The solar energy balance

Our Earth’s heating system: the Sun

Our planet has an excellent source of energy: Sun. When solar radiation (i.e. the Sun’s rays) reaches the Earth’s atmosphere, part of it is reflected back outwards and part of it is absorbed by the atmosphere. The rest reaches the Earth’s surface, where once again, part is reflected back and part is absorbed. The atmosphere and the surface (both land and ocean) are heated, and thus emit infrared radiation out towards space. The overall difference between incoming and outgoing energy is known as the net radiation or radiation balance, and it can be stated for a region or for the planet as a whole.

Hot in the tropics, cold at the poles

The Sun does not heat all parts of the Earth to the same extent; the Equator receives more energy than the poles. This is because the Earth is round and spins leaning over in relation to the Sun. So at the poles, the Sun’s rays hit the Earth at a very flat angle and they also have to pass through more of the atmosphere so more of their energy is absorbed. Lastly, the heat input is shared over a larger surface.

Two cold poles

In winter, the nights in the polar regions are very long. Without the Sun’s heat, it is very cold. Furthermore, the Earth continues to emit infrared radiation outwards so it continues to lose heat. In summer, the white snow and ice act as a giant mirror, reflecting back the Sun’s rays and absorbing very little (the albedo effect). Over a year, the polar regions lose more energy than they receive.

Heat exchanges around the globe

In theory, the polar regions should keep getting colder and the tropics hotter, but a lot of the excess heat received by tropical zones is absorbed by the oceans and the atmosphere, so the ocean currents and winds transport it to cooler regions, helping to establish an overall balance. However, the Earth’s overall radiation balance (or net radiation) can be positive, which means the planet is warming up, or negative, which means it is getting colder.

Visible (light) and invisible (infrared) radiation

Most of the radiation emitted by the Sun is short-wave (<4µm) electromagnetic radiation, which includes visible light. Part of this energy is re-emitted back outwards by the Earth in the form of long-wave (>4µm) electromagnetic radiation, known as infrared or heat radiation. The Earth, like our bodies when they are hot, radiates heat outwards.
Currently, the total energy received from the Sun, as measured at the edge of the Earth’s atmosphere several dozen kilometres up, is on average 340 Watts per square metre (W/m2). In fact, this would be 1,368 W/m2 if the Earth was a flat surface perpendicular to the Sun, but the Earth is really a globe. The atmosphere and the Earth’s surface reflect outwards about 30% of this radiation. The rest, about 240 W/m2, is absorbed and converted into heat, serving to maintain the planet’s temperature and to drive circulation in the atmosphere and the oceans, to evaporate water, etc., thus powering the “heat machine” we live on and regulating its climate.
Approximately the same amount of the Sun’s incoming energy (240 W/m2) is emitted back into space.

Polar summer and winter

In the upper latitudes there is no solar radiation in winter. Night lasts for 6 months at the poles, 4 months at 80°N/S and 2 months at 70°N/S. This means it gets very cold: not only is there very little heat input but emissions of infrared heat radiation continue (albeit somewhat reduced by the snow and the pack ice which form an insulating mantle).
In summer, on the other hand, the poles receive solar radiation 24 hours a day, which has a marked warming effect. For a period of about 2 months, around the summer solstice, the number of calories per square metre received each day is actually greater at the poles than at 40°N/S. The effect of this is accentuated by other factors (orientation, etc.), so that temperatures are sometimes quite high during a short period of the year. However, the sun stays quite low in the sky, thus reducing radiation (150 W/m2) and the snow and ice reflect up to 80% of it (albedo effect). So the amount of energy reflected back by the Earth into space is always quite large at the poles, even in summer. Because of this, the ice caps always remain and the ice pack melts quite late in the season.

The Sun’s energy reflected back by the Earth

The greenhouse effect and net radiation

If the atmosphere was totally transparent (i.e. letting it all through) to the infrared radiation (IR) emitted by the Earth, our planet would be much colder (-18°C on average), but in fact the atmosphere (made up of gases, water vapour, etc.) absorbs part of the infrared radiation and also reflects some back to Earth (this is what is known as the greenhouse effect) thus maintaining an average temperature of +15°C. The temperature on the Earth’s surface is determined by a combination of factors: outgoing IR of about 390 W/m2; of this, 240 W/m2 will be lost in space and 150 will be retained by the atmosphere (greenhouse effect); input of 100 W/m2 of non-radiative energy convecting up from the centre of the Earth; the 80 W/m2 that was absorbed from the initial 240 W/m2 of solar radiation reaching the surface. The result is that 330 W/m2 is “trapped” in the atmosphere and radiates towards the surface (inwards IR heat).
Any increase in the greenhouse effect tends to increase the net radiation, because less is being re-emitted right out into space.

Satellites watching over the Earth

Our climate is impacted by the planet’s radiation balance, and it is essential to find out precisely what that net radiation situation is if we are to monitor climate change. Satellites can measure the net radiation (in W/m2) at the outer edge of the atmosphere. When the net radiation is zero, incoming and outgoing radiation balances and there is a heat balance. If the net radiation is positive, the Earth is warming up, and if it is negative, our planet is cooling down.
All the latest data suggests that the net radiation is positive.

Physicist’s corner

Every physical body emits heat energy (W, in Watts) equivalent to that of a “black body”, depending on its absolute temperature (T in degrees Kelvin) and in accordance with Stephan’s law, W=sT4, where s is a constant. This is true of the Sun, but also of the Earth and its oceans, ice, etc…

Did you know ?

Planet Earth captures only a very small proportion (a billionth) of the energy emitted by the Sun. Nevertheless, this amount is 10,000 times as much as the energy consumed by humankind (175 million megawatts). The geothermal energy coming from the central section of the Earth is 4,000 times less than this (0.09 W/m2).

The polar sea ice is capable of stopping almost all heat exchange between the ocean and the atmosphere. Sometimes it is referred to as a heat shield. But the “heat-tightness” of the ice depends on its area and thickness. However, the ice does allow a certain amount of solar radiation in the form of visible light to penetrate, so that marine organisms can develop under the ice.