Volatile Organic Compounds And Climate
It is known that tree leaves evaporate many dilferent 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 monotcrpenoids and isoprcne, which arc 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.
Mow 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 arc 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 arc emitted in greater quan-
lities 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 VVETAMC 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 arc.
The haze from VOCs does not normally travel more than a few hundred kilo-
meters, 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 arc 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 chic 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 CII4 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 1 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 Oil 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 cither increased or warming tropical climate where higher temperatures tend to decrease the photosynthesis and growth of plants. Any remaining fragments of the rainforest ecosystem would quite likely be wiped out by this warming, and it would not be much good for the croplands either.
Conversely, cutting down the world's rainforests might actually help preserve the mid and higher latitudes against global warming. Since latent heat from the rainforests helps keep the higher latitudes warm, cutting off this heat source might help to counteract some of the warming from the greenhouse effect. However, this would be of no benefit to tropical countries which would have to sweat out the climatic consequences of losing their forest cover, and we must also consider all the extra carbon from the destroyed forests that would enter the atmosphere as C02, adding to the warming everywhere in the world. All things considered, the mid-latitudes might actually end up warmer rather than cooler overall if the tropical forests were clcared.
On the other hand, what if world forest cover becomes more widespread in the future? This will in itself tend to cause a warming of global climate irrespective of any greenhouse effect warming, because forest has a lower albedo than cleared land. If all the world's deforested land were allowed to return to its natural forest cover, it might warm the world overall by 1.3°C due to its effects on albedo, according to a rcccnt modeling study by Gibbard and colleagues. Because tropical forests soak up so much heat in transpiration, this change would barely be felt in the tropics and the warming would mostly occur at higher latitudes (part of the warming there would be a result of latent heat reaching them from the increased tropical forests). On a global scale 1.3 C is a large warming - not very much less than some of the forecasts for the greenhouse effect over the next century, and larger than the warming that has occurred since the mid-lSOOs that has caused so many changes around the world. In addition, as well as transporting more latent heat around, more forest will mean more water vapor in the atmosphere at any one time, and water vapor is a potent greenhouse gas that might further add to the warming. This scenario of increased water vapor in the atmosphere from more forest cover is another factor that needs to be fully considered in climate modeling, if we are to carefully weigh all the options. There is at present a lot of interest in planting forests to soak up carbon dioxide, to prevent some of the warming from the greenhouse effect (see Chapter 7). Yet, as Roger Pielke and his co-authors have pointed out, with all the vegetation-climate feedbacks this increased forest cover might actually warm the planet by more than the C02 that it soaks up!
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