ENERGY BALANCE OF CLIMATE

Abstract

The Sun is the ultimate energy source for climate. The Sun radiates toward Earth at an almost constant intensity of about 1360 watts per square meter (W/ m2), as measured above the Earth ’s atmosphere. Most of this radiation takes place in the short ultra- violet and visible light wavelengths. We refer to it as incoming short- wave radiation (ISWR; the wavelengths are short because the Sun radiates at very high temperatures of about 5500°C). Earth is not a two- dimensional disk, but a 3- dimensional sphere. Its day- side faces the Sun and receives radiation, while its night- side is directed away from the Sun and does not receive solar radiation. As a result, the global average energy received from the Sun per square meter of Earth surface is the energy received by the day- side of Earth averaged over the surface area of the entire sphere. When we do the mathematics, this gives an average input of solar radiation into every square meter of Earth, at the top of the atmosphere, of 340 W/ m2 (Box 3.1). That is the value that things work out to when considering the ISWR from the Sun in a continuous and globally equally “smeared out” sense, and that is what matters when we are working out the balance between energy gained and lost by Earth (Box 3.2). Many people are puzzled by the fact that we talk only about energy from the Sun. They then especially wonder why we ignore heat input from the deep Earth, and in particular from volcanoes, which after all are very hot. But in spite of the spectacular shows of heat, steam, gases, and primordial mayhem that volcanoes put on display, they turn out to be almost negligible in terms of heat flow into the climate system. Compared with the global average solar energy gain of 340 W/ m2, recent assessments show that total heat outflow from the Earth’s interior is not even 0.09 W/ m2.