A point of no return?

Does climate change have a 'tipping point', or a level at which the momentum for change becomes unstoppable? More simply put, how warm is too warm?

A gradual awareness is building in the scientific community that our stressed ecosystems are perched on a cliff edge. Given the right nudge, they are capable of slipping rapidly from a seemingly stable state to flood or drought. An Earth that is just a few degrees Celsius hotter may push the planet's climate system past critical thresholds.

A recent study, 'Tipping elements in the Earth's climate system', published in the Proceedings of the National Academy of Sciences, reviews 14 earth systems to assess how small changes can have large long-term consequences on human and ecological systems.

Potential tipping points
Tipping element	Feature of change	Feature of change	Transition timescale	Key impacts
Source: Proceedings of the National Academy of Sciences
Arctic summer sea-ice	Areal extent(-)	Unidentified	~10yr	Amplified warming, ecosystem change
Greenland ice sheet	Ice volume (-)	+~3°C	>300yr	Sea level +2-7 m
West Antarctic ice sheet	Ice volume (-)	+~5-8°C	>300yr	Sea level +5 m
Atlantic thmohaline circulation	Overturning (-)	+0.1-0.5 sverdrup	~100yr	Regional cooling
El Niño - Southern Oscillation	Amplitude (+)	Unidentified	~100yr	Drought in SE Asia
Indian summer monsoon	Rainfall (-)	0.5	~1yr	Drought
Sahara/Sahel and West African monsoon	Vegetation fraction (+)	100 mm/yr	~10yr	Increased carrying capacity
Amazon rainforest	Tree fraction (-)	1,100 mm/yr	~50yr	Biodiversity loss, decreased rainfall
Boreal forest	Tree fraction (-)	+~7°C	~50yr	Biome switch
Antarctic Bottom Water	Formation (-)	+100 mm/yr	~100yr	Ocean circulation, carbon storage
Tundra	Tree fraction (+)	Missing	~100yr	Amplified warming
Permafrost	Volume (-)	Missing	<100yr	CH4 and CO2 release
Marine methane hydrates	Hydrate volume (-)	Unidentified	103 to 105yr	Increased carrying capacity
Ocean anoxia	Ocean anoxia (+)	+~20%	~104yr	Marine mass extinction
Arctic ozone	Column depth (-)	195 K	<1yr	Increased UV at surface

Melting ice

The table shows that Arctic sea-ice and the Greenland Ice Sheet (GIS) are regarded as the most sensitive tipping elements with the smallest uncertainty.

• While sea ice is very reflective and acts as a mirror to the sun, the ocean surface is dark and soaks up incoming sunlight. Therefore, melting sea ice can lead to absorption of more solar energy, thus increasing temperatures further. Recent Intergovernmental Panel on Climate Change (IPCC) models significantly overestimated the amount of sea ice, indicating that a tipping point for stable loss of arctic sea ice may be closer than first thought.
• Unlike sea ice melting, which does not contribute to sea level rise, melting of land-based ice sheets leads to sea level changes. The GIS appears to be changing quicker than models predict, which if continued, would lead to faster sea level rises. Complete melting may be possible within 300 years.

The West Antarctic Ice Sheet (WAIS) is probably less sensitive as a tipping element, but projections of its future behaviour have large uncertainty. Complete WAIS collapse is unlikely within 300 years, but it may thin, contributing to faster sea level rise.

More wind, fewer trees?

There is also large uncertainty over the Amazon rainforest and Boreal forest -– a nearly continuous belt of coniferous trees across North America and Eurasia -- the El Niño phenomenon, and the West African monsoon (WAM):

• Amazon deforestation is expected to reduce precipitation by 20-30%.
• Climate change is expected to lead to permafrost melting and fire increases in the Boreal forest. 
• A changing climate is expected to affect El Niño's intensity, with large-scale societal impacts. A tipping point may be reached this century.
• If the WAM decreases, climate change is expected eventually to increase rainfall in the Sahel, a thin belt of tropical savanna in Africa: a case where climate change largely could benefit the local population.

A political solution

Recent analysis of climate tipping points adds a new conceptual framework to climate change policy, where gradual change is viewed as the possible trigger of disaster. Many tipping elements could reach their critical point this century, and although it is possible to push events in the other direction, restoring a system depends on the art of the possible. Al Gore, the Jeremiah of global warming, may have been wrong when he declared climate change "not a political issue".