Influence on nucleation on aerosol-climate forcing

UKCA simulations have highlighted the importance of new particle formation in determining the climate forcing from changes in aerosol since the pre-industrial era.

Two pairs of nudged UKCA runs with pre-industrial and present-day IPCC AR5 emissions were carried out with a control pair of runs including the usual source of secondary particles from new particle formation in the upper troposphere, and a sensitivity pair of runs having nucleation switched off, leaving only primary aerosol particles.

The control runs produce a total all-sky aerosol forcing (direct and indirect effects) of -2.0 W/m2 relative to pre-industrial, whereas the sensitivity runs finds the forcing to be -1.5 W/m2. Futher analysis reveals that the clear-sky direct forcing is around -1.0 W/m2 for both pairs of runs, which suggests the indirect forcing is approximately halved once nucleation is accounted for.

The findings, suggest that the indirect climate forcing from aerosols (one of the largest uncertainties in climate assessments) may be considerably reduced once changes in secondary particles from nucleation are considered.

UKCA present-day global distribution of the key climate-relevant aerosol properties and forcings for the control File:UKCA forcing withBHN.pdf and no-nucleation File:UKCA forcing noBHN.pdf pair of runs.

In each plot, shown are 550nm Aerosol Optical Depth (top left), Cloud droplet number concentration (bottom left), clear-sky TOA direct aerosol forcing relative to pre-industrial (top right) and All-sky TOA total aerosol forcing relative to pre-industrial (bottom-right).