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

Abstract

Abstract. Quantifying forcings from anthropogenic perturbations to the Earth system (ES) is important for understanding changes in climate since the pre-industrial (PI) period. Here, we quantify and analyse a wide range of present-day (PD) anthropogenic effective radiative forcings (ERFs) 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, quantifying ERFs that include rapid adjustments within a full ESM enables the role of various chemistry–aerosol–cloud interactions to be investigated. Global mean ERFs for the PD (year 2014) relative to the PI (year 1850) period for carbon dioxide (CO2), nitrous oxide (N2O), ozone-depleting substances (ODSs), and methane (CH4) are 1.89 ± 0.04, 0.25 ± 0.04, −0.18 ± 0.04, and 0.97 ± 0.04 W m−2, respectively. The total greenhouse gas (GHG) ERF is 2.92 ± 0.04 W m−2. UKESM1 has an aerosol ERF of −1.09 ± 0.04 W m−2. A relatively strong negative forcing from aerosol–cloud interactions (ACI) and a small negative instantaneous forcing from aerosol–radiation interactions (ARI) from sulfate and organic carbon (OC) are partially offset by a substantial forcing from black carbon (BC) absorption. Internal mixing and chemical interactions imply that neither the forcing from ARI nor ACI is linear, making the aerosol ERF less than the sum of the individual speciated aerosol ERFs. Ozone (O3) precursor gases consisting of volatile organic compounds (VOCs), carbon monoxide (CO), and nitrogen oxides (NOx), but excluding CH4, exert a positive radiative forcing due to increases in O3. However, they also lead to oxidant changes, which in turn cause an indirect aerosol ERF. The net effect is that the ERF from PD–PI changes in NOx emissions is negligible at 0.03 ± 0.04 W m−2, while the ERF from changes in VOC and CO emissions is 0.33 ± 0.04 W m−2. Together, aerosol and O3 precursors (called near-term climate forcers (NTCFs) in the context of AerChemMIP) exert an ERF of −1.03 ± 0.04 W m−2, mainly due to changes in the cloud radiative effect (CRE). There is also a negative ERF from land use change (−0.17 ± 0.04 W m−2). When adjusted from year 1850 to 1700, it is more negative than the range of previous estimates, and is most likely due to too strong an albedo response. In combination, the net anthropogenic ERF (1.76 ± 0.04 W m−2) is consistent with other estimates. By including interactions between GHGs, stratospheric and tropospheric O3, aerosols, and clouds, this work demonstrates the importance of ES interactions when quantifying ERFs. It also suggests that rapid adjustments need to include chemical as well as physical adjustments to fully account for complex ES interactions.

Atmospheric methane and nitrous oxide: challenges alongthe path to Net Zero

“…The recent growth in methane has been accompanied by a concurrent isotopic trend in the global methane burden, now sustained for well over a decade, towards more negative δ 13 steadily more positive due to anthropogenic emissions from anthracitic coal use, oil and gas, and fires. But since 2007, concurrently with the sharp recent rise in the methane burden (figure 2), atmospheric δ 13 C CH4 has steadily been shifting negative (figure 3) [4,58].…”

The Evaluation of the North Atlantic Climate System in UKESM1 Historical Simulations for CMIP6

“…Finally, although we highlight some deficiencies in UKESM1, it is important to emphasize that the representation of the North Atlantic climate system is largely comparable to GC3.1, the physical climate model that forms the basis of UKESM1. Given UKESM1 was specifically developed to include as many prognostic cross‐domain and cross‐phenomena couplings as possible (e.g., Archibald et al., 2019; Mulcahy et al., 2018; O'Connor et al., 2020; Sellar et al., 2019), and thereby increasing the model degrees of freedom, this similarity of performance is encouraging. A priority for future development of UKESM1 is to better constrain such cross‐domain and phenomena coupling, using observations and detailed process models.…”

U.K. Community Earth System Modeling for CMIP6

“…Factors contributing to these discrepancies are briefly discussed in Andrews et al. (2020) with the role of aerosols being important (e.g., Archibald et al., 2019; Mulcahy et al., 2019; O'Connor et al., 2020).…”