The current concentration of carbon dioxide in the atmosphere is of 370 parts per million (ppm). Scientific models indicate that 450 ppm CO2e is the maximum tolerable concentration in order to prevent the rise of global temperatures from exceeding 2 degrees Celsius in comparison with pre-industrial levels. According to the Intergovernmental Panel on Climate Change (IPCC) and the United Nation Framework Convention on Climate Change (UNFCCC), this is the limit to which we should aim for, in order to avoid disastrous consequences of climate change.
However, according to the Stern Review, the stocks of hydrocarbons that are profitable to extract, under current policies, are more than enough to increase the levels of greenhouse-gas (GHG) in the atmosphere well beyond 750ppm CO2e, with very dangerous consequences to the climate system, the society, and the economy.
Without strong coordinated international action to limit GHG emissions, the concentration in the atmosphere will easily exceed 450ppm CO2e, the scenario that gives a 50% chance of avoiding the worst effects of climate change.
Even the Stern Review analysis of the economics of climate change recommends aiming for stabilisation somewhere within the range 450 - 550ppm CO2e. Higher levels would substantially increase the risks of very harmful impacts, while reducing the costs of mitigation by comparatively little. On the other hand, the above mentioned review states that aiming to lower concentrations would impose very high mitigation costs in the short term with small gains, and might not even be feasible given the past delays of the international community in taking strong action against climate change.
Global coordinated action to prevent the world from going beyond the 450 ppm CO2e is needed urgently. It matters a great deal.
"STERN REVIEW: The Economics of Climate Change" available at http://webarchive.nationalarchives.gov.uk/+/http:/www.hm-treasury.gov.uk/sternreview_index.htm