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The BBC discusses changes in the sun - and why they don’t mean an ice age is on the way

  • 09 Aug 2013, 12:15
  • Roz Pidcock

Could changes deep within in the sun soon see us skating on a frozen-over Thames? The question of the sun's role in climate change - and the prospect of an icy future - pops up in the media from time to time.

This morning, it was the BBC's turn on the Today Programme. The slot did a pretty good job of explaining that although changes in the sun have affected climate in the past, greenhouse gases are now the dominant factor driving temperatures up.

The programme's topic was the imminent flip in the sun's magnetic field, the force responsible for driving sunspot activity. Steve Tobias, professor of maths at Leeds University, explained how this switch happens about every 11 years, when the sun's output is at the highest point in a natural cycle.

Asked whether the sun's waxing and waning contributes to climate change. Professor Tobias explained:

"It does feed in but it's a very small effect. The sun's total irradiance actually varies by a fraction of a percent, so the direct forcing is very small … it may have indirect effects for the climate but I think I should stress current global warming trends are not really due to changes in the sun's activity."

Presenter Justin Webb pressed a bit more, saying "but years ago the sun created an ice age, didn't it?"

The simple answer is yes. But as Tobias explained, human activity is now the main driver climate change, not the sun - a mistake that has led some parts of the media to suggest we're headed for a ' mini ice age'. Here's a closer look at what the sun has to do with our climate.

Solar changes

Scientists know changes in the sun's activity have consequences for earth's climate. Chris Rapley, professor of climate science at University College London, told Carbon Brief recently:

"Climate science shows that the sun does have an influence on climate; this is not controversial. The planet responds to changes in the flux of energy that it intercepts from the Sun."

Just this week, a paper in the journal Nature described how variations in the amount of solar energy reaching earth over the last million years have been a major contributor to the ebb and flow of earth's vast ice sheets.

On shorter timescales, the sun's energy also rises and falls on an 11-year cycle. The graph below shows how solar activity has changed in the last four centuries.

Sunspot _activity

Source: NASA

"Mini ice age"

As Tobias explains, the sun went through a period of prolonged low solar activity beginning in the late 17th Century - known as the Maunder Minimum. This lasted for around 70 years and coincided with the beginning of what's known as the Little Ice Age (LIA).

During the LIA, parts of the northern hemisphere cooled by as much as two degrees Celsius - causing the River Thames to freeze over completely.

Could it happen again? The sun's activity has been in slow decline for the last few decades. Scientists have speculated that the trough as part of the next 11-year cycle (called 'cycle 25') could be a particularly low one - perhaps even another Maunder-style minimum.

This has led some commenters and parts of the media of the media to conclude that we are on the verge of another LIA. There are two reasons why this is wrong.

The sun is not the only factor

Recent research suggests the Maunder Minimum was only one factor in bringing about the LIA. In fact, it may be more closely linked to unusually intense volcanic activity and interactions between sea ice and ocean currents during the period.

At a recent conference, scientists explained how a major atmospheric circulation known as the North Atlantic Oscillation (NAO) was in a negative phase at the onset of the LIA, which amplified the cooling effect of a reduction in solar output and volcanic activity.

This, in turn, affected the distribution of sea ice in the Arctic and disrupted a major ocean circulation that distributes heat across the globe, known as the Atlantic Meridional Overturning Circulation ( AMOC).

In other words, a fall in temperature to rival the LIA would need more than a drop in solar activity.

Bigger human influence

Even if we did see a return to a Maunder-style low in solar activity, earth's climate is very different now to how it was in the 17th century.

Scientists have modelled the expected temperature drop over the 21st century due to waning solar activity - and they found that the change is likely to be dwarfed by the much bigger warming effect of greenhouse gases in the atmosphere.

This echoes the IPCC's last report on the science underpinning climate change. It states:

"In today's atmosphere, the radiative forcing from human activities is much more important for current and future climate change than the estimated radiative forcing from changes in natural processes."

In other words, changes in the sun's activity might have a small but noticeable effect on earth's temperature, if man-made greenhouse gases weren't having a much bigger one.

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