Climate Change: We’ve Been Here Before
by Paul Wignall
The world’s climate is always changing and always has. Even during the past few centuries we have seen substantial variations, but only recently have we begun to blame ourselves for them. But how much natural variability is there, and just how extreme can climate change be? To gain some longer-term perspective on the climate’s variability we can look back through geological time, particularly at catastrophic events known as mass extinctions. In my recent book, The Worst of Times, I focus on an 80 million year interval when life on Earth suffered one disaster after another. These catastrophes included the Permo-Triassic mass extinction, the worst crisis that life has ever faced. It is not very reassuring to find that these extinctions all coincide with intervals of rapid global warming.
So, are we all going to hell in a hand basket? Well, probably not just yet. The story from the past is much more nuanced than this and I believe there is substantial hope that all is not so bad today. The reason is that the worst 80 million years happened a long time ago and more recently (in the past 100 million years) things have got a lot better. At one time all the world’s continents were joined together into a single supercontinent called Pangea. This seems to have created a global environment that was very fragile. Every time there was a phase of giant volcanic eruptions in Pangea, climates changed rapidly, the oceans stagnated and life began to suffer. The cause seems to be not the actual lava flows themselves, although these were very large, but the gases that bubbled out of them, especially carbon dioxide, everyone’s (not so) favorite greenhouse gas. As I explain in my book the effects of these gases on climate and oceans changed global environments in a disastrous way. Rapid increases in global temperature were part of the story and the results were some of the hottest climates of all time. The results for life were profound; dominant groups went extinct and new groups appeared only to have their brief hegemony terminated by the next disaster. By the time these waves of extinction were over the dinosaurs were the newest kids on the block. They went on to thrive and get very large whilst scurrying around at their feet were a group of small furry creatures. These were the mammals and they would have to wait a long time for their turn.
Dinosaurs were the dominant animals on Earth for over 140 million years and it is often thought that they were somehow competitively successful but I think they were just very lucky. They appeared at a time when the Earth was rapidly getting better at coping with climatic changes caused by giant volcanism. There were plenty of episodes of large-scale eruptions during the time of the dinosaurs and none caused major extinctions. The key thing was that Pangea was splitting up and separate continents were forming – the familiar continents of today’s world. Such a world seems better able to cope with rapid increases in atmospheric gases because feedback mechanisms are more effective. In particular rainfall is more plentiful when the continents are small and nowhere is too far away from the sea. Rain scrubs the atmosphere and thus alleviates the problems.
However, the $64,000 question is how quickly this feedback can happen. The world seems better at doing this today than it was in deep time but maybe we are adding the carbon dioxide too fast to our atmosphere, maybe we are swamping the system? This is a hard question to answer, we’re not sure how much gas came out during the giant eruptions of the past and so it’s hard to directly compare with the present day pollution rates. What we do know is that past mega-eruptions have been remarkably damage-free. For over 100 million years, our world has been a benign place.
Oh, except for a remarkably large meteorite impact that was bad news for the dinosaurs, but that’s another story.
Paul B. Wignall is professor of palaeoenvironments at the University of Leeds. He has been investigating mass extinctions for more than twenty-five years, a scientific quest that has taken him to dozens of countries around the world. The coauthor of Mass Extinctions and Their Aftermath, he lives in Leeds.