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Maunder minimum, solar activity and the Little Ice Age: new research

  • 24 Aug 2011, 00:00
  • Verity Payne

This summer the skeptic notion that changing solar activity has caused global warming has been pedalled widely in the mainstream media. Consider, for example, notorious US skeptic Joe Bastardi's wild claims on Fox News earlier this month:

"We have warmed up overall over the last 20 to 30 years, or the last 200 years because of sunspot cycles, you can trace it to the sunspot cycle."

The promotion of this misconception is not limited to the US. Skeptic commentators in the British press have proclaimed that a new ice age is nearly upon us thanks to an approaching 'solar minimum' - suggestions that have been soundly rejected by scientists, as we reported  here.

A previous period of low sunspot activity, the Maunder Minimum, lasted 70 years in the late 17th to early 18th century and coincided with part of the 'Little Ice Age' - a period of cooling affecting parts of the globe that lasted around 300 years.

In spite of evidence that a new solar minimum cannot significantly counteract the current man-made warming, some commentators incorrectly assumed that an approaching Maunder-style solar minimum meant we were on the verge of a new ice age. 

The new research tries to ascertain exactly how the sun was behaving during the Little ice Age. In a paper titled "Are the most recent estimates for Maunder Minimum solar irradiance in agreement with temperature reconstructions?" Dr Georg Feulner of the Potsdam Institute examined two estimates of the amount of sunlight reaching the Earth (Total Solar Insolation - TSI) during the Maunder Minimum. He used these estimates to simulate temperatures over the last thousand years, and then compared the simulated temperatures with the actual temperature record to see which which estimate of TSI gave the most accurate temperature reconstuction.

The two TSI estimates Feulner looked at used different approaches and came up with very different results. The first, conducted by US scientists Schrijver et al., suggested that solar conditions during the recent 2008/2009 solar minimum might give a good idea of solar conditions during the Maunder Minimum. The second study, from Swiss researchers Shapiro et al., tried to reconstruct solar behaviour during the Maunder Minimum from long-term 'proxies' for solar activity - things which vary as the sun's activity levels do - and suggested that TSI during the Maunder minimum was around 5.8 watts per square metre (W/m2) below the 2008/2009 solar minimum. 

These two estimates clearly didn't agree. Dr. Feulner's research found that the lower Shapiro TSI estimate gave simulated temperatures that were low in comparison with the temperature record. On the other hand, using the Schrijver TSI gave a good fit between simulated temperatures and the temperature record.

This suggests that solar activity cannot have been reduced by as much as Shapiro suggests during the Maunder Minimum. TSI was instead only moderately reduced - on a par with 2008/2009 - which suggests that solar activity was not the dominant driver for the Little Ice Age.

What does this mean? The finding supports current understanding of climate science, that although energy from the sun provides almost all of the energy to power the Earth's climate system, it is not the dominant climate forcing causing recent warming. A number of independent studies have shown that over the last 35 years solar activity has waned, yet global average temperature has continued to rise. Something other than solar activity is causing the warming.

However, our understanding of how the sun influences Earth's climate system is not yet complete. Research published in 2010 by Haigh et al. added further complexity to the relationship between solar activity and climate, finding that the amount of solar energy reaching Earth during the current solar cycle actually decreased as solar activity (and TSI) increased. This unexpected result could mean that the sun's influence on atmospheric temperature and ozone is different than previously thought. Dr Mike Lockwood, space physicist at the University of Reading, UK clarifies:

"At face value, the data seem incredibly important. If solar activity is out of phase with solar radiative forcing, it could change our understanding of how processes in the troposphere and stratosphere act to modulate Earth's climate."

However, the findings of Haigh still do not mean the sun can be responsible for global warming over the last 35 years. The study's principal author Dr Joanna Haigh, atmospheric physicist at Imperial College London, UK, explains:

"If the climate were affected in the long term, the Sun should have produced a notable cooling in the first half of the twentieth century, which we know it didn't."

Our knowledge of the affect of solar activity on Earth's current and past climate is improving, but there is clearly still more to uncover. Given current evidence, there is no reason to think that warming over the last 35 years was down to the sun, or that the occurrence of the Little Ice Age undermines the idea that more recent climate change is man made.

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