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.