Sun's Surprise: Even As It Relaxes, It May Heat Earth's Climate
A solar eruption.

As the ultimate source of all the energy on Earth, the sun has an inextricable hand in driving our planet's climate and atmosphere. But a new look at the sun's connection to Earth's climate has returned some surprising results.

The study finds that during the most recent lull in the sun's weather cycle, the amount of energy that reached Earth increased, instead of decreasing as predicted. The planet may have experienced a slight warming effect as well, researchers said.

Breaking down the radiation

The study, led by Joanna Haigh, a professor of atmospheric physics at the Imperial College London,?analyzed the types of radiation that reach Earth from the sun, and the various effects they have on our planet's atmosphere.

Haigh and her colleagues used satellite measurements taken from 2004 to 2007, the declining phase of the latest 11-year solar weather cycle.

As the sun becomes less active, it typically releases less energy in the form of radiation. Previously, this was understood as a decrease in the total amount of radiation that reaches the top of the Earth's atmosphere.

In examining solar emissions during this declining phase, however, the researchers found that a large decrease in ultraviolet radiation was roughly compensated for, by an increase in visible radiation.

"Visible radiation is the only kind that, in any substantial quality, gets to the Earth's surface and heats the lower atmosphere," Haigh told "We found that as the sun's activity declined from 2004 to 2007, more of this radiation was entering into the lower atmosphere."

Ultraviolet radiation is largely absorbed in the stratosphere, where it combines with ozone molecules to form what is known as stratospheric ozone. As stratospheric ozone depletes, more UV radiation is able to pass through to the Earth's surface.

Visible radiation, on the other hand, more readily penetrates into the Earth's lower atmosphere. So, if more visible radiation reaches the Earth's surface, the heating of our planet's lower atmosphere results in a warming of the climate.

"In just over three years of observation, we conclude that the visible radiation was going to be warming the planet as the solar activity declined," Haigh said.

This may seem counterintuitive, and the researchers are careful to note that their findings cannot be generalized without more extensive study of these processes. Furthermore, they said, their observations were made over a relatively short period of time during a potentially anomalous solar cycle.

Their research is detailed in the Oct. 7 issue of the journal Nature.

Understanding the solar cycle

An extremely long stretch of low solar activity in recent years has baffled scientists, and the expected minimum of solar activity between 2008 and 2009 was unusually quiet.

The ebb and flow of the sun's magnetic activity, and the amount of energy it puts out, make up the solar cycle. Typically, a cycle lasts about 11 years, taking roughly 5.5 years to move from a solar minimum to a solar maximum.

The total energy that reaches Earth from the sun varies by only 0.1 percent across the solar cycle, and atmospheric physicists and meteorologists have struggled to link such a small variation to the ups and downs of Earth's natural weather and climate patterns.

"In the past, it was thought that the changes were too small to do anything," Haigh said. "People knew there was a UV component that was heating the stratosphere, but it was thought to be unimportant to the climate."

The findings of this new study, however, could be a step toward piecing together the puzzle.

"The sun has been behaving very strangely over the past few years," Haigh said. "We need to know more about how strange it is before we extrapolate the findings to other periods of time. But it does suggest that our previous understanding of how the sun affects the Earth's climate may be in need of revision."

It doesn't end there

There is more work to be done, Haigh said.

Now that the sun has presumably awoken from its solar minimum, scientists are keen to observe the star as it ramps up its activity.

"It'll be very interesting," Haigh said. "If the visible radiation starts to decline as solar activity goes up, that would be very, very interesting."

These types of studies will be an important part of the discourse on climate change. Being able to gauge the influence of solar activity on our climate and atmosphere will be crucial to the ongoing debate, Haigh said.

"It's quite clear that we have to understand what the sun does to our climate and how much more or less solar activity affects the atmosphere," she said. "We need to work out the solar component of climate change."

"This is not at all to suggest that the sun is causing climate change, but I do think we need to accurately know what the sun is doing, so that we can better assess the human component."