Abstract
The historical observational record offers a way to constrain the relationship between cumulative carbon dioxide emissions and global mean warming. We use a standard detection and attribution technique, along with observational uncertainties to estimate the all-forcing or ‘effective’ transient climate response to cumulative emissions (TCRE) from the observational record. Accounting for observational uncertainty and uncertainty in historical non-CO2 radiative forcing gives a best-estimate from the historical record of 1.84°C/TtC (1.43–2.37°C/TtC 5–95% uncertainty) for the effective TCRE and 1.31°C/TtC (0.88–2.60°C/TtC 5–95% uncertainty) for the CO2-only TCRE. While the best-estimate TCRE lies in the lower half of the IPCC likely range, the high upper bound is associated with the not-ruled-out possibility of a strongly negative aerosol forcing. Earth System Models have a higher effective TCRE range when compared like-for-like with the observations over the historical period, associated in part with a slight underestimate of diagnosed cumulative emissions relative to the observational best-estimate, a larger ensemble mean-simulated CO2-induced warming, and rapid post-2000 non-CO2 warming in some ensemble members.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'.
Highlights
The use of Earth System Models (ESMs), full complexity climate models which explicitly simulate important biogeochemical cycles in the climate system, have enabled an improved understanding of the link between emissions of climatically important gases and realized climate change [1,2]
Given the large uncertainties in TCRE arising from ESM simulations, this paper investigates to what extent useful information can be derived from the historical record to further inform estimates of the TCRE, and constrain estimates of remaining carbon budgets
We have analysed the implications of the observational record of warming and cumulative CO2 emissions to update previous observational constraints on the TCRE
Summary
The use of Earth System Models (ESMs), full complexity climate models which explicitly simulate important biogeochemical cycles (such as carbon) in the climate system, have enabled an improved understanding of the link between emissions of climatically important gases and realized climate change [1,2]. We compare these observational effective TCRE estimates with those simulated over the historical period in ESMs. In §3b, we discuss inferences about uncertainty in the CO2-only TCRE over the historical period and compare with estimates of ESM-simulated historical CO2-induced and non-CO2-induced. Time-series of both warming and cumulative emissions for all RCP ESM simulations are smoothed with a 10 year centred moving average to remove modes of unforced natural variability and help isolate the externally forced signal (1%/yr ESM simulations are smoothed with a 15 year running mean)
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