Science Centres: Energy

A surprising source of methane

Last month, the prestigious science journal Nature published the remarkable discovery that terrestrial plants emit methane. The results came from a series of carefully controlled experiments by European researchers led by Dr Frank Keppler of Germany’s Max Planck Institute for Nuclear Physics.

Nature asked NIWA atmospheric chemist Dr David Lowe to provide expert commentary. Here Dave does the same for us.

How significant is this finding?

Methane is a powerful greenhouse gas. It traps more than 20 times as much heat per molecule as carbon dioxide. Dr Keppler and his colleagues used an established technique to extrapolate their measurements to the whole world. The numbers are uncertain but staggering: 10-30% of the annual total of methane entering the atmosphere could be coming from living plants.

Are plants still an important carbon sink?

Yes. It looks like plants' absorption of carbon by photosynthesis vastly outweighs their release of methane. The range of estimates puts the global warming effect of methane from plants at 1-10% of the positive benefits from their uptake of carbon dioxide.

Does the research ring true?

The study needs to be duplicated and confirmed, but it appears to explain several previous puzzles, including satellite measurements of inexplicably large plumes of methane above tropical forests, and the current slowing of the global growth rate of atmospheric methane (probably due to tropical deforestation).

Why have people not seen this before?

Firstly, you won't find any textbook suggesting that green plants in oxygen produce methane. Secondly, the current estimates for identified methane sources and sinks are quite inexact, so finding another source does not necessarily 'blow out' the numbers. Some of the methane emitted from plants is probably already being counted and wrongly attributed to microbial activity in wetlands, including swamps and rice paddies.

What does it mean for New Zealand?

There are many questions to answer. As a first step, NIWA and our colleagues at Landcare Research are hoping to measure the emissions from a North Island pine forest. It will be very important to measure the size of this source compared to photosynthetic uptake of carbon under New Zealand conditions.