For the earth, the sun is a source of energy and the engine of our life and climate. It emits a wide range of energy in the form of electromagnetic radiation. This ranges from X-rays to ultraviolet radiation, visible light, and infrared radiation, all the way to microwave radiation. Radiation heats the earth's atmosphere and surface – sometimes more, sometimes less, depending on the season, geographical location and local weather. As a result, storms occur, water circulates in the oceans, deserts expand their territory, and torrential rains fall.

The upper section of the earth’s atmosphere receives on average 1361 watts of energy from sunlight per square meter. However, only a quarter of what reaches each square meter then reaches the earth’s surface. Another 30 per cent is bounced back into space, particularly off clouds and the earth's surface, with the result that the natural fluctuations of solar radiation influence our upper atmosphere much more than layers closer to the earth’s surface. Only about 70 percent of those 1361 watts are ultimately absorbed by the earth’s surface and atmosphere as heat.

But the radiation that the sun sends to the earth varies, even as it is emitted. What happens to this radiation is observed by Dr. Natalie Krivova at the Max Planck Institute for Solar System Research in Göttingen. The astronomer creates sunlight models to find out whether fluctuating sunlight possibly contributes to the warming or cooling of the earth. On Krikova’s office wall is a whiteboard, where in a corner, she has drawn a smiling red sun. One important question that plagues Natalie Krivova and her team was posed by climate change. “We wonder whether changes to sunlight hitting earth contributes to climate change,” explains Krivova. “Maybe reduced solar activity even acts against global warming?” Krivova and her team have simulated solar activity over the last few centuries. “If we want to know to what extent greenhouse gases are responsible for the warming of the earth, we also need to know what influence the sun has,” said the astronomer. The radiation that reaches the Earth's atmosphere fluctuates in a regular eleven-year cycle. This can be seen in the changes of sunspots on the surface of the star. When larger numbers of sunspots emerge, the radiation on the earth is at its highest, increasing by about one watt. That sounds small, but has a major impact on life on earth. Such a fluctuation resulted in a particularly cold period in 17th century Europe. Ice and snow covered the landscape well into spring. Artists such as the Dutch painter Hendrick Avercamp captured this phenomenon in pictures. His image IJsvermaak (Ice pleasure) shows people on a frozen canal in the Netherlands during the cold winter of 1608. Today, however, Dutch canals are usually ice-free in winter.

These eleven-year fluctuations to sunlight also occupy Hauke Schmidt from the Max Planck Institute for Meteorology in Hamburg. “It is also interesting for us atmospheric scientists that the solar spectrum fluctuates very unevenly. While in the visible range, the variation is below 0.1 percent, in the ultraviolet range it is several percent, rising to even shorter wavelengths in the direction of X-rays,” explains Schmidt.

There is evidence that sunlight on earth has increased by around one watt over the past 300 to 400 years. “However, we cannot rule out a decrease in sunlight in the next few decades,” says Krivova. “And we must not trivialize the fact that greenhouse gases have contributed significantly more to global warming than the sun has.”