The impact of Earth system feedbacks on carbon budgets and climate response

Abstract

A number of studies have examined the size of the allowable global cumulative carbon budget compatible with limiting twenty-first century global average temperature rise to below 2°C and below 1.5°C relative to pre-industrial levels. These estimates of cumulative emissions have a number of uncertainties including those associated with the climate sensitivity and the global carbon cycle. Although the IPCC fifth assessment report contained information on a range of Earth system feedbacks, such as carbon released by thawing of permafrost or methane production by wetlands as a result of climate change, the impact of many of these Earth system processes on the allowable carbon budgets remains to be quantified. Here, we make initial estimates to show that the combined impact from typically unrepresented Earth system processes may be important for the achievability of limiting warming to 1.5°C or 2°C above pre-industrial levels. The size of the effects range up to around a 350 GtCO budget reduction for a 1.5°C warming limit and around a 500 GtCO reduction for achieving a warming limit of 2°C. Median estimates for the extra Earth system forcing lead to around 100 GtCO and 150 GtCO, respectively, for the two warming limits. Our estimates are equivalent to several years of anthropogenic carbon dioxide emissions at present rates. In addition to the likely reduction of the allowable global carbon budgets, the extra feedbacks also bring forward the date at which a given warming threshold is likely to be exceeded for a particular emission pathway.This article is part of the theme issue 'The Paris Agreement: understanding the physical and social challenges for a warming world of 1.5°C above pre-industrial levels'.

Estimating and tracking the remaining carbon budget for stringent climate targets

“…Unfortunately, all that glitters is not gold. Over the past five years, a plethora of studies have been published 12,[30][31][32][33][34][35][36][37][38][39][40][41][42][43][44] further exploring and estimating the size of carbon budgets while in some way accounting for non-CO 2 climate forcing. These studies most often focus on requirements for holding warming to the internationally agreed 1.5 °C or 2 °C limits [14][15][16] .…”

“…These feedback processes have typically been related to permafrost thawing [40][41][42]85 and the associated long-term release of CO 2 and CH 4 . However, other Earth system feedback sources that can affect remaining carbon budgets have been identified 42 , including changes in vegetation CO 2 uptake linked to nitrogen availability [86][87][88] . If unrepresented feedback results in a direct CO 2 emission from an ecosystem, the translation to the E Esfb term is direct.…”

“…Several estimates also infer the chance of limiting warming to 1.5 °C from ad hoc frequency distributions of model results, instead of from a formal representation of the uncertainty in TCRE. In addition, studies typically do not include all currently identified Earth system feedback, although the impact of some has been described in dedicated studies [40][41][42]85 .…”

“…E Esfb is estimated based on literature that explicitly quantifies the effect of permafrost thawing on additional CO 2 release 40,41,85,93 and that translates the effect of other unrepresented feedback into a CO 2 -equivalent correction 42 . It is estimated to reduce the remaining carbon budget by about 100 Gt CO 2 over the course of the twenty-first century, but this estimate has very low confidence attached to it ( Table 1).…”

The global and regional impacts of climate change under representative concentration pathway forcings and shared socioeconomic pathway socioeconomic scenarios

“…Projections of the increase in global mean surface temperature for each of the RCP forcings ( figure 3(a)) were made using a probabilistic implementation of version 4.2 of the MAGICC energy balance model (Lowe et al 2009, Lowe andBernie 2018). This probabilistic implementation samples across 1863 combinations of feasible values of equilibrium climate sensitivity (ECS), ocean diffusivity and carbon cycle feedback strength to produce 1863 projections of global mean temperature over time (each with a relative probability derived from the probability distributions of the ocean diffusivity and carbon cycle feedback strength parameters).…”