The hemispheric contrast in cloud microphysical properties constrains aerosol forcing

Area of Science:

• Atmospheric Science
• Climate Science
• Earth Science

Background:

• Aerosol-cloud interactions significantly influence Earth's albedo and climate sensitivity.
• Uncertainty in these interactions is a major challenge in understanding historical climate change.
• Cloud droplet number concentration is a key variable controlling aerosol-cloud interactions.

Purpose of the Study:

• To use the present-day hemispheric contrast in cloud droplet number concentration as a proxy for anthropogenically driven changes.
• To constrain the radiative forcing from aerosol-cloud interactions since 1850.
• To evaluate the suitability of Southern Ocean droplet number concentration as a proxy for preindustrial conditions.

Main Methods:

• Utilizing remotely sensed data to estimate cloud droplet number concentration changes.
• Comparing satellite-derived estimates with in situ measurements.
• Analyzing discrepancies between climate model predictions and observational data.

Main Results:

• Remotely sensed estimates constrain the change in droplet number concentration between 8 cm⁻³ and 24 cm⁻³.
• Radiative forcing from aerosol-cloud interactions is constrained to -1.2 W⋅m⁻² to -0.6 W⋅m⁻².
• Climate models systematically underpredict cloud droplet number concentration over the Southern Ocean, especially near Antarctica.

Conclusions:

• The assumption of Southern Ocean droplet number concentration as a proxy for preindustrial levels requires further investigation.
• High droplet number concentrations observed in the pristine Southern Ocean suggest higher preindustrial aerosol concentrations than typically modeled.
• Detailed process studies on aerosol production and cloud interactions in pristine environments are crucial.