It is known that tree leaves evaporate many different organic compounds (VOCs) out into the air around them, especially when they are heated under a hot sun. There are several groups of compounds, including monoterpenoids and isoprene, which are thought to play some sort of protective role within the leaves, though no-one is quite sure what (it might, for example, be against insects, fungi or heat). The rate at which these chemicals are emitted depends on the particular forest type, and also the temperature conditions. Generally, the broadleaved forests of warmer climates (such as tropical rainforests) emit more isoprene, while conifer forests at high latitudes emit the most monoterpenoids.
How might these compounds affect climate? VOCs oxidize to produce a bluish natural haze in the atmosphere, and it is noticeable that many mountain regions in forested areas of the world have names that refer to this haze. A couple of examples are the Blue Ridge and the Smokey Mountains of the heavily forested southeastern USA. Analogous names occur in different languages in many different parts of the world where there is extensive forest cover. Haze tends to reflect sunlight back into space, cooling the lower atmosphere. Given that VOCs are emitted in greater quantities at warmer temperatures, if the climate warms (either naturally or due to human effects), more VOCs will be emitted, damping the warming. So far, no climate modeling has been attempted to quantify this effect, but it might turn out to be significant if studied.
VOCs may also help to make rain by forming clouds, with the oxidized particles derived from VOCs acting as nucleation centers for the cloud droplets. Observations of cloud formation over the Amazon Basin by an international team of scientists in the WETAMC study suggested that VOCs might be responsible for the formation of "shallow" clouds that are very effective at yielding rain. Thus, VOCs may help to promote rapid recycling of rainwater within a particular rainforest area, keeping the local climate wetter than it would otherwise be. It is possible that VOCs might have similar effects in other forested areas of the world, but so far there are no observations or models elsewhere that might give clues as to how important they are.
The haze from VOCs does not normally travel more than a few hundred kilometers, but VOCs can have far-reaching indirect effects on global climate because they are easily oxidized by hydroxide (OH) radicals in the atmosphere. OH is a sort of chemical vacuum cleaner that breaks down many different organic molecules in the air. Because VOCs from plants are so easily oxidized, they tend to "mop up'' OH that could otherwise react with and destroy methane, an important greenhouse gas produced mainly by swamps. By in effect preserving methane from being broken down (because it produces VOC that uses up the OH), an increase in global forest area might slightly increase the warming that occurs due to the greenhouse effect. Increases in VOCs are also expected to increase the amount of ozone gas in the lower atmosphere, and as ozone is a greenhouse gas this could likewise warm the atmosphere.
Here then are some of the possible changes resulting from VOC emission by the leaves of trees. Note that 1. and 3. work in opposite directions, and it is not certain whether the cooling or warming effect predominates overall:
1. Leaves emit VOC ^ VOC oxidized by OH ^ less OH to oxidize CH4 ^ more CH4 in the atmosphere ^ warmer climate.
2. Leaves emit VOC ^ VOC oxidized to give particles ^ particles promote cloud formation ^ more rain.
3. Leaves emit VOC ^ VOC oxidized to give particles ^ particles promote cloud formation ^ more sunlight reflected into space ^ cooler climate.
It is possible that the broad changes in forest cover that followed from climate swings in the Quaternary themselves damped these changes. A study my colleagues and I carried out suggested that, due to the lower temperatures and reduced forest cover, there may have been 30-50% less VOC emission in the world under glacial conditions, soaking up less OH and thus tending to lower the methane content of the atmosphere (which is in fact what occurred, though probably due to a combination of factors such as less methane production too). Future changes in forest cover due to deforestation or forest-planting might also indirectly lead to either increased or decreased breakdown of methane, either lessening or increasing the greenhouse effect due to this gas.
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