[Vision2020] New Maunder Minimum: Cooling of 0.3°C by 2100 Offset by Anthropogenic Warming of 4°C.

[Ted Moffett]

http://www.realclimate.org/index.php/archives/2011/06/what-if-the-sun-went-into-a-new-grand-minimum/#more-7928

Guest commentary by Georg Feulner<http://www.pik-potsdam.de/members/feulner>

During a meeting <http://astronomy.nmsu.edu/SPD2011/> of the Solar Physics
Division <http://spd.aas.org/> of the American Astronomical
Society<http://aas.org/>,
solar physicists have just
announced<http://www.nso.edu/press/SolarActivityDrop.html>a prediction
that the Sun might enter an extended period of low activity (a
‘grand minimum’) similar to the Maunder Minimum in the 17th century. In this
post I will explore the background of this announcement and discuss
implications for Earth’s climate.

It has been known for a long time that solar activity shows a very regular
pattern. Every 11 years the Sun is particularly active, and numerous dark
sunspots are visible on its surface. These maxima of solar activity are
separated by times of low activity when only few (if any) sunspots appear.

One could think that the Sun emits less light during a solar maximum because
of the many dark spots. In fact it is the other way round, since active
regions around the sunspots emit more radiation than is “lost” in the cooler
sunspot areas. This effect can be best seen in ultraviolet images of the
Sun.

An analysis of historic sunspot observations shows that the 11-year solar
activity cycle was interrupted during the late 17th century.
This period of time, during which the Sun appeared without sunspots most of
the time, was called the Maunder Minimum by Jack
Eddy<http://en.wikipedia.org/wiki/John_A._Eddy>in his famous Science
paper <http://www.sciencemag.org/content/192/4245/1189.short>.
(Alliteratively named after Edward
Maunder<http://en.wikipedia.org/wiki/Edward_Maunder>,
although it was actually first discovered by Gustav
Spörer<http://en.wikipedia.org/wiki/Gustav_Spörer>
.)

The Maunder Minimum falls within the climatically cooler period of the
“Little Ice Age”, during which temperatures were particularly low over
continents in the Northern hemisphere (especially in winter). It has long
been suspected that the low solar activity during the Maunder Minimum was
one of the causes of the Little Ice Age, although other factors like a small
drop in greenhouse gas concentrations around 1600 and strong volcanic
eruptions during that time likely played a role as well.

Solar physicists do not yet understand how an extended solar-activity low
like the Maunder Minimum arises. Yet there is recent observational evidence
for an unusual behavior of the Sun during the current cycle 24, including a
missing zonal wind flow within the Sun, decreasing magnetic field strength
of sunspots and lower activity around the poles of the Sun. These
observations prompted Frank Hill and
colleagues<http://www.economist.com/node/18833483>to suggest that the
Sun might enter a new Maunder-like minimum after the
current 11-year cycle ends (i.e. after 2020 or so).

It remains to be seen whether this prognosis turns out to be true (there
have been some doubts
expressed<http://dotearth.blogs.nytimes.com/2011/06/15/a-solar-scientist-rebuts-a-cool-sunspot-prediction/>),
but since grand minima of solar activity did occur in the past, it is
certainly interesting to explore what effects such a minimum might have on
21st century climate if it did occur. This is precisely the question Stefan
Rahmstorf and I investigated in a
study<http://www.agu.org/pubs/crossref/2010/2010GL042710.shtml>published
last year (see also our press
release<http://www.pik-potsdam.de/news/press-releases/weakening-sun-would-hardly-slow-global-warming>.
(Earlier estimates for the size of this effect can be found
here<http://rsta.royalsocietypublishing.org/content/330/1615/547.short>and
here<http://www.cambridge.org/gb/knowledge/isbn/item1173464/?site_locale=en_GB>.)
In our study we find that a new Maunder Minimum would lead to a cooling of
0.3°C in the year 2100 at most – relative to an expected anthropogenic
warming of around 4°C. (The amount of warming in the 21st century depends on
assumptions about future emissions, of course).

According to these results, a 21st-century Maunder Minimum would only
slightly diminish future warming. Moreover, it would be only a temporary
effect since all known grand solar minima have only lasted for a few
decades. Critics of this result might argue that the solar forcing in these
experiments is only based on the estimated change in total irradiance, which
might be an underestimate, or that does not include potential indirect
amplifying effects (via an ozone response to UV changes, or galactic cosmic
rays affecting clouds). However, our model reproduces the historic Maunder
minimum with these estimates of solar irradiance. Furthermore, even if one
multiplied the solar effects by a huge factor of 5 (which is unrealistic),
no absolute cooling would take place (the temperatures would be temporarily
cooler than the base scenario, but the trends would still be warming).

It is clear that if a grand minimum were to happen it would be a
tremendously exciting opportunity for solar physicists, however it is
unlikely to be very exciting for anyone else.

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Vision2020 Post: Ted Moffett
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