Sat 18 May 2019
Experts agree that global heating of 4C by 2100 is a real possibility. The effects of such a rise will be extreme and require a drastic shift in the way we live
Drowned cities; stagnant seas; intolerable heatwaves; entire nations uninhabitable… and more than 11 billion humans. A four-degree-warmer world is the stuff of nightmares and yet that’s where we’re heading in just decades.
While governments mull various carbon targets aimed at keeping human-induced global heating within safe levels – including new ambitions to reach net-zero emissions by 2050 – it’s worth looking ahead pragmatically at what happens if we fail. After all, many scientists think it’s highly unlikely that we will stay below 2C (above pre-industrial levels) by the end of the century, let alone 1.5C. Most countries are not making anywhere near enough progress to meet these internationally agreed targets.
Climate models predict we’re currently on track for a heating of somewhere between 3C and 4C for 2100, although bear in mind that these are global average temperatures – at the poles and over land (where people live), the increase may be double that. Predictions are tricky, however, as temperatures depend on how sensitive the climate is to carbon dioxide (CO2). Most models assume that it is not very sensitive – that’s where the lower 3C comes from – but a whole new set of models to be published in 2021 finds much greater sensitivity. They put heating at around 5C by the end of the century, meaning people could be experiencing as much as 10C of heating over land.
Such uncertainty isn’t ideal, but for our purposes let’s plump for an entirely feasible planetary heating of 4C by the end of the century. If that seems a long time away, consider that plenty of people you know will be around then. My children will be in their 80s, perhaps with middle-aged children and grandchildren. We are making their world and it will be a very different place.
Four degrees may not sound like much – after all, it is less than a typical temperature change between night and day. It might even sound pleasant, like retiring from the UK to southern Spain. However, an average heating of the entire globe by 4C would render the planet unrecognisable from anything humans have ever experienced. The last time the world was this hot was 15m years ago during the miocene, when intense volcanic eruptions in western North America emitted vast quantities of CO2. Sea levels rose some 40 metres higher than today and lush forests grew in Antarctica and the Arctic. However, that global heating took place over many thousands of years. Even at its most rapid, the rise in CO2 emissions occurred at a rate 1,000 times slower than ours has since the start of the Industrial Revolution. That gave animals and plants time to adapt to new conditions and, crucially, ecosystems had not been degraded by humans.
Things look considerably bleaker for our 2100 world. Over the past decade, scientists have been able to produce a far more nuanced picture of how temperature rise affects the complexities of cloud cover and atmospheric and oceanic circulation patterns and ecology. We’re looking at vast dead zones in the oceans as nutrients from fertiliser runoff combine with warmer waters to produce an explosion in algae that starve marine life of oxygen. This will be exacerbated by the acidity from dissolved CO2, which will cause a mass die-off, particularly of shellfish, plankton and coral. “We will have lost all the reefs decades before 2100 – at somewhere between 2C and 4C,” says Johan Rockström, director of the Potsdam Institute for Climate Impact Research in Germany.
Sea levels will be perhaps two metres higher and, more worryingly, we will be well on our way to an ice-free world, having passed the tipping points for the Greenland and west Antarctic ice sheets, committing us to at least 10 metres of sea-level rise in coming centuries. That’s because as ice sheets melt, their surface drops to a lower altitude where it is warmer, speeding up melting in a runaway feedback loop. Eventually, dark, heat-absorbing land is exposed, speeding the melting process even more. By 2100, we will also have lost most low-latitude glaciers, including two-thirds of the so called third pole of the Hindu Kush-Karakoram-Himalayan mountains and Tibetan plateau that feeds many of Asia’s important rivers.