Given the importance of aerosol climate forcing and climate sensitivity

 

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Given the importance of aerosol climate forcing and climate sensitivity,Footnote28 there is a crying need for global monitoring of aerosol and cloud particle microphysics and cloud macrophysicsFootnote29 to help sort out climate forcings and feedbacks.Footnote30 Global monitoring of aerosol/cloud microphysical properties and cloud macrophysics from a dedicated small satellite mission has been proposed, but not implemented.Footnote31 The need for such data will only increase in coming decades as the world recognizes growing consequences of climate change and tries to chart a course to restore Holocene-level global climate. NASA’s 2024 PACE satellite missionFootnote32 includes polarimeters capable of measuring aerosol and cloud microphysics including aerosol and cloud droplet number concentrations, which could be a step toward a dedicated, long-term aerosol mission to monitor global aerosol and cloud properties as required to calculate climate forcings and feedbacksFootnote33 (analogous to greenhouse gas monitoring that permits calculation of greenhouse gas forcing). In the absence of that data, we now explore less direct evidence of aerosol climate forcing.

Evidence of Aerosol Climate Forcing

Paleoclimate data suggest the important role of aerosols in global climate. In the past 6,000 years, known as the late Holocene, atmospheric CO2 and CH4 increased markedly, likely as a result of deforestation and methane from rice agriculture,Footnote34 causing greenhouse gas (GHG) climate forcing to increase more than 0.5 W/m2,1 yet global temperature during the late Holocene held steadyFootnote35 or decreased slightly.Footnote36 This divergence of GHG forcing and global temperature is a strong anomaly; CO2 is a tight control knob on global temperature at other times in the ancient paleo record (see  in Note 1 at end), as anticipated on theoretical grounds.Footnote37 Aerosols, the other large human-made climate forcing, is a suggested solutionFootnote38 for this “Holocene conundrum.” Aerosols increased in recent millennia as burning of wood and other biofuels provided fuel for a growing global population. Moving to recent, preindustrial, times, the required magnitude of the implied (negative) aerosol forcing from biofuel burning reached at least 0.5 W/m2. Biofuel aerosol forcing has likely increased since then, as the biofuel energy source is widespread in developing countries and continues in developed countries.1

Recent restrictions on ship emissions provide a great opportunity to investigate aerosol forcing. The International Maritime Organization (IMO) introduced limits on the sulfur content of ship fuels in stages, with the greatest global restriction effective January 2020 (Sidebar 5). Change of global aerosol forcing from this limit on ship emissions, based on IPCC’s formulation of aerosol forcing,18 is calculatedFootnote39 as 0.079 W/m2. Forster et al.,Footnote40 updating IPCC’s aerosol forcing, estimate the ship aerosol forcing change as +0.09 W/m2; they also note that this ship aerosol forcing would be offset by negative aerosol forcing from increased forest fires and other biomass burning. A reviewFootnote41 of five ship aerosol modeling studies finds a range 0.07 to 0.15 W/m2, with mean 0.12 ± 0.03 W/m2. A recent model resultFootnote42 of 0.2 W/m2 refers to ocean area and is thus a global forcing of 0.14 W/m2. None of these modeled Ship Aerosol Forcings would have much effect on global temperature because GHG forcing currently is increasing 0.4-0.5 W/m2 per decade.

However, if the aerosol effect is highly nonlinear (i.e., if aerosols emitted into polluted air have much less effect on clouds than aerosols emitted into a pristine atmosphere), decreased ship emissions may have a large effect on Earth’s albedo (reflectivity). The largest effect should be in the North Pacific and North Atlantic, where ship emissions dominate over natural sulfate aerosols (Sidebar 5). Fortunately, Earth’s albedo has been monitored for almost a quarter of a century by the CERES (Clouds and the Earth’s Radiant Energy System) satellite instrument,Footnote44 which reveals a stunning darkening of Earth ().Footnote45 Earth’s albedo decreased about 0.5% (of 340 W/m2), which is 1.7 W/m2 additional heating of Earth since 2010! Such albedo change is equivalent to an increase of CO2 by 138 ppm, from the 419 ppm actually measured at the beginning of 2024 to 557 ppm. However, the 1.7 W/m2 increase in energy absorbed by Earth is not all climate forcing; it is partly climate feedback – cloud changes and reduced ice and snow cover caused by global warming. Our task is to apportion the 1.7 W/m2 between aerosol forcing and climate feedbacks, accomplishing this in the absence of adequate aerosol and cloud measurements.