Trees are cool!

There is a climate anomaly in the south eastern states of the USA that, until recently, scientists have been unable to explain. While the rest of the country has suffered from rapidly rising temperatures, these anomalous areas have either flatlined or cooled. What is going on?

A new study has found that long-term reforestation over enormous areas of the eastern USA seems to be the major reason for this inconsistency.

Right from the days when America was still a British colony, settlers cleared woodland for farming and to provide material for building. Vast tracts of native forest were cut down over a period of more than two centuries. However, as the USA industrialised at the beginning of the 20th century, people began to leave its agricultural areas and travel into the cities to find work. As the people moved out and abandoned marginal land, the trees swiftly moved back in.

And in addition the government undertook an intensive, century-long, programme of tree-planting in the eastern states, a response to unsustainable farming practises that had damaged the land.

How are these trees cooling the area? Trees transpire, which means that they draw up water from the ground into their leaves from where it evaporates as water vapour. Evaporation is a process that uses energy, cooling the surrounding air. While a single transpiring leaf, or even a lone tree, makes no measurable difference to the ambient temperature, millions of hectares of trees do.

The re-established forests cool the eastern US by 1°C to 2°C each year. The effect is strongest on the hottest days when temperatures can be 2°C to 5°C lower.

The researchers warn that reforestation is probably not the only reason for the cooling effect, and that we mustn’t use their findings as an excuse to turn our attention away from the more complex and difficult solutions to the problems of global warming. We know that fossil fuel use, in all its manifestations, is what is driving the emergency we are hoping to live through and we mustn’t let a good news story about trees distract us.