Potential Tipping Points from Global Warming
February 24, 2020
(All comments assume mainstream interpretation of mainstream data and models, primarily as expressed by the Intergovernmental Panel on Climate Change, IPCC, www.ipcc.ch .)
The IPCC defines a tipping point, or abrupt climate change, as "a large-scale change in the climate system that takes place over a few decades or less, persists (or is anticipated to persist) for at least a few decades, and causes substantial disruptions in human and natural systems.”
However, beyond noting that tipping points are possible, the discussion about tipping points by the IPCC is generally limited to one section of its lengthy reporting. An article in The Guardian specifically takes the IPCC to task for not mentioning tipping points in its recent report comparing the effects of 1.5⁰ C increase in atmospheric temperature and an increase of 2.0⁰ C.
Tipping points are unlikely; tipping points are difficult to model and predict.
However, the probability of reaching a climate tipping point is greater than zero. And, if the more extreme scenarios of how global warming unfolds are correct, some tipping points discussed in this article become inevitable.
Let’s take a look at the serious discussion that is out there.
In an article in Nature Commentary in 1987, Professor Wally Broecker of Columbia University said that paleoclimate data suggests that the “Earth’s climate does not respond to forcing in a smooth and gradual way. Rather, it responds in sharp jumps which involve large-scale reorganisation of Earth’s system.”
Early in the discussion about tipping points, a group of researchers, led by Tim Lento at Exeter University, warned in a 2008 paper that there were potential tipping points from anthropogenic climate change. They identified two primary tipping points, one each associated with Arctic sea ice and the Greenland ice sheet. They noted that there are five other potential “surprises.” These surprises include significant changes in the Atlantic THC (simplistically, think, Gulf Stream), disintegration of the WAIS (West Antarctic Ice Sheet), Amazon rainforest dieback, dieback of boreal forests, and shift of the El Niño-La Niña cycle (ENSO) to a mean state. These researchers thought that tipping points would arise when global warming exceeded 5⁰ C (9⁰ F) above pre-industrial levels. However, in a recent paper, Lenton led a group that said that some tipping points may already have been reached at 1⁰ C of warming – ie, now.
Since that time, there have been discussions about tipping points in scientific papers and in the media. Much of the discussion in the media is overwrought and borders on hysteria. However, tipping points are entirely possible.
In my book, The Only Ten Things You Need To Know About Global Warming (free copy available at www.tenthingswarming.com ), I said that tipping points were possible, but were generally not included in mainstream discussions. I discussed two possible tipping points, noting that there were others:
James Hansen’s “albedo flip.” Dr. Hansen is a former NASA scientist and is active in the climate change arena. His beliefs are outside the mainstream in that his predictions are generally more urgent and more dire than those in the mainstream. For example, he believes that “the goals of limiting human-made warming to 2⁰ C and CO2 to 450 ppm [parts per million] are prescriptions for disaster.” He believes that a concentration of 350 ppm is the “safe level” (as compared with a concentration of 413 ppm, and increasing, in January 2020, or a concentration of 450 ppm which is generally associated with the 2⁰ C increase.). He attempts to predict the future by comparing it to past conditions. Albedo flip (in this case) is a process by which melting ice results in darker water underneath which reflect less light, including infrared or heat, than the brighter ice did. This warms the water, which warms the atmosphere, creating more melting, exposing more darker water … The result is a much-more rapidly rising of sea levels, creating increases of multiple meters in a century – as much as 5 meters (16.4 feet) - instead of the current rate of about one foot per century. “BAU [business as usual] scenarios result in global warming of the order of 3-6°C. It is this scenario for which we assert that multi-meter sea level rise on the century time scale are not only possible, but almost dead certain.”
The melting of “methane ice.” Very large amounts of methane hydrate (methane is also called “natural gas”) are trapped in sediments of marine continental margins and permafrost areas. Methane is 30 times more potent as a greenhouse gas than CO2, but is absorbed by the environment much more rapidly (say, a decade) than CO2 (perhaps 200 years or more). There is a great deal of uncertainty about how much methane hydrate there is, how much warming would be required to release a meaningful amount and what the effect of a release of methane hydrates might be. An article in World Ocean Review summarizes the state of play: “Today it is assumed that in the worst case, with a steady warming of the ocean of 3 degrees Celsius, around 85 per cent of the methane trapped in the sea floor could be released into the water column. Other, more sensitive models predict that methane hydrates at great water depths are not threatened by warming. According to these models, only the methane hydrates that are located directly at the boundaries of the stability zones would be primarily affected. At these locations, a temperature increase of only 1 degree Celsius would be sufficient to release large amounts of methane from the hydrates. The methane hydrates in the open ocean at around 500 metres of water depth, and deposits in the shallow regions of the Arctic would mainly be affected.” The majority of methane that is released will likely be broken down during its rise. “Rough estimates suggest that anaerobic and aerobic oxidation of methane together currently convert around 90 per cent of the methane produced in the sea floor before it can reach the atmosphere. The more slowly methane migrates through the sea floor or through the water column, the more effective the microbes are in converting it. … No one can yet say with certainty how the methane release in the Arctic will develop with global warming, either in the ocean or on the land. This research is still in its infancy.” The methane that would ultimately make it into the atmosphere would add to the other emissions, including methane emissions from the flaring of oil and natural gas wells. The IPCC concludes, with low confidence, that it is “possible that permafrost will become a net source of atmospheric greenhouse gases,” during the 21st century.
