Study Reveals Industrialization has Reduced “Coniferous Forest Effect”

Greenhouse gas emissions cause a warming effect on the climate, while small airborne particles in the atmosphere serve as a cooling mechanism.

However, in a new study from Lund University in Sweden, it has been revealed that the tiniest aerosols are increasing at the expense of the standard sized and somewhat larger aerosols—and it is only the second type that has a cooling effect.

The atmosphere is filled with small airborne particles called aerosols. A few are naturally synthesized, while others are produced when burning fuel. Some are detrimental to health, while other aerosols reflect sunlight.

One of the key natural sources of aerosols is the aromatic terpenes from coniferous forests. For instance, the boreal coniferous forest area, “the taiga,” that extends like a ribbon across the entire world makes up for 14% of the world’s vegetation coverage. Thus, it is the largest coherent land ecosystem in the world.

Through chemical reactions with the ozone in the air, the terpenes are converted into extremely oxygenated organic molecules that stick to aerosol particles already present in the air. This results in more cloud droplets, as each cloud droplet is created through steam condensing on an adequately large aerosol particle. More cloud droplets result in more dense clouds and lower insolation.

However, the new research, which was reported in Nature Communications, reveals that this “coniferous forest effect” has reduced because of industrialization.

Ammonia emissions from agriculture and sulfur dioxide from fossil fuels alter the parameters of the game: the terpenes and also other organic molecules are separated into many more, but smaller, aerosol particles. Since very minute aerosols have a diameter smaller than the wavelength of light, the particles are incapable of reflecting light.

Despite being gases, ammonia and sulfur dioxide produce new particles through chemical reactions in the air.

“Paradoxically, a larger number of aerosol particles can lead to the cooling effect from the organic molecules released from the forests being reduced or even eliminated,” says Pontus Roldin, researcher in nuclear physics at Lund University in Sweden and first author of the article.

In collaboration with a global research team, he designed a model that for the first time exposes the process behind new particle formation of these aerosols.

The heavily oxidised organic molecules have a significant cooling effect on the climate. With a warmer climate it’s expected that forests will release more terpenes and thus create more cooling organic aerosols. However, the extent of that effect also depends on the emission volumes of sulphur dioxide and ammonia in the future.

Pontus Roldin, Study First Author and Nuclear Physics Researcher, Lund University

Roldin continued, “It’s very clear, though, that this increase in organic aerosols cannot by any means compensate for the warming of the climate caused by our emissions of greenhouse gases.”

This research can help mitigate the ambiguity surrounding the effect of aerosol particles on clouds and the climate.

There has already been a substantial reduction of sulfur dioxide emissions in the United States and Europe since the 1980s. Furthermore, steps in the correct path have now also been observed in China.

Relatively simple technical solutions are required to reduce sulphur dioxide, for example, cleaning of exhaust gases from ships and coal-fired power plants, etc. It’s much harder to reduce ammonia, as it’s released directly from animals and when soil is fertilized.

Pontus Roldin, Study First Author and Nuclear Physics Researcher, Lund University

Predictions have been made that in future, worldwide meat production will increase significantly as wealth in undeveloped countries, largely in Asia, increases. Currently, the consequences of these changes are unknown, but a prediction necessitates the use of comprehensive models such as the one developed in this study.

In a few years, Pontus Roldin will work within a research project that will add knowledge to next-generation climate models, like EC-Earth.

We already know that the forest is a significant carbon sink. However, other factors, such as the cooling effect of aerosols, types of vegetation and emissions, affect the climate. Hopefully, our results can contribute to a more complete understanding of how forests and climate interact.

Pontus Roldin, Study First Author and Nuclear Physics Researcher, Lund University