Climate

Climate

In terms of the carbon cycle, the ocean is probably one of the biggest controls of the carbon dioxide in the atmosphere. The [oceans’] volume is holding about 50 times the amount of carbon that the atmosphere holds.

Cliff Law, a NIWA scientist, in an interview about the interactions between the oceans and the atmosphere, broadcast in Our Changing World, National Radio, 16 February 2006

The coastal cliffs at Baring Head, south-east of Wellington, are a popular spot for rock climbers, but this is also the site of New Zealand’s longest-running continuous measurement of atmospheric carbon dioxide (CO₂).

When the wind blows from the south, the air at Baring Head has travelled a long way across the Southern Ocean, which means that the concentrations of gases such as CO₂, methane and nitrous oxide are uncontaminated by sources on land and representative of a large part of the Southern Hemisphere.

Back in the early 1970s, National Institute of Water and Atmospheric Research scientist Dave Lowe set up the equipment on the wind-swept hills, initially focusing on CO₂. Levels recorded here are slightly lower than in the Northern Hemisphere, but follow the same upward trend. Methane measurements began in 1989 and nitrous oxide followed in 1996.

Planet fever

Energy from the sun beats down on Earth, and while some of it is reflected back into space, the rest is absorbed by the atmosphere surrounding our planet, warming Earth enough to support life. Atmospheric greenhouse gases such as CO₂, methane, nitrous oxide and water vapour are responsible for trapping the heat – and this greenhouse effect is a natural process. However, an unprecedented increase in human-made emissions of greenhouse gases is causing additional global warming. As the greenhouse gases increase, temperatures will continue to rise, causing sea-level rises and a greater incidence of extreme weather events, such as storms, heat waves, droughts and floods. Scientists expect that the world will continue to warm over the next century.

New Zealand has ratified the Kyoto Protocol, an international agreement setting targets for industrialised countries to cut their greenhouse gas emissions to 1990 levels in the time period between 2008 and 2012.

Sink or source

New Zealand’s shrublands have been identified as potentially valuable assets for meeting the commitments under the Kyoto Protocol, and a team of Landcare Research scientists, led by David Whitehead, has been investigating the flow of carbon dioxide in and out of indigenous forests, shrublands and pasture.

Plants take up CO₂ during photosynthesis, but they also emit the gas during the night, so the team’s main question is to determine how much CO₂ plants soak up from the atmosphere and, in turn, how much of the carbon stored in trees, leaf litter, grass and soil is released back into it.

The team studies the carbon balance of several contrasting types of vegetation: rimu-dominated rainforest, kanuka shrubland and pasture reverting back to bush.

Although the carbon exchange in pasture is small, it could well become the most important ecosystem because it represents New Zealand’s most dominant form of land use.

New Zealand is unique among industrialised nations because its main greenhouse gas is not CO₂, but methane, which is 21 times more potent. The most important emitters of methane are cattle and sheep, which produce methane during rumination, and several research groups are investigating ways to monitor and reduce methane production in grazing animals.

Nitrous oxide, another potent greenhouse gas with 300 times the warming effect of CO₂, is emitted from soils as a byproduct of the microbial processing of nitrogen. A Lincoln University team recently developed a soil treatment that reduces both emissions of nitrous oxide and the leaching of nitrate.

Taking the pulse of the ocean

More than 70 percent of the planet is covered by water, and the top 2.5m of the world’s oceans store as much heat as the entire atmosphere. Global warming would be much more rapid were it not for the vast expanse of oceans, as they absorb nearly half of the anthropogenic CO₂. This is due partly to physics as cold ocean surface water allows CO₂ to dissolve more readily and sink to depth (solubility pump), and partly to biology, as minute algae absorb CO₂ when they grow (biological pump). When these minute plants die, they sink, carrying carbon to the bottom of the ocean.

Over the past few years, NIWA scientists have been dropping hundreds of smart floats into the Southern Ocean as part of an international project called ARGO. In total, almost 3000 floats are now cruising the world’s oceans, each diving between the surface and 2000m to measure the speed of ocean currents, temperature and salinity. Every time a float comes back to the surface, it beams the data and its position back to a central computer via satellite.

Already, there is a noticeable increase in ocean temperatures, and the maximum increase has occurred at latitudes of around 40 degrees south, particularly in the South Pacific.

This warming is accompanied by an acceleration of the South Pacific gyre, a giant circular motion extending across the southern part of the Pacific Ocean.

By Veronika Meduna

Medals and awards

David Whitehead: Distinguished Service Award, IUFRO

Further reading and websites

National Institute of Water and Atmospheric Research (NIWA) website

Manāki Whenua Landcare Research website 

ARGO website

Image courtesy of NIWA

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