Aerosol and Cloud Particle Microphysics

 

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A huge submarine volcanic eruption on 15 January 2022 – Hunga in the Pacific Ocean, east of Australia, near the dateline – blasted about 150 million tons of water vapor and 1 million tons of SO2 into Earth’s stratosphere. It was much less SO2 than for Pinatubo, and any cooling effect was partly offset by warming from the added stratospheric water vapor (a GHG).16 Nevertheless, we need to estimate the possible effect of Hunga on global temperature in 2022-2023 to see if it had a significant effect on the unprecedented warming that followed. Although it is difficult to disentangle Hunga effect on Earth’s measured energy balance from natural variability (due mainly to cloud variability), analysis shows that aerosol cooling dominated over water vapor warming16 and the net volcanic forcing declined to a small fraction of its peak value by two years after the eruption. We approximate the Hunga forcing based on the Pinatubo forcing (), but with peak forcing −0.3 W/m2. Like the solar forcing, the Hunga forcing is small.

Given that we know precisely the natural climate forcings – volcanic aerosols and solar irradiance – as well as the human-made and natural greenhouse gas forcings, it is obvious that human-made aerosol forcing is the elephant in the climate forcing story that receives too little attention. Aerosol forcing occurs in part from the direct effect of human-made aerosols as they reflect and absorb incoming sunlight, but also from the indirect aerosol effect as the added aerosols modify cloud properties as discussed below. IPCC estimates the indirect aerosol forcing based largely on mathematical models.Footnote18 We suggest that this modeling fails to fully capture the fact that human-made aerosols have a larger impact on clouds when the aerosols are injected into relatively pristine air in places that are susceptible to cloud changes. Later in this paper, we use spatial and temporal changes of climate and Earth’s energy balance to explore this indirect aerosol forcing. First, however, we discuss aerosol and cloud particle microphysics.

Aerosol and Cloud Particle Microphysics

Climate forcing by aerosols depends on aerosol and cloud processes on minute scales. Aerosol composition matters, both for the direct effect of aerosols on radiation and the indirect effect on clouds. Indirect aerosol forcing arises because aerosols are condensation nuclei (tiny sites of water vapor condensation or “cloud seeds”) for cloud drops. More nuclei yield more cloud particles and brighter clouds that reflect more sunlight and cause cooling.Footnote19 More aerosols also increase cloud cover, as shown by cloud trails behind ships (“ship tracks”).Footnote20 Observations to quantify these effects are challenging because human-made aerosols must be distinguished from changes of natural aerosols. Thus, there is large uncertainty in the overall net aerosol forcing.Footnote21,Footnote22

Simultaneous with human-caused aerosol and cloud changes, clouds also change as a climate feedback. [Climate feedbacks – response of the climate system (such as change of clouds or sea ice) to climate change – can be either amplifying or diminishing. Amplifying feedbacks increase climate change, tending to produce instability, while diminishing feedbacks decrease climate change, promoting stability.] Cloud feedback is the main cause of uncertainty in climate sensitivity, the holy grail of climate research.Footnote23 Climate sensitivity is defined as global temperature response to a standard forcing. Observations reveal that the sizes and locations of zones with different characteristic clouds are changing – the intertropical convergence zone (encircling the Earth near the thermal equator) is shrinking, the subtropics are expanding, and the midlatitude storm zone (not near the poles or the equator) is shifting polewardFootnote24 – with associated changes of Earth’s energy balance that constitute potentially powerful, but still inadequately understood, climate feedbacks. Some of the difficulties in climate modeling include cloud microphysics, such as the need to realistically simulate mixed phase (both ice and water) clouds.Footnote25 As cloud modeling has become more complex and realistic, several global climate models have found higher climate sensitivity correlated with more realistic cloud distributions (Sidebar 4).