Sunday, September 30, 2007

"Potential Anthropogenic Tipping Elements in the Earth System"

(#s 1&16-tipped / #2-in limbo / the rest stable)
From the The Potsdam Institute for Climate Impact Research (PIK)

1 Arctic Sea Ice Loss
As sea ice melts in a warming climate, it exposes a dark ocean surface, which absorbs more solar radiation and thus amplifies the warming. Over the last 30 years the area covered by sea ice has decreased significantly. This is also bad news for many species, like seals or polar bears, which depend on that ice for hunting and breeding. Time Frame: ~ 100 yr.

2 Melting of Greenland Ice Sheet
Greenland’s ice sheet is melting due to the extraordinary warming of the Arctic region. Recent observations sug- gest an accelerated destabilization also due to melt-water lubrication effects. The complete collapse of the Greenland ice sheet would cause a global sea level rise of 7 m. Time Frame: Unknown due to highly non-linear processes. Current estimates: 300–1000 yr.

3 Methane Escape from Thawing Permafrost Regions and Continental Shelves
Huge amounts of methane, which is a highly potent greenhouse gas, could be released by global warming. On the one hand, terrestric methane will emanate from thawing permafrost areas in Siberia and Northern America. On the other hand, ‘methane ice’ assembled by natural processes over millions of years off many coasts might be activated by changing ocean temperatures and currents. Time Frame: ~ 1000 yr.

4 Boreal Forest Dieback
Northern boreal forests account for almost one third of the global forest inventory. They are declining in a warming climate because of enhanced disturbance stress through fires, pests, and storms. At the same time, their regenerative capabilities are diminished by temperature and water stress as well as direct human interference (logging, fragmentation, etc.). The dieback would trigger massive release of carbon dioxide, which in turn enhances climate change as well as significant losses in biodiversity. Time Frame: ~ 50–100 yr.

5 Suppression of Atlantic Deep Water Formation
The warm Atlantic surface ocean current is responsible for the benign climate in Northwestern Europe. This gre- at ‘conveyor belt’ is ultimately driven by cold and dense water sinking to the bottom of the North Atlantic off the coasts of Greenland and Labrador. A warming climate leads to an increased freshwater flow into the ocean, thus decreasing the water’s density and slowing down the deep water formation. Time frame: ~ 100–500 yr.

6 Climatic Change-Induced Ozone Hole over Northern Europe
Particularly Northern Europe could face a climate change-induced ozone hole. Global warming at the bottom of atmospheric strata implies cooling in the stratospheric “roof”. This cooling induces ice cloud formation which in turn provides a catalyst for ozone destruction. Time Frame: ~ 10–1000 yr.

7 Darkening of the Tibetan Plateau
As the snow cover of the Tibetan territory melts due to global warming, the exposed dark rock surface will amplify regional warming through increased absorption of solar radiation. As a side effect, the freshwater supply for many Asian countries, which depend on glacier melt water, will be reduced. Moreover, it is possible that the darkening of the Tibetan plateau could affect the Indian monsoon system. Time Frame: ~ 50–100 yr.

8 Disruption of Indian Monsoon
Up to 90% of India’s precipitation is provided by the regular summer monsoon. Carbon dioxide as well as aerosols play a key role in this highly variable system. Air pollution, land-cover change and greenhouse gas emissions could bring about a roller-coaster succession of intensified and weakened monsoons in South Asia causing extreme droughts and floods. Time Frame: 30–100 yr.

9 Re-Greening of the Sahara and Sealing of Dust Sources
Vegetation could re-appear due to higher precipitation in the Sahel region, but this greening of the Sahara may be overridden by intensive land-use, especially grazing. If the re-greening happened, it could seal major sources of dust that is blown across the Atlantic and fertilizes South American ecosystems. Time Frame: ~ 50 yr.

10 West African Monsoon Shift
The West African monsoon is affected both by heavy deforestation in coastal areas and increasing sea-surface temperatures. The future of this monsoon system is still uncertain. Global warming may bring about a doubling of dry years in the Sahel by the end of the century or a complete monsoon collapse, both of which would have profound large-scale impacts. Time Frame: ~ 50–100 yr.

11 Dieback of Amazon Rainforest
A large fraction of precipitation in the Amazon basin is recycled evaporation water. The reduction of regional rainfall in a warming climate, intimately connected to El Niño/Southern Oscillation, as well as forest fragmentation due to human activity could bring the forest cover to a critical threshold. Amazon dieback would have profound influence on the global climate and at the same time result in a huge loss of biodiversity. Time Frame: ~ 50–100 yr.

12 Change in Southern Pacific Climate Oscillation
Although uncertainties are large, some climate models predict an increased frequency and/or intensity of El Niño conditions in the Southern Pacific. The impacts of such a change in the oceanic oscillation patterns would be felt around the globe, especially in the form of droughts in South-East Asia and many other regions. Time Frame: Rapid changes possible in 10-100 yr.

13 Disruption of Marine Carbon Pump
This “pump” acts as a sink for both natural and anthropogenic excess CO2. There is a risk of a decline of this sink caused by increased ocean acidification and stratification owing to rising atmospheric CO2 levels. The acidification impedes floating and fixed organisms, such as plankton algae and corals, to build their skeletons, which bind carbon. Time Frame: unknown.

14 Suppression of Antarctic Deep Water Formation and Nutrients Upwelling
Similar to the North Atlantic, convection of water masses in the Southern ocean can be suppressed by freshwater inflow from melting ice. If there is a critical threshold, it has not been assessed so far. The resulting decline of nutrients would reduce krill, which marks the basis of the marine food chain. Time Frame: ~ 100 yr?

15 Collapse of the West Antarctic Ice Sheet
Although assumed to be not as vulnerable as the Green- land Ice Sheet, a collapse of the West Antarctic Ice Sheet could be initiated within this century. Warming oceans result in melting of offshore ice shelves, which currently impede the out-flow of the continental ice masses be- hind. Furthermore, the warm water could be undercutting the ice sheet and yield further separation from the bedrock, thus accelerating the decay. The complete ice sheet collapse would raise the global sea level by 4-5 m. Time frame: ~ 300–1000 yr.

16 Antarctic Ozone Hole
Already strongly perturbed by humanity’s emissions of chlorofluorocarbons in the past, the protective ozone layer is believed to be regenerating after these chemicals have been banned. Yet strong interactions between stratospheric ozone depletion and global warming may widen the ozone hole over the Antarctic once again. Time Frame: ~ 10–100 yr.

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