Assessment of pre-industrial to present-day anthropogenic climate forcing in UKESM1

Abstract

<p><strong>Abstract.</strong> Quantifying forcings from anthropogenic perturbations to the Earth System (ES) is important for understanding changes in climate since the pre-industrial period. In this paper, we quantify and analyse a wide range of present-day (PD) anthropogenic climate forcings with the UK's Earth System Model (ESM), UKESM1, following the protocols defined by the Radiative Forcing Model Intercomparison Project (RFMIP) and the Aerosol and Chemistry Model Intercomparison Project (AerChemMIP). In particular, by quantifying effective radiative forcings (ERFs) that include rapid adjustments within a full ESM, it enables the role of various climate-chemistry-aerosol-cloud feedbacks to be quantified.</p> <p>Global mean ERFs are 1.83, 0.13, −0.33, and 0.93 W m<sup>−2</sup> at the PD (Year 2014) relative to the pre-industrial (PI; Year 1850) for carbon dioxide, nitrous oxide, ozone-depleting substances, and methane, respectively. The PD total greenhouse gas ERF is 2.89 W m<sup>−2</sup>, larger than the sum of the individual GHG ERFs.</p> <p>UKESM1 has an aerosol forcing of −1.13 W m<sup>−2</sup>. A relatively strong negative forcing from aerosol-cloud interactions and a small negative instantaneous forcing from aerosol-radiation interactions are partially offset by a substantial forcing from black carbon absorption. Internal mixing and chemical interactions mean that neither the forcing from aerosol-radiation interactions nor aerosol-cloud interactions are linear, making the total aerosol ERF less than the sum of the individual speciated aerosol ERFs.</p> <p>Tropospheric ozone precursors, in addition to exerting a positive forcing due to ozone, lead to oxidant changes which in turn cause an indirect aerosol ERF, altering the sign of the net ERF from nitrogen oxide emissions. Together, aerosol and tropospheric ozone precursors (near-term climate forcers, NTCFs) exert a global mean ERF of −1.12 W m<sup>−2</sup>, mainly due to changes in the cloud radiative effect. There is also a negative PD ERF from land use (−0.32 W m<sup>−2</sup>). It is outside the range of previous estimates, and is most likely due to too strong an albedo response. In combination, the net anthropogenic ERF is potentially biased low (1.61 W m<sup>−2</sup>) relative to other estimates, due to the inclusion of non-linear feedbacks and ES interactions.</p> <p>By including feedbacks between greenhouse gases, stratospheric and tropospheric ozone, aerosols, and clouds, some of which act non-linearly, this work demonstrates the importance of ES interactions when quantifying climate forcing. It also suggests that rapid adjustments need to include chemical as well as physical adjustments to fully account for complex ES interactions.</p>

Effective radiative forcing and adjustments in CMIP6 models

“…The physical models show ERFs around 0.2 W m −2 greater than the ESMs, a greater IRF and a smaller stratospheric temperature adjustment. Additionally, for UKESM1-0-LL, large and compensating ERFs from CH 4 (+0.93 W m −2 ) and halocarbons (−0.33 W m −2 ), resulting from interactive chemistry, bring the total WMGHG ERF closer to the nochemistry ERFs total from HadGEM3-GC31-LL (O'Connor et al, 2020).…”

“…The inter-model spread is larger, both in relative and absolute terms, in the total anthropogenic forcing than it is for any of its individual components, suggesting that individual models respond very differently to the same combinations of forcing. In the absence of nonlinearities between forcing components, the residual ERF of +0.21 W m −2 from the land use, aerosol and WMGHG components compared to the total anthropogenic would mostly be comprised of ozone forcing, although the sum of individual forcings does not necessarily equal the total forcing in some models (Thornhill et al, 2020;O'Connor et al, 2020). As for the aerosol forcing experiment, there is no significant correlation between total anthropogenic forcing and ECS or TCR.…”

Minimal Climate Impacts From Short‐Lived Climate Forcers Following Emission Reductions Related to the COVID‐19 Pandemic

“…The SLCFs include sulfur dioxide (SO 2 ), nitrogen oxides (NO and NO 2 , which together form NO x ), carbon monoxide (CO), and organic carbon and black carbon (OC and BC, respectively). Such species perturb the oxidant balance of the atmosphere (O'Connor et al, 2020), the ozone budget (Young et al, 2018), and aerosol burden (Karset et al, 2018) and thus the radiative balance of the atmosphere and climate (Myhre et al, 2013). This paper aims to assess how the perturbations to atmospheric composition arising from changes to emissions of SLCFs due to the COVID-19 pandemic affect parameters important for climate.…”

The Impacts of Aerosol Emissions on Historical Climate in UKESM1

“…Contrarily, the positive aerosol IRF (+0.74 W/m 2 ), shown in ECC seems to be due to a stronger absorbing effect of the BC (+2.98 W/m 2 ) than the SO 2 cooling effect (−2.55 W/m 2 ). Thus, the main reason for less effective IRF in ECC is that the cooling effect of scattering aerosols is almost offset by BC warming, as shown by O'Connor et al (2020) [45].…”

“…stronger absorbing effect of the BC (+2.98 W/m ) than the SO2 cooling effect (−2.55 W/m ). Thus, the main reason for less effective IRF in ECC is that the cooling effect of scattering aerosols is almost offset by BC warming, as shown by O'Connor et al (2020) [45]. Lastly, the spatial distribution of aerosol ERFs and the main contributing factors were investigated.…”