Model: IPPR peak and decline emissions model results

IPPR Policy Paper: “Designing emissions pathways to reduce the risk of dangerous climate change”

Summary:

The report estimates the emissions policies necessary to reduce temperature and impacts risks to any chosen level. It finds that:

● Even if CO2 is reduced to 60% below 1990 levels by 2050 there's still a 16-43 % risk of exceeding the 'dangerous threshhold of 2ºC.

● If CO2 is reduced to 80% below 1990 levels by 2050 there's still a 9-26 % risk of exceeding 2ºC

The most widespread interpretation of ‘dangerous climate change’ has been the definition of the ‘2ºC threshold’. The goal of holding global average temperature increase to less than 2ºC above the pre-industrial level has been a stated objective of the European Union, including the UK government, for a number of years.

Based Baer 2005 and Retallack 2005 the Report suggests that the likely and possible consequences of exceeding the 2ºC threshold warrant seeking a high to very high likelihood of staying below the 2ºC threshold. This is interpreted in this Report, quantitatively, as requiring no more than a 10 to 25 per cent likelihood of exceeding the 2ºC threshold.

Our primary focus in this study, then, is to develop estimates of emissions pathways that lead to a ‘peak and decline’ in both CO2 concentrations and in the net equivalent CO2 concentration (including other GHGs) and that have a high likelihood of keeping the average surface temperature below the 2ºC threshold.

The 2006 CO2 concentration is 380ppm.

The report discusses the risk associated with a non peak and decline scenario - the familiar stabilisation scenario in which CO2 concentrations reach 450 ppm and are held at that level indefinitely. In this example the Report finds that the likely risk of exceeding temperature thresholds in the next 200 years are as follows:

● Risk of exceeding 2ºC: between 46 and 85 per cent

● Risk of exceeding 2.5ºC: between 21 and 55 per cent

● Risk of exceeding 3ºC: between 11 and 24 per cent

● Risk of exceeding 3.5ºC: between 4 and 11 per cent

Scenarios in which CO2 concentrations reach 500 or 550 ppm have a correspondingly greater risk of exceeding 2°C: 70-95 per cent and 78-99 per cent respectively.

In the six peak and decline scenarios modelled in th Report, global emissions peak between 2010 and 2014, and the maximum annual rate of emissions reductions – between three and five per cent (depending on the scenario) – are reached between 2015 and 2020. (We discuss the modelling of non-CO2 GHGs and aerosols in the text.) Here we show the results for the highest and lowest of the scenarios and one in between. The results for global CO2 emissions are sobering:

● Peak in 2014, three per cent maximum annual rate of decline, 48 per cent reduction below 1990 levels by 2050: 20-49 per cent risk of exceeding 2ºC.

● Peak in 2010, three per cent maximum annual rate of decline, 57 per cent reduction below 1990 levels by 2050: 16-43 per cent risk of exceeding 2ºC.

● Peak in 2010, five per cent maximum annual rate of decline, 81 per cent reduction below 1990 levels by 2050: 9-26 per cent risk of exceeding 2ºC.

The message should already be clear: while very rapid reductions can greatly reduce the level of risk, it nevertheless remains the case that, even with the strictest measures we model, the risk of exceeding the 2ºC threshold is in the order of 10 to 25 per cent.

What these calculations show is that, if the 2ºC threshold is taken seriously, our situation is indeed very urgent.

In addition to our modelling of global emissions scenarios, we also make estimates of the allowable emissions for the UK under our peak-and-decline scenarios. Assuming that global per capita emissions of CO2 converge no later than 2050, we calculate that the UK’s fair global allocation in 2050 would be in the order of 88 to 94 per cent below 1990 levels, compared with the 60 per cent cuts that have been proposed by the UK government.

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