A February 2020 article in CarbonBrief outlined “Nine ‘tipping points’ that could be triggered by climate change,” mostly summarizing a section in a report by the IPCC. The definition used in the article is, “where a small change makes a big difference and changes the state or the fate of a system.” The article notes that there are natural tipping points in the climate system, not induced by global warming, such as ice ages.
Nine “abrupt changes” were summarized by the IPCC as follows (this list is almost, but not quite the same as CabonBrief’s list):
Shutdown of the Atlantic Meridional Overturning Circulation. This is also referred to as the Atlantic THC by Lento, of which the Gulf Stream is a part. The idea is that cooling water from melting Arctic ice would change the dynamics of the system, slowing it down or terminating it altogether, having a significant effect on climate, including cascading tipping points. No one knows what the shutdown point is, although some research has indicated that the circulation has weakened by about 15% since the middle of the 20th century. One expert indicated that “most people think that to trigger a real shutdown would require substantial global warming – like 3C or 4C above pre-industrial levels.” The IPCC’s report on 1.5⁰ C of warming concludes, while “it is very likely that the AMOC will weaken over the 21st century,” there is “no evidence indicating significantly different amplitudes of AMOC weakening for 1.5C versus 2C of global warming, or of a shutdown of the AMOC at these global temperature thresholds.”
Ice sheet collapse. This is the WAIS disintegration referred to by Lento. This chunk of Antarctic ice is based on rock that is under water and therefore subject to compromise by a warming ocean. Ice that is under water, if melted, would not increase sea levels for the reason that the water level in your glass does not rise (much) when the ice cubes in it melt. However, loss of the WAIS could trigger destabilization of land-based ice in Antarctica, which could result in dramatic changes in sea level. There is evidence of current instability in the WAIS. However, effects will likely play out over long timeframes. The IPCC concluded that it is “exceptionally unlikely that either Greenland or West Antarctic ice sheets will suffer near complete disintegration (during the 21st century).”
Permafrost carbon release. This was discussed, above.
Tropical forest dieback. Primarily referring to the Amazon forests, the concern is that tropical forests absorb CO2. If such forests die back, the CO2 that would have been absorbed would remain in the atmosphere, increasing the global warming effect. A dieback in forests would have other effects, such as changing rain cycles and amounts. While the IPCC has “low confidence in the projections of the collapse of large areas of tropical forests,” during the 21st century, political changes in Brazil have led to the burning of large areas of the Amazon forests. It has been projected that deforesting 40% of the Amazon would lead to dieback. A recent study indicates that perhaps 20% of the Amazon has already reached its tipping point. There are concerns that increases in atmospheric temperature will trigger dieback in the forests. Some scientists have projected that it would require 4⁰ C of global warming above pre-industrial levels to trigger degrading of the savannahs and forests. Deforestation and global warming would reinforce each other, exacerbating the dieback from just one of the factors.
West African / Indian monsoon shift. The IPCC also has “Low confidence in projections of a collapse in monsoon circulation” during the 21st century. Basically, global warming would change the atmospheric dynamics in the regions, shifting the area in which the monsoon occurs.
Coral reef die-off. (not included in the IPCC’s list, but included in CarbonBrief’s list.) As CO2 increases in the atmosphere, some of that is absorbed into the ocean. In usual discussion, this is referred to as making the oceans “more acidic,” when it is strictly making the oceans less alkaline. In any event, this lowering of alkalinity / increase in acidity creates conditions under which corals become less healthy and often die. This process, also called “bleaching,” has been seen in coral reefs, worldwide. This process is complex and may be reversed. A 2007 study in Science concluded that atmospheric concentrations “above 500ppm appears extremely risky for coral reefs and the tens of millions of people who depend on them directly, even under the most optimistic circumstances.” The IPCC, in its 1.5⁰ C study, was more pessimistic, concluding, “Even achieving emissions reduction targets consistent with the ambitious goal of 1.5C of global warming under the Paris Agreement will result in the further loss of 70-90% of reef-building corals compared to today, with 99% of corals being lost under warming of 2C or more.”
Boreal forest shift – Boreal forests are found in the cold climates of the northern hemisphere high latitudes, accounting for 30% of the world’s forests. Global warming would place pressure on these forests, including dieback, particularly at the southern edges. The IPCC has “low confidence in projections of the collapse of large areas of boreal forest” during the 21st century.”
Disappearance of summer Arctic sea ice – (mentioned in CarbonBrief under the category of “other tipping points”) the IPCC concludes that it is “likely that the Arctic Ocean becomes nearly ice-free in September before mid-century under high forcing scenarios such as RCP8.5.” Projections by the IPCC are made based on several scenarios of what the future looks like in terms of population, carbon control, energy generation, etc. RCP8.5 is the “worst case” of the model results reported by the IPCC. The loss of Arctic sea ice has no effect on sea levels since Arctic ice is floating in the water.
Long-term droughts. (Not mentioned in CarbonBrief) The IPCC concludes that it has “low confidence in projections of changes in the frequency and duration of megadroughts” during the 21st century.
In summary, many of these tipping points seem unlikely to occur during the 21st century. I highlighted the two I thought were most likely, albeit unlikely, in my book, as I discussed, above. The threats to the Amazon forests and to corals are real and happening in the present.