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"Nature": Global Warming Reduces Cooling Effect of Forests

Jülich scientists discover changes in the emissions of trees

[17. September 2009]

Jülich, 17 September 2009: Increasing global temperatures are affecting the degree to which forests can cool the climate. A team from Forschungszentrum Jülich have discovered that changes in the emissions of trees inhibit the formation of new particles. Since these particles function as cloud condensation nuclei, the result will be fewer clouds which will lead to increased solar radiation on the Earth’s surface. The Jülich scientists’ findings have been published in the current edition of the high-impact journal "Nature".

The typical smell of a forest emanates from monoterpenes, which are the main components of essential oils. These volatile organic compounds are thought to play an important role in the formation of aerosols – particles in air – which in turn are involved in the formation of clouds. The fact that trees emit more monoterpenes at higher temperatures has led researchers in the past to believe that increasing temperatures meant increased cloud formation – particularly above the forests in the northern cold-temperate climatic zones in Scandinavia, Siberia and Canada. These extra clouds would then regulate global warming by preventing as much solar radiation from hitting the Earth’s surface.

The new findings of the team headed by Jülich physicist Dr. Astrid Kiendler-Scharr, however, contradict this conviction. The scientists investigated the effect of a compound that is also emitted by trees in increasing quantities with increasing temperatures: isoprene. They discovered that isoprene reacts with hydroxyl radicals, thus decreasing the concentration of hydroxyl radicals in air. Hydroxyl radicals are formed in the atmosphere from ozone and water molecules under the influence of UV radiation. They degrade a number of pollutant trace gases in the lower atmospheric layer. Kiendler-Scharr’s team succeeded in demonstrating that these very hydroxyl radicals are also needed for the formation of aerosols. Without these radicals, monoterpenes cannot form aerosols.

The researchers concluded that isoprene is depriving the monoterpenes of hydroxyl radicals. This causes a decrease in the production of aerosols, which in turn leads to a decrease in the formation clouds. Increasing temperatures will lead to a situation where forests will emit more isoprenes than monoterpenes. Based on their findings, the Jülich scientists projected a 20 % decrease in aerosol production by 2100. This would then reduce the cooling effect of clouds by 12 %.

With their discovery, the scientists have also clarified a phenomenon that has puzzled researchers in the past. Due to the fact that the trees emit more monoterpenes with warmer temperatures, it was therefore assumed that aerosol production over forests was at a maximum in summer. In actual fact however, peak production is in spring and autumn. The reasons for this can now be explained by Kiendler-Scharr and her colleagues. The amount of isoprene emitted by the trees in relation to the corresponding quantity of monoterpenes is highest in summer. This is why increased monoterpenes levels in summer do not actually mean anything – the hydroxyl radicals required for aerosol production are not available because they have been exhausted by isoprenes.

The scientists conducted their experiments in a plant aerosol chamber containing birch, beech, spruce and pine trees. They specifically chose these trees because they emit very little isoprene. This allowed the researchers to vary the concentration of isoprene in the plant aerosol chamber by feeding controlled amounts into the chamber. The scientists conducted experiments with and without isoprene addition on alternate days in order to observe the impact of isoprene on aerosol production. This daily alternation also reflected the naturally fluctuating monoterpene emission of trees.

In order to verify their findings, the scientists added an oak tree to the trees in the chamber. Oak trees emit high amounts of isoprene. As expected, this almost brought aerosol production to a complete standstill.

Original publication

"Nature" vol. 461 no. 7262, pp. 381-384
New particle formation in forests inhibited by isoprene emissions, Astrid Kiendler-Scharr, Jürgen Wildt, Miikka Dal Maso, Thorsten Hohaus, Einhard Kleist, Thomas F. Mentel, Ralf Tillmann, Ricarda Uerlings, Uli Schurr, Andreas Wahner
Nature Website


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