[Vision2020] "Solar Spectral Stumper": Maunder Minimum Not Cause of Little Ice Age?

[Ted Moffett]

http://www.realclimate.org/index.php/archives/2010/10/solar-spectral-stumper/#more-5140

Solar spectral stumper
Filed under: Climate Science Climate modelling Sun-earth connections—
gavin @ 7 October 2010
It’s again time for one of those puzzling results that if they turn
out to be true, would have some very important implications and upset
a lot of relatively established science. The big issue of course is
the “if”. The case in question relates to some results published this
week in Nature by Joanna Haigh and colleagues. They took some ‘hot off
the presses’ satellite data from the SORCE mission (which has been in
operation since 2003) and ran it through a relatively complex
chemistry/radiation model. These data are measurements of how the
solar output varies as a function of wavelength from an instrument
called “SIM” (the Spectral Irradiance Monitor).

It has been known for some time that over a solar cycle, different
wavelengths vary with different amplitudes. For instance, Lean (2000)
showed that the UV component varied by about 10 times as much as the
total solar irradiance (TSI) did over a cycle. This information (and
subsequent analyses) have lent a lot of support to the idea that solar
variability changes have an important amplification via changes in
stratospheric ozone (Shindell et al (2001), for instance). So it is
not a novel finding that the SIM results in the UV don’t look exactly
like the TSI. What is a surprise is that for the visible wavelengths,
SIM seems to suggest that the irradiance changes are opposite in sign
to the changes in the TSI. To be clear, while the TSI has decreased
since 2003 (as part of the descent into the current solar minimum),
SIM seems to indicate that the UV decreases are much larger than
expected, while irradiance in visible bands has actually increased!
This is counter to any current understanding of what controls
irradiance on solar cycle timescales.

What are the implications of such a phenomena? Well, since the UV
portion of the solar input is mostly absorbed in stratosphere, it is
the visible and near-IR portions of the irradiance change that
directly influence the lower atmosphere. Bigger changes in the UV also
imply bigger changes in stratospheric ozone and temperature, and this
influences the tropospheric radiative forcing too. Indeed, according
to Haigh’s calculations, the combination of the two effects means that
the net radiative forcing at the tropopause is opposite in sign to the
TSI change. So during a solar minimum you would expect a warmer
surface!

Much of the longer term variance in solar output has been hypothesised
to follow what happens over the solar cycle and so if verified, this
result would imply that all current attributions to solar variability
of temperature changes in the lower atmosphere and surface ocean would
be of the wrong sign. Mechanisms elucidated in multiple models from
multiple groups would no longer have any validity. It would be
shocking stuff indeed.

Conceivably, there might be another missing element (such as a
cosmic-ray/cloud connection) that would counteract this physics and
restore the expected sign of the change, but no-one has succeeded in
finding any mechanism that would quantitatively give anything close
the size of effect that would now be required (see our previous posts
on the subject).

So is this result likely to be true? In my opinion, no. The reason why
has nothing to do with problems related to the consequences, but
rather from considerations of what the SIM data are actually showing.
This figure gives a flavour of the issues:

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courtesy Judith Lean). Estimates of irradiance in three bands are
given in each panel, along with the raw measurements from various
satellite instruments over the last 30 years. The SIM data are the
purple dots in the third panel. While it does seem clear that the
overall trend from 2003 to 2009 is an increase, closer inspection
suggests that this anti-phase behaviour only lasts for the first few
years, and that subsequently the trends are much closer to
expectation. It is conceivable, for instance, that there was some
undetected or unexpected instrument drift in the first few years. The
proof of the pudding will come in the next couple of years. If the SIM
data show a decrease while the TSI increases towards the solar
maximum, then the Haigh et al results will be more plausible. If
instead, the SIM data increase, that would imply there is an
unidentified problem with the instrument.

In the meantime, this is one of those pesky uncertainties we
scientists love so much…

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Vision2020 Post: Ted Moffett