Accelerated Global Warming Could Lock Earth Into a Hothouse Future

If you think of Earth’s climate system as a backyard swing that’s been gently swaying for millennia, then human-caused global warming is like a sudden shove strong enough to disrupt the usual arc and buckle the chains.

And if humans keep heating the planet with greenhouse gas pollution, the climate swing could lock Earth into a hothouse trajectory, as parts of the system feed on their own momentum, even if emissions are reduced later, an international team of scientists warned Wednesday in a new paper published in the journal One Earth. 

Their analysis covers 16 key Earth systems, including oceans, ice sheets and forests, that are likely to destabilize if the planet continues to warm. If large parts of the Amazon rainforest and tropical coral reefs die, they absorb less carbon dioxide, triggering a dangerous chain reaction of warming.

If Earth’s climate starts on a hothouse trajectory, it would represent a “global tipping point” as the heating sustains itself even if greenhouse gas emissions drop, said lead author William Ripple, a distinguished professor of ecology at Oregon State University and a leading researcher on climate tipping points.

In the backyard, that’s the moment when the push is so hard that the swing hesitates at the top, just long enough to show that the ride may not be under control anymore and the chains are being tested.

“What typically took thousands of years is now happening in decades,” Ripple said, adding that human-caused warming is already nudging the climate system out of 11,000 years of relative stability with good conditions for farming and societal development.

Earth could be entering a period of unprecedented climate change on a one-way trajectory, in which processes such as ice-sheet collapse can continue even if the average global temperature is stabilized, he said.

The authors of the new paper stressed that a self-sustaining hothouse trajectory is not the same as a Hothouse Earth state, which would be when the global climate rebalances at a much hotter average temperature.

No Good Analog Climates

Instead of offering a single new climate forecast, the paper synthesizes decades of research revealing how different parts of the climate system influence one another. When one part of the system is destabilized, they wrote, it can amplify stress in others, pushing the planet along a self-reinforcing warming pathway. 

Earth has had hothouse climates in the ancient geological past. But the authors of the new paper said there may not be a parallel to what’s happening now, at least not during the past 3 million years, co-author Johan Rockström, co-director of the Potsdam Institute for Climate Impact Research, said via email. 

“The reason is that our starting point is a WARM state. So, we are going from WARM to HOT,” he wrote. This may mean “getting stuck” at a global mean surface temperature of 4 to 6 degrees Celsius above pre-industrial levels, he added. 

That amount of warming goes beyond current expectations and would devastate ecosystems and communities globally. Many other current climate projections suggest that, under current policies, warming would level off somewhere between 2.7 and 3 degrees Celsius.

Human-caused warming is happening much faster than any other warming documented in the paleoclimate record, and it’s also unprecedented because it’s driven by a single dominant force, Rockström added: human greenhouse gas emissions. Under these conditions, research has documented that Earth is already losing some of the natural buffers that dampened climate swings in recent millennia.

“We now see worrying signs that the Earth system is losing resilience,” Rockström said. Recent extremes, he added, are a sign that the climate system “may respond more strongly to the same amount of warming than it did before.”

The authors wrote that the magnitude and pace of recent climate extremes “have surprised scientists, raising questions about how well current climate projections capture risk.”

One warning sign is the recent acceleration of warming, from about 0.18 degrees Celsius per decade from 1970 to 2014, to about 0.26 degrees Celsius in the last decade; another is the reduced carbon uptake in tropical, temperate and boreal forests. And, Rockström added, “Earth is getting darker, due to multiple factors,” including melting ice, tree lines moving closer to the poles in the Northern and Southern hemispheres and changing cloud patterns due to increased evaporation and moisture in the atmosphere.

The recent acceleration of warming was also noted recently by climate scientist James Hansen, a former NASA researcher who has accurately projected the planet’s global warming trajectory for several decades.

In a climate bulletin published last week, Hansen wrote that a current shift toward a warm tropical Pacific Ocean phase could push Earth to a new temperature record this year or next, potentially surpassing 2 degrees Celsius above the pre-fossil-fuel era benchmark sometime in the 2030s.

“Don’t be too pessimistic as the evidence for high climate sensitivity grows,” Hansen wrote in his Feb. 6 update. “Realistic understanding of the climate situation, and public recognition of that, is the essential first step toward successfully addressing climate change.” 

The science shows that climate stability is no longer guaranteed, Rockström said. Choices made this decade, he said, could shape the Earth System for generations.