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Scientists have identified many systems that will be affected by global warming. They have also warned that there may be "tipping points" that trigger large changes. Probably the best known example is that the Greenland Ice Sheet could begin a slow but irreversible meltdown if global temperature passes (and remains above) a certain threshold.
Last week, scientists published a paper that expands on the concept of tipping points. It highlights the future impacts that are most policy-relevant and summarizes what we know about them.
What’s on the list, and how do experts rank the threats?
The authors introduce a new term, "tipping element", to describe a large-scale Earth system that can be switched to an entirely new state by tiny changes in some control parameter. The tipping point of each element is the critical point at which the system switches to a new state. For example, frozen water switches states (solid to liquid) when its control parameter (temperature) reaches the critical point (water’s melting point, or 32 °F).
The authors identified 15 large Earth systems that qualify as tipping elements. They then shortened the list to those they deemed most policy relevant. A tipping element made the short list if tipping it would affect many people and decisions made in the next 100 years could affect whether the element’s tipping point is reached.
For the nine tipping elements that made the cut, the authors estimated the elements’ sensitivity to change and assessed scientists’ knowledge of each system.
Based on these assessments, they concluded that "the greatest (and clearest) threat is to the Arctic" followed by a meltdown of the Greenland Ice Sheet. Scientists know less about the other tipping elements, so these systems are "candidates for surprising society".
Here’s a rundown of the tipping elements highlighted in the report. Just to give a sense of scale for the critical points, global average temperature is already committed to further warming of about 0.6 °C.
Arctic Summer Sea Ice
Critical Point: 0.5 – 2 °C above present (global average)
Expert Assessment: High sensitivity/Small uncertainty
Sea ice extent is already decreasing, but total loss would be a huge change for Arctic ecosystems. For example, polar bears use summer sea ice to hunt. Sea ice loss can also affect temperatures 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".
Greenland Ice Sheet (GIS)
Critical Point: 1 -2 °C above present (global average)
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 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 current models predict.
West Antarctic Ice Sheet (WAIS)
Critical Point: 3 – 5 °C above present (global average)
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.
Atlantic Thermohaline Circulation (THC)
Critical Point: 3 – 5 °C above present (global average)
Expert Assessment: Low sensitivity/Medium uncertainty
Significant slowing or a complete shutoff of circulation patterns in the Atlantic Ocean would affect regional climate patterns, since the THC transports heat from the tropics to Northern Europe. That doesn’t mean Europe’s temperatures would plunge, though – the IPCC estimates that it would simply warm less quickly. The study’s authors call this element "a source of concern", though it presents a "less immediate threat" than other tipping elements.
El Niño – Southern Oscillation
Critical Point: 3 – 6 °C above present (global average)
Expert Assessment: Medium sensitivity/Large uncertainty
This study concluded that El Niño amplitude could increase. Among other effects, stronger El Niño events would 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 involved in this new study disagree, although they note that "the existence and location of any threshold is particularly uncertain".
Indian Summer Monsoon
Critical Point: Albedo above 0.5
Expert Assessment: Large uncertainty
A decrease in monsoon precipitation could spell disaster for India, which depends on the rainfall for irrigation. The control parameter here is not temperature, but reflectivity (also called "albedo", which increases as a surface gets brighter). In this case, land use change and aerosol pollution can increase regional albedo, which weakens the monsoon. The authors of this study argue that the IPCC, which predicted an increase in monsoon precipitation, underestimated the effects of land use change and aerosols.
West African Monsoon
Critical Point: 3 – 5 °C above present (global average)
Expert Assessment: Medium sensitivity/Large uncertainty
In a rare example of a potential benefit from global warming, rainfall in Africa’s Sahara/Sahel could increase, leading to greening.
Amazon Rainforest
Critical Point: 3 -4 °C above present (global average)
Expert Assessment: Medium sensitivity/Large uncertainty
Warming could decrease precipitation and lengthen the Amazonian dry season, leading to forest dieback. Deforestation also leads to regional drying, and the authors warn that land use change alone could bring the forest to a critical threshold.
Boreal Forest
Critical Point: 3 – 5 °C above present (global average)
Expert Assessment: Medium sensitivity/Large uncertainty
Earth’s vast boreal forests cover the northern latitudes of Canada, Alaska, Europe and Russia. They store carbon and support many rare and beautiful species. Boreal regions may become drier as temperatures rise, leaving these forests vulnerable to disease and fire.
Take-home Message
We’re getting ever closer to massive changes in some key Earth systems, and other tipping elements could surprise us. I think the paper gives us nine very clear reasons for cutting emissions as much, and as soon, as possible.
Further Reading
- The PNAS paper is freely available (PDF)
- Chapter 19 of the IPCC's Working Group 2 report summarizes key vulnerabilities and risks from climate change
- You can find more posts on climate change science, policy, and news on Climate 411