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Link found between tropical rainfall and Earth's magnetic field

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 artists 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.'

Graph showing how rainfall is closely linked to the Earth's magnetic field

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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You might be interested in this paper.

Climate Change and the Earth's Magnetic Poles, A Possible Connection
Author: Kerton, Adrian K.
Source: Energy & Environment, Volume 20, Numbers 1-2, January 2009 , pp. 75-83(9)
Publisher: Multi-Science Publishing Co Ltd
Many natural mechanisms have been proposed for climate change during the past millennia, however, none of these appears to have accounted for the change in global temperature seen over the second half of the last century. As such the rise in temperature has been attributed to man made mechanisms. Analysis of the movement of the Earth's magnetic poles over the last 105 years demonstrates strong correlations between the position of the north magnetic, and geomagnetic poles, and both northern hemisphere and global temperatures. Although these correlations are surprising, a statistical analysis shows there is a less than one percent chance they are random, but it is not clear how movements of the poles affect climate. Links between changes in the Earth's magnetic field and climate change, have been proposed previously although the exact mechanism is disputed. These include: The Earth's magnetic field affects the energy transfer rates from the solar wind to the Earth's atmosphere which in turn affects the North Atlantic Oscillation. Movement of the poles changes the geographic distribution of galactic and solar cosmic rays, moving them to particularly climate sensitive areas. Changes in distribution of ultraviolet rays resulting from the movement of the magnetic field, may result in increases in the death rates of carbon sinking oceanic plant life such as phytoplankton.

Document Type: Research article
DOI: 10.1260/095830509787689286

adrian kerton, Bristol UK
Wednesday, 1 April 2009 - 12:37


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