Ronald Reagan is widely quoted as having said trees cause more pollution than automobiles do.
Not true of course, but not entirely without foundation either. Trees and other vegetation, particularly conifers, emit terpenes, aromatic chemicals whose generic name is derived from turpentine. The delightful aroma of pine resin in conifer woodlands comes from chemicals such as α-pinene and β-pinene.
Yet according to the National Physical Laboratory :-
- In addition to anthropogenic emissions, the earth’s natural vegetation releases huge amounts of organic compounds into the air.
- An estimated 1300 Tg C of terpenes a year are emitted, 10 times more than anthropogenic emissions.
As far as climate is concerned, the NPL has this to say:-
- Terpene emissions are expected to rise sharply as global temperatures rise.
- As carbon dioxide levels increase, the earth will warm and higher levels of terpenes will be emitted.
- This will increase cloud formation, which will increase the optical thickness of clouds resulting in an increase in the reflection of sunlight back into space.
- Terpenes constitute a significant potential for feedback mechanisms in the climate.
- Terpenes also mediate the generation of ozone in the lower atmosphere.
Dramatic stuff - even somewhat over-dramatic. Yet all this does not imply Reagan was correct because terpenes are not pollutants. They may be involved in the photochemical reactions which give that attractive haze over distant forests, but haze isn’t smog and doesn’t have the same effect on lungs and mucous membranes.
Without oxides of nitrogen from, for example vehicle exhaust emissions, tree terpenes alone would not cause the notorious photochemical smog which first appeared in Los Angeles in the 1940s and subsequently other large, sunny cities.
To my mind, this is why electric vehicles such as trams make sense in large cities, especially those where photochemical smogs are a problem. The issue isn’t CO2 emissions as many now seem to suppose, but old-fashioned air pollution such as oxides of nitrogen, unburned fuel and particulates. Sunlight just adds to the problem but that was there long before we decided to whizz around in metal boxes.
Electric vehicles are not pollution-free modes of transport of course. They are effectively powered by whatever is used to recharge their batteries, but any polluting effects are moved away from the city to the power station and subject to simpler and more stringent regulation.
Reagan pointed the finger at trees, climate alarmists point to CO2 and as so often a fog of dramatic misdirection hits the headlines.