A look into the Anthropocene

The private, non-profit National Research Council issued a report by its Committee on Stabilization Targets for Atmospheric Greenhouse Gas Concentrations entitled “Climate Stabilization Targets: Emissions, Concentrations, and Impacts over Decades to Millennia.” The report announces that we have entered into a new geological epoch, the Anthropocene, during which human activities will largely determine the Earth’s climate. They conclude that carbon dioxide emissions specifically will determine how high peak warming will reach and for how long. The length of the warming will in turn trigger other “amplifying” effects (such as deep ocean warming, which in turn would release carbon based in ocean sediments) further increasing temperatures.

What makes this study interesting is that it takes a relatively long-term view of the effects of the decisions we make today. The discussion of the impact over thousands of years gives a sobering perspective on what is at stake over policy decisions being made (so far by failing to make any) now. The committees approach is to discuss the impacts by degree (Celcius) of temperature increase rather than carbon concentrations. It does this for practical reasons but gives a chart showing the likely range (with 66% confidence) of atmospheric carbon concentration attributable to each degree of average warming increase (p3). A fact, no less stunning for each time it is related, is that carbon dioxide concentrations in the atmosphere have increased by 35% since the dawn of the industrial era in 1750. Although this concentration is the highest on Earth for 800,000 years, the Committee projects that the concentration could double or triple by the end of this century. Because the natural carbon cycle takes so long to remove carbon from the atmosphere, effects of carbon dioxide released into the atmosphere now can have effects lasting 100,000 years.

In the near term (the next few decades and centuries) the committee estimates the following effect for every single degree (ºC) increase of warming:

  • 5-10% less rainfall in SW US, Mediterranean and southern Africa
  • 5-10% more rainfall in Alaska and high altitude areas of the Northern Hemisphere
  • 5-15% decrease yield of corn crops
  • 15% reduction in Arctic sea ice on average and 25% reduction in the month of September
  • 200-400% increase in the areas lost to wildfires in certain parts of western US

With an increase of 3º C, they project a loss of 250,000 square km of wetlands and drylands with serious increases of coastal flooding. A 4º C increase will result on average that 9 out of 10 summers will be warmer than any ever experienced in the 20th century. A 5º C increase brings with it the possibility of doubling food prices world-wide.

For the long term the committee concludes that increased temperatures will take very long times to reverse even after man-made carbon emissions have stopped altogether. It will take an 80% reduction in carbon emissions to stabilize concentrations at any given level. The longer before such a reduction takes place, the more reduction will be necessary to achieved the stabilization.

Over the long term (several millennia) increased atmospheric carbon concentrations could lead to large (4 meter) sea level rises from arctic melts alone. The melting of the Greenland ice sheet could add an additional 7 meter rise (p14). Added to these effects would be the triggering of amplifiers such as the release of carbon from methane hydrates in deep oceans due to deep ocean warming, and the release of organic carbon in soils and permafrosts.

Whatever is done by way of stabilizing carbon concentrations, the effect will not have immediate effect, because of the delay in climate increases. If a certain level of carbon dioxide concentration is stabilized, global warming will continue for several centuries (p15). Among the specific impacts of future climate changes considered are ocean acidification, infrastructure impacts including the increased demand for electricity and other power supplies to offset the warming effect and various ecological impacts.

In the end the committee agrees (pp185-86) with the assessment of the IPCC that:

“There is high confidence that neither adaptation nor mitigation alone can avoid all climate change impacts; however, they can complement each other and together can significantly reduce the risks of climate change. Adaptation is necessary in the short and longer term to address impacts resulting from the warming that would occur even for the lowest stabilization scenarios assessed. Unmitigated climate change would, in the long term, be likely to exceed the capacity of natural, managed and human systems to adapt. The time at which such limits could be reached will vary between sectors and regions.” (Quoting Intergovernmental Panel on Climate Change, “Contribution of Working Groups I, II and III to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change,” in Climate Change 2007: Synthesis Report, edited by R. K. Pachauri and A. Reisinger (NY: 2007), p19).

A pre-publication copy of the 243 page report is available for free download at the website of the Division of Earth & Life Sciences of the National Academies.

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