Concerned that humans might push Earth into major climate shifts, a team of experts has published a study that lists nine "tipping elements," or areas of concern for policymakers.
Arctic sea-ice and the Greenland Ice Sheet, both of which have shown significant melt, were regarded as the most sensitive tipping elements with the smallest uncertainty.
"Society may be lulled into a false sense of security by smooth projections of global change," the scientists at British, German and U.S. institutes wrote in a report saying there were many little-understood thresholds in nature.
"The greatest and clearest threat is to the Arctic with summer sea ice loss likely to occur long before, and potentially contribute to, Greenland Ice Sheet melt," they wrote in the journal Proceedings of the National Academy of Sciences.
The experts coined the term "tipping element" to describe those components of the climate system that are at risk of passing a "tipping point," which was defined as a critical threshold at which a small change in human activity can have large, long-term consequences for the Earth’s climate system.
"These tipping elements are candidates for surprising society by exhibiting a nearby tipping point," the authors added.
"Many of these tipping points could be closer than we thought," said lead author Timothy Lenton, of the University of East Anglia in England.
"Our findings suggest that a variety of tipping elements could reach their critical point within this century under human-induced climate change," he added. "The greatest threats are tipping of the Arctic sea-ice and the Greenland ice sheet, and at least five other elements could surprise us by exhibiting a nearby tipping point."
Other sudden changes linked to climate change, stoked by human use of fossil fuels, included a dieback of northern pine forests, or a stronger warming of the Pacific under El Nino weather events that can disrupt weather worldwide, they wrote.
A possible greening of parts of the Sahel and the Sahara, if monsoon rains in West Africa were disrupted, was one of the few positive abrupt shifts identified by the scientists.
Even a moderate warming could set off a thaw of Greenland's ice sheet that could then vanish in 300 years — raising sea levels by 20 feet and threatening coasts, Pacific islands and cities from Bangkok to Buenos Aires.
The report also identified risks such as damage to northern pine forests — widely exploited by the pulp industry — because of factors such as more frequent fires and vulnerability to pests in warmer, drier conditions.
But it played down some other fears, such as of a runaway melt of Siberian permafrost, releasing stores of methane which is a powerful greenhouse gas.
And it said a shutdown of the Gulf Stream in the Atlantic Ocean that brings warm water north to Europe "appears to be a less immediate threat."
The experts based the list on a 2005 workshop of 36 leading climate scientists, a survey of 52 other experts, and a review of scientific literature.
In a statement released with the study, the authors summarized the nine tipping elements by assigning them to categories tied to sensitivity and uncertainty, and estimating the time it would take each to undergo a major transition:
Highly sensitive, smallest uncertainty
Greenland Ice Sheet (more than 300 years): "Warming over the ice sheet accelerates ice loss from outlet glaciers and lowers ice altitude at the periphery, which further increases surface temperature ... The exact tipping point for disintegration of the ice sheet is unknown, since current models cannot capture the observed dynamic deglaciation processes accurately. But in a worst case scenario local warming of more than three degrees Celsius could cause the ice sheet to disappear within 300 years. This would result in a rise of sea level of up to seven meters."
Arctic sea-ice (approximately 10 years): "As sea-ice melts, it exposes a much darker ocean surface, which absorbs more radiation than white sea-ice so that the warming is amplified. This causes more rapid melting in summer and decreases ice formation in winter. Over the last 16 years ice cover during summer declined markedly. The critical threshold global mean warming may be between 0.5 to 2 degrees Celsius, but could already have been passed. One model shows a nonlinear transition to a potential new stable state with no arctic sea-ice during summer within a few decades."
Intermediately sensitive, large uncertainty
West Antarctic Ice Sheet (more than 300 years): "Recent gravity measurements suggest that the ice sheet is losing mass. Since most of the ice sheet is grounded below sea level the intrusion of ocean water could destabilize it. The tipping point could be reached with a local warming of five to eight degrees Celsius in summer. A worst case scenario shows the ice sheet could collapse within 300 years, possibly raising sea level by as much as five meters."
Boreal forest (approximately 50 years): "The northern forests exhibit a complex interplay between tree physiology, permafrost and fire. A global mean warming of three to five degrees Celsius could lead to large-scale dieback of the boreal forests within 50 years. Under climate change the trees would be exposed to increasing water stress and peak summer heat and would be more vulnerable to diseases. Temperate tree species will remain excluded due to frost damage in still very cold winters."
Amazon rainforest (approximately 50 years): "Global warming and deforestation will probably reduce rainfall in the region by up to 30 percent. Lengthening of the dry season, and increases in summer temperatures would make it difficult for the forest to re-establish. Models project dieback of the Amazon rainforest to occur under three to four degrees Celsius global warming within fifty years. Even land-use change alone could potentially bring forest cover to a critical threshold."
El Niño Southern Oscillation (approximately 100 years): "The variability of this ocean-atmosphere mode is controlled by the layering of water of different temperatures in the Pacific Ocean and the temperature gradient across the equator. During the globally three degrees Celsius warmer early Pliocene ENSO may have been suppressed in favor of persistent El Niño or La Niña conditions. In response to a warmer stabilized climate, the most realistic models simulate increased El Niño amplitude with no clear change in frequency."
Africa rainfall (approximately 10 years): "The amount of rainfall is closely related to vegetation climate feedback and sea surface temperatures of the Atlantic Ocean. Greenhouse gas forcing is expected to increase Sahel rainfall. But a global mean warming of three to five degrees Celsius could cause a collapse of the West African monsoon. This could lead either to drying of the Sahel or to wetting due to increased inflow from the West. A third scenario shows a possible doubling of anomalously dry years by the end of the century."
Indian summer monsoon (approximately 1 year): "The monsoon circulation is driven by a land-to-ocean pressure gradient. Greenhouse warming tends to strengthen the monsoon since warmer air can carry more water. Air pollution and land-use that increases the reflection of sunlight tend to weaken it. The Indian summer monsoon could become erratic and in the worst case start to chaotically change between an active and a weak phase within a few years."
Lowly sensitive, intermediate uncertainty
Atlantic thermohaline circulation (approximately 100 years): "The circulation of sea currents in the Atlantic Ocean is driven by seawater that flows to the North Atlantic, cools and sinks at high latitudes. If the inflow of freshwater increases, e.g. from rivers or melting glaciers, or the seawater is warmed, its density would decrease. A global mean warming of three to five degrees Celsius could push the element past the tipping point so that deep water formation stops. Under these conditions the North Atlantic current would be disrupted, sea level in the North Atlantic region would rise and the tropical rain belt would be shifted."