Short-term solar radiation shields such as sulphur aerosols. Space sunshade technology. Dissemination of ocean iron filings aimed at increasing fertilisation by plankton and algal blooms. Temperature exchange through vertical ocean pipe systems. "Sodium trees."
These are some of the options being seriously considered in efforts to restrain human-induced climate change.
An understanding of the scientific knowledge indicates that attempts at returning the climate to anywhere near its original condition will require an effective planetary defence effort on the scale spent on the military -- which has totalled more than $20 trillion since World War II.
This kind of action is not presently occurring. Atmospheric carbon dioxide is triggering amplifying feedbacks, and therefore efforts at reduction in atmospheric CO2 emissions are no longer sufficient to prevent further global warming. Efforts need to be undertaken in an attempt to reduce atmospheric CO2 levels from their current level of near 400 parts per million (ppm) to well below 350 ppm.
The scale and rate of modern climate change have been greatly underestimated. The release of a total of over 560 billion tonnes of carbon through emissions from industrial and transport sources, land clearing and fires, has raised CO2 levels from about 280 ppm in pre-industrial periods to 397-400 ppm, reaching a current CO2 growth rate of about 2 ppm per year
These developments are shifting the Earth’s climate toward Pliocene-like (5.2-2.6 million years ago; mean global temperatures of 2-3 degrees above pre-industrial temperatures) and possibly toward mid-Miocene-like (approximately 16 million years ago; mean global temperatures 4 degrees above pre-industrial temperatures) conditions within a few centuries. That's a geological blink of an eye.
The current CO2 level generates amplifying feedbacks, including the reduced capacity of warming water to absorb CO2 from the atmosphere, released from fires, droughts, loss of vegetation cover, disintegration of methane released from bogs, permafrost and methane-bearing ice particles and methane-water molecules.
With CO2 atmospheric residence times in the order of hundreds to tens of thousands
of 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 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 (from approximately 10,000 years ago) 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 20cm since 1880 and about 6cm since 1990.
For temperature rise of 2.3 degrees, to which the climate is committed if sulphur aerosol emission discontinues
, sea levels would reach Pliocene-like levels of 25 metres plus or minus 12 metres, with lag effects due to ice sheet inertia.
With global atmospheric CO2 equivalent (a value which includes the effect of methane) above 470 ppm, just under the upper stability limit of the Antarctic ice sheet
, with current rate of CO2 emissions from fossil fuel combustion, cement production, land clearing and fires of about 9.7 billion tonnes of carbon in 2010, global civilisation faces the following alternatives:
- With carbon reserves sufficient to raise atmospheric CO2 levels to above 1000 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, abrupt decrease in carbon emissions may no longer be sufficient to prevent current feedbacks (melting of ice, methane release from permafrost, fires). Attempts to stabilise the climate require global efforts 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.
As indicated in Table 1, the use of short-term solar radiation shields such as sulphur aerosols cannot be regarded as more than a band aid, with severe deleterious consequences in terms of ocean acidification and retardation of the monsoon and of precipitation over large parts of the Earth. By 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 defence project by NASA.
Dissemination of ocean iron filings aimed at increasing fertilisation by plankton and algal blooms
, or temperature exchange through vertical ocean pipe systems
, are unlikely to constitute effective means of transporting CO2 to storage at relatively safe water depths.
By contrast to these methods, CO2 sequestration through fast track reforestation, soil carbon, biochar and possible chemical methods such as "sodium trees"
may be effective, provided these are applied on a global scale.