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Correlation ain't causation.
But for some time I have been noticing that the correlations
between certain aspects of solar activity and certain aspects of
climate are getting really rather impressive -- far more so than
anything relating to carbon dioxide.
Carbon dioxide certainly can affect climate, but so for sure can
other things, and in explaining the ups and downs of past climate,
before industrialisation, variations in the sun are looking better
and better as an explanation. That does not mean the sun causes
current climate change, but it certainly suggests that it is at
least possible that forcings more powerful than carbon dioxide
could be at work.
I am not yet a solar partisan in this debate, nor do I plan to
become one. But I find the hypothesis that solar variation has been
stronger than carbon dioxide in recent decades sufficiently
intriguing that I do not see why it should be dismissed yet.
Here are some of the correlations that have impressed me. Some
may be wrong, or misleading. Some come from more
trustworthy causes sources than others. Some
might have been smoothed or otherwise manipulated. I don't really
know. But it's interesting to lay them out.
First the Parana river flow and sunspot number:
Second the Nile river flow and sunspot
correlation between cosmic ray flux (as measured by beryllium
and carbon isotope levels) and climate in Greenland. Cosmic ray
flux varies inversely with solar activity:
Fourth, cloud cover and sunspot number:
Fifth, carbon 14 and climate in Austrian
speleothems (can't get a good picture off the pdf).
Sixth, solar magnetic variation and rate of
change of temperature (interannual variation):
Now, I repeat, correlation does not mean causation, and I stress
that no individual data set can be considered reliable till
thoroughly checked and replicated. So I am not saying all climate
change is caused by the sun, nor that recent climate change must
have been caused by the sun or anything like that.
1, Some correlations exist
2. Plausible mechanisms exist (stronger solar activity leads to
better shielding of the earth from cosmic rays leads to less cloud
leads to more warmth)
3. The order of magnitude is not wrong. Carbon partisans will
argue that total solar irradiance varies by tenths of a percent
over the solar cycle, but UV output and magnetic field strength
vary by far greater amounts.
4. The only reason for ruling it out a priori that I have come
across is that solar activity has not increased markedly in recent
decades while climate has warmed. You hear this used as a final
dismissal of solar theories in the most emphatic way (here's an example:
And the results were clear. Warming in the
last 20 to 40 years can't have been caused by solar activity," said
Dr Piers Forster from Leeds University, a leading contributor to
this year's Intergovernmental Panel on Climate Change (IPCC).)
-- and it nearly always goes back to one paper, by Lockwood and Frohlich:
Here we show that over the past 20 years, all
the trends in the Sun that could have had an influence on the
Earth's climate have been in the opposite direction to that
required to explain the observed rise in global mean
But this argument assumed a lag between forcing and
equilibration (ie, how long before the earth stops warming after a
warming stimulus stops changing) of less than ten years. Here is
what Lockwood toldAlec Rawls, whose interesting post on this
subject caught my attention last week:
In the attached paper, we cite a paper by
that discusses and quantifies the heat capacity of the oceans
relevant to GMAST changes and so what the relevant response time
constant is. It is a paper that has attracted some criticism but I
think it is a good statement of the issues even if the numbers may
not always be right. In a subsequent reply to comments he arrives
at a time constant of 10 years. Almost all estimates have been in
the 1-10 year range.
In the attached paper we looked at the effect
of response time constants between 1-10 years and showed that they
cannot be used to fit the solar data to the observed GMAST rise.
Put simply. The peak solar activity in 1985 would have caused peak
GMAST before 1995 if the solar change was the cause of the GMAST
rise before 1985.
So solar activity peaked in 1985 and the earth should have
stopped warming by 1995.
Two immediate problems with this argument are immediately
obvious. First, the earth did stop warming around 2000, so there is
not a lot of difference. As Rawls puts it:
If the solar activity peak shifts five years
then Mike's temperature response formula says peak GMAST should
have occurred by 2000, which is pretty close to when it did
Second, Rawls points out that short equilibration times don't
suit the carbon partisans, because they leave them with too little
unfelt warming in the pipeline. Therefore:
Gavin Schmidt recently had occasion
comment on the time to equilibrium:
``Oceans have such a large heat capacity that
it takes decades to hundreds of years for them to equilibrate to a
This is not an unconsidered remark. Schmidt
was one of co-authors of The Team's response to Schwartz. Thus Mike
Lockwood's suggestion that "[a]lmost all estimates have been in the
1-10 year range," is at the very least passé. The clearly increased
realism of the two reservoir model makes it perfectly plausible
that the actual speed of equilibration-especially in response to a
long period forcing-could be quite slow.
So the rejection of the solar hypothesis rests on an assumption
that causes real problems for the carbon dioxide hypothesis. Either
the earth is quick to adjust to a warming factor, in which case the
mild warming of the past few decades shows that earth's sensitivity
to carbon dioxide is much lower than the models assume, or there is
still lots of warming still to come, in which case it might have
been caused by the sun's ramp-up of its activity prior to 1985.
Either way, it's wholly premature to say that the solar
hypothesis has been investigated and disproved.
There is a startlingly good brief talk by Vincent
Courtillot here. It is moderate in tone, impeccably
rational and translucently clear.
In it he argues that the actual empirical support for the IPCC's
climate models is poor, while the empirical support for a powerful
role for variations in the solar magnetic field as a driver of
climate change on earth is good.
He argues that the solar effect could vary by as much as 8 watts
per squer metre, or roughly double the forcing of carbon
Is he wrong?