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Sun causes 'La Nina-like effect' on earth


The effect of radiation from the sun is amplified through feedback mechanisms on earth, researchers say (Source: NASA)

Changes in the sun's radiation cause a small La Niña-like effect on earth, an international team has found.

They say this together with another process involving ozone in the upper atmosphere, amplifies the effect of the sun on earth's climate.

Australian climate scientist, Julie Arblaster of the Bureau of Meteorology in Melbourne and colleagues report their findings in today's issue of the journal Science.

"It's a small signal but it does explain some of our climate variability," says Arblaster.

Scientists have long known that more sunspots increase the amount of radiation that reaches the earth.

The total energy that reaches earth from the sun varies by around 0.1% across an approximate 11-year solar cycle, say Arblaster and colleagues.

But what has long puzzled researchers is that the influence of this variation on the earth's climate has been more than two times greater than would be expected.

Arblaster and colleagues have investigated a number of theories as to why this is the case and think they may have solved the mystery.

"We're trying to understand the mechanism of how this change in the solar output amplifies in the climate in terms of temperature and rainfall," says Arblaster.

Amplification effect

Arblaster says the effect of the sun is amplifed by two processes, the first of which is called the "bottom-up" effect.

In tropical areas where there are no clouds to reflect back the sun's radiation, the sun heats the lower atmosphere, says Arblaster.

She says this increases winds, which cool the earth's surface in the tropics and increase rainfall in subtropical areas.

"It's similar to a La Niña pattern," says Arblaster.

She says the effect is about half the size of the regular La Niña, and differs in the pattern of rainfall it brings to Australia.

The La Niña-like pattern triggers a positive feedback because as the wind blows away clouds from the tropics, more of the sun's radiation is able to warm the lower atmosphere.

Arblaster says this combines with a "top-down" effect in which extra radiation forms more ozone, which warms the upper atmosphere.

By combining the effect of both of these processes, Arblaster says the new model will help better climate predictions and understanding of past climate variability.

Arblaster and colleagues suggest their findings should not be used to support the views of scientists who argue the sun has more influence on the earth's climate than humans.

"This response ... cannot be used to explain recent global warming because the 11-year solar cycle has not shown a measurable trend over the past 30 years," they say.

Regional variability

Dr Marc Duldig, a solar terrestrial physicist with the Australian Antarctic Division says the new research makes an "important contribution" in explaining how the sun's impact on earth's climate can be amplified.

He says it will reduce the uncertainty in climate change predictions and make it easier for policy makers to plan for the future.

Duldig says this kind of work could be useful in developing models that explain regional variations in climate.

"It might be able to tell you how the whole northern part of Australia's rainfall might vary through the 11-year [solar] cycle, and how sea surface temperatures in the regions around northern Australia would vary, which would have an impact on fishing," he says.

Duldig says these kinds of interactions may even one day help explain the seventy-year European cold snap that occurred in the seventeenth century during an extended solar minimum, known as the Maunder Minimum.

He says during that time, cold winds from the north of Russia hit Europe instead of Siberia and the River Thames froze over.

Feedback mechanisms

Former senior CSIRO scientist, Dr Ken McCracken, who has expertise in solar and cosmic ray physics, says the new research shows that climate feedback mechanisms can have a very significant effect.

He says Arblaster and colleagues' research shows there is roughly a two and half fold amplification of the sun's energy.

"It's a bloody big effect," says McCracken, who is now affiliated with the University of Maryland in the US.

He believes such amplification effects have been underestimated by the Intergovernmental Panel on Climate Change.

Tags: climate-change, physics, ozone, stars