Beyond climate tipping points
Greenhouse gas levels exceed the stability limit of the
Greenland and Antarctic ice sheets
Greenhouse gas levels exceed the stability limit of the
Greenland and Antarctic ice sheets
by Andrew Glikson
Abstract
The pace of global warming has been grossly underestimated. As the world keeps increasing its carbon dioxide (CO₂) emissions, rising in 2018 to a record 33.1 billion ton of CO₂ per year, the atmospheric greenhouse gas level has now exceeded 560 ppm (parts per million) CO₂-equivalent, namely when methane and nitrous 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 human 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 CO₂ levels (measured at Mauna Loa, Hawaii) reached 414.66 ppm, growing at a rate of 3.42 ppm/year, well above the highest rate recorded for the last 65 million years. The total CO₂, methane (CH₄) and nitrous oxide (N₂O) expressed as CO₂-equivalents has reached at least 560.3 ppm (Table 1) (at a very low forcing value for methane ¹), the highest concentration since 34 - 23 Million years ago, when atmospheric CO₂ ranged between 350 and 500 ppm.
Table 1. Total atmospheric CO2e from CO2, CH4 and N2O
CO2
|
CO2 rate
|
CH4
|
CH4 rate
|
N2O
|
414.66 ppm
|
3.42 ppm/year
|
1865.4 ppb
|
9.2 ppm/year
|
332ppb
|
CO2 ppm
rise/year
|
CH4 ppb
rise/year
|
N2O forcing = 298 CO2e
| ||
CO2 ppm
414.7
|
(equivalent)
|
N2O ppm forcing
0.332 x 298 = 99 ppm CO2e
(equivalent)
|
Total CO2e: 414.7+46.6+99 = >560.3 ppm CO2e
¹A methane forcing value of 25 x CO2 is very low. Higher forcing values are more appropriate.Plus: SF₆, CHF3, CH2F2, CF4, C2F6, C3F8, C4F10, C4F8, C5F12, C6F1
Figure 1. Projected CO₂ levels for IPCC emission scenarios |
The current rise of the total greenhouse gas levels to at least 560 ppm CO₂-equivalent, twice the pre-industrial CO2 level of 280 ppm, implies that global warming has potentially reached +2°C to +3°C above pre-industral 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 +1.5°C (Berkeley Earth institute).
The threshold for 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 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 closely correlates with the temperature peak at ~16 million tears ago (Figures 2 and 5), when the Greenland ice sheet did not exist and large variations affected the Antarctic ice sheet (Gasson et al., 2016).
Figure 2. Updated Cenozoic pCO₂ and stacked deep-sea benthic foraminifer oxygen isotope curve for 0 to 65 Ma (Zachos et al., 2008) converted to the Gradstein timescale (Gradstein et al., 2004). ETM2 = Eocene Thermal Maximum 2, PETM = Paleocene/Eocene Thermal Maximum. |
Transient melt events
As the glacial sheets disintegrate, cold ice-melt water flowing into the ocean ensue 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 the north Atlantic Ocean south of Greenland and in the Southern Ocean fringing Antarctica (Figures 3 and 4). 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 “Beast from the East event”.
Warming of +3°C to +4°C above pre-industrial levels, leading to enhanced ice-sheet melt, would raise sea levels by 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 the amplifying feedbacks, prolonged Greenland and Antarctic melting (Figures 3 and 4) and a consequent freeze event may ensue, lasting perhaps as long as two to three centuries.
Figure 3. (A) Global warming map (NASA 2018). Note the cool ocean regions south of Greenland and along the Antarctic. Credits: Scientific Visualization Studio/Goddard Space Flight Center; (B) 2012 Ocean temperatures around Antarctica, (NASA 2012). |
21st–23rd centuries uncharted climate territory
Modelling of climate trends for the 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 water 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:
- The albedo-flip in melting sea ice and ice sheets and the increase of the water surface area and thereby the sequestration of CO₂. Hudson (2011) estimates a rise in radiative forcing due to removal of Arctic summer sea ice of 0.7 Watt/m², a value close to the total of methane release since 1750.
- Reduced ocean CO₂ intake due to lesser solubility of the gas with higher temperatures.
- 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 a differential feedbacks from vegetation as the globe warms (Notaro et al. 2007).
- An increase in wildfires, releasing greenhouse gases.
- Release of methane from permafrost, bogs and sediments and other factors.
Figure 4. Model Surface-air temperature (°C) for 2096 relative to 1880–1920 (Hansen et al. 2016). The projection portrays major cooling of the North Atlantic Ocean, cooling of the circum-Antarctic Ocean and further warming in the tropics, subtropics and the interior of continents, including Siberia and Canada. |
Summary and conclusions
- Global greenhouse gases have reached a level exceeding the stability threshold of the Greenland and Antarctic ice sheets, melting at an accelerated rate.
- The current growth rate of atmospheric greenhouse gas of 3.42 ppm CO₂/year is the fastest recorded for the last 55 million years.
- Allowing for the transient albedo enhancing effects of sulphur dioxide and other aerosols, mean global temperature has reached about 2 degrees Celsius above pre-industrial temperatures.
- Due to hysteresis the large ice sheets outlast their melting temperatures.
- Cold ice melt water flowing from the ice sheets at an accelerated rate will reduce the temperature of large ocean tracts in the North Atlantic and circum-Antarctic. Strong temperature contrasts between cold polar-derived air and water masses and tropical air and water masses would result in extreme weather events, retarding agriculture in large parts of the world.
- Humans will survive in relatively favorable parts of Earth, such as sub-polar regions and sheltered mountain valleys, where hunting of surviving fauna may be possible.
- In the wake of partial melting of the large ice sheets, the Earth climate would shift to polarized conditions including reduced polar ice sheets and tropical to super-tropical regions such as existed in the Miocene (5.3 - 23 million years ago) (Figure 5).
Andrew Glikson |
Dr Andrew Glikson
Earth and climate scientist
Australian National University
Canberra, Australian Territory, Australia
Australian National University
Canberra, Australian Territory, Australia
geospec@iinet.net.au
Books:
The Archaean: Geological and Geochemical Windows into the Early Earth
The Asteroid Impact Connection of Planetary Evolution
Asteroids Impacts, Crustal Evolution and Related Mineral Systems with Special Reference to Australia
Climate, Fire and Human Evolution: The Deep Time Dimensions of the Anthropocene
The Plutocene: Blueprints for a Post-Anthropocene Greenhouse Earth
Evolution of the Atmosphere, Fire and the Anthropocene Climate Event Horizon
From Stars to Brains: Milestones in the Planetary Evolution of Life and Intelligence
From Stars to Brains: Milestones in the Planetary Evolution of Life and Intelligence |
The Plutocene: Blueprints for a Post-Anthropocene Greenhouse Earth |
Added below is a video with an August 6, 2019, interview of Andrew Glikson by Guy McPherson and Kevin Hester, as edited by Tim Bob.