TAG 166 - Geological Society of Australia

Special Report 3Global warming and geoengineering– an Earth Science perspectiveThe scale and rate of modern global warming have beenunderestimated. The release to date of a total of over500 billion tonnes (GtC) of carbon through emissions, landclearing and fires has raised CO 2 levels to 397–400 ppm and near470 ppm CO 2 -e (a value including methane) at a rate of about 2 ppmCO 2 per year (IPCC AR4; Global Carbon Project; State of the PlanetDeclaration; undated).This is shown in the two figures accompanying this article. Thesedevelopments are shifting the Earth’s climate toward Pliocene-like(5.2–2.6 Ma, +2–3°C) conditions and possibly mid-Miocene-like(ca 16 Ma; +4°C) conditions (Zachos, 2001; Beerling & Royer, 2011;PRISM USGS Pliocene Project, undated), within a couple of centuries— a geological blink of an eye.The current CO 2 level generates amplifying feedbacks from theice–water transformation and albedo loss; from methane releasefrom permafrost, methane clathrates and bogs; from droughtsand loss of vegetation cover; from fires; and from reduced CO 2sequestration by warming water.With CO 2 atmospheric residence times in the order of thousandsto tens of thousands years (Eby et al, 2008), protracted reduction inemissions, either flowing from human decision or due to reducedeconomic activity in an environmentally stressed world, may nolonger be sufficient to arrest the feedbacks.Four of the large mass extinction events in the history of Earth(end-Ordovician, end-Devonian, Permian–Triassic, end-Triassic,K–T boundary) have been associated with rapid perturbations of thecarbon, oxygen and sulfur cycles on which the biosphere depends,at rates to which species could not adapt (Keller, 2005; Glikson,2005; Ward, 2007).Mean CO 2 level from ice cores, Mauna Loa observatory and marine sites. Inset — Climate forcing 1880–2003 (Hansen et al, 2011). Aerosol forcingincludes all aerosol effects, including indirect effects on clouds and snow albedo. Greenhouse gases (GHGs) include O 3 and stratospheric H 2 O inaddition to well-mixed GHGs.36 |TAG March 2013