Arctic News: The portent of runaway greenhouse warming

Wednesday, December 11, 2019

The portent of runaway greenhouse warming

by Andrew Glikson
Earth and climate scientist
Australian National University

Figure 1. Relations between CO₂ levels in the atmosphere and mass extinctions of genera.

Data cited from D. Royer et al. (2002), from G. Keller (2016) and P. Wignall et al. (2002).

Carbon, the essential element underpinning photosynthesis and life, is transformed into toxic substances in the remnants of plants and organisms buried in sediments. Once released to the atmosphere in the form of CO₂, CO and methane, in large quantities these gases become lethal and have been responsible for mass extinctions of species (Fig. 1).

Figure 2. Potential heating, Carana (2019)

Given amplifying feedbacks from land and oceans triggered by rising temperatures, the concept of an upper limit of warming determined by limitation on carbon emissions alone is unlikely, since, under a rising high greenhouse gas concentration, amplifying feedbacks triggered by methane release, bushfires, warming oceans and loss of reflectivity of melting ice, temperatures would keep rising. As an example, findings show that warmer ocean water is melting hydrates and releasing methane into the sediment and waters off the coast of Washington state, at levels that reach the same amount of methane from the Deepwater Horizon blowout. Carana (2019) finds a potential for abrupt warming of 18°C or 32.4°F (Fig. 2).

Attempts at CO₂ drawdown (sequestration), if urgently applied on a global scale, may conceivably be able to slow down further warming. This article refers to natural methane reservoirs and human-induced methane emissions, indicating that, once temperatures supersede a critical level, a further rise in methane release would result regardless of restrictions of emissions.

According to Kelley (2003) a planetary “runaway greenhouse event” may be triggered when a planet overheats due to absorption of more solar energy than it can give off to retain equilibrium. As a result, the oceans may boil filling its atmosphere with steam, which leaves the planet uninhabitable, as Venus is now. Planetary geologists think there is good evidence that Venus was the victim of a runaway greenhouse effect which turned the planet into the boiling hell we see today. According to Hansen (2010): “If we burn all fossil fuels, the forcing will be at least comparable to that of the PETM, but it will have been introduced at least ten times faster. [. .] The warming ocean can be expected to affect methane hydrate stability at a rate that could exceed that in the PETM, where the rate of change was driven by the speed of the methane hydrate climate feedback, not by the nearly instantaneous introduction of all fossil fuel carbon.” In a critical review of the theory of runaway greenhouse warming, Goldblatt and Watson (2012) state: “We cannot therefore completely rule out the possibility that human actions might cause a transition, if not to full runaway, then at least to a much warmer climate state than the present one.”

The concentration of fossil carbon deposits in the form of coal, oil, natural gas, coal seam gas, permafrost methane, ice clathrates, shale oil, and oil sands, once released to the atmosphere in large quantities, generates powerful feedbacks from land, ocean, atmosphere and cryosphere. This includes further release of greenhouse gases, warming oceans, loss of reflectivity of melting ice, and bushfires, pushing temperatures further upward. With carbon dioxide concentrations rising at a rate of 2–3 parts per million (ppm) per year (October 2018: 406.00 ppm; October 2019: 408.53 ppm) and the Earth heating-up by 0.98°C since 1951-1980, the ultimate consequences of this trend belong to the unthinkable.

Through 2012, total accumulated emissions are estimated to have reached 384 GtC, with an annual amount of 43.1 billion tonnes of carbon dioxide expected to be added in 2019.

A 2016 IPCC analysis found that no more than 275 GtC of the world’s reserves of fossil fuels of 746 GtC could be emitted, if the global temperature rise is to be restricted to 2°C above pre-industrial temperatures, an impossible target since amplifying carbon feedbacks would push temperatures upwards.

According to Heede and Oreskes (2016), global reserves of oil (~171 GtC), natural gas (~95) and coal (479 GtC) add up to a total of 746 GtC. Hansen et al. (2013) estimates that recoverable fossil fuel reserves include ~120 GtC gas, ~80 GtC oil, >10,000 GtC coal, >2000 GtC unconventional gas, and ~700 GtC unconventional oil, adding up to a total of ~13,000 GtC (Fig. 3).

Figure 4. Vulnerable carbon pools. (A) Land: Permafrost ~900 GtC; High-latitude peatlands ~400 GtC;
Tropical peatlands ~100 GtC; Vegetation subject to fire and/or deforestation ~650 GtC;
(B) Oceans: Methane hydrates ~10,000 GtC; Solubility pump ~2700 GtC; Biological pump ~3300 GtC;
Total (A) + (B): ~18,050 GtC (Canadell 2007)

The amount of unstable methane deposits in permafrost and methane hydrates (clathrates) in ocean sediments is of a similar order of magnitude as the amount of fossil fuel reserves. Vulnerable carbon pools include methane hydrates in sediments (~10,000 GtC), solubility and biological pump (~6000 GtC), permafrost methane (~900 GtC), and peatlands and vulnerable vegetation (~1150 GtC), adding up to a total of ~18,050 GtC (Fig. 4).

