Climate Science with GERB

	Understanding the processes which control the natural stability and variability of the climate system, and which dictate how man's activities might modify the climate balance is one of the most difficult and challenging, yet critically important, scientific problems facing mankind today. GERB (together with SEVIRI - the primary multi-channel instrument on MSG), will enable studies of water vapour and cloud forcing feedback. These are two of the most important (and poorly understood) feedback processes in climate prediction.
	The Earth's Radiation Budget (ERB) is the balance between the incoming radiation from the sun and the outgoing reflected and scattered solar radiation plus the thermal infrared emission to space. Observations from space have a central role in understanding the ERB since they are quasi-global. Measurements need to be to very high absolute accuracy,  ~ 1 Wm-2.
	An accurate ERB radiometer on a geostationary satellite will exploit the essentially perfect temporal sampling possible from geostationary orbit, and will provide a unique and important complement to polar orbiting measurements. It will permit much more stringent tests of climate models (eg Figures 1 and 5) allowing essential progress in our capability to understand the natural balance of the climate, and how we might perturb it.
	The development, integration and flight of GERB on MSG is being supported by EUMETSAT and the European Space Agency (ESA). For a relatively small extra cost, European countries can achieve an important 'first' in space-borne climate research. This is a unique opportunity for us to lead in a crucial area of science with strong practical relevance to global climate change, food production and natural disaster prediction.
	GERB will measure the shortwave and longwave radiation from the Earth every 15 minutes, and will provide strong synergy with planned instruments (polar orbiters) such as CERES, SCaRaB, and with the SEVIRI sensor on MSG. This is extremely important: a thorough understanding of our climate system will only be achieved by merging accurate observations from both geostationary and polar orbiting satellites.

Figure 1: Clear-sky Outgoing Longwave Radiation (OLR)		Figure 2: Zonal and annual averages of the 'forcing' of climate by clouds Much more detailed studies will be possible using GERB data because clouds are strongly affected by diurnal variations.
Figure 3: A calculation of the greenhouse effect of the Earth. Curve E shows the emission assumed from the Earth's surface in this calculation, as a function of wavenumber. Curve G shows E minus the OLR at the top of the atmosphere (a measure of the effectiveness of the greenhouse effect). It can be seen that in absolute terms (G), the greenhouse effect is dominated by water vapour absorption between 100 and 500 cm-1, carbon dioxide absorption between 600 and 800 cm-1. Oxygen at about 1050 cm-1 and water vapour again between 1300 and 2000 cm-1. Absorption and emission by water vapour 'continuum' can be inferred between 750 and 1250 cm-1. The link to the SEVIRI water vapur channel on MSG is critical to the understanding of the water vapour feedback process.		Figure 4: The radiation balance of the Earth, showing incoming shortwave radiation in the parallel solar beam being matched by outgoing infrared radiation cooling to space from all parts of the Earth. (The incoming and outgoing fluxes are about 240 Wm-2; the effect of clouds on the surface radiation budget can be as large as 100 Wm-2 but the equivalent effect of global warming through CO2 doubling is only about 4Wm-2; therefore we need to understand the ERB to about 1 Wm-2).

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