Environmentalists have long warned that human-induced global warming could unleash a runaway effect, with feedback mechanisms continuing to cook the planet even after humans stop emitting greenhouse gasses. A new paper from leading climate scientists indicates the danger is real and at temperatures not much higher than where we are now.
The Earth’s climate is a complex system. When a factor disturbs it, either by adding extra heat or taking some away, it sets off multiple feedback loops. Some of these work against the new effect, keeping global temperatures in balance. Others amplify the initial alteration – for example, we've often heard about the way changes in ice coverage affect how much heat the planet reflects or absorbs. Climate scientists spend their lives trying to understand these feedback mechanisms.
Most feedbacks are smaller than the initial impetus (even if they add to it), but scientists are troubled by several that could potentially turn modest changes in temperature into something much larger. In Proceedings of the National Academy of Sciences, Australian National University’s Professor Will Steffen and 15 co-authors consider the 10 most dangerous examples and what could occur if we trip them off.
High-latitude threats include thawing permafrost or ocean floor hydrates, the loss of summer sea ice in the Arctic and Antarctic, melting ice sheets, and a reduction in winter snow cover. Elsewhere, the dieback of the Amazon or boreal forests, weakening of carbon sinks on land and sea, and increased respiration by ocean bacteria rank as possible dangers.
“For some of the tipping elements, crossing the tipping point could trigger an abrupt, nonlinear response (e.g., conversion of large areas of the Amazon rainforest to a savanna or seasonally dry forest),” the paper notes. “While for others, crossing the tipping point would lead to a more gradual but self-perpetuating response (large-scale loss of permafrost).”
By definition, the non-linear responses are almost impossible to predict, but the paper doesn't foresee any of these having a large impact individually over the course of this century. However, the combination is anticipated to add up to almost 0.5ºC (0.9ºF) to the direct effects of human emissions.