Unoxidized metastable deposits of methane and methane hydrates, accumulated during the Pleistocene glacial-interglacial cycles and vulnerable to temperature rise, are already leaking as indicated by atmospheric concentrations which have risen from 1988 (~1700 ppb CH₄) to 2019 (~1860 ppb CH₄) at a rate of ~5.2 ppb/year, a rise of more than 4 ppm CO₂-equivalent at GWP25xCO₂ or 24 ppm CO₂-e at GWP150xCO₂.

Meinshausen et al. (2011) estimated global-mean surface temperature increases, applying a climate sensitivity of 3°C per doubling of CO₂, resulting by 2100 in a temperature rise of between 1.5°C to 4.5°C relative to pre-industrial levels. By 2300, under constant emissions, CO₂ concentrations would rise to ~2000 ppm, methane to 3.5 ppm and nitrous oxide to 0.52 ppm (Fig. 5). Amplifying feedbacks are taken into account, but the effects of tipping points and of cold ice-melt pools formed in the oceans near Greenland and Antarctica ice sheets are unclear.

Given the estimated total of exploitable hydrocarbon resources (~13.000 GtC) and of vulnerable carbon pools (~18,050 GtC), the amount released under different future climate conditions is subject to estimates:

Assuming mean global temperature of +2°C (above pre-industrial), with allowance made for the masking effects of sulphur aerosols, the combustion of ~2% of the fossil fuel reserves (13,000 GtC), i.e. ~260 GtC, would raise CO₂ concentration by ~130 ppm (100 GtC = 50 ppm CO₂) (Fig. 3). Combustion of ~5% of the fossil fuel reserve would raise CO₂ concentration by ~325 ppm.
Under +2°C above pre-industrial, release of CO₂ from fires and other feedback effects such as melting of permafrost and release of methane would raise atmospheric carbon by at least 1 percent of vulnerable carbon pools (~18,050 GtC).
The flow of ice melt water from Greenland and Antarctica into the oceans would create large regions of cold water capable of absorption of atmospheric CO₂.

Hansen (2010) concludes: “if we burn all reserves of oil, gas, and coal, there's a substantial chance that we will initiate the runaway greenhouse. If we also burn the tar sands and tar shale, I believe the Venus syndrome [runaway greenhouse warming] is a dead certainty”. Stephen Hawking (2017) appears to agree with Hansen’s warning, stating: “if the US pulls out of the Paris climate agreement it may lead to runaway global warming, eventually turning Earth's atmosphere into something resembling Venus”. Goldblatt and Watson (2012) wrote: “The ultimate climate emergency is a ‘runaway greenhouse’: a hot and water-vapor-rich atmosphere limits the emission of thermal radiation to space, causing runaway warming … This would evaporate the entire ocean and exterminate all planetary life … We cannot therefore completely rule out the possibility that human actions might cause a transition, if not to full runaway, then at least to a much warmer climate state than the present one … However, our understanding of the dynamics, thermodynamics, radiative transfer and cloud physics of hot and steamy atmospheres is weak.”

An analysis by Carana (2013) suggests that accelerated release of methane from permafrost and methane hydrates (clathrates) could trigger runaway global warming (Fig. 6). A polynomial trend for the Arctic shows temperature anomalies of +4°C by 2020, +7°C by 2030 and +11°C by 2040, threatening major feedbacks, further albedo changes and methane releases leading to global temperature anomalies of 20°C+ by 2050.

Figure 6. A polynomial 2 trend line points at global temperature anomalies (Carana 2013). A polynomial function is a function such as a quadratic, a cubic, a quartic, and so on, involving only non-negative integer powers of x.

The magnitude of the runaway greenhouse effect that now threatens to eventuate becomes evident when looking at the geological record. For example, the 55 million years-old PETM event (Paleocene-Eocene Thermal Maximum), lasting for about 100,000 years, driven by CO₂ levels as hugh as 1700 ppm, does not appear to have triggered a runaway greenhouse process. The PETM is attributed to ¹³C-depleted methane (Zeebe et al. 2009), reaching 5 - 8°C and leading to a mass extinction of 35-50% of benthic foraminifera. By sharp contrast, the current Anthropocene hyperthermal event, commencing with the industrial age and re-accelerating since about 1975, constitutes a temporally abrupt development exceeding the rate of geological hyperthermal events (Fig. 7), a rate which does not allow biological adaptation and thereby enhances a mass extinction of species (Barnosky et al. 2011).

Figure 7. A comparison of Cenozoic CO₂ rise rates and temperature rise rates,
highlighting the extreme rise rates in the Anthropocene. From an earlier post.

As Australia burns, the IPCC maintains there is time left to consume a carbon budget and to keep handing out offsets and carbon credits; at the 25th meeting of the Conference of the Parties to the United Nations Convention on Climate Change in Madrid, Australia is seeking to use "carry-over credits" to meet its pledged emissions reductions. The situation is illustrated by Sam Carana in the image below.