Climate: The vanishing solar factor
United Press International
A weekly series by UPI examining the potential human impact on global climate change.
BOULDER, Colo., JULY 26 2004 (UPI) -- It should be no mystery to anyone who has ever been outdoors that the
sun influences the climate. But recent scientific evidence indicates solar forcing of climate change actually may be less
than previously thought.
Conventional wisdom follows the line pursued in a recent article in the London Daily Telegraph, headlined,
"Hotter-burning sun warming the planet." The story quoted Sami Solanki, director of the Max Planck Institute for Solar System
Research in Gottingen, Germany.
"The sun has been at its strongest over the past 60 years and may now be affecting global temperatures," Solanki
told the newspaper. This offers a possible explanation for global warming "that needs to be weighed when proceeding with expensive
efforts to cut emissions of greenhouse emissions," the Telegraph story said.
Solanki himself only partially subscribes to this conclusion, however.
In the 2002 Harold Jeffreys Lecture to the Royal Astronomical Society in London, Solanki said: "After 1980,
however, the Earth's temperature exhibits a remarkably steep rise, while the sun's irradiance displays at the most a weak
secular trend. Hence the sun cannot be the dominant source of this latest temperature increase, with man-made greenhouse gases
being the likely dominant alternative."
This line of reasoning found more support in a 2003 paper that analyzed beryllium-10 concentrations in ice
As found by Solanki and Ilya G. Usoskin of the Sodankyla Geophysical Observatory at the University of Oulu
in Finland: "The reconstruction shows reliably that the period of high solar activity during the last 60 years is unique throughout
the past 1,150 years. The current high level of solar activity may also have an impact on the terrestrial climate," the authors
"We note a general similarity between our long-term sunspot number reconstruction and different reconstructions
of temperature," the pair wrote, although they did not estimate how much of the observed increase in global temperature might
be accounted for by solar activity.
Beryllium-10 -- or 10Be, as it is commonly known -- is an isotope of the metal produced by the effects of
cosmic radiation in the atmosphere.
"In the absence of any cosmic ray flux, this production rate should be constant," Caspar Ammann, a paleoclimatologist
at the National Center for Atmospheric Research in Boulder, told United Press International. The place to find 10Be is in
When the sun is very active, producing a lot of sunspots -- which it does in roughly 11 years cycles -- it
forms a kind of windshield across the Earth, reducing the number of cosmic rays reaching the surface. Fewer cosmic rays means
less 10Be in the ice cores. It functions as a tidy proxy for solar activity.
The Usoskin and Solanki paper shows a neat graph with a sharp spike in a Greenland ice core that seems to
indicate stronger solar activity over the last 60 years than any time in the last 1,200 years or so.
The devil, as they say, mucks around in the details, however. In the first place, the Solanki-Usoskin paper
relies on a single ice core record from Greenland and one from the Antarctic.
"The reconstruction based on 10Be from Southern Greenland is in my opinion very problematic," Raimund Muscheler,
a geologist at Lund University in Sweden and an expert on cosmic ray-generated isotopes -- also known as cosmogenic radionucleides
-- told UPI.
"(Beryllium 10) is not only influenced by changes in solar activity and geomagnetic field changes," he continued,
"but it can also be influenced by local effect due to changes in atmospheric transport and deposition."
The Greenland record, for example, does not always agree with 10Be from Antarctica, as well as with certain
tree ring records, Muscheler explained. He added that Usoskin and Solanki did not use the last 100 years of the South Pole
In other words, the beryllium proxy is not clean and foolproof. There may already be a signal from a warming
"My conclusion about past solar activity based on radionucleide records would be the following: Solar activity
was relatively high during the last 50 years, but there were similar periods during the last 1,000 years," Muscheler said.
Ammann added: "If you would take averages of all the ice cores, you would not get this increase in (solar
activity) in the last 50 years, but it would stay relatively flat. It is only one core that shows the rise. This is not the
common feature of all of them."
A second fundamental problem is new research is showing that many long-held assumptions about solar forcing
of the climate might be incorrect. Following this part of the story requires going back to the Maunder Minimum, a period of
virtually no sunspot activity from 1645 to 1715, when Earth was undergoing a little ice age.
The sun has an 11-year cycle, in which it goes from zero sunspots (and its lowest energy) to about 150 sunspots,
on average, at its highest energy. During the Maunder Minimum, this cycle essentially disappeared for 70 years, and the sun
just floated out there at its minimum energy output.
"People say that during the Maunder Minimum, the sun must have been very unusual, because we had a cold climate
around the world," Ammann said. "They suggested that we have a lower energy coming from the sun."
