Climate heating just makes things hotter still

Climate heating has a way of making the globe even hotter. As higher temperatures kick in, there is a pronounced planetary tendency to send the mercury rising even further.

And − on the evidence of the last 66 million years − this is a process that doesn’t even need human help. Some kind of warming bias seems to have been baked into the climate machinery, according to a new study.

And if the warming process gets a bit of help from humankind in the form of greenhouse gases from fossil fuel combustion, natural planetary ecosystems and global geochemistry could augment the process and take it a lot further.

“The northern hemisphere’s ice sheets are shrinking, and could potentially disappear as a long-term consequence of human actions,” said Constantin Arnscheidt of the Massachusetts Institute of Technology, who led the research outlined in the journal Science Advances.

“Our research suggests that this may make the Earth’s climate fundamentally more susceptible to extreme, long-term global warming events such as those seen in the geologic past.”

Continuous cycle

It’s not a new idea. Researchers have been saying for decades that as the polar ice retreats, more open sea and rock is exposed. Ice and snow reflect radiation, dark rock and blue sea absorb it, to amplify warming and accelerate climate change.

As the permafrost thaws, long-buried plant remains begin to surrender methane and carbon dioxide into the atmosphere, to speed up the thaw even more. As forests − vast stores of atmospheric carbon − become hotter and more drought-stricken, they are at risk of fire, which puts even more greenhouse gas  back into the atmosphere to warm the world even more intensely.

But these have all been warnings: what two researchers at MIT did was to look at the tell-tale patterns of climate change deep in prehistory. Because fossil evidence tells a reliable story of past climates, they could study changes in the composition of the shells of foraminifera preserved in ocean sediments.

These identified a continuous cycle of temperature rise and fall and, in particular, the way temperature rose during the aeons that followed the extinction of the dinosaurs at the close of the Cretaceous.

The researchers found the curve of warming and cooling was more skewed to warm events than cool ones: warm events tended to be of a more extreme temperature than the most extreme cool spells. Sometimes the planetary climate changed dramatically, to bring crocodiles to Arctic waters, and forests to Antarctica.

“This may make the Earth’s climate fundamentally more susceptible to extreme, long-term global warming events such as those seen in the geologic past”

The researchers call this a multiplier effect: increases in temperature bias the climate system towards even more increases.

The study may throw additional light on an enduring climate puzzle: the pattern of temperature for much of the Earth’s history can be matched to the pattern of cyclic shifts in the planetary orbit around the sun, over hundreds of thousands of years.

But these changes are themselves tiny. The multiplier effect could explain why they jolt the planet’s climate into a new regime.

“Climate warms and cools in synchrony with orbital changes, but the orbital cycles themselves would predict only modest changes in climate. But if we consider a multiplicative model, then modest warming, paired with this multiplier effect, can result in extreme events that tend to occur at the same times as these orbital changes,” said Daniel Rothman, a geophysicist at MIT, and a co-author.

And Constantin Arnscheidt said: “Humans are forcing the system in a new way. And this study is showing that, when we increase extreme temperature, we’re likely going to interact with these natural, amplifying effects.”