Dr Nicholas Leach

Abstract:

Some of the differences between recent estimates of the remaining budget of carbon dioxide (CO2) emissions consistent with limiting warming to 1.5 °C arise from different estimates of the level of warming to date relative to pre-industrial conditions, but not all. Here we show that, for simple geometrical reasons, the combination of both the level and rate of human-induced warming provides a remarkably accurate prediction of remaining emission budgets to peak warming across a broad range of scenarios, if budgets are expressed in terms of CO2-forcing-equivalent emissions. These in turn predict CO2 emissions budgets if (but only if) the fractional contribution of non-CO2 drivers to warming remains approximately unchanged, as it does in some ambitious mitigation scenarios, indicating a best-estimate remaining budget for 1.5 °C of about 22 years’ current emissions from mid-2017, with a ‘likely’ (1 standard error) range of 13–32 years. This provides a simple, transparent and model-independent metric of progress towards an ambitious temperature stabilization goal that could be used to inform the Paris Agreement stocktake process. It is less applicable to less ambitious goals. Alternative definitions of current warming and scenarios for non-CO2 drivers give lower 1.5 °C budgets. Lower budgets based on the MAGICC simple modelling system widely used in integrated assessment studies reflect its relatively high simulated current warming rates.

Abstract:

The relationship between cumulative CO2 emissions and CO2-induced warming is determined by the Transient Climate Response to Emissions (TCRE), but total anthropogenic warming also depends on non-CO2 forcing, complicating the interpretation of emissions budgets based on CO2 alone. An alternative is to frame emissions budgets in terms of CO2-forcing-equivalent (CO2-fe) emissions – the CO2 emissions that would yield a given total anthropogenic radiative forcing pathway. Unlike conventional ‘CO2-equivalent’ emissions, these are directly related to warming by the TCRE and need to fall to zero to stabilise warming: hence CO2-fe emissions generalise the concept of a cumulative carbon budget to multi-gas scenarios. Cumulative CO2-fe emissions from 1870-2015 inclusive are found to be 2900 ± 600GtCO2-fe, increasing at a rate of 67 ± 9.5GtCO2-fe/year. A TCRE range of 0.8–2.5° Cper 1,000 GtC implies a total budget for 0.6° C of additional warming above the present decade of 880–2,750 GtCO2-fe, with 1,290 GtCO2-fe implied by the CMIP5 median response, corresponding to 19 years' CO2-fe emissions at the current rate.