In a message dated 3/13/2007 11:19:34 A.M. Eastern Daylight Time,
RLAMONT@providence.edu writes:
was totally unaware that global climate models incorporate the sunspot
cycle and the associated variability in solar output into the modeling
process. I have seen arguments presented outside the global climate
models that argue that solar variability is not sufficient to explain
the global temperature changes that occurred in the last century
(although I find the data that Baliunas presents more convincing), but I
didn't think that this was explicitly incorporated in the models
themselves.
SOLAR FORCING OF CLIMATE (ARE INDIRECT SOLAR EFFECTS AFFECTING CLIMATE?)
A common alternative explanation to anthropogenic greenhouse gas climate
forcing in the 20th century is that our Sun is responsible for observed
temperature increases. Have not there been significant temperature variations during
the Holocene (last 10,000 years) when there were no fossil fuels being
burned?_3_ (http://www.thescientificworld.com/ViewHTML.asp?artID=37713047#e3) If
the Sun's variability caused those, why could not it be causing the present
warming? Perhaps a more focused question would be: "How much of the observed
warming in the 20th century is caused by changes in solar activity?"
Much work has gone into looking for correlations between northern hemisphere
(NH) temperature variations and proxy determinations of solar variability
(because land surface, which predominates in the NH, responds to solar
variation more than sea surface).3 There seems no other way to explain such
historical events as warming in medieval times and cooling in both the 17th and 19th
Centuries excepting by solar variations and resulting feedbacks (with the
complication of assorted large volcanic eruptions)[_49_
(http://www.thescientificworld.com/ViewHTML.asp?artID=37713047#r49) ].
The Sun is a variable star. In fact other stars like our Sun are seen to
vary also[_50_
(http://www.thescientificworld.com/ViewHTML.asp?artID=37713047#r50) ]. In a cycle that varies in length from 10 to 12 years, sunspots come and
go, the solar wind strengthens and weakens, and other associated phenomena
are observed changing. Also the longer-term level of solar activity changes
with time - now getting stronger with more sunspots, now getting weaker with
fewer (see sunspot numbers plotted in _Fig. 12_
(http://www.thescientificworld.com/ViewHTML.asp?artID=37713047#f12) ). In fact in the 1600s there were no
sunspots at all! This has been called the Maunder Minimum and occurred at
roughly the same time as the so-called "Little Ice Age" when temperatures around
the planet were cooler than normal.
If one looks at the record of sunspot numbers vs. time, the 10- to 12-year
cycle is obvious, but also obvious is that the maximum number of sunspots in
each cycle varies. These appear loosely to be modulated by an 80 year
envelope, called the Gleissberg Cycle._4_
(http://www.thescientificworld.com/ViewHTML.asp?artID=37713047#e4) Comparison of the ups and downs at the maximum of
each activity cycle with NH temperature estimates over the past few hundred
years shows a correlation, but it is not strong largely because at times it lags
the temperature changes by up to two decades. However, another correlation
was found that was much better. Instead of correlating with activity
amplitude, Friss-Christensen and Lassen[51a,b] correlated NH temperature change with
the length of the solar cycles. Surprisingly, this gave a very good
correlation, particularly because its ups and downs in the 20th century were some 20
years on average ahead of a similar pattern found in the solar activity
amplitude record, allowing it to coincide with temperature changes. Extension of
this proxy back to 1550 seemed to give equally good results (_Fig. 11a_
(http://www.thescientificworld.com/ViewHTML.asp?artID=37713047#f11a) ) - but another
attempt to repeat this work arrived at less good correlations (_Fig. 11b_
(http://www.thescientificworld.com/ViewHTML.asp?artID=37713047#f11b) ).
To give these correlations a more physical basis, all that was needed was a
quantitative measure of irradiance change with solar activity, because,
unless solar irradiance was varying with solar activity, there was no accepted
mechanism for how these changes could affect Earth's climate. Satellites began
measuring the solar flux in 1979, and so by the early 1990s we had those
numbers._5_ (http://www.thescientificworld.com/ViewHTML.asp?artID=37713047#e5)
Armed with this 20-year quantitative measure, several groups attempted to
calibrate the changes in solar irradiance in the past few hundred years. Most
quoted are two works: (1) Hoyt and Schatten[_52_
(http://www.thescientificworld.com/ViewHTML.asp?artID=37713047#r52) ] reasoned that changes in convective
activity were the root cause of changes in the length of the solar cycle. They
related irradiance changes to this and developed a proxy record based on length
of the cycles that could be followed back to 1700. Lean et al.[_53_
(http://www.thescientificworld.com/ViewHTML.asp?artID=37713047#r53) ] developed a
similar reconstruction but based on activity amplitude of the solar cycles that
could be traced back to 1610 (_Fig. 12_
(http://www.thescientificworld.com/ViewHTML.asp?artID=37713047#f12) ).
To assess how much of the proxy temperature variations over this time
interval could be attributed to solar activity variation, energy balance models
using these proxy irradiance reconstructions and assuming feedback mechanisms
similar to those found for GHG forcing were employed. These showed that solar
variations could account for most of the NH temperature changes until midway
in the 20th century. They suggested that increases in solar irradiance could
account for 30 to 50% of the 20th century warming (_Fig. 13_
(http://www.thescientificworld.com/ViewHTML.asp?artID=37713047#f13) ). Some attempts to
reproduce these results with better models have tended to side with the lower part
of this range[_54_
(http://www.thescientificworld.com/ViewHTML.asp?artID=37713047#r54) ], and recent studies[_48_
(http://www.thescientificworld.com/ViewHTML.asp?artID=37713047#r48) ,_55_
(http://www.thescientificworld.com/ViewHTML.asp?artID=37713047#r55) ] and suggest only about 25%. (These included the
cooling effects of volcanic eruptions and used the most recent temperature
reconstructions [_Fig. 9_
(http://www.thescientificworld.com/ViewHTML.asp?artID=37713047#f9) ] which show less temperature variation than earlier work.)
However, other recent detailed studies using both H&S and Lean et al. forcing[_56_
(http://www.thescientificworld.com/ViewHTML.asp?artID=37713047#r56) ,_57_
(http://www.thescientificworld.com/ViewHTML.asp?artID=37713047#r57) ,_58_
(http://www.thescientificworld.com/ViewHTML.asp?artID=37713047#r58) ] show that, in
order to explain the 20th century record well, one needs 30 to 45% solar and
the rest a combination of AGHG positive forcing and aerosol negative forcing.
So the answer to our original question is that, yes, some of the warming in
the 20th century was caused by increases in solar activity-related
irradiance, but not all, apparently not even half. See the recent excellent summary
article on this by Lean and Rind[_59_
(http://www.thescientificworld.com/ViewHTML.asp?artID=37713047#r59) ].
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