Projected effects of declining aerosols in RCP4.5: unmasking global warming? 1Centre for Australian Weather and Climate Research, CSIRO Marine and Atmospheric Research, Aspendale, Vic, Australia
07 Nov 2013
2Department of Science, Information Technology, Innovation and the Arts, Dutton Park, Qld, Australia
3Centre for Australian Weather and Climate Research, Bureau of Meteorology, Melbourne, Vic, Australia
Received: 21 June 2013 – Published in Atmos. Chem. Phys. Discuss.: 11 July 2013 Abstract. All the representative concentration pathways (RCPs) include declining
aerosol emissions during the 21st century, but the effects of these declines
on climate projections have had little attention. Here we assess the global
and hemispheric-scale effects of declining anthropogenic aerosols in RCP4.5
in CSIRO-Mk3.6, a model from the Coupled Model Intercomparison Project Phase
5 (CMIP5). Results from this model are then compared with those from other
Revised: 01 October 2013 – Accepted: 11 October 2013 – Published: 07 November 2013
We calculate the aerosol effective radiative forcing (ERF, including indirect
effects) in CSIRO-Mk3.6 relative to 1850, using a series of atmospheric
simulations with prescribed sea-surface temperatures (SST). Global-mean aerosol ERF
at the top of the atmosphere is most negative in 2005
(−1.47 W m−2). Between 2005 and 2100 it increases by
1.46 W m−2, i.e., it approximately returns to 1850 levels.
Although increasing greenhouse gases (GHGs) and declining aerosols both exert
a positive ERF at the top of the atmosphere during the 21st century, they
have opposing effects on radiative heating of the atmosphere: increasing GHGs
warm the atmosphere, whereas declining aerosols cool the atmosphere due to
reduced absorption of shortwave radiation by black carbon (BC).
We then compare two projections for 2006–2100, using the coupled
atmosphere-ocean version of the model. One (RCP45) follows the usual RCP4.5;
the other (RCP45A2005) has identical forcing, except that emissions of
anthropogenic aerosols and precursors are fixed at 2005 levels. The
global-mean surface warming in RCP45 is
2.3 °C per 95 yr, of which almost
half (1.1 °C) is caused by declining
aerosols. The warming due to declining aerosols is almost twice as strong in
the Northern Hemisphere as in the Southern Hemisphere, whereas that due to
increasing GHGs is similar in the two hemispheres.
For precipitation changes, the effects of declining aerosols are larger than
those of increasing GHGs due to decreasing atmospheric absorption by black
carbon: 63% of the projected global-mean precipitation increase of
0.16 mm per day is caused by declining aerosols. In the Northern
Hemisphere, precipitation increases by 0.29 mm per day, of which
72% is caused by declining aerosols.
Comparing 13 CMIP5 models, we find a correlation of –0.54 (significant at
5%) between aerosol ERF in the present climate and projected
global-mean surface warming in RCP4.5; thus, models that have more negative
aerosol ERF in the present climate tend to project stronger warming during
2006–2100. A similar correlation (–0.56) is found between aerosol ERF and
projected changes in global-mean precipitation.
These results suggest that aerosol forcing substantially modulates projected
climate response in RCP4.5. In some respects, the effects of declining
aerosols are quite distinct from those of increasing GHGs. Systematic efforts
are needed to better quantify the role of declining aerosols in climate
Citation: Rotstayn, L. D., Collier, M. A., Chrastansky, A., Jeffrey, S. J., and Luo, J.-J.: Projected effects of declining aerosols in RCP4.5: unmasking global warming?, Atmos. Chem. Phys., 13, 10883-10905, doi:10.5194/acp-13-10883-2013, 2013.