Thursday 21 November 2013

More greenery = less carbon dioxide? Not always

By Feng Zengkun, The Straits Times, 20 Nov 2013

HAVING more greenery does not always help to protect against climate change, a team of Singapore scientists has found.

A three-year study in Telok Kurau yielded the surprising fact that the greenery there emits more carbon dioxide (CO2) than it absorbs, the scientists said.

While the plants, trees and grass in the neighbourhood take in more CO2 than they give out, the soil they grow on also emits the gas.

Singapore-MIT Alliance for Research and Technology (SMART) research scientist Erik Velasco said the vegetation types in Telok Kurau were part of the problem.

Turfgrass and palm trees make up about 30 per cent of the greenery, but these absorb less CO2 due to their small biomass compared with larger, woody trees, he said.

Hence, the net effect is that the greenery there contributes to, rather than reduces, emissions to the atmosphere.

The study suggests that to better reduce CO2 emissions and protect against climate change, "large trees should not be replaced by young trees and palms, as is the tendency along secondary roads in Singapore". The larger trees would better offset the soil's CO2 emissions. Soil's CO2 emissions also need to be studied further, said Dr Velasco.

The scientists' work started in 2010 and was published in the Atmospheric Chemistry and Physics science journal last month. They were from SMART and the National University of Singapore (NUS).

According to the research paper, they chose Telok Kurau because the low-rise buildings there make it easier to measure CO2 emissions to the atmosphere. NUS built a 21m-tall tower there in 2006 to track CO2 floating from the ground.

The scientists got CO2 measurements from the tower. They also did their own estimates by getting data about the area's population, traffic, soil, buildings and vegetation from the authorities and through manual surveys.

This data was compared to previous research on CO2 emissions and the relationship between trees and the gas, to come up with the estimates.

Dr Velasco stressed, however, that the Telok Kurau findings cannot be extrapolated to the whole of Singapore due to the wide variety in building and vegetation types, and more research is needed.

"But it shows that green spaces can act as emission sources rather than CO2 sinks, as is widely believed."


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