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.
Most of these feedbacks operate on a longer time scale, over which they could easily tip us into a “hothouse world” where sea levels rise by 15 meters (50 feet) and many areas become incapable of supporting human life.
Crucially, the paper notes, most of these forces are almost impossible to stop once they get started. Although these threats have been considered before individually, the authors focused on the neglected aspect of how they could interact.
“The real concern is these tipping elements can act like a row of dominoes. Once one is pushed over, it pushes Earth towards another. It may be very difficult or impossible to stop the whole row of dominoes from tumbling over,” Steffen said in a statement.
The paper notes that the burning of fossil fuels is far from the only human activity that can make these tipping points more likely. For example, tropical rainforests benefit from being surrounded by other rainforests. Chop some down and there is less rain for nearby forests, increasing the danger of dry-season fires.
The authors admit our knowledge of the temperatures at which these tipping points will be crossed is poor, and in some cases, the rate of warming is as important as the absolute amount. Nevertheless, they argue we can't be confident of keeping the Earth in a stabilized temperature state if temperatures rise by 2ºC (3.6ºF) compared to pre-Industrial levels. Terrifyingly, we are already halfway there.