If there is one thing politicians do a lot of, it is quick fixes. These almost never turn out to be good fixes because, by definition, the ramifications have not been thought through.

Planting forests to combat global warming seemed a good idea at the time. Now we know that its impact on carbon dioxide levels will be extremely short-term. Another factor not taken into account was that trees, especially fast-growing trees, emit 'volatile organic compound' (VOC)* scents. Sweet-smelling as these can be for humans, many VOCs react with the human-made Nitrogen Oxide (NOx) gases pumped out of vehicle exhausts to create poisonous ground ozone.

New research at Princeton University (US) showed that the volume of VOCs from new plantations and scrubland increased 17% in the US during the ’80s and ’90s and by 2004 was three times larger than the VOC reductions made by industry. [1] Determined reforestation worldwide could actually increase levels of ground ozone unless human-made NOx emissions are drastically reduced.
Before politicians blame trees for ground ozone and use this as a reason not to reforest or, worse, as a reason to deforest, lead researcher Drew Purves wants to make it very clear that:

• the volume of ground ozone created by a reaction between natural NOx (from, for instance, soil microbes and lightning) and natural VOCs is tiny relative to the volumes produced when human-made NOx and human-made VOCs interact

• interactions between VOCs, NOx and ozone are complex and that it would be premature to base environmental policy on studies of VOCs alone

• when human-made NOx levels are low, natural VOCs react in the opposite way and actually reduce ozone concentrations

To conclude, planting trees will not reduce global-warming carbon dioxide levels long-term, but will increase levels of ground ozone in some areas unless substantial efforts are made to reduce NOx emissions:

• most fast-growing trees are heavy emitters of VOCs

• the levels of VOCs produced by trees are highest in hot weather

• the ozone-producing reaction of VOCs and NOx gases happens most readily in hot weather

• global warming is currently heating up the planet

* Organic compounds are chemicals that contain carbon and are found in all living things. Volatile organic compounds (VOCs) are organic compounds that easily become vapours or gases.

The Intergovernmental Panel on Climate Change (IPCC) said that massive tree-planting to absorb carbon dioxide will at best only buy time and, at worst, accelerate global warming. It claimed that the new trees would soon be saturated with CO2 and start returning most of it to the atmosphere. Although trees absorb carbon dioxide through photosynthesis, they also release it back into the air when their plant matter breaks down the sugars they have made. This is called ‘respiration’, and respiration increases in response to temperature rises, which are triggered by rises in CO2 levels.

Scientists fear that recent rises in temperatures may cause respiration to accelerate, turning forests from ‘carbon sinks’ into carbon (dioxide) sources. Scientists had not spotted this possibility before because, although CO2 take up is instantaneous, the warming which triggers respiration has a built-in delay of around 50 years due to the slow warming of the oceans. Planting trees, therefore, may buy a little time now but cause very serious problems downstream.

Ed.- The only solution is to reduce carbon emissions themselves - a course that takes more political will and responsibility than most politicians possess.

US research on pine forests suggests that they are not the answer to global warming.

Although, when young, trees are net consumers of carbon dioxide (CO2), over their entire life their contribution to carbon dioxide level reduction is insignificant. When needles fall to the ground and turn to mulch almost as much carbon dioxide is returned to the atmosphere. Also, much has been made of the idea that trees grow more vigorously, absorbing more CO2, as overall atmospheric carbon dioxide levels increase. The scientists found that, after three years, the pines tended to adapt to the new levels and returned to normal growth rates.

If the same is true of other types of forest (e.g. rainforest or deciduous forest) trees cannot be part of climate change negotiations. These must be based on human-based emissions reduction.