Climate thresholds are being passed now – each scenario causes conflict.

It is tempting to think of climate change as a gradual, linear process that follows the steady accumulation of greenhouse gasses in the atmosphere. The truth may be scarier. Evidence is mounting that critical components of the Earth system could be approaching tipping points that, when crossed, will bring about massive, nonlinear shifts that will themselves accelerate climate change, with disastrous and potentially irreversible consequences for nature and humanity. Faced with this prospect, national governments must cooperate to identify, prepare for and, where possible, head off or mitigate these abrupt and calamitous shifts.

As the Intergovernmental Panel on Climate Change made clear in its recent assessment report, the long-foreseen climate emergency is upon us. For the past 12,000 years, a period geologists call the Holocene, humanity has enjoyed remarkable climate stability. That benign era, which permitted the emergence of civilization, is over. The global environment is changing in ways unprecedented in human history. Atmospheric carbon dioxide levels are the highest they’ve been in 800,000 years, and the past decade has included nine of the 10 hottest years on record. Glaciers and arctic sea ice are retreating; the oceans are warming and acidifying; climatic zones are shifting poleward; sea levels are rising; species and ecosystems are disappearing; and searing heat waves, punishing droughts, heavy precipitation and tropical cyclones are increasing in frequency and intensity.

The coming decades will bring more of the same, but how much more will depend on the degree to which average global temperatures rise. Unfortunately, the world is poised to blow past the Paris Agreement target of limiting that increase to 2 degrees Celsius, and ideally no more than 1.5 degrees Celsius, above preindustrial levels. Even accounting for the pledges countries made at the United Nations COP26 Climate Summit in Glasgow last November, global temperatures are on track to reach 2.7 degrees Celsius above preindustrial levels by 2100.

More alarming still, rising temperatures could soon breach thresholds in important natural systems, a possibility emphasized in the IPCC report. Higher global warning levels could, for instance, trigger a rapid dieback in the Amazon rainforest or in boreal forests, a precipitous collapse of the Greenland and Antarctic ice sheets or the disruption of the Indian Ocean’s monsoon system. They could also accelerate the melting of permafrost, which would in turn release massive stores of methane and carbon dioxide that would accelerate global warming even further. Or they could cause a precipitous shutdown of the Atlantic Meridional Overturning Circulation, an system of ocean currents that helps keep Northern Europe temperate and habitable. The AMOC has already been slowing, but its total collapse would render Europe much colder and would be impossible to reverse within a human lifetime.

These transitions, if they occur, may not unfold all at once, as Peter Cox, a professor of climate system dynamics at the University of Exeter, told me during an event at the Council on Foreign Relations. An Amazon rainforest dieback, for instance, could occur within the next few decades, but a shift in the AMOC or the melting of ice sheets are likely to progress more gradually. Unfortunately, scientists haven’t yet developed adequate models to determine just how close humanity is to any of these cliff edges—and whether the world may be able to reverse any of these changes when and if it reaches net-zero carbon emissions. Scientists speculate that it might be possible, in a decarbonized world, to return natural systems that pass tipping points more slowly to their previous state, whereas abrupt transitions are likely to be irreversible. But even this is just a working hypothesis.

As with climate change more generally, these climatic tipping points will damage some parts of the world far more than others. The disruption of Indian Ocean monsoons, coming on top of intense warming, could well leave the Indian subcontinent in a state of perpetual environmental crisis. An Amazon dieback could do the same to Brazil and its neighbors, while collapse of the AMOC would obviously be devastating for Europe.

Given the unequal distribution of these consequences, the nearing of any particular tipping point would likely have geopolitical reverberations, increasing diplomatic tensions as countries blame one another for allowing climate change to reach this point in the first place. The governments of countries under threat could well take matters into their own hands, with destabilizing consequences—for instance, by launching last-ditch, unilateral solar climate intervention, a scenario the U.S. National Intelligence Council warned about in October.