CliSAP successfully finished in 2018. Climate research continues in the Cluster of Excellence "CLICCS".

How thick the cloud cover was in 1632? Tree rings hold the answer


News from Climate Science: Once a month, climate researchers report on their latest findings in the newspaper "Hamburger Abendblatt." Dr. Eduardo Zorita works as an expert for paleoclimatology at the Helmholtz-Zentrum Geesthacht. He explains how tree rings provide climate relevant information.

Imagine this: a forester fells an old tree and examines the cross-section in detail. Shortly afterwards he announces that in 1632 the sky must have been particularly cloudy. A fairytale? Yes, of course. But with a bit of technical help, we can actually use tree rings to determine what the cloud cover was like in a given year. How?

My aim is to understand the climate of the last 2,000 years, as it can provide us with valuable information. A major question mark surrounds cloud behavior: Will global warming mean thicker or thinner average cloud cover? This is hardly unimportant, since a thin layer lets more sunlight through and increases global warming. A thick layer on the other hand could even slow down global warming.

At the KlimaCampus, my colleagues and I have conducted research into a new technique that yields valuable information on past cloud cover. For example we know that there was the “Little Ice Age,” which happened about 300 years ago. What’s interesting for us is whether the clouds then were thick, or whether the sky was often clear.

Here trees can help us – they are to some extent gourmets, and not all “food” is the same to them. When they take in carbon or carbon atoms during photosynthesis, there are two stable forms in the surrounding air: 12C and 13C atoms. However, 12C simply “tastes” better, so trees prefer it and convert it into biomass to grow. 

In strong sunlight, however, the situation is different: Now plants have the chance to photosynthesize as rapidly as possible, which lets them grow particularly intensively. But during these periods of extreme growth, there’s a shortage of the preferred atom, and the tree is forced to fall back on 13C atoms. We take advantage of this effect!

To be specific, we’ve tested this technique for Scandinavia, where samples were taken from around 50 trees in various locations and used the tree rings to determine the age of the respective tree. We then used a device called a mass spectrometer to measure the level of 13C atoms. When we found more of them, it told us there was plenty of solar penetration, which means the cloud layer was thin – an accurate indicator for the years in question.

To sum it up: During the “Little Ice Age,” Scandinavians often enjoyed clear blue skies in summer. Our study shows that, for the entire 300-year cold spell, the average cloud cover was thinner. That allows us to infer that, for this region, rising temperatures produce a thicker cloud layer – which could in turn lessen future global warming on a local scale.

But every new method has to first be validated, which is why we were very glad to see similar results in our next study, conducted in the Pyrenees. That means these vegetable “eyewitnesses” now offer a valuable tool that helps us better determine the connections between sun, cloud cover and local climate.

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