Global warming causes more UV rays

Does the sun affect the earth's climate?

The radiation from the sun warms the air and allows water to evaporate - and thus drives the weather on earth. But the sun doesn't always shine in the same way: every eleven years on average, dark, cool spots cover the face of our central star. And other, longer-term changes in solar activity are superimposed on this cycle. Does the total radiation of the sun change with the number of dark sunspots? And do such variations also influence the earth's climate?

Scientists now generally accept that the strength of solar radiation on the earth influences the climate. But this influence is not due to changes in the sun itself, but to the movement of the earth. As the Serbian astrophysicist Milutin Milankovic showed in 1920, the precession of the earth's axis, changes in the inclination angle of the earth's axis and variations in the eccentricity of the earth's orbit lead to periodic or almost periodic fluctuations in solar radiation on the earth, which are reflected in the long-term climate, especially in the ice ages . From paleoclimatological studies, however, climatic periods are also known that do not coincide with these astronomical cycles. The variations in the earth's movement are apparently only one factor among many that have an impact on the climate - plus volcanism and continental drift, among other things.

More stains, more radiation

Number of sunspots over the past 400 years

And what about changes in the sun itself? In fact, the energy output of the sun changes with the activity - albeit differently than one might initially assume: more sunspots on the surface of our central star do not lead to a decrease, but on the contrary to an increase in the total radiation of the sun. Because with strong solar activity there are not only more dark spots, but also more bright torch areas at the same time. All in all, the sun shines around 0.1 percent brighter in the activity maximum than in the activity minimum. Statistical studies indicate that these changes in solar radiation on earth lead to global temperature fluctuations in the range of 0.1 to 0.2 degrees. However, this fluctuation is so small that it is very difficult to record and does not cause any long-term climate change or influence on the weather.

But the sun's activity is also subject to long-term fluctuations. Historical data show that in the period from 1645 to 1715 the cycle of spots came to a complete standstill. During the entire time the face of the sun remained almost free of spots. This so-called Maunder minimum falls exactly in the coldest phase of the “Little Ice Age”, a period of bad weather lasting from 1450 to 1850 with cool, rainy summers and extremely cold winters, in which the Baltic Sea was sometimes frozen over. The role that the long-term reduced solar activity played for the Little Ice Age is controversial among climate researchers. It is conceivable, for example, that a globally small reduction in the average temperature due to a change in the wind currents in the high atmosphere led to a particularly strong cooling in the northern hemisphere. On the other hand, there are also indications of an above-average number of volcanic eruptions during this time - so it is possible that two effects were at work here, the climatic effects of which reinforced each other.

Sunspots and global warming

Could current global warming also - at least in part - be caused by the sun? The sunspot data indicate that the activity of our central star increased in the first half of the last century - in line with an increase in temperature observed during this time. Thereafter, however, the solar activity remained almost constant, while the temperature rise tended to accelerate. Satellites have been measuring solar radiation with great accuracy since the 1970s - and have no longer been able to detect an increase. But the global temperature has continued to rise by around 0.2 degrees per decade since then. The vast majority of climate researchers therefore agree that the current rise in temperature is not caused by the sun, but by the greenhouse gases produced by humans.