Standard climate projections represent future volcanic eruptions by a constant forcing inferred from 1850-2014 volcanic forcing. Using the latest ice-core and satellite records to design stochastic eruption scenarios, we show that there is a 95% probability that explosive eruptions could emit more sulfur dioxide (SO2) into the stratosphere over 2015-2100 than current standard climate projections (i.e., ScenarioMIP). Our simulations using the UK Earth System Model with interactive stratospheric aerosols show that for a median future eruption scenario, the 2015-2100 average global-mean stratospheric aerosol optical depth (SAOD) is double that used in ScenarioMIP, with small-magnitude eruptions (< 3 Tg of SO2) contributing 50% to SAOD perturbations. We show that volcanic effects on large-scale climate indicators, including global surface temperature, sea level and sea ice extent, are underestimated in ScenarioMIP because current climate projections do not fully account for the recurrent frequency of volcanic eruptions of different magnitudes.

A document by Yusuf A. Bhatti. Click on the document to view its contents.

We assess the effective radiative forcing due to ozone-depleting substances using models participating in the Aerosols and Chemistry Model Intercomparison Project (AerChemMIP). A large inter-model spread in this globally averaged quantity necessitates an “emergent constraint” approach whereby we link the radiative forcing to the amount of ozone depletion simulated during 1979-2000, excluding two volcanically perturbed periods. During this period ozone-depleting substances were increasing, and several merged satellite-based climatologies document the ensuing decline of total-column ozone. We use these analyses to come up with effective radiative forcing magnitudes. For all of these satellite climatologies we find an effective radiative forcing outside or on the edge of the previously published “likely” range given by the 5th Assessment Report of IPCC, implying an offsetting effect of ozone depletion and/or other atmospheric feedbacks of -0.4 to -0.25 Wm-2, which is in absolute terms is larger than the previous best estimate of -0.15 Wm-2.

The pre-industrial (Year 1850) to present-day (Year 2014) increase in methane from 808 to 1831 ppb leads to an effective radiative forcing (ERF) of 0.97±0.04 Wm‑2 in the United Kingdom’s Earth System Model, UKESM1. The direct methane contribution is 0.54±0.04 Wm‑2. It is better represented in UKESM1 than in its predecessor due to the inclusion of shortwave absorption, updates to the longwave spectral properties, and no interference from dust. An indirect ozone ERF of 0.13-0.20 Wm-2 is largely due to the radiative effect of the tropospheric ozone increase outweighing that of the stratospheric ozone decrease. An indirect water vapor ERF of 0.07±0.05/0.02±0.04 Wm‑2 is consistent with previous estimates based on the stratospherically-adjusted radiative forcing metric. The methane increase also leads to a cloud radiative effect of 0.12±0.02 Wm‑2 from aerosol-cloud interactions and thermodynamic adjustments. The aerosol-mediated contribution (0.28‑0.30 Wm‑2) arises because methane-driven oxidant changes alter the rate of new particle formation (-8 %), causing a change in the aerosol size distribution towards fewer larger particles. There is a resulting decrease in cloud droplet number concentration and an increase in cloud droplet effective radius. There are additional shortwave and longwave contributions of 0.23 and ‑0.35 Wm-2 to the cloud forcing which are dynamically-driven. They arise from radiative heating and stabilization of the upper troposphere, resulting in a reduction in global cloud cover and convection. These results highlight the importance of chemistry-aerosol-cloud interactions and dynamical adjustments in climate forcing and can explain some of the diversity in multi-model estimates of methane forcing.

Quantifying Effective Radiative Forcing due to aerosol-cloud interactions (ERFACI) remains a largely uncertain process, and the magnitude remains unconstrained in general circulation models. Previous studies focus on the magnitude of ERFACI arising from all cloud types, or examine it in the framework of dynamical regimes. Aerosol forcing due to aerosol-cloud interactions in the HadGEM3-GA7.1 global climate model is decomposed into several global observational cloud regimes. Regimes are allocated to model gridboxes and forcing due to aerosol-cloud interactions is calculated on a regime-by-regime basis with a 20-year meaning period. Patterns of regime occurrence are in good agreement with satellite observations. ERFACI is then further decomposed into three terms, representing radiative changes within a given regime, transitions between different cloud regimes, and nonlinear effects. The total global mean ERFACI is -1.8 Wm-2. When decomposed, simulated ERFACI is greatest in the stratocumulus regime (-0.75 Wm-2).