9 Dangerous "Tipping Elements"

This post is by Lisa Moore, Ph.D., a scientist in the Climate and Air program at Environmental Defense.

The term "tipping point" refers to a critical threshold at which a small change can qualitatively alter the state of a system. For example, when temperature reaches 32°F, ice changes into water. There also are "tipping points" in global warming. The best known is the Greenland Ice Sheet, which could begin a slow, irreversible meltdown if global temperature passes a certain threshold.

Last week, climate researchers published a paper that examines Earth systems in danger of passing tipping points due to human activity. They call these "tipping elements", and highlight nine such systems from around the world. They say the greatest threat is to the Arctic, followed by the Greenland Ice Sheet. Here’s the list.

1. Arctic Summer Sea Ice
   Critical Point: 0.5-2°C above present
   Expert Assessment: High sensitivity/Small uncertainty

   Sea ice extent is already decreasing, but total loss would devastate Arctic ecosystems. For example, polar bears, which use summer sea ice to hunt, would starve. Sea ice loss also can accelerate warming because ice is more reflective than open water. The authors warn that "a summer ice-loss threshold, if not already passed, may be very close and a transition could occur well within this century."

2. Greenland Ice Sheet (GIS)
   Critical Point: 1-2°C above present
   Expert Assessment: High sensitivity/Small uncertainty

   A total meltdown of the GIS would eventually raise sea level about 20 feet. This study’s range for the critical point is both lower and smaller than what’s given in the IPCC Fourth Assessment Report, which estimated 1.3-4°C above present. The new estimate takes into account more recent research, including observations that both sea ice and ice sheets are melting faster than models predict.

3. West Antarctic Ice Sheet (WAIS)
   Critical Point: 3-5°C above present
   Expert Assessment: Medium sensitivity/Large uncertainty

   A collapse of the WAIS would eventually raise sea level about 15 feet. Although the estimated critical point for the WAIS is higher than for the GIS, the authors note that the range is "clearly accessible within this century" and that the WAIS is more likely to cause rapid sea level rise than the GIS.

4. Atlantic Thermohaline Circulation (THC)
   Critical Point: 3-5°C above present
   Expert Assessment: Low sensitivity/Medium uncertainty

   THC refers to ocean circulation driven by density differences due to temperature and salinity (see this fact sheet for more). Significant slowing or a complete shutoff of circulation patterns in the Atlantic Ocean would affect regional climate patterns since THC transports heat from the tropics to Northern Europe. That doesn’t mean Europe’s temperatures would plunge – the IPCC estimates it would simply warm less quickly – but it’s still a concern.

5. El Niño – Southern Oscillation
   Critical Point: 3-6°C above present
   Expert Assessment: Medium sensitivity/Large uncertainty

   El Niño causes warm water to move eastward so the Pacific Ocean releases more heat (see our previous post). Stronger El Niño events due to global warming would, for example, bring drought to Southeast Asia. This tipping element is interesting because the IPCC said there wasn’t enough information to predict any particular change in El Niño patterns. The experts in this new study disagree, though they note that "the existence and location of any threshold is particularly uncertain."

6. Indian Summer Monsoon
   Critical Point: Albedo above 0.5
   Expert Assessment: Large uncertainty

   The control parameter here is not temperature, but reflectivity (also called "albedo") which increases as a surface gets brighter. Land use change and aerosol pollution (for example, the sulfate particles Bill discussed in his post on geoengineering) can increase regional albedo, which weakens the monsoon. A decrease in monsoon precipitation could spell disaster for India, which depends on the rainfall for irrigation. The authors argue that the IPCC, which predicted an increase in monsoon precipitation, underestimated the effects of land use change and aerosols.

7. West African Monsoon
   Critical Point: 3-5°C above present
   Expert Assessment: Medium sensitivity/Large uncertainty

   Rainfall in Africa’s Sahara and Sahel regions could increase, leading to greening. This might harm plants and animals adapted to current conditions, but the societal impact could be very positive – a rare potential benefit of climate change.

8. Amazon Rainforest
   Critical Point: 3-4°C above present
   Expert Assessment: Medium sensitivity/Large uncertainty

   Warming could decrease precipitation and lengthen the Amazonian dry season, leading to forest dieback. Deforestation can accelerate warming, exacerbating the problem. The authors warn that land use change alone could bring the forest to a critical threshold.

9. Boreal Forest
   Critical Point: 3-5°C above present
   Expert Assessment: Medium sensitivity/Large uncertainty

   Earth’s vast boreal forests cover the northern latitudes of Canada, Alaska, Europe and Russia. They store carbon, filter water, and support many rare and beautiful species. Boreal regions may become drier as temperatures rise, leaving these forests vulnerable to fire and disease.

This new study warns that we’re getting ever-closer to massive changes in key Earth systems, and that other tipping elements could surprise us. It gives us nine compelling reasons to cut emissions as much, and as soon, as possible.