Organic aerosols from forests could help cool the planet

Researchers in Finland have observed a plant-induced cooling effect in the atmosphere, which strengthens as temperatures increase. The team, led by Taina Yli-Juuti at the University of Eastern Finland, studied the negative feedback mechanism through both ground- and space-based observations of a Finnish forest. Their results could provide important guidance for climate models that include the influence of aerosols in the atmosphere.

Organic aerosols are tiny particles that include dust, ash, and pollen. They cool Earth’s climate by reflecting sunlight back into space and by accelerating the formation of water droplets in clouds, which can also have a cooling effect. As climate scientists predict future global temperatures, it is crucial for their models to fully account for these cooling processes. However, the properties of aerosol particles are very diverse and therefore there is significant uncertainty in how they should be incorporated into climate models.

One proposed climate feedback mechanism involves organic aerosols emitted by plants at concentrations that are known to increase with temperature. The idea is that as the biosphere responds to a warming atmosphere, concentrations of these particles will increase, influencing the climate in turn.

Land and space observations

To study this effect, Yli-Juuti’s team took measurements from two different instruments. On the ground, they used the University of Helsinki’s Hyytiälä Forestry Field Station, which is in taiga (boreal forest) about 200 km north of Helsinki. Over a seven-year period, the station has monitored concentrations of organic aerosols while taking temperature measurements simultaneously. From space, they used the MODIS instrument, aboard NASA’s Aqua satellite, to monitor the properties of clouds forming above the forest over the same period.

The observations revealed a clear increase in organic aerosol concentrations in years with higher summer temperatures. These changes came simultaneously with increasing concentrations of cloud nucleation centres: particles which form through chemical reactions with organic aerosols, which provide surfaces on which vapour in the surrounding air can condense – allowing liquid cloud droplets to form far more readily. Both of these effects were strongly associated with a clear increase in cloud reflectivity above the forest, resulting in cooler temperatures on the ground.

Altogether, the team’s results provided the first observational evidence of this negative climate feedback mechanism. As global temperatures increase, Yli-Juuti and colleagues predict that the effect will become increasingly prevalent in boreal forest environments. By incorporating the mechanism into their models, the researchers hope that climate scientists will attain more accurate predictions of future changes to Earth’s climate.

The research is described in Nature Communications.

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