Aerosol forcing in the Climate Model Intercomparison Project (CMIP5) simulations by HadGEM2-ES and the role of ammonium nitrate

Abstract

[1] The latest Hadley Centre climate model, HadGEM2-ES, includes Earth system components such as interactive chemistry and eight species of tropospheric aerosols. It has been run for the period 1860-2100 in support of the fifth phase of the Climate Model Intercomparison Project (CMIP5). Anthropogenic aerosol emissions peak between 1980 and 2020, resulting in a present-day all-sky top of the atmosphere aerosol forcing of −1.6 and −1.4 W m −2 with and without ammonium nitrate aerosols, respectively, for the sum of direct and first indirect aerosol forcings. Aerosol forcing becomes significantly weaker in the 21st century, being weaker than −0.5 W m −2 in 2100 without nitrate. However, nitrate aerosols become the dominant species in Europe and Asia and decelerate the decrease in global mean aerosol forcing. Considering nitrate aerosols makes aerosol radiative forcing 2-4 times stronger by 2100 depending on the representative concentration pathway, although this impact is lessened when changes in the oxidation properties of the atmosphere are accounted for. Anthropogenic aerosol residence times increase in the future in spite of increased precipitation, as cloud cover and aerosol-cloud interactions decrease in tropical and midlatitude regions. Deposition of fossil fuel black carbon onto snow and ice surfaces peaks during the 20th century in the Arctic and Europe but keeps increasing in the Himalayas until the middle of the 21st century. Results presented here confirm the importance of aerosols in influencing the Earth's climate, albeit with a reduced impact in the future, and suggest that nitrate aerosols will partially replace sulphate aerosols to become an important anthropogenic species in the remainder of the 21st century.

Preferred response of the East Asian summer monsoon to local and non-local anthropogenic sulphur dioxide emissions

“…Emissions of sea-salt, mineral dust, and ocean-based DMS, are computed interactively. HadGEM2-ES participates in the CMIP5 simulations and the validation of model aerosols and their radiative forcing has been documented in Bellouin et al (2011). Figure 1 shows the annual mean emissions of SO 2 in 2000 (Lamarque et al 2010).…”

“…The model used is the atmospheric component of the UK Met Office Hadley Centre Earth system model HadGEM2-ES (Collins et al 2011;Jones et al 2011;Bellouin et al 2011). The atmospheric resolution is N96 (1.875° by 1.25°) with 38 vertical levels with the model top at ∼39 km.…”

“…For the second indirect effects, the large scale precipitation scheme uses the CDNC to compute the autoconversion rate of cloud water to rainwater. The detailed descriptions of the aerosol module are given in Bellouin et al (2011). The historical emissions for tropospheric aerosols and aerosol precursors are described by Lamarque et al (2010).…”

Aerosol and ozone changes as forcing for climate evolution between 1850 and 2100

“…According to RCP projections, the ammonia emissions increase in all the RCP scenarios during the twentyfirst century with 2100 values corresponding to a 50 % increase of the 2000 emissions. According to the recent Bellouin et al (2011) study, nitrate aerosols could partially replace sulfate aerosols in the climate system in the remainder twenty-first century. The combining of tropospheric and stratospheric ozone fields is also a weakness of this work and a single model including both chemical schemes would allow more consistency for the upper troposphere-lower stratosphere ozone distribution and stratosphere/troposphere exchanges.…”

A steady-state analysis of the temperature responses of water vapor and aerosol lifetimes

“…This is attributed by the authors to the increased precipitation, but also to differences in the simulated seasonalities of precipitation and aerosol concentrations. A study with HadGEM2-ES (Bellouin et al, 2011; not listed in Table 1) computes a sulfate lifetime that varies from 3.8 days in 1850 to 3 days in 1960 and then to 4 days at the end of the 21st century. The simulated decrease of the sulfate lifetime between 1850 and 1960 is due to the fact that anthropogenic activities caused a relocation of the main sulfur dioxide emissions from the (sub-)tropics to extratropical regions, thus situating them closer to the northern hemispheric storm tracks where precipitation is abundant.…”