The impact of greenhouse gases (GHGs)
Increasing atmospheric concentrations of ‘greenhouse gases’ (GHGs), of which carbon dioxide (CO2) is the most important, is a primary cause of anthropogenic climate change.
The global atmospheric CO2 concentration has risen by over a third from pre-industrial levels of about 280 ppm in 1750 to 383ppm in 2007, far exceeding the maximum of the natural range over the past 650,000 years of 300 ppm. Some recent estimates (GCP, 2008) indicate that the global atmospheric CO2 concentration is currently rising at over 2ppm a year.
Aggregate atmospheric GHG concentrations, often measured in carbon dioxide equivalents (CO2e), currently exceed 430ppm CO2e. It is thought that they will have to be stabilised around 450ppm CO2e if global warming is to be limited to below 2°C and dangerous climate change prevented.
Establishing a target stabilisation level
As the extent of warming associated with different atmospheric concentrations of GHGs is uncertain, the target stabilisation level depends upon a balance of expected probabilities and attitudes to risk.
An atmospheric concentration of 750ppm CO2e by 2100 might be a conservative estimate of the consequences of ‘business as usual’ emissions (Stern, 2009, p.26), a level which Hadley Centre models have suggested may be associated with a 9% probability of exceeding a 7°C temperature increase (Stern Review, 2006, Box 8.1, p.220).
The impact of higher temperatures
The higher the temperature increase, the more severe the expected impacts.
Climate sensitivity estimates drawing upon probability density functions from scientific papers covered by the IPCC (2007, Table 9.3) and climate feedback effects might be interpreted as suggesting a 5% probability of a greater than 10°C change in mean global surface temperature and a 1% probability of a greater than 20°C change in around the next 200 years compared to pre-industrial revolution levels if mitigation measures are only gradually introduced.
Weitzman (2009) notes that in either case such rapid warming would lead to mass extinctions and biosphere ecosystem disintegration, destroying life on Earth as we know it.
Forests and carbon: valuation, discounting and risk management (PDF-1168K)
Reviewing methods to value carbon over time, examining approaches for dealing with risk and considering approaches that could be used in extending standards to forestry more generally in voluntary carbon markets in the UK.
For further information contact Gregory Valatin