Here PETER SOMERVILLE provides more detail on how a UK carbon budget could be set, and discusses some problems with the Climate Change Committee (CCC) budgets. This is the second of two articles, the first is this overview of the CCC’s Sixth Carbon Budget
A global carbon budget is the total amount of carbon dioxide emissions that human activities across the world can be allowed to generate, in order to avoid excessive global warming.
Budgets vary, according to the degrees of temperature increase that are judged to be allowable, and according to how sensitive the climate is judged to be in response to carbon emissions: the greater the sensitivity, the smaller the budget has to be.
Unfortunately, we do not know exactly how sensitive the climate is to carbon emissions, so budgets are calculated across the range of possible sensitivities.
The IPCC Special Report on Global Warming of 1.5 degrees provided a range of figures for the remaining global carbon budget in 2018 (Table 2.2 on page 108).
On the basis of the median climate sensitivity, the budget to limit warming to 1.5°C above pre-industrial levels was stated as 580 billion tonnes of carbon dioxide (580 GtCO2). That means the world has a mere 50:50 chance of staying below 1.5°C.
Arguably, however, a higher level of climate sensitivity is required, to give the world at least a 66% chance of reaching the 1.5°C target. At this level, the carbon budget in 2018 was 420 GtCO2.
All economic and other human activity in the world currently emits approximately 40 GtCO2 per year, so the remaining budget today in 2021 is closer to 300 GtCO2. At this rate the budget would be fully spent before 2029.
The task here is then to calculate what might count as a fair share of this budget to be allocated to the UK.
The first problem is that the global budget is for carbon dioxide only: other greenhouse gases (GHGs) such as methane and nitrous oxide are calculated separately.
Methane has minimal long-term effect on the climate, but it is a powerful greenhouse gas in the short-term, which needs to be reduced to zero as soon as possible in order to minimise its contribution to peak warming (see CCC Sixth Carbon Budget report, page 372). Arguably, therefore, a fair carbon budget for the UK should take account of all GHGs.Read the rest of this entry »