Beyond climate tipping points: greenhouse gas levels exceed the stability limit of the Greenland and Antarctic ice sheets

Jun 27, 2019

The pace of global warming has been grossly underestimated. As the world keeps increasing its carbon emissions, rising in 2018 to a record 33.1 billion ton CO2 per year, the atmospheric greenhouse gas level has now exceeded 560 ppm (parts per million) CO2equivalent, namely when methane and nitric oxide are included. This level surpasses the stability threshold of the Greenland and Antarctic ice sheets. The term “climate change“ is thus no longer appropriate, since what is happening in the atmosphere-ocean system, accelerating over the last 70 years or so, is an abrupt calamity on a geological dimension threatening nature and civilization. Ignoring what the science says, the powers-that-be are presiding over the sixth mass extinction of species, including humanity.

As conveyed by leading scientists “Climate change is now reaching the end-game, where very soon humanity must choose between taking unprecedented action or accepting that it has been left too late and bear the consequences” (Prof. Hans Joachim Schellnhuber) …“We’ve reached a point where we have a crisis, an emergency, but people don’t know that … There’s a big gap between what’s understood about global warming by the scientific community and what is known by the public and policymakers” Prof. James Hansen.

Rising greenhouse gases and temperatures

By May 2019 the level of CO2 (measured at Mouna Loa, Hawaii) has reached 414.66 ppm, growing at a rate of 3.42 ppm/year, well above the highest growth rate recorded for the last 56 million years. The total of CO2, methane (CH4) and Nitric oxide (N2O), expressed as CO2-equivalents, has reached at least 563 ppm (depending on the greenhouse forcing value of methane), the highest concentration since 34-23 million years ago, when atmospheric CO2 ranged between 300-530 ppm.

The current rise of the total greenhouse gas level to at least 560 ppm CO2-equivalent, twice the pre-industrial level or 280 ppm, implies global warming has potentially reached +2oC to +3oC above pre-industrial temperature. Considering the mitigating albedo/reflection effects of atmospheric aerosols, including sulphur dioxide, dust, nitrate and organic carbon, the mean rise of land temperature exceeds +1.5oC (Berkeley Earth Institute).   

The threshold of collapse of the Greenland ice sheet is estimated in the range of 400-560 ppm CO2 at approximately 2.0-2.5 degrees Celsius above pre-industrial temperatures, and is retarded by hysteresis (where a physical property lags behind changes in the effect causing it). The threshold for the breakdown of the West Antarctic ice sheet is similar. The greenhouse gas level and temperature conditions under which the East Antarctic ice sheet formed about 34 million years ago are estimated as ~800–2000 ppm at 4 to 6 degrees Celsius above pre-industrial values. Based mainly on satellite gravity data there is evidence the East Antarctic ice sheet is beginning to melt in places (Jones, 2019), with ice loss rates of approximately 40 Gt/y (Gigaton of ice per year) in 1979–1990 and up to 252 Gt/y in 2009–2017 (Rignot et al., 2019).

The cumulative contribution to sea-level rise from Antarctic ice melt was 14.0 ± 2.0 mm since 1979. This includes 6.9 ± 0.6 mm from West Antarctica, 4.4 ± 0.9 mm from East Antarctica, and 2.5 ± 0.4 mm from the Antarctic Peninsula (Rignot et al., 2019). Based on the above, the current CO2-equivalent level of at least 560 ppm is close to the temperature peak at ~16 million years ago, when the Greenland ice sheet did not exist and large variations affected the Antarctic ice sheet (Glasson et al., 2016).

Transient melt events

As the glacial sheets disintegrate, cold ice-melt water flowing into the ocean results in large cold water pools, a pattern recorded through the glacial-interglacial cycles of the last 450,000 years , manifested by the growth of cold regions in north Atlantic Ocean south of Greenland  and in the Southern Ocean fringing Antarctica.  The warming of the Arctic is driven by the ice-water albedo flip (where dark sea-water absorbing solar energy alternate with high-albedo ice and snow) and by the weakening of the polar boundary and jet stream. Penetration of Arctic-derived cold air masses through the weakened boundary results in extreme weather events in North America, Europe and northern Asia, such as the recent “Beast from the East” event.

Warming of +3oC to +4oC above pre-industrial levels, leading to enhanced ice-sheet melt, would raise sea levels by at least 2 to 5 meters toward the end of the century, and likely by 25 meters in the longer term. Golledge et al. (2019) show meltwater from Greenland will lead to substantial slowing of the Atlantic overturning circulation, while meltwater from Antarctica will trap warm water below the sea surface, increasing Antarctic ice loss. The effects of ice sheet-melt waters on the oceans were hardly included in IPCC models. Depending on amplifying feedbacks, prolonged Greenland and Antarctic melting and a consequent freeze event may ensue, lasting perhaps as long as two to three centuries.

21st–23rd centuries’ uncharted climate territory

Modelling of climate trends for 2100-2300 by the IPCC AR5 Synthesis Report, 2014 portrays predominantly linear models of greenhouse gas rise, global temperatures and sea levels. These models however appear to take little account of amplifying feedbacks from land and ocean and of the effects of cold ice-melt on the oceans. According to Steffen et al. (2018) “self-reinforcing feedbacks could push the Earth System toward a planetary threshold” and “would lead to a much higher global average temperature than any interglacial in the past 1.2 million years and to sea levels significantly higher than at any time in the Holocene”.

Amplifying feedbacks of global warming include:

  1. The albedo-flip of melting sea ice and ice sheets and the increase of the water surface area and thereby sequestration of CO2. Hudson (2011) estimates a rise in radiative forcing due to removal of Arctic summer sea ice as 0.7 Watt/m2, a value close to the total of methane release since 1750.
  2. Reduced ocean CO2 intake due to lesser solubility of the gas with higher temperatures.
  3. Vegetation desiccation and loss in some regions, and thereby reduced evaporation with its cooling effect. This factor and the increase of precipitation in other regions lead to differential feedbacks from vegetation as the globe warms (Notary et al. 2007).
  4. An increase in wildfires, releasing greenhouse gases.
  5. Release of methane from permafrost, bogs and sediments and other factors.

To summarize, linear temperature models do not appear to take into account the effects of ice-melt water flowing from the large ice sheets into the oceans.  This includes the possibility of a major freeze event such as has already commenced in ocean regions fringing Greenland and Antarctica. In the shorter term sea level rises may be caused by melting of the Greenland ice sheet (6-7 meter sea level rise) and West Antarctic ice sheet melt (4.8 meter sea level rise). Referring to major past melt events, a prolonged breakdown of parts of the Antarctic ice sheet could result in major sea level rise and extensive cooling of northern and southern latitudes (Hansen et al.. 2016). The clash between polar-derived cold weather fronts and tropical air masses is bound to lead to a rise in extreme weather events, echoed in Storms my grandchildren (Hansen, 2010).

Andrew Glikson is an earth and paleo-climate scientist.

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