Tipping Points Could Be Closer Than We Thought

Cross-posted on Reuters, USA Today

An international team of experts has submitted a report that lists nine tipping elements -- areas of concern for lawmakers -- that quantify how much time is left to address their impending impact.

Glacial Melt Lakes in Bhutan

Produced by scientists from the U.K, Germany and the U.S., the study states: "Society may be lulled into a false sense of security by smooth projections of global change," and goes on to predict the 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 report states. "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."

The report lists nine tipping elements and the projected timing within which they could begin (with the caveat that the models could change in certainty as conditions change):
Indian Summer Monsoon (within ~one 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."

Africa Rainfall (within ~ten 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."

Arctic Sea-Ice (within ~ten 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."

Boreal Forest (within ~fifty 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 (within ~fifty 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 (within ~one hundred 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."

Atlantic Thermohaline Circulation (within ~one hundred 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."

Greenland Ice Sheet (within ~three hundred 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."

West Antarctic Ice Sheet (within ~three hundred 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."

The report plays down the fears of a sudden permafrost melt releasing excessive methane, and that 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 caveat there is that the report was based on a 2005 workshop of 36 leading climate scientists, a survey of 52 other experts, and a review of scientific literature. The information will require reevaluation as the models change to determine changes in sensitivity and certainty.

Journal article: Lenton, T. M., Held, H., Kriegler, E., Hall, J. W., Lucht, W., Rahmstorf, S. and Schellnhuber, H. J. (2008). Tipping elements in the Earth's climate system. Proceedings of the National Academy of Sciences, Online Early Edition. February 4, 2008.

Potsdam Institute for Climate Impact Research (2008, February 7). Tipping Elements In Earth's Climate System. ScienceDaily. Retrieved February 6, 2008, from http://www.sciencedaily.com­ /releases/2008/02/080204172224.htm