Most discussions around international climate change - the Kyoto Protocol, the Lieberman-Warner Bill introduced on the floor of the US Senate last month, and the G8 discussions taking place this week - include targets for emissions of carbon dioxide (CO2) and other greenhouse gases (GHG). A more direct question was addressed at the Tallberg Forum in Sweden last week: regardless of emissions, how much CO2 can actually accumulate in the Earth's atmosphere without causing enormous and irreversible impacts on human society and the natural environment?

According to the experts at Tällberg, CO2 concentrations in the atmosphere need to be stabilized at 350 parts per million (ppm), lower than both current atmospheric levels (382 ppm) and current stabilization targets set by the European Union (around 400 ppm). In the June 23rd editions of the Financial Times, the International Herald Tribune, the New York Times, and several other media outlets, the Forum launched a campaign to publicize both the target and the critical need of an international agreement to reach it.

Tipping points and Earth's natural boundaries

NASA Chief Scientist James Hansen and eight of his colleagues initially proposed this goal last April in a paper entitled 'Target Atmospheric CO2: Where Should Humanity Aim?' A combination of paleoclimate data and sensitivity analysis leads to their conclusion that 'if humanity wishes to preserve a planet similar to that on which civilization developed and to which life on Earth is adapted...CO2 will need to be reduced from its current 385 ppm to at most 350 ppm.'

According to their new conclusions, in order to restore sea levels, Arctic sea ice extent, glaciers, and other pre-industrial planetary conditions, it may well require even lower levels of CO2 - as low as 300 to 320 ppm - in the long-term. As an initial global target, 350 ppm represents the upper limit of a 'safe zone.' Hansen said he was 99 percent certain about the validity of this target, but that the largest uncertainties arise from the influence of non-CO2 greenhouse gases on the atmosphere.

The forum's leading scientists stressed that the Earth’s climate is a complex system in which interactions can amplify effects and lead to so-called climate 'tipping points': boundaries that, once crossed, can catalyze cascades of non-linear, abrupt changes or slow, long-term, irreversible changes beyond human control.

The Forum emphasized the moral imperative to act on aggressive climate targets and a range of other issues, including ozone depletion, biodiversity, water resources, and land use. All of these systems are approaching boundaries and limitations, but the goal of reducing CO2 emissions remains one of the most challenging.

From 2 Degrees to 350 ppm

The European Union (EU) has established a goal of keeping global temperatures from increasing more than 2ºC above pre-industrial levels. The Intergovernmental Panel on Climate Change (IPCC), in its Fourth Assessment Report, indicated that achieving the EU's 2ºC target will mean stabilizing GHG concentrations in the atmosphere at about 445 to 490 ppm CO2-equivalent. This includes not only carbon dioxide, but rather a host of other greenhouse gases. To achieve a stabilization of 350 ppm CO2, the EU’s current target would need to be reduced by 10 to 20 percent, to 400 ppm CO2-equivalent.