Producing quantitative estimates of radiative forcing

By Will Davies

Last year the Paris climate conference agreed to an action plan to limit global warming to below 2 degC – preferably 1.5 degC. Various initiatives are measuring performance against this target – such as the global warming index which provides an index of human-induced global warming relative to pre-industrial times, and the Copernicus Atmosphere Monitoring Service (CAMS) which will deliver operational services including near-real-time analyses and forecasts of atmospheric composition, and estimates of instantaneous radiative forcing (RF).

The difference in the amount of radiation received from the sun and the amount that is radiated back into space is referred to as the Earth’s energy budget. RF measures the imbalance in this budget when the climate system is perturbed by components such as greenhouse gases, aerosols and clouds.

Here in Reading I am part of a CAMS team producing quantitative estimates of RF with respect to pre-industrial times (PI) using PI concentrations provided in the Fifth Assessment Report (AR5) of the Intergovernmental Panel on Climate Change (IPCC) from year 1750. The production chain being developed will use a CAMS global reanalysis dataset which will include data assimilated from recent satellite launches such as Sentinel-3 in order to produce improved RF estimates. CAMS consolidates previous research such as MACC and so the CAMS production chain has been prototyped using MACC reanalysis data.

The CAMS 74 production chain uses a radiative transfer (RT) code that is based on the standalone version of the Rapid Radiative Transfer Model for General circulation models (RRTMG) as used in the European Centre for Medium-Range Weather Forecasts (ECMWF)’s Integrated Forecast System (IFS). The development of this RT code will include stratospheric temperature adjustment using the seasonally-evolving fixed dynamical heating approximation.

Early versions of the code have been run on MACC reanalysis data – see Figures 1,2 and 3.

2016 10 13 Will Davies - Fig1

Figure 1. The 2007 annually-averaged RF for the aerosol-radiation interaction (ari) short wave (SW) RF, the aerosol-cloud interaction (aci) SW RF, the all sky methane (CH4) long wave (LW) RF and the all sky carbon dioxide (CO2) SW+LW RF

2016 10 13 Will Davies - Fig2.pngFigure 2. The mean global distribution of the methane RF at the top of atmosphere (TOA) for a clear sky on 21 June 2009, showing the effect that meteorological conditions have on the methane RF.2016 10 13 Will Davies - Fig3Figure 3. The CO2 TOA LW clear sky yearly instantaneous RF from 2003 to 2009 which shows the steady increase in RF and hence global warming caused by CO2 emissions.

Many scientists see the Paris 2015 target as ambitious. The radiative forcing products provided by the CAMS monitoring service will clarify this and will help to highlight the scale of the challenge that we face.



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