Published online 3 October 2008 | Nature | doi:10.1038/news.2008.1148

Column: Muse

Identifying the dripping taps of climate change

Seeking out the subtle effects of carbon dioxide can lead to some bizarre conclusions, says Philip Ball.

What is the connection between rain in Tibet, sunspots, Earth's magnetic field, iron filings, cosmic rays and insects?

The answer is that all have been proposed as agents of climate change. Some of them now look fairly well established as such; others remain controversial, or have been largely discounted.

HimalayasMonsoons over the Himalayas may have increased chemical weathering, which consumes carbon dioxide from the atmosphere, cooling the climate.Punchstock

But the factors are awfully hard to investigate. In every case, their effects on the global environment look minuscule, but over geological timescales the effects can really add up.

This issue goes to the heart of the debate over climate change. It's not hard to imagine that a 10-kilometre-wide meteorite hitting the planet might have consequences of global significance. But tiny variations in the geo-, bio-, hydro- and atmospheres that trigger dramatic environmental shifts — the dripping taps that eventually flood the building — are tough for scientists to evaluate, or even to spot in the first place (see 'Iron fertilization', 'Cenozoic uplift', 'Plant growth' and 'Ozone depletion').

Two papers published in Geophysical Research Letters now propose new 'trickle effects' connected with atmospheric levels of the greenhouse gas carbon dioxide that are both subtle and hard to assess. One suggests that CO2 levels shift as the strength of Earth's magnetic field changes1, the other that they may alter the ambient noisiness of the oceans2.

Scandalously sketchy

What can a trace gas have to do with noise? Peter Brewer and his colleagues at Monterey Bay Aquarium Research Institute in Moss Landing, California, point out that the transmission of low-frequency sound in seawater is known to be dependent on the water's pH: between 0.1 and 10 kilohertz, greater acidity reduces sound absorption. And as atmospheric CO2 rises, more is absorbed in the oceans and seawater gets more acid through the formation of carbonic acid.

whaleAre whales being deafened as the oceans acidify?Punchstock

The explanation - invoking an imbalance in dissolved ions that absorb vibrations at acoustic frequencies - seems almost scandalously sketchy. But the effect of acidity on sound transmission through water is well documented. Brewer and his colleagues calculate that the ocean absorbs at least 12% less sound now than it did in pre-industrial times, and that by 2050 low-frequency sound might travel up to 70% farther.

Indeed, low-frequency ambient noise in the ocean is 9 decibels louder off the Californian coast than it was in the 1960s, and not all of this rise can be explained by human activity. How the changes might affect marine mammals that use long-distance acoustic communication is a question left hanging.

Hard to credit

Uptake of atmospheric CO2 by the oceans is also central to the proposal by Alexander Pazur and Michael Winklhofer of the Ludwig-Maximilian University of Munich in Germany that changes in Earth's magnetic field could affect climate. They claim that for every 1% decrease in magnetic field, CO2 solubility drops by 0.5%.

plantsPlants may grow faster when they have more carbon dioxide available for photosynthesis, an important negative feedback that could mitigate climate change.Punchstock

This decrease in solubility would release ten times more CO2 than all the gas currently emitted from subsea volcanism. That's tiny compared with present inputs from human activities, but it would change the atmospheric concentration by 1 part per million per decade, which is important over a long timescale.

Once you start to think about it, the list of possible interactions such as these seems endless. How to know which are worth pursuing? The effect claimed by Pazur and Winklhofer does seem a trifle hard to credit, although their hypothesis that it acts via ions adsorbed on the surfaces of tiny bubbles of dissolved gas is plausible. But there are good arguments why such effects are unlikely to be significant at field strengths as weak as Earth's3. Moreover, the researchers measure the solubility changes indirectly, via the effect of tiny bubbles on light scattering – but bubble size and coalescence is itself sensitive to dissolved salt in complicated ways4. In any event, the effect vanishes in pure water.

A devil to discern

The broader issue is that anticipating and assessing the importance of these effects it is distressingly hard. Climate scientists have been saying for decades that feedbacks in the biogeochemical cycles that influence climate are a devil to discern and probe, which is why the job of forecasting future change is so fraught with uncertainty.

And of course every well-motivated proposal of some subtle modifier of global change — such as cosmic rays — tends to be commandeered to spread doubt about whether global warming is driven by human activity.

Perhaps this is a good reason to embrace the metaphor of 'planetary physiology' proposed by James Lovelock. We are all used to the idea that tiny quantities of chemical agents can produce all kinds of surprising, nonlinear and non-intuitive transformations in our bodies. One doesn't have to buy into the arid debate about whether or not our planet is 'alive'; maybe we need only reckon that it might as well be. 

  • References

    1. Pazur, A. & Winklhofer, M. Geophys. Res. Lett. 35, L16710 (2008). | Article |
    2. Hester, K. C., Peltzer, E. T., Kirkwood, W. J. & Brewer, P. G. Geophys. Res. Lett. 35, L19601 (2008). | Article |
    3. Kitazawa, K., Ikezoe, Y., Uetake, H. & Hirota, N. Physica B 294, 709-714 (2001). | Article |
    4. Craig, V. S. J., Ninham, B. W. & Pashley, R. M. Nature 364, 317-319 (1993). | Article | ChemPort |
    5. Martin, J. Paleoceanography 5, 1–13 (1990).
    6. Raymo, M. E. & Ruddiman, W. F. Nature 359, 117–122 (1992).
    7. Woodward, F. I. Nature 327, 617–618 (1987).
    8. Farman, J. C., Gardiner, B. G. & Shanklin, J. D. Nature 315, 207–209 (1985).
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