hampshire in December. Subsequent stopovers included Scarborough, where the local council cancelled New Year’s Eve celebrations so they did not frighten Thor, who gratefully responded by masturbating. Then he moved on to Blyth, in Northumberland, before heading home to the Arctic. Walruses are rare visitors to Britain and the public’s positive reaction to his arrival is encouraging. Wildlife has never been more under threat and the fact so many people gathered to see Thor has to be treated as good news. As to the reason for his visit, most experts conclude it was caused by climate change, triggered by rising fossil fuel emissions, which is heating the planet and reducing the sea ice cover upon which walruses rest, hunt and digest their food. As Molly Gray, of British Divers Marine Life Rescue (BDLMR) put it: “It’s not very normal to see walruses down here so we imagine it is because of climate change.” This interpretation is not shared by everyone, however. Indeed, it was flatly rejected by climate change sceptic Matt Ridley in the Times. Everything is fine and dandy in the Arctic, he insisted. Sea ice levels are not badly affected and, far from being a threatened species, walrus numbers are rising, along with those of polar bears. Why don’t you report the good news about the environment? he demanded. Needless to say, Ridley’s assertion is hotly disputed. Yes, numbers of walruses have been recovering slowly since 1952 – when Norway made it illegal to hunt them. That does not mean they are thriving in a warming climate, says Bob Ward, policy director at the Grantham Research Institute on Climate Change. “In fact, in 2016, the walrus was classified as vulnerable on the IUCN red list of threatened species.” Ridley’s claims that the reduction in the Arctic sea ice has not been significant is not supported by data, which shows a 700,000 sq kilometres reduction over 20 years. Thor’s visit to Britain clearly highlights a shift in the climate wars. Deniers cannot now claim – without looking idiotic – that the world is not heating. It clearly is. Instead, they argue there may be some warming but it is having little impact. The world, like the walrus, is doing fine. This claim is simply not true. The planet is changing and all its inhabitants are in danger, as David Attenborough said. “The lives of walruses, like those of polar bears and seals, are changing. All are living at the frontier of climate change and all are suffering as a result.” ● Robin McKie is the Observer’s Science Editor