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Thursday, January 22, 2009

Link found between tropical rainfall and Earth's magnetic field

Link found between tropical rainfall and Earth's magnetic field

PlanetEarth online, 20 January 2009

Climate records stretching back 5000 years seem to show a strong link between rainfall in the tropics and changes in the Earth's magnetic field, according to new research.

Magnetic field

Computer simulation showing the Earth's magnetic field. Researchers say they have found a correlation between the strength of the magnetic field and tropical rainfall.

The Earth's magnetic field varies considerably and in the last 1500 years it has weakened. The paper, published in the journal Geology, links this variability, and more recently, decline, to changes in rainfall patterns around two sites in the tropics.

The researchers, led by Dr Mads Faurschou Knudsen while working as a NERC Research Fellow at the University of Oxford, reconstructed the Earth's magnetic field over the last 50,000 years and compared this to stalagmites from two caves in Oman and China. The ratio of oxygen isotopes in stalagmites rising from the floors of limestone caves contains information about past climates including rainfall in the local area.

'In the last 5000 years, the correlation between changes in the Earth's magnetic field and precipitation data from Dongge Cave in Southern China is striking,' says, Knudsen, now at the University of Aarhus. 'The Oman cave is less good.'

Knudsen stresses that they can't say anything about global rainfall or climate patterns and the data still need to be verified elsewhere.

Slight changes in Earth's orbit around the sun and minor shifts on its axis affect climate. Since the end of the last ice age, around 11,500 years ago, these changes have altered climate: a band of low pressure around the equator known as the Intertropical Convergence Zone, which has a big affect on rainfall, has moved farther south.

When Knudsen and his colleague, Dr Peter Riisager from the Geological Survey of Denmark and Greenland, removed climate effects caused by variations in Earth's orbit, they found a direct link between the changing magnetic field and rainfall.

'The only mechanism we can think of for this is modulation by cosmic rays,' says Knudsen.

Cosmic rays

An artist's impression of cosmic rays.

Cosmic ray theory

A decade ago Henrick Svensmark, a senior scientist at the Center for Sun-Climate Research at the National Space Institute, Denmark, suggested cosmic rays help cloud formation.

Charged particles, or cosmic rays, constantly bombard the Earth. We are shielded from these rays by the sun's and Earth's magnetic fields. The Earth's field is weaker than the sun's. When these fields are strong, fewer cosmic rays reach Earth.

Clouds don't spring from thin air. Water vapour in the atmosphere forms droplets by condensing on small particles. There are some suggestions electrically charged particles in the atmosphere, or ions, are an important part of this nucleation process. Svensmark reckoned cosmic rays are the source of the charge.

But Knudsen says, 'We don't understand all aspects of this process. More low-lying clouds should lead to less rainfall. But this is not what we see. Several studies have, however, shown a link between the concentration of aerosols [small particles] in the atmosphere and precipitation'.'


The relationship between rainfall and Earth's magnetic field over the last 5000 years, as seen in China's Dongge Cave. The left-hand axis shows oxygen isotope ratio (blue line) -- an indicator of rainfall. The right-hand axis shows the Earth's magnetic field (black line). The x-axis shows time from 5000 years to present.

The researchers say this work has little bearing on climate change induced by rising greenhouse gas levels.

'The Chinese correlation is striking.' Dr Mads Faurschou Knudsen, former NERC Fellow, now University of Copenhagen.

'Greenhouse gases have a major effect on climate. If our correlation is correct, then we are saying that the Earth's magnetic field may have some effect on rainfall, and the sun's field should have a larger effect on climate,' says Knudsen. A full understanding of all natural causes of climate variation will improve climate models.

The Earth's magnetic field is very dynamic. In the last 10,000 years it has changed quite abruptly several times. While the team recreated the Earth's magnetic field for the last 50,000 years, they only found a correlation in the last 5000 years.

'Our reconstruction of the Earth's magnetic field beyond 6000 years is less accurate,' explains Knudsen. 'And the climate data only goes back 10,300 years at one site and 8850 years at the other.'

He add that when you go beyond 11,000 years, large changes in climate caused by other factors dwarf any shifts in rainfall patterns linked to Earth's magnetic field making any kind of correlation difficult.

'It is only because climate has been relatively stable recently that we can see a pattern,' explains Knudsen.

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