Ice Sheet Tipping Points
Scientists Fear Irreversible Changes from West Antarctic Ice Sheet Collapse
Most projections of climate change presume that impacts will happen incrementally as greenhouse gas pollution increases and temperatures warm. However, scientists are increasingly studying the presence of “tipping points” in the climate. Pushing global temperatures past these thresholds can trigger irreversible changes even if we do not add any further CO2 to the atmosphere. These changes may be abrupt or may take hundreds of years, but once the threshold is passed they cannot be reversed.
The longer we continue with business-as-usual emissions of greenhouse gas pollution, the greater the risk that we will push parts of our climate system past these tipping points. Now, two new studies (Cole et al. and Joughin et al.) of the West Antarctic Ice Sheet (WAIS) show that we may have already passed a tipping point of crucial importance to the stability of this massive ice sheet.
Ice sheet collapse will have major consequences.
The studies’ revelation comes from new mapping of the bedrock beneath the ice. The West Antarctic Ice Sheet’s glaciers typically flow along the bedrock into the ocean, and melt from below when they come into contact with warmer ocean water. Since the 1930s, the upper 1,000m of the Southern Ocean that surrounds Antarctica has warmed significantly in conjunction with climate warming. Scientists even estimate the subsurface water in the Southern Ocean may be warming faster than any other part of the ocean. Warmer ocean water has led to the recent acceleration of WAIS glacier melt.
When glaciers melt they become lighter and flow faster. Researchers had hypothesized that if hills or obstructions existed in the bedrock beneath the ice, these could snag the glaciers and stabilize the ice sheet. But the new satellite maps show that these hoped-for obstructions are nowhere to be found, meaning the pattern of melt has likely already gone too far to be reversed.
The WAIS contains enough ice to raise sea levels 10 to 14 feet, which has major implications for all of the world’s coastal cities. In New York, New Jersey, and Florida alone, more than $1.2 trillion of property currently lies within the zone that would be flooded. Full collapse is projected to take more than 200 years, but we will see severe impacts long before collapse is complete. For perspective, modern sea levels have risen by just eight inches and the impacts are already proving costly. For more on the science of how climate change is impacting Antarctica, see the Climate Nexus backgrounder.
Tipping points affect IPCC projections.
In the fall of 2013 and spring of 2014, the Intergovernmental Panel on Climate Change (IPCC) released a series of reports detailing the state of scientific knowledge on climate change and projecting future impacts. The IPCC projected a high of three feet of sea level rise by 2100, but this projection did not include a significant contribution from the WAIS. One scientist commented to NBC that future IPCC estimates "will almost certainly be revised, and revised upwards." Instead of a high-end estimate, a rise of three feet by 2100 might end up being the middle-of-the-road projection.
Tipping points also affect other important IPCC findings, a fact that they acknowledge explicitly. For example, the IPCC calculated that an additional temperature increase of 2°C would incur costs of up to 2% of annual global income, but states that this does not include the possibility for tipping points or catastrophic changes (WGII SPM pg.19). Real costs are likely to be higher, and the WAIS collapse may be just the first of many tipping points we pass.
Reinforcing this, the IPCC notes: “The precise levels of climate change sufficient to trigger tipping points… remain uncertain, but the risk associated with crossing multiple tipping points… increases with rising temperature” (WGII SPM pg.14).
Other tipping points will add to impacts.
Other factors besides the collapse of the WAIS have been identified as possible tipping points that could significantly impact our future climate. Uncertainty obscures the exact timing for these events: in some cases we may have already passed the tipping points, while some may be farther in the future. There’s a chance that (just as we’ve now done for the WAIS) we will only definitively confirm the existence of the tipping points after it’s too late. This makes it all the more critical to slow the progress of warming and avoid triggering these dangerous events.
Arctic sea ice collapse: Warmer Arctic temperatures have caused Arctic summer sea ice to plummet over the past decade, far faster than predicted. The 2007 IPCC report estimated that the Arctic might not be ice-free until after 2100, but the 2013 report moved this date to 2050 (WGI SPM pg.25), and many scientists think the ice will collapse still sooner. The loss of Arctic sea ice has serious consequences for the planet, in that ice normally reflects sunlight back into space. Melting ice accelerates warming by replacing a reflective surface (ice) with a dark absorbent surface (ocean water). While the loss of the Arctic summer sea ice is still ongoing, evidence is already emerging that the loss of this ice may have global effects on extreme weather by altering the course of the jet stream.
Greenland ice sheet collapse: It is possible that the warming to date has already passed the tipping point that eventually leads to major losses from the Greenland Ice Sheet. Recent trends show that Greenland’s melt is accelerating, even in parts of the ice sheet once thought to be stable. A 2014 study found that a large region in the northeast of the ice sheet was stable until 2003, then changed abruptly to melting at a rate of 10 gigatons per year. The melt rate is now 15 to 20 gigatons per year and rising. The IPCC has found that the warming threshold for near-complete loss of the Greenland Ice Sheet is somewhere between 1-4°C (1.8-7.2˚F), but the uncertainty is so great that a likely range cannot be established (WGI TS pg.72). It is likely that during the last interglacial period, when the global mean temperature never exceeded 2°C (3.6˚F) above recent pre-industrial conditions, the Greenland Ice Sheet contributed up to 10 feet to sea levels (WGI Ch.13 pg.36). Full melt would contribute sea level rise of about 23 feet (WGI TS pg.72).
Arctic sub-sea methane release: Frozen methane in the shallow shelves of the Arctic Ocean represents an unlikely but potentially catastrophic feedback loop in a warming climate. Methane is a short-lived but extremely potent greenhouse gas that is 25 times more powerful than CO2. While the release of these methane deposits is likely to be slow, the deposits are large and vulnerable as warming continues (WGI Ch.6 pg.65). The amount of methane hydrates in the Arctic could be 10 times greater than the methane presently in the atmosphere (WGI Ch.6 pg.77). It’s likely the melting of frozen methane has been a mechanism in past abrupt warming events when global temperatures spiked by as much as 6.1°C (11˚F). The IPCC estimates the likelihood of a large release of methane to the atmosphere this century at between 0 and 33% (WGI Ch.6 pg.65).
Ecosystem collapses: The National Academy of Sciences has reported that the rate of climate change is now likely as fast as any warming period in the past 65 million years, and it is projected to move even faster in the coming decades (pg.4). With the added pressure of climate change, mass extinction comparable to the extinction that wiped out the dinosaurs could occur before 2100 (pg.4). For example, the World Bank has reported a significant risk that the Amazon rainforest will be lost in an abrupt transition as the climate warms past a tipping point as low as 3°C (5.4˚F), and that the die-off would be permanent (Section 7). In this die-off the Amazon could become a significant source of CO2, creating another feedback loop to drive additional warming.
Release of gases from permafrost: The carbon stored in Arctic permafrost is the single largest terrestrial carbon pool (WGI Ch.6 pg.56). The melting of frozen methane in thawing permafrost is judged likely to have been one of the mechanisms in past abrupt warming events when global temperatures spiked by as much as 6.1°C (11˚F). However, most climate models do not take methane release from permafrost into account (WGI TS pg.103; WGI Ch.6 pg.56). If we pass a tipping point for the release of these stored gases, warming could proceed much faster than models predict.