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PLANETARY BOUNDARIES: THE CO2 <350 PPM UPPER LIMIT OF HUMAN HABITATS

October 13th, 2009

Andrew Glikson

THE RISE OF ATMOSPHERIC CO2 ABOVE 350 PPM AT THE CURRENT RATE OF 2 PPM/YEAR IS TRANSCENDING ENVIRONMENTAL CONDITIONS WHICH ALLOWED THE DEVELOPMENT OF HUMAN CIVILIZATION AND, WITH LAG EFFECTS, IS LEADING TOWARD AN ICE-FREE EARTH AND A MASS EXTINCTION OF SPECIES

Climate change is tracking toward levels which transcend the planetary boundaries which allowed the development of humans over the last 3 million years [1]. These limits have already been crossed in terms of the rise in greenhouse gases (CO2, methane, Nitric oxide) and extensive loss of species [1]. Given lag effects, looming threats include (A) ocean acidification and phosphorous flux, collapse of coral reefs and the marine food chain; (B) availability of freshwater; (C) conversion of natural forests to cropland, i.e. the Amazon; (D) ozone depletion; (E) atmospheric aerosol loading and (F) chemical pollution by metals, plastics, radioactive nuclei etc.

The rate of climate change since the mid-1970s, at up to ~2 ppm CO2 per-year, reaching 388 ppm CO2 and ~460 ppm CO2-equivalent (including methane), is leading toward ~1.5 degrees C mean global temperature rise relative to pre-industrial time. This results in carbon cycle and ice/water feedback processes, with consequent (A) extreme rates of polar ice melting, including the Arctic Sea, Greenland, West and East Antarctica [2], which threatens accelerated sea level rise; (B) a progressive shift of climate zones toward the poles, extending the tropics as indicated by intensified cyclones and floods, and enlarging desert regions as manifested by extreme droughts and fires, including in Australia.

The consequences for human habitats include loss of arable land, fresh water supplies and extreme weather events. The loss of Himalayan snow and thereby decreased river flow, coupled with a failure of the monsoon and sea level rise, threatens more than one billion people in south and southeast Asia. As the polar regions warm [3], the release of methane from the many hundreds of billions of tons of carbon stored in permafrost and shallow lakes and seas, is underway.

Reports by the world’s leading climate research organizations (Hadley-Met, Tyndall, NASA/GISS, Potsdam, NSIDC, CSIRO, BOM), and in thousands of papers in the peer-reviewed scientific literature, demonstrate the anthropogenic origin of climate change since the industrial revolution, accelerating since the mid-1970s, beyond reasonable doubt [4]. The Australian continent, dominated by subtropical arid zones, is in particular danger from extending tropical floods in the north and progressive desertification and fires in the south.

Humans and species can adapt to gradual changes in the environment, and our prehistoric ancestors were able to migrate over much of the world through extreme glacial-interglacial changes. This is not the case with the 6 billion members of present-day civilization, anchored as they are to coastal and valley agricultural lands. The consequences of the extreme rise rate of CO2 at 2 ppm/year will greatly complicate adaptation.

In my view an upper limit of 450 ppm CO2, proposed by a range of reports by government organizations, including the Garnaut Review [5] and the Australian Government White Paper [6], can not be sustained, for the following reasons:

  1. The atmosphere has already transcended the CO2-equivalent (including the forcing of methane) level of 460 ppm.
  2. A level of 450 ppm CO2 is a mere ~40 ppm below the upper boundary of ~500 ppm, which is the upper limit of stability of the Antarctic ice sheet, formed about 34 million years ago. In the Pliocene, 3 million years ago, a CO2 level of 400 ppm led to temperature rise of about 2–3 degrees C and sea level rise of 25+/-12 meters.
  3. There is no evidence that the climate can be “stabilized” at such high level of greenhouse-induced forcing. Due to carbon cycle feedback loops and feedbacks related to ice melt/water interaction, CO2 level of 450 ppm may lead to yet higher greenhouse levels, high temperature levels and possible tipping points.
  4. Not taken into account in many projections are looming emissions of methane, which are already taking place under atmospheric CO2 levels of 388 ppm, or CO2-e levels of 460 ppm.

In the view of leading US climate scientists there is no alternative to attempts at reducing atmospheric CO2 levels to below 350 ppm as soon as possible [8]. In my view, only a combination of (A) deep urgent cuts in carbon emissions; (B) fast-track development of clean renewable energy systems; (C) an intensive reforestation campaign; (D) application of a range of biosequestration measures, including chemical sequestration and carbon draw-down methods, may be able to prevent further carbon cycle and ice melt feedback effects from triggering dangerous tipping points [9] with tragic consequences.

1. Schellnhuber, Oxford meeting, 28-30.10.09 http://www.eci.ox.ac.uk/4degrees/programme.php
2. British Antarctic Survey, 23.9.09
http://www.antarctica.ac.uk/press/press_releases/press_release.php?id=989
3. Polar regions have warmed by a mean of up to 4 degrees Celsius since the mid-20th century (NASA/GISS). http://data.giss.nasa.gov/gistemp/
4. Contrary arguments, by a handful of climate change denialists, are unreferenced or derived by deceptive alteration of scientific data.
5. Garnaut Review. http://www.garnautreview.org.au/domino/Web_Notes/Garnaut/garnautweb.nsf
6. White Paper/CPRS http://www.climatechange.gov.au/whitepaper/index.html
7. Copenhagen Synthesis Report http://www.anu.edu.au/climatechange/content/news/copenhagen-synthesis-report-released-today/
8. Hansen et al. 2008. Target CO2: Where Should humanity aim? http://www.columbia.edu/~jeh1/2008/TargetCO2_20080407.pdf
9. Lenton et al., 2008. Tipping points in the Earth climate system. http://www.sciencedaily.com/releases/2008/02/080204172224.htm

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Andrew Glikson
Earth and paleoclimate scientist
Australian National University

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