It’s not the fall that will kill you, but the suddenness of the stopping. Just as it is with a plane crash, so it is with global heating. Changes in the climate don’t in themselves represent a significant risk – the Earth’s climate has been changing for billions of years after all – but it is the abrupt changes that could spell disaster for us.

New research suggests that climate models which predict greater global warming in the future also have more volatile warming trends. If that is the case, and the climate is as sensitive to our carbon dioxide emissions as many of the latest models suggest, this will seriously threaten our ability to adapt. And it is that which will put a significant fraction of humanity in jeopardy.  

Many scientists focus on how much warmer the Earth’s climate will become because of the extra carbon dioxide humans have put into the atmosphere. The technical term for this is Equilibrium Climate Sensitivity (ECS). Equilibrium because it takes years, decades or centuries for the climate system to respond to extra heating.

Think how much energy it takes to heat a bathtub full of water and how long it can stay warm for. The amount of energy required to heat the trillions of litres of the Earth’s oceans is enormous.

Consider that bath of water again. Imagine the tap is dripping because it’s not entirely closed, so the water level in the bath is increasing, but only very slowly. Trying to track changes in water level just by eye would be as difficult as seeing the march of time in the hour hand of a watch. Now picture someone is in the bath. Unless the person stays absolutely still, they will have an impact on the water level as their movements will make waves. Depending on how much they move, these waves can be plain to see and would easily mask the very slow water level increase.

ECS tells us where the climate system will end up – what the final water level will be. But it may not tell us a great deal about how we will get there. Will we have a smooth increase or a much bumpier ride? How much will the climate fluctuate as it responds to being heated up? How big will the “waves” in temperatures be? These are the answers that this recent research has addressed and why on reading it my first response was “Oh no. This isn’t good”.

Because what the team lead by University of Exeter PhD researcher Femke Nijsse found, is that more sensitive climates have higher fluctuations. Using extensive climate simulations, they discovered that if a climate system reacts strongly to increased greenhouse gases, then it is also more likely to have decades when temperatures are much higher or sometimes much lower than the longer-term average. In fact, more sensitive climates may have a run of years that are cooler than less sensitive climates.

Because the warming trend of the Earth’s real climate between 2002-2012 was a bit less than what it should have been given how much we have been increasing greenhouse gases. Some people leapt at that as being evidence that the climate is less sensitive than was initially feared. They argued we shouldn’t decarbonise too rapidly, or even at all, because there is no urgency to do so.

But, this period of depressed warming is consistent with the Earth’s climate having higher fluctuations. What this means is that at some point in the future we may see temperatures swing across to much faster warming trends. This period of hyper-warming could swamp whatever adaptation measures we are currently putting in place.

Exploring the relationship between sensitivity and fluctuations is very complex, and no single study can be considered to provide the last word on the matter. But this new research has kicked away another support against the collapse into despair about how we are affecting the climate.

Because, remember, it’s not the fall, but the sudden stop. And the mess our civilisation will make if climate change sends us plummeting hard and fast to the floor will not be pretty.