Excerpt:
Earth’s warming could trigger sweeping changes in the natural world that would be hard, if not impossible, to reverse.
Right now, every moment of every day, we humans are reconfiguring Earth’s climate bit by bit. Hotter summers and wetter storms. Higher seas and fiercer wildfires. The steady, upward turn of the dial on a host of threats to our homes, our societies and the environment around us.
We might also be changing the climate in an even bigger way.
For the past two decades, scientists have been raising alarms about great systems in the natural world that warming, caused by carbon emissions, might be pushing toward collapse. These systems are so vast that they can stay somewhat in balance even as temperatures rise. But only to a point.
Once we warm the planet beyond certain levels, this balance might be lost, scientists say. The effects would be sweeping and hard to reverse. Not like the turning of a dial, but the flipping of a switch. One that wouldn’t be easily flipped back.
Mass Death of Coral Reefs
When corals go ghostly white, they aren’t necessarily dead, and their reefs aren’t necessarily gone forever. Too much heat in the water causes the corals to expel the symbiotic algae living inside their tissues. If conditions improve, they can survive this bleaching. In time, the reefs can bounce back. As the world gets warmer, though, occasional bleaching is becoming regular bleaching. Mild bleaching is becoming severe bleaching.
Scientists’ latest predictions are grim. Even if humanity moves swiftly to rein in global warming, 70 percent to 90 percent of today’s reef-building corals could die in the coming decades. If we don’t, the toll could be 99 percent or more. A reef can look healthy right up until its corals start bleaching and dying. Eventually, it is a graveyard.
This doesn’t necessarily mean reef-building corals will go extinct. Hardier ones might endure in pockets. But the vibrant ecosystems these creatures support will be unrecognizable. There is no bouncing back anytime soon, not in the places corals live today, not at any scale.
When it might happen: It could already be underway.
Abrupt Thawing of Permafrost
In the ground beneath the world’s cold places, the accumulated remains of long-dead plants and animals contain a lot of carbon, roughly twice the amount that’s currently in the atmosphere. As heat, wildfires and rains thaw and destabilize the frozen ground, microbes get to work, converting this carbon into carbon dioxide and methane. These greenhouse gasses worsen the heat and the fire and the rain, which intensifies the thawing.
Like many of these vast, self-propelling shifts in our climate, permafrost thaw is complicated to predict. Large areas have already come unfrozen, in Western Canada, in Alaska, in Siberia. But how quickly the rest of it might defrost, how much that would add to global warming, how much of the carbon might stay trapped down there because the thawing causes new vegetation to sprout up on top of it — all of that is tricky to pin down.
“Because these things are very uncertain, there’s a bias toward not talking about it or dismissing the possibility, even,” said Tapio Schneider, a climate scientist at the California Institute of Technology. “That, I think, is a mistake,” he said. “It’s still important to explore the risks, even if the probability of occurrence in the near future is relatively small.”
When it might happen: The timing will vary place to place. The effects on global warming could accumulate over a century or more…
