Figure 2: Relations between CO2 rise rates and mean global temperature rise rates during warming periods, including the Paleocene–Eocene Thermal Maximum, Oligocene, Miocene, glacial terminations, Dansgaard–Oeschger cycles and the post-1750 period.
With CO2 atmospheric residence times in the order of thousands to tens of thousands years, protracted reduction in emissions, either flowing from human decision or due to reduced economic activity in an environmentally stressed world, may no longer be sufficient to arrest the feedbacks.
Four of the large mass extinction of species events in the history of Earth (end-Devonian, Permian–Triassic, end-Triassic, K–T boundary) have been associated with rapid perturbations of the carbon, oxygen and sulphur cycles, on which the biosphere depends, at rates to which species could not adapt.
Since the 18th century, and in particular since about 1975, the Earth system has been shifting away from Holocene (approximately 10,000 years to the pre-industrial time) conditions, which allowed agriculture, previously hindered by instabilities in the climate and by extreme weather events. The shift is most clearly manifested by the loss of polar ice. Sea level rises have been accelerating, with a total of more than 20 cm since 1880 and about 6 cm since 1990.
For a temperature rise of 2.3 oC, to which the climate is committed if sulphur aerosol emission discontinues (see Figure 1), sea levels would reach Pliocene-like levels of 25 meters plus or minus 12 meters, with lag effects due to ice sheet hysteresis (system inertia).
With global atmospheric CO2-equivalents (a value which includes the effect of methane) above 470 ppm (just under the upper stability limit of the Antarctic ice sheet, with the current rate of CO2 emissions from fossil fuel combustion, cement production, land clearing and fires of ~9.7 billion ton of carbon in 2010), global civilization faces the following alternatives:
- With carbon reserves sufficient to raise atmospheric CO2 levels to above 1,000 ppm, continuing business-as-usual emissions can only result in advanced melting of the polar ice sheets, a corresponding rise of sea levels on the scale of meters to tens of meters, on a time scale of decades to centuries, and high-to-extreme continental temperatures rendering agriculture and human habitat over large regions unlikely.
- With atmospheric CO2 at about 400 ppm, an abrupt decrease in carbon emissions may no longer be sufficient to prevent current feedbacks (melting of ice, methane release from permafrost, fires). Attempts to stabilize the climate require global efforts at CO2 draw-down, using a range of methods, including global reforestation, extensive biochar application, chemical CO2 sequestration (using sodium hydroxide, serpentine and new innovations), as well as burial of CO2.
In contrast, retardation of solar radiation through space sunshade technology may allow time for CO2 draw-down. Unlike sulphur dioxide injections, this will not have ocean acidification effects – an effort requiring a planetary defense project by NASA.
Dissemination of ocean iron filings aimed at increasing fertilization by plankton and algal blooms, or temperature exchange through vertical ocean pipe systems, are unlikely to constitute effective means of transporting CO2 to relatively safe water depths.
In contrast to these methods, CO2 sequestration through fast-track reforestation, soil carbon, biochar and possible chemical methods such as “sodium trees” and serpentine (combining Ca and Mg with CO2) may be effective, provided these are applied on a global scale.
Such efforts will require an effective planetary defense effort on the scale currently expended on military spending (totaling more than $20 trillion since WWII).
It is likely that a species which decoded the basic laws of nature, split the atom, placed a man on the moon and ventured into outer space should also be able to develop the methodology for fast sequestration of atmospheric CO2. The alternative, in terms of global heating, sea level rise, extreme weather events, and the destruction of the world’s food sources is unthinkable.
Good planets are hard to come by.