The sun is the one thing big enough and
significant enough to affect climate at all.
That is a little detail assiduously ignored by most in the climate game
since it looks a lot like a constant.
Yet minor variations in spectrum and flux can plausibly trigger
significant accelerators that do impact wildly and enough to disturb our world.
We learn here that EUV goes crazy during a sunspot maximum and that the
long term trend is negative. Thus we are
leaning toward a direct link between solar activity or lack there off to the sudden
cooling that has been experienced in the past.
I am also reminded that we
really need is a century of good satellite data.
Solar Variability and Terrestrial Climate
Jan. 8, 2013: In the galactic scheme of things, the Sun is a remarkably constant star. While some stars exhibit dramatic pulsations, wildly yo-yoing in size and brightness, and sometimes even exploding, the luminosity of our own sun varies a measly 0.1% over the course of the 11-year solar cycle.
There is, however, a dawning
realization among researchers that even these apparently tiny variations can
have a significant effect on terrestrial climate. A new report issued by
the National Research Council (NRC), "The Effects of Solar Variability on
Earth's Climate," lays out some of the surprisingly complex ways that
solar activity can make itself felt on our planet.
These six extreme UV images of
the sun, taken by NASA's Solar Dynamics Observatory, track the rising level of
solar activity as the sun ascends toward the peak of the latest 11-year sunspot
cycle.More
Understanding the sun-climate
connection requires a breadth of expertise in fields such as plasma physics,
solar activity, atmospheric chemistry and fluid dynamics, energetic particle
physics, and even terrestrial history. No single researcher has the full
range of knowledge required to solve the problem.
To make progress, the NRC had to assemble
dozens of experts from many fields at a single workshop. The report
summarizes their combined efforts to frame the problem in a truly
multi-disciplinary context.
One of the participants, Greg
Kopp of the Laboratory for Atmospheric and Space Physics at the University of
Colorado, pointed out that while the variations in luminosity over the 11-year
solar cycle amount to only a tenth of a percent of the sun's total output, such
a small fraction is still important. "Even typical short term
variations of 0.1% in incident irradiance exceed all other energy sources (such
as natural radioactivity in Earth's core) combined," he says.[ please note this particularly – Arclein ]
Of particular importance is
the sun's extreme ultraviolet (EUV) radiation, which peaks during the years
around solar maximum. Within the relatively narrow band of EUV
wavelengths, the sun’s output varies not by a minuscule 0.1%, but by whopping
factors of 10 or more. This can strongly affect the chemistry and
thermal structure of the upper atmosphere.
Space-borne measurements of
the total solar irradiance (TSI) show ~0.1 percent variations with solar
activity on 11-year and shorter timescales. These data have been corrected for
calibration offsets between the various instruments used to measure TSI.
SOURCE: Courtesy of Greg Kopp, University
of Colorado
Several researchers discussed
how changes in the upper atmosphere can trickle down to Earth's surface.
There are many "top-down" pathways for the sun's influence. For
instance, Charles Jackman of the Goddard
Space Flight
Center
Isaac Held of NOAA took this
one step further. He described how loss of ozone in the stratosphere
could alter the dynamics of the atmosphere below it. "The cooling of
the polar stratosphere associated with loss of ozone increases the horizontal
temperature gradient near the tropopause,” he explains. “This alters the flux
of angular momentum by mid-latitude eddies. [Angular momentum is
important because] the angular momentum budget of the troposphere controls the
surface westerlies." In other words, solar activity felt in the
upper atmosphere can, through a complicated series of influences, push surface
storm tracks off course.
How incoming galactic cosmic
rays and solar protons penetrate the atmosphere. SOURCE: C. Jackman, NASA Goddard
Space Flight
Center
Many of the mechanisms
proposed at the workshop had a Rube Goldberg-like quality. They relied on
multi-step interactions between multiples layers of atmosphere and ocean, some
relying on chemistry to get their work done, others leaning on thermodynamics
or fluid physics. But just because something is complicated doesn't mean
it's not real.
Indeed, Gerald Meehl of the National Center
The solar cycle signals are so
strong in the Pacific, that Meehl and colleagues have begun to wonder if
something in the Pacific climate system is acting to amplify them. "One of
the mysteries regarding Earth's climate system ... is how the relatively small
fluctuations of the 11-year solar cycle can produce the magnitude of the
observed climate signals in the tropical Pacific." Using
supercomputer models of climate, they show that not only "top-down"
but also "bottom-up" mechanisms involving atmosphere-ocean
interactions are required to amplify solar forcing at the surface of the
Pacific.
Composite averages for
December-January-February for peak solar years. SOURCE: G.A. Meehl, J.M.
Arblaster, K. Matthes, F. Sassi, and H. van Loon, Amplifying the Pacific
climate system response to a small 11 year solar cycle forcing, Science
325:1114-1118, 2009; reprinted with permission from AAAS.
In recent years, researchers
have considered the possibility that the sun plays a role in global warming.
After all, the sun is the main source of heat for our planet. The NRC
report suggests, however, that the influence of solar variability is more
regional than global. The Pacific region is only one example.
