Current level and rate of warming determine emissions budgets under ambitious mitigation

Abstract

Some of the differences between recent estimates of the remaining budget of carbon dioxide (CO 2) 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 CO 2-forcing-equivalent emissions. These in turn predict CO 2 emissions budgets if (but only if) the fractional contribution of non-CO 2 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 with a 'likely' (1-standard-error) range of 13-32 years. This provides a simple, transparent, and modelindependent metric of progress towards an ambitious temperature stabilisation goal that could be used to inform the Paris Agreement stocktake process. It is less applicable to less ambitious goals. Alternate definitions of current warming and scenarios for non-CO 2 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. Recent discussion 1,2,3,4 of the "carbon budget" to limit warming to below 1.5°C relative to preindustrial levels highlights the impact of different geophysical constraints on emissions consistent with ambitious mitigation goals, including the definition of "preindustrial", 5 current level of warming, 6 committed warming due to past emissions, 7 interpretation of "exceedance", 8 and contribution of non-CO 2 forcing. 9,10 Many methods compute remaining carbon budgets as a residual, calculating an emission budget relative to some reference period and then subtracting estimated emissions since that time, which does not make optimal use of proximity to the target temperature. Differences in the estimated current level of warming explain discrepancies between budgets based on full-complexity climate models, but not between these budgets and those based on the MAGICC simple modelling system 11,12 which, by expressing temperatures relative to a recent reference period, simulates a current level of warming close to that observed. This has led to the suggestion 13 that the entire carbon budget concept is too uncertain to be relevant to very ambitious mitigation scenarios, and attention should focus instead on the pathway to achieving net zero emissions. This seems paradoxical, because the remaining Threshold Exceedance Budget 14 (TEB, or emissions to the time a

Non-CO2 forcing changes will likely decrease the remaining carbon budget for 1.5 °C

“…Lastly, we wanted to explore the impact of assuming proportionality between CO 2 induced forcing to net non-CO 2 forcing. This is an assumption, made by several recent studies 9,10 , of a constant future ratio between the net non-CO 2 forcing and the CO 2 induced forcing. For this implementation, we used the observed ratio of the net non-CO 2 forcing to the total CO 2 forcing for the 20 years period from 1995 to 2015, which gives a value of 0.26.…”

“…In a recent overview of studies assessing the 1.5°C carbon budget, Rogelj et al 8 showed that 9 out of 14 studies did not use non-CO 2 warming that is consistent with the assumed net-zero CO 2 emissions pathway. This includes some analyses that assumed a proportionality of future CO 2 and non-CO 2 forcing (i.e., that the relative contribution to future warming from non-CO 2 emission remains similar to today) 9,10 , as well as others who prescribed non-CO 2 forcing from one or more representative concentration pathway (RCP) scenarios [11][12][13] . Both approaches are problematic.…”

Cross-cutting Issues

“…The current GHG emission rate, if it persists, will result in continuation of the current rate of global temperature increase of ~0.2°C per decade (e.g. Haustein et al 2017), crossing the 1.5°C Paris Agreement target by the 2040s (Leach et al 2018).…”

Why setting a climate deadline is dangerous

“…5 o C becomes 'likely'. For example, Leach et al[11] introduced a new metric-an 'adaptation/mitigation timescale'-to capture this thinking, i.e. calculating the remaining time until a given temperature target is exceeded if the current rate of warming continues.…”

“…calculating the remaining time until a given temperature target is exceeded if the current rate of warming continues. Instead of inferring from carbon budgets estimated by model simulations, Leach et al[11] used observational data alone, an approach claimed to be more scientifically rigorousthan relying on models (see also ref. 12).…”