Gas exchange and climate

Murray Smith and the SAGE Team went to sea to investigate the connection between wind, waves, and CO₂.

Global warming is now widely recognised as being caused by the accumulation of greenhouse gases such as carbon dioxide (CO₂) in the atmosphere, and it is generally accepted that human activity is causing these high concentrations of gases. However, 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 oceanic physics and partly to oceanic biology. On the physics side of things, cold ocean surface water allows CO₂ to dissolve more readily, and also sinks to depth. The combined effects act to pump CO₂ into the ocean interior – the ’solubility pump'. On the biology side, phytoplankton absorb CO₂ when they grow. When they die, they sink, carrying that carbon to the bottom of the ocean – the 'biological pump'.

The air-sea interface

A limiting step in this transport is the transfer of CO₂ across the air-sea interface. This transfer is highest in regions of the ocean where surface concentrations of CO₂ are lowest, often as a result of elevated biological activity (photosynthesis), and where windspeeds are highest. These conditions are satisfied on our doorstep in the Southern Ocean, providing New Zealand scientists with optimal conditions to examine ocean-atmosphere exchange.

The processes at the interface involve breaking waves, turbulence, and diffusion, and are too complex to allow us to estimate transfer rates from a purely theoretical basis. We can, however, carry out measurements to determine exchange under different conditions. This was one of our objectives in the SOLAS SAGE experiment (see box below), which took us to sea for a month in the subantarctic waters of the Bounty Trough.

Tracing the air-sea transfer

The key part of the physics side of the experiment was to follow the same patch of water for several weeks and measure how the concentrations of two tracer gases changed according to different wind and wave conditions. The two tracers we used – sulphur hexafluoride (SF₆) and an isotope of helium (³He) – are found naturally in the ocean in only minute quantities. The tracers went into the ocean together with an iron sulphate solution, which was added in an attempt to stimulate the growth of plankton and thereby enhance the uptake of CO₂ (see Plankton, iron, and climate).

Since SF₆ and ³He transfer across the ocean-atmosphere interface at different rates, we were able to separate the effects of the escape of gas to the atmosphere from the spreading of the patch. From these measurements we were able to calculate the gas exchange rate at the high wind speeds typical of the Southern Ocean. In fact, exchange rates were measured at the highest windspeeds to date. (This was good for the experiment, but not so good for the scientists and crew!)

Knowing the rate of transfer of gases between atmosphere and ocean is a crucial step in modelling and predicting the world’s changing climate. Armed with this information, we are now re-evaluating the amount of carbon being taken up globally by the oceans.

Dr Murray Smith is a physicist based at NIWA in Wellington. Other members of the SAGE Team are listed on the SAGE web page: www.niwascience.co.nz/rc/atmos/sage