Prominent climate scientists warn urgently of an almost irreversible warming of the Earth if humanity does not quickly stop burning coal, gas and oil, as well as deforesting forests and drying out peatlands. “More than 11,000 years ago, the Earth’s climate stabilized after millions of years alternating between warmer periods and ice ages, making agriculture and complex societies possible,” says Willian Ripple, who researches at the Oregon State University College of Forestry in the United States. “Now we are moving away from this stability and could enter an era of unprecedented climate change.”
In a commentary published on Wednesday in the scientific journal OneEarth, an international team of researchers describes how various factors could come together to transform Earth into a so-called Hothouse. Among those involved were, among others, former head of the Potsdam Institute for Climate Impact Research (PIK), Hans Joachim Schellnhuber, as well as the current PIK president Johan Rockström.
As a Hothouse state, Earth’s climate would be 6 to 8 degrees hotter than today and sea level would be higher by many meters. That was the case at times in Earth’s history, including when the dinosaurs lived. Returning from such a state to the current climate would be nearly impossible, the researchers write.
The scientists point out that a number of consequences of warming further amplify climate change. For example, methane and CO₂ released from melting permafrost would further heat the Earth.
Moreover, individual Earth system components can “tip”, i.e., beyond a certain point they can no longer be returned to their original state. This includes, among others, the Atlantic Meridional Overturning Circulation AMOC, which provides Europe with a temperate climate, the Greenland ice sheet, or the Amazon rainforest. Many of these tipping points would also make the Earth hotter should they occur.
Physicists: Models Represent Feedbacks Very Well
If these feedbacks amplify each other or even trigger tipping points, the Earth could move toward a Hothouse and heat up even faster than current models predict, warn the researchers.
The physicist Niklas Boers from the Technical University of Munich and the PIK notes that “the positive and negative feedbacks, for example the methane stored in permafrost, are very well represented in the models.” Boers was not involved in the commentary. Tipping systems like the Greenland ice sheet or the AMOC are, by contrast, “unfortunately still not well represented” in the models.
One figure in the commentary, however, portrays the path to the Hothouse state as if the Earth would fall inexorably toward the Hothouse state once a tipping point is reached. Boers prefers another picture. He likens the effect of tipping points on temperature to a staircase, with the individual tipping points forming the steps: if you snap one tipping point, you may not be able to return. But stepping down one step does not necessarily mean you will step down to the next step: “I don’t think there is a single global tipping point beyond which the Earth would unavoidably warm further.”
Despite these caveats, Boers does not downplay the warning of the scientists in OneEarth: “Comments like this are important because they draw attention to worst-case scenarios that we cannot rule out, and that therefore must be studied more closely.” Many important scientific papers have been sparked by comments.
That the research on feedbacks and tipping points is not finished is also admitted by the authors of the commentary: “Are we risking crossing planetary tipping points and entering a path toward a Hothouse state? Science has no clear answer, but the question urgently calls for research.”
