Evidence from ice core data indicates that the current concentration of atmospheric carbon dioxide (CO2) is unprecedented in the past 800,000 years, with data from boron-isotope ratios in ancient planktonic shells suggesting that it is likely to be at its highest level for about 23 million years.
Rise of carbon emissions
Anthropogenic carbon emissions rose by 70% between 1970 and 2004, from 29 to 49 thousand million tonnes of carbon dioxide equivalent (GtCO2e) per year, with global emissions rising by 3% a year since 2000.
The current atmospheric concentration of CO2 of over 390 parts per million (ppm), which is around two-fifths higher than the pre-industrial level of about 280 ppm, is currently rising at an annual rate around 2 ppm.
As atmospheric CO2 concentrations have increased over the past 150 years, the mean global temperature has risen. In the absence of new policy action, annual world greenhouse gas (GHG) emissions could rise by a further 70% by 2050, and lead to a rise of 4°C, or possibly 6°C, above the pre-industrial global mean temperatures by the end of the century, with greater temperature rises likely in some regions, including the Arctic.
Impact of rising global temperature
Likely adverse impacts associated with exceeding a 1.5-2.5°C temperature increase include increased risk of extinction of around 20-30% of plant and animal species, with many millions more people expected to be at risk of floods due to sea level rise by the 2080s.
Warming could lead to positive feedbacks that magnify temperature changes. These could include potential dieback of Amazon rainforest if warming exceeds 3°C. Thawing of the permafrost and subsequent soil decomposition could lead to the further release of up to 380 GtCO2e under a high warming (7.5°C increase) scenario by the end of the century.
Recent evidence shows that warming of the Arctic is occurring faster than had been predicted, with sea level rising more rapidly than expected.
Preventing dangerous climate change
In order to prevent ‘dangerous climate change’, international agreements reached at Cancun and under the Copenhagen Accord call for limiting the average global temperature rise to no more than 2°C above pre-industrial levels, with consideration of adopting a limit of 1.5°C. To be confident of limiting the mean global temperature rise to between 2°C to 2.4°C, is thought to require stabilisation of atmospheric GHG concentrations in the 445 ppm to 490 ppm range, with reductions in annual global carbon emissions occurring no later than 2015, and emissions 50-85% below 2000 levels by 2050.
However, some scientists have argued that even the existing GHG atmospheric concentration, which, including the effect of other GHGs, is equivalent to around 430 ppm CO2e, is too high for the temperature rise to stay below the 2°C threshold.
Some have argued that the increase in atmospheric GHGs since pre-industrial times to date probably commits the world to a warming of 2.4°C (1.4°C to 4.3°C) above the pre-industrial level during the current century, and have recommended a rapid reduction from the current concentration by around 10%.
Further information on climate change is available on the Met Office website.