Forests a key contributor to cloudiness in the pre-industrial climate

Emissions from trees could be playing a more important role in the atmosphere than previously thought, according to two new studies involving LEAF scientists.

fir tree forest in sunny day

When particles in the atmosphere reach a certain size they are able to act as seeds for cloud droplets to form around. This means that understanding where particles in the atmosphere come from, and the processes controlling their growth, is vital to predicting how cloudy the planet will be.

For a long time, atmospheric scientists thought that sulphuric acid (formed from emissions of sulphur dioxide) was required to form new particles in the atmosphere. However, in recent years – evidence has emerged that different chemical compounds, such as ammonia and amines, are able to contribute to this process too.

Other possible contenders are gases, called terpenes, which are emitted by trees and give forests their characteristic piney smell. Terpenes react rapidly in the atmosphere to form highly oxidised multifunctional organic compounds some of which were thought to combine with sulphuric acid and go on to form particles.

However, new work from the CLOUD project at CERN suggests that the compounds emitted by trees are able to form new particles by themselves, without the help of sulphuric acid. The researchers also found that the rate at which these particles would form was enhanced in the presence of a beam of cosmic rays, which may link this process to variations in the Sun’s output.

“The CERN CLOUD experiment is uniquely capable of measuring how individual clusters of molecules form and grow into particles that eventually affect clouds and climate. We have shown that particles can form just from the organic molecules emitted by natural vegetation. This means that sulphur-containing pollutants are not needed”, explained Professor Ken Carslaw from the Institute for Climate & Atmospheric Science (ICAS) and an author on the study.

blue sky background with clouds

In a second study, the CLOUD team found that some of these same organic molecules are helping in the very early stages of particle growth. They were also able to work out precisely which type of organic compounds are helping out in which stages of growth. This is important because particles have to grow to around 50 times their initial newly-formed size before they are able to help form cloud droplets.

In the present day atmosphere, there is so much sulphuric acid around that it is difficult to observe these processes happening – but in pre-industrial times, before humans began polluting the atmosphere (i.e. around 1750 AD), the role of these biogenic emissions may have been much more important.

Understanding what the pre-industrial atmosphere was like is crucial when trying to calculate by how much humans have altered the climate. New knowledge about the pre-industrial atmosphere therefore improves our ability to project how the climate will change in the future.

Kamalika Sengupta, a PhD student from ICAS who contributed to the studies said, “The results of the CLOUD experiment mark an important milestone in understanding how organic compounds, released by vegetation, impact the climate. We demonstrated the critical contribution these organic vapours make in the initial stages of particle formation. Organic materials constitute as much as 90% of total fine particulate mass in some tropical regions and a significant fraction in continental mid-latitudes. Hence these studies contribute significantly towards understanding atmospheric particles and their relation to climate change”.

If the pre-industrial atmosphere was in fact cloudier, then the pollution that humans have added may have made less of an impact on the climate than previously thought; Jasper Kirkby from the CLOUD project explains more in the TEDEd video below.

Read the original articles published in Nature here and here.

Post by Cat Scott – University of Leeds.