Sustainability Issue #4 August 2010

Terrestrial ecosystems produce, apart from carbon dioxide and methane, a large amount of carbon compounds which are often referred to as BVOC, biogenic volatile organic compounds. These volatile hydrocarbons contain a large number of different molecules which, in turn, perform many different functions for plants. All the functions are not yet known, but it may be mentioned as an example that the hydrocarbons attract pollinators through spreading flower scent, and that they protect  the plant from herbivores and environmental effects such as ozone and short term heat.

Emits isoprenes. Some trees emit more terpenoids than others. Oaks emit isoprenes, while pines emit monoterpenes. Photographer: Stefan Rosengren.

Terpenoids

Once they have been emitted into the atmosphere, BVOC also affect the climate and the composition of the atmosphere. This is particularly true in the case of a group of volatile hydrocarbons called terpenoids. It is mainly forests and forest vegetation that emit terpenoids.

In polluted air, volatile hydrocarbons may be the cause of ozone formation. This is the case for air with high concentrations of reactive nitrogen compounds (where NO and NO2 together form NOx), for example from road traffic.  In the lower part of the atmosphere, the troposphere, ozone occurs as a strong greenhouse gas, and it is also highly toxic for humans, animals and plants.
Light from clouds

Terpenoids are also important with  regard to the growth of secondary organic particles, SOA, which are formed in the atmosphere through chemical reactions. These particles disperse and absorb radiation, and they also affect the life of clouds and their reflecting properties.

It is therefore evident that emissions – through different processes and different forms of interaction – can play an important part in the climatic system, both today and in future. A future warmer climate may considerably increase the emission of volatile hydrocarbons. This is already known, since we can see highly elevated emissions when we measure the BVOC content from a leaf that has been exposed to a higher temperature.

Heat and carbon dioxide

But climate change is not the only factor that has to be considered. There are also many other processes that will affect future emissions of hydrocarbons.

For example, there is the increased concentration of carbon dioxide which is the main cause of climate change. Laboratory experiments have shown that the emission of certain terpenoids from leaves actually decreases when plants are exposed to higher concentrations of carbon dioxide.
When models of future emissions of BVOC are constructed, there are two processes which have to be taken into account, and these are processes that act in completely different directions:

• A  warmer climate increases emissions.
• Higher carbon dioxide concentrations decrease emissions.

It is impossible to say with certainty which of these will be the dominant factor.

What controls hydrocarbons?

It is important to enhance our understanding and knowledge of what it is that controls volatile hydrocarbons in their different forms, since elevated emissions of BVOC can have either a warming or cooling effect on the climate.

Since ozone is a powerful greenhouse gas, the interaction of greater emissions of BVOC from the forest in a carbon dioxide contaminated environment will contribute to a warmer climate.

On the other hand, secondary organic particles probably  have a cooling effect since these particles disperse and reflect sunlight, and probably cause clouds to reflect light to a higher degree.

Feedback loops

It is very important to understand the interactions so that we may be able to predict future climate changes, since they are a part of what are called feedback loops. Such loops can either accelerate or retard climate changes caused by human action. It is at present uncertain what a feedback between climate change and biological processes implies for climate changes and for the policies that are to alleviate the effects of climate changes and modify energy use.

But it is also important to understand how closely climate change and air pollution interact, since ozone is a strong polluter, and it is formed with the help of BVOC in environments contaminated by nitrogen oxide.

Emissions in July. Emissions of isoprene (at left) and monoterpenes (at right) in relation to potential natural vegetation (at top) and present land use (bottom) in July, average results for 1981-2000. The figure shows that emission of hydrocarbons has decrased in landscapes where agriculture has replaced forests in Europe, compared with potential emissions if the whole Europe had natural forest vegetation.

Crops

So far we have only discussed the effects of climate change in relation to the emission of terpenoids by the forest.

But what is the position in relation to crops?

Broadly speaking, crops emit groups of BVOC different from those from forests. There is less of the terpenoid type and more of OVOC, oxygenated volatile organic compounds. Such oxygenated volatile hydrocarbons, and their significance for the atmosphere, have not been the subject of as much research as terpenoids.  Nor have we any data regarding the quantity of OVOC emitted from agricultural vegetation regionally or globally, and the way these emissions will react to changes in climate and atmospheric CO2 concentration.

But it is evident that, in order to be better able to predict the atmosphere and climate of the future, and to better understand the relationship between climate change and air pollution, we must also take account of land used for forest and for crops and the respective sizes of these areas.

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