PARIS - Even if humanity slashes greenhouse gas emissions in line with Paris climate treaty goals, the planet could overwhelm such efforts and irretrievably tip into a hellish “hothouse” state, top scientists warn.
Under such a scenario, Earth’s average temperature would stabilize at 4 or 5 degrees Celsius (7 to 9 degrees Fahrenheit) above pre-industrial levels, rather than the cap of 1.5 to 2 C called for in the 196-nation pact.
As it is, the world is struggling to curb the man-made carbon pollution that — even with only 1 degree Celsius of warming so far — amplifies the likelihood and intensity of deadly heat waves, droughts and superstorms.
Simply put, climate change continues to outpace the transition to a green, clean global economy.
But that challenge will become exponentially more difficult if Earth itself gets in on the act, the researchers reported in the U.S. Proceedings of the National Academy of Sciences.
No fewer than 10 distinct facets of what scientists call the “Earth System” could switch from neutral or helpful to harmful, eventually dumping more carbon dioxide and methane into the atmosphere than all human activity combined.
Most have temperature “tipping points” beyond which the release of these planet-warming gases would be irreversible, at least on a human time scale.
“The feedback process becomes self-perpetuating after a critical threshold is crossed,” the study said.
“The Earth System may be approaching a planetary threshold that could lock in a continuing, rapid pathway towards much hotter conditions — Hothouse Earth.”
Weakened carbon ‘sinks’
Earth’s forests and oceans have together absorbed more than half of the carbon pollution emitted over the last several decades, even as those emissions grew.
But forests are shrinking and oceans are showing signs of carbon dioxide saturation, according to recent studies.
In other words, these carbon sponges, or “sinks,” may be weakening.
(Not so) permafrost
Methane and carbon dioxide trapped in the increasingly misnamed permafrost of Russia, Canada and northern Europe is roughly equivalent to 15 years of emissions at today’s levels.
The release of these gases — negligible so far — would speed climate change and, in effect, hasten their own escape, in what scientists call positive feedback.
Similarly, rock-like formations in shallow ocean waters called methane hydrates — prime suspects for episodes of rapid global warming millions of years ago — are also vulnerable to global warming, but the threshold at which this would be seen remains unknown.
Global warming of 3 degrees Celsius could condemn 40 percent of the Amazon forests to die back, a process that would reach well into the next century, according to recent research.
Accidental or land-clearing fires — not accounted for in these models — could hasten this destruction.
In Canada, forests that gained carbon dioxide-absorbing biomass for most of the 20th century began to lose it around 1970, due mainly to climate-related insect infestations and fires.
Taken together, these forest die-offs would release billions of tons of carbon into the air.
Less snow = more heat
Dramatically shrinking polar sea ice, especially in the Arctic, means the deep-blue ocean water that takes its place absorbs as much of the sun’s radiative force — about 80 percent — as was previously reflected back into space by snow’s mirror-like surface.
The Arctic will likely see its first ice-free summer before midcentury — and in a 2-degree world could remain that way every 1 in 4 years.
Over the last four decades, minimum sea ice extent has dropped by about 40 percent.
Ice sheets, sea level
Experts disagree on how much climate change it will take to condemn the West Antarctic and Greenland ice sheets, and how quickly they would melt, but all agree that such a tipping point exists, with estimates ranging from 1 C to 3 C.
The consequences for humankind would be catastrophic: Two-thirds of the world’s mega-cities are less than 10 meters about sea level, as is much of the agricultural land that feeds them.
Together, West Antarctica’s and Greenland’s frozen reservoirs would lift the ocean by 13 meters.
Another 12 meters of potential sea level rise is locked in parts of the East Antarctic Ice Sheet that are far more susceptible to climate change than once thought.
All of these processes are interconnected, the authors note, and the collapse of one could trigger another.
“The risk of tipping cascades could be significant at a 2 C temperature rise, and could increase sharply beyond that point.”
“This cascade of events may tip the entire Earth system into a new mode of operation,” said co-author Hans Joachim Schellnhuber, Director of the Potsdam Institute for Climate Impact Research.
The “carrying capacity” of a 4- or 5-degree degree world, he has said previously, could drop to just a billion people.