Caspar Amman of NCAR noted in
the report that "When Earth's radiative balance is altered, as in the case
of a chance in solar cycle forcing, not all locations are affected
equally. The equatorial central Pacific is generally cooler, the runoff
from rivers in Peru is
reduced, and drier conditions affect the western USA
Raymond Bradley of UMass, who
has studied historical records of solar activity imprinted by radioisotopes in
tree rings and ice cores, says that regional rainfall seems to be more affected
than temperature. "If there is indeed a solar effect on climate, it
is manifested by changes in general circulation rather than in a direct
temperature signal." This fits in with the conclusion of the IPCC
and previous NRC reports that solar variability is NOT the cause of global
warming over the last 50 years.
Much has been made of the
probable connection between the Maunder Minimum, a 70-year deficit of sunspots
in the late 17th-early 18th century, and the coldest part of the Little Ice
Age, during which Europe and North America
were subjected to bitterly cold winters. The mechanism for that
regional cooling could have been a drop in the sun’s EUV output; this is,
however, speculative.[ Do I need
to remind all that we lack a good explanation?
I have been playing with a long cycle change in ocean currents but also
suspect that it needs help - Arclein]
The yearly averaged sunspot
number for a period of 400 years (1610-2010). SOURCE: Courtesy of NASA Marshall Space Flight
Center
Dan Lubin of the Scripps
Institution of Oceanography pointed out the value of looking at sun-like stars
elsewhere in the Milky Way to determine the frequency of similar grand minima.
“Early estimates of grand minimum frequency in solar-type stars ranged from 10%
to 30%, implying the sun’s influence could be overpowering. More recent
studies using data from Hipparcos (a European Space Agency astrometry
satellite) and properly accounting for the metallicity of the stars, place the
estimate in the range of less than 3%.” This is not a large number,
but it is significant.
Indeed, the sun could be on
the threshold of a mini-Maunder event right now. Ongoing Solar Cycle
24 is the weakest in more than 50 years. Moreover, there is
(controversial) evidence of a long-term weakening trend in the magnetic field
strength of sunspots. Matt Penn and William Livingston of the National
Solar Observatory predict that by the time Solar Cycle 25 arrives, magnetic
fields on the sun will be so weak that few if any sunspots will be formed.
Independent lines of research involving helioseismology and surface polar
fields tend to support their conclusion. (Note: Penn and Livingston were not
participants at the NRC workshop.)
“If the sun really is entering
an unfamiliar phase of the solar cycle, then we must redouble our efforts to
understand the sun-climate link,” notes Lika Guhathakurta of NASA’s Living with
a Star Program, which helped fund the NRC study. “The report offers some good
ideas for how to get started.”
In a concluding panel
discussion, the researchers identified a number of possible next steps.
Foremost among them was the deployment of a radiometric imager. Devices
currently used to measure total solar irradiance (TSI) reduce the entire sun to
a single number: the total luminosity summed over all latitudes,
longitudes, and wavelengths. This integrated value becomes a solitary
point in a time series tracking the sun’s output.
In fact, as Peter Foukal of
Heliophysics, Inc., pointed out, the situation is more complex. The sun
is not a featureless ball of uniform luminosity. Instead, the solar disk
is dotted by the dark cores of sunspots and splashed with bright magnetic froth
known as faculae. Radiometric imaging would, essentially, map the surface
of the sun and reveal the contributions of each to the sun’s luminosity.
Of particular interest are the faculae. While dark sunspots tend to
vanish during solar minima, the bright faculae do not. This may be why
paleoclimate records of sun-sensitive isotopes C-14 and Be-10 show a faint
11-year cycle at work even during the Maunder Minimum. A radiometric
imager, deployed on some future space observatory, would allow researchers to
develop the understanding they need to project the sun-climate link into a
future of prolonged spotlessness.
Some attendees stressed the
need to put sun-climate data in standard formats and make them widely available
for multidisciplinary study. Because the mechanisms for the sun’s
influence on climate are complicated, researchers from many fields will have to
work together to successfully model them and compare competing results.
Continued and improved collaboration between NASA, NOAA and the NSF are keys to
this process.
Hal Maring, a climate scientist
at NASA headquarters who has studied the report, notes that “lots of
interesting possibilities were suggested by the panelists. However, few,
if any, have been quantified to the point that we can definitively assess their
impact on climate.” Hardening the possibilities into concrete,
physically-complete models is a key challenge for the researchers.
Finally, many participants
noted the difficulty in deciphering the sun-climate link from paleoclimate
records such as tree rings and ice cores. Variations in Earth’s magnetic
field and atmospheric circulation can affect the deposition of radioisotopes
far more than actual solar activity. A better long-term record of the
sun’s irradiance might be encoded in the rocks and sediments of the Moon or
Mars. Studying other worlds might hold the key to our own.
The full report, “The Effects
of Solar Variability on Earth’s Climate,” is available from the National
Academies Press at http://www.nap.edu/catalog.php?record_id=13519
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