Foreword: International Space Science Institute (ISSI) Workshop on Observing and Modeling Earth’s Energy Flows

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The Earth's climate, as well as planetary climates in general, is broadly regulated by three fundamental parameters: the total solar irradiance, the planetary albedo and the planetary emissivity. Observations from series of different satellites during the last three decades indicate that these three quantities are generally very stable. The total solar irradiation of some 1,361 W/m 2 at 1 A.U. varies within 1 W/m 2 during the 11-year solar cycle (Frohlich 2012). The albedo is close to 29 % with minute changes from year to year but with marked zonal differences (Stevens and Schwartz 2012). The only exception to the overall stability is a minor decrease in the planetary emissivity (the ratio between the radiation to space and the radiation from the surface of the Earth). This is a consequence of the increase in atmospheric greenhouse gas amounts making the atmosphere gradually more opaque to long-wave terrestrial radiation. As a consequence, radiation processes are slightly out of balance as less heat is leaving the Earth in the form of thermal radiation than the amount of heat from the incoming solar radiation. Present space-based systems cannot yet measure this imbalance, but the effect can be inferred from the increase in heat in the oceans where most of the heat accumulates. Minor amounts of heat are used to melt ice and to warm the atmosphere and the surface of the Earth. The reverse is happening when there is a volcanic eruption that emits particles that reflect solar radiation. In this case less heat is entering the Earth's system than leaving it and a cooling take place, as the system is moving toward another equilibrium. In reality the Earth's system is never in equilibrium as it is continuously exposed to disturbances in its energy balance. In addition to external and anthropogenic effects there are natural pro- cesses in the Earth's system caused by variations in the cloud cover, in the exchange of heat between ocean and atmosphere, in the surface conditions such as snow and ice cover, and in changes in the three-dimensional distribution of water vapor. Other changes in the energy balance include land vegetation that affects both the surface albedo and the surface fluxes of heat and water. Some of the processes now affecting the Earth's climate system are relatively well understood, and others less so. The radiative effect of the greenhouse gases added to the