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Roz Pidcock

Roz Pidcock

29.10.2013 | 5:00pm
ScienceClimate scientists don’t think we’re heading for another “Little Ice Age”
SCIENCE | October 29. 2013. 17:00
Climate scientists don’t think we’re heading for another “Little Ice Age”

From time to time, we’re told by parts of the media that earth is headed for another ‘little ice age’. Today was the turn of The Daily Express, in an  article urging us to “get ready” for erratic and extreme weather in the UK.

The paper claims experts warn Britain “faces a new mini-Ice Age with decades of severe Siberian winters and washout summers”. But the scientist the paper cites tells us he feels “very misrepresented”.

Inside out

The piece is loosely based on comments made by Professor Mike Lockwood from the University of Reading to BBC weatherman Paul Hudson for last night’s Inside Out programme.

The BBC programme looks back over recent cold winters in the UK and opens with the claim, “Scientists are warning that we could be heading towards a mini-ice age”.

Hudson wrote up his take on the interview here, beginning:

“It’s known by climatologists as the ‘Little Ice Age’, a period in the 1600s when harsh winters across the UK and Europe were often severe. The severe cold went hand in hand with an exceptionally inactive sun, and was called the Maunder solar minimum.

Now a leading scientist from Reading University has told me that the current rate of decline in solar activity is such that there’s a real risk of seeing a return of such conditions.”

Having spoken with Professor Lockwood, it’s clear he isn’t warning of a “new mini-ice age.” He tells us a decline in solar activity would have “nothing more than a minor effect” on global temperature.

Maunder Minimum

Both the BBC programme and the Express focus on a period beginning in the late 17th Century in which the sun went through a period of prolonged low solar activity – known as the Maunder Minimum.

This period of low solar activity lasted for around 70 years, and coincided with the beginning of what’s known as the Little Ice Age (LIA), when parts of the northern hemisphere cooled by as much as two degrees Celsius.

 Sunspot _activity

Source: NASA

25 to 30 per cent chance

Could we see another period of particularly low solar activity? As well as rising and falling on an 11-year cycle, scientists know the sun’s activity has been in  decline since about 1985.

In a paper from 2010, Lockwood put the chances of seeing a return to a Maunder Minimum-style low in solar activity in the next 50 years at about eight per cent. But a particularly fast decline in solar output in the last few years means he now thinks it’s more like a 25 to 30 per cent chance, he told the BBC’s Inside Out programme.

Does this mean we’re on course for a return to Little Ice Age conditions? Lockwood tells us he “could not have been clearer” in the interview that the answer is a resounding no.

Nevertheless, to varying degrees, this is something both the BBC blog post and The Express suggest. Warning of an imminent ice age is a perennial story. But there are two reasons why it isn’t really on the cards.

The sun is not the only factor

The latest IPCC report, published last month, explains that the Maunder Minimum was only one factor in bringing about the LIA.

It may turn out that the onset of the LIA was 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).

So a fall in temperature to rival the LIA would need more than a drop in solar activity.

Bigger human influence

Perhaps more importantly, even if we did see a return to a Maunder-style low in solar activity, earth’s climate is very different now from how it was in the 17th century.

In a paper from 2012, Lockwood and colleagues 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.

The paper says:

“[T]he likely reduction in the warming by 2100 is found to be between 0.06 and 0.1 [degrees Celsius], a very small fraction of the projected anthropogenic warming.”

Another paper from 2010 by Stefan Rahmstorf and Georg Feulner comes up with a slightly higher estimate, but still “no more than [a drop of] 0.3 degrees Celsius in the year 2100”.

That would be significant, but on the other hand we’ve already seen 0.85 degrees of human-caused warming over the industrial period, with more to come.

The IPCC’s recent report [Chp 8, page 34] says:

“[E]ven if there is such decrease in the solar activity, there is a high confidence that the Total Solar Irradiance (TSI) variations will be much smaller in magnitude than the projected increased forcing due to GHG.”

So a less active sun could have a small but noticeable cooling effect on earth’s temperature – but only if greenhouse gases weren’t having a much bigger warming influence.

The BBC piece includes the caveat that “most scientists believe long term global warming hasn’t gone away. Any global cooling caused by this natural phenomenon would ultimately be temporary, and if projections are correct, the long term warming caused by carbon dioxide and other greenhouse gases would eventually swamp this solar-driven cooling.”

But it also claims that “global temperatures may fall enough, albeit temporarily, to eliminate much of the warming which has occurred since the 1950s.”

UK specific

While we’re not on course to see a return to Little Ice Age conditions globally, there is some evidence of a regional effect of low solar activity in the UK.

Research by Lockwood and colleagues suggests colder than average winters in the UK occur more commonly during low solar activity. He tells the BBC programme:

“We think lower solar activity does seem to tie up with more cold winters in central Europe and the UK.”

Lockwood’s paper proposed a few possible mechanisms, but the authors are careful to stress that “this is a regional and seasonal effect relating to European winters and not a global effect.”

Lockwood describes the effect as a “redistribution of temperature around the Northern hemisphere, not a global effect.”

The weather we get in the UK and northern Europe is already influenced by a number of factors. So the effect of a drop in solar activity could be hard to disentangle from the natural swings in temperature from one year to the next. It’s a very complicated picture – take emerging research suggesting Arctic sea ice loss could be increasing the chances of cold winters, for example.

Irrespective, it’s clear from speaking to Professor Lockwood that he doesn’t think a decline in solar activity would push us into a mini ice age, given the current backdrop of human-induced warming.

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