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Roz Pidcock

30.04.2013 | 4:30pm
ScienceWhy changes in solar activity don’t mean a ‘mini ice age’ is on the way
SCIENCE | April 30. 2013. 16:30
Why changes in solar activity don’t mean a ‘mini ice age’ is on the way

From time to time, we’re told by parts of the media that earth is headed for a ‘mini ice age’. Despite decades of data to the contrary, the theory is linked to the mistaken belief that the sun is driving climate change, not human activity.

The Daily Mail repeated the claim today in an article urging us to “forget global warming …the Earth may soon be plunged into a 250-year cooling period”. The piece follows a radio interview with scientists from the Russian Pulkovo observatory.

But the vast majority of scientists do not agree that the sun is the main driver of climate change – or that we’re on course for a mini ice age. Here’s why.

Solar changes and the Little Ice Age

Scientists know the sun has some impact on 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.”

The sun’s energy fluctuates, rising and falling on an 11-year cycle. The graph below shows how solar activity has changed over time.

Sunspot _activity

Source: NASA

Beginning in the late 17th Century, the sun went through a period of prolonged low solar activity – 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 next solar minimum (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 irradiance 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.

Dr Peter Stott, who leads the Met Office’s climate monitoring and attribution unit, told us recently the idea that the sun could override the effects of human activity on the climate is at odds with the broader scientific understanding. He says:

“There is a strong scientific consensus that human-induced greenhouse gas emissions are the main cause of global warming over the last fifty years and it is misleading to the public that other theories, such as that most of the warming is caused by solar changes, carry equal weight.”  

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.”

A less active sun would probably have a small cooling effect on earth’s temperature, if man-made greenhouse gases weren’t having a much bigger warming influence.



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