A change in the lowered energy of the sun at the Earth's surface is very small and very hard to measure. The
sun's energy is expressed as watts per meter squared, and the solar constant, as it is called, is 1,367 watts per meter squared.
The variation has been measured at about 0.1 percent -- one thousandth -- or 1.36 watts per meter squared.
In order for the sun to force the climate to the little ice age observed during the Maunder Minimum, the change
in the solar constant had to be about twice what has been observed during modern, zero-sunspot periods. In other words, the
zero of the Maunder Minimum has to be lower than a modern zero at the bottom of the 11-year solar cycle. Climate models that
include solar forcing do, in fact, include this larger influence, making them more sensitive to solar forcing.
In the 1970s, astronomers observed about 10 sun-like stars, six of which had 11-year cycles like the sun's,
and four of which did not. Based on this small sample, they concluded the assumption about the low-energy Maunder Minimum
Since then, however, astronomers have surveyed many more sun-like stars, and recent work indicates the energy
output at the low end of the solar cycle does not warrant this assumption.
In another line of research that supports this conclusion, Peter Foukal of Heliophysics Inc., compared sunspot
cycles and ultraviolet irradiance. He found "a factor (three times to five times) lower than expected to produce a significant
global warming contribution based on present-day climate model sensitivities."
"For the 20th Century," Ammann said, "there should be a pretty big change based on the earlier assumption.
But they raise significant doubts on the existence of any low-frequency additional change. We have no physical basis to expect
that the zeros (of the Maunder Minimum) are any different than the zeros that we measure today."
Ammann has conducted numerous experiments on his own climate models and found they are, if anything, oversensitive
to solar forcing, based on what now known about the observed physical processes.
"It is making a relatively small forcing even smaller," he said.
Dan Whipple covers the environment for UPI Science News. E-mail firstname.lastname@example.org
Copyright 2004 United Press International
An unexplained anomaly in the climate seems to have been the result
of bad data
August 11, 2005
Get article background
CLIMATOLOGY is an inexact science at the best of times. Unfortunately it has
become, over the past couple of decades, a politically charged one as well. As the debate about global warming—and what,
if anything, to do about it—has gathered pace, uncertainties in the data that would be of merely academic interest in
other disciplines have acquired enormous practical significance. And one of the most curious uncertainties of all is the apparent
discrepancy between what is happening to temperatures at the Earth's surface and what is happening in the troposphere—the
lowest layer of the atmosphere, and thus the part that is in contact with that surface.
The troposphere is where most of the air is found and where most of the weather
occurs. Computer models predict that, if global warming is really happening, temperatures in the troposphere should rise along
with those on the surface. Recorded surface temperatures are, indeed, rising. However, both data from weather balloons and
observations made by satellites suggest that temperatures in the troposphere have remained constant since the 1970s. Over
the tropics they may even have dropped. This counter-intuitive result has caused sceptics to question how much warming, if
any, is actually going on.
There are, of course, three possibilities. One is that the sceptics are right.
A second is that the models are wrong. And the third is that there is something wrong with the data. Three papers published
in this week's issue of Science suggest that the third possibility is the correct one.
The first of these studies, conducted by Steven Sherwood of Yale University
and his colleagues, examined data from weather balloons. For the past 40 years, weather stations around the world have released
these balloons twice a day at the same time—midday and midnight Greenwich Mean Time. Each balloon carries a small, expendable
measuring device called a radiosonde that sends back information on atmospheric pressure, humidity and, most importantly for
this study, temperature.
Unfortunately, data from radiosondes come with built-in inaccuracies. For
example, their thermometers, which are supposed to be measuring the temperature of the air itself (that is, the temperature
in the shade) are often exposed to, and thus heated by, the sun's rays. To compensate for this, a correction factor is routinely
applied to the raw data. The question is, is that correction factor correct?
Dr Sherwood argues that it is not. In particular, changes in radiosonde design
intended to reduce the original problem of over-heating have not always been accommodated by reductions in the correction
factors for more recently collected data. Those data have thus been over-corrected, reducing the apparent temperature below
the actual temperature.
Dr Sherwood and his colleagues hit on a ruse to test this idea. Because weather
stations around the world release their balloons simultaneously, some of the measurements are taken in daylight and some in
darkness. By comparing the raw data, the team was able to identify a trend: recorded night-time temperatures in the troposphere
(night being the ultimate form of shade) have indeed risen. It is only daytime temperatures that seem to have dropped. Previous
work, which has concentrated on average values, failed to highlight this distinction, which seems to have been caused by over-correction
of the daytime figures. When the team corrected the erroneous corrections, the result agreed with the models of the troposphere
and with records of the surface temperature. The improvement was particularly noticeable in the tropics, an area that had
previously appeared to have high surface temperatures but far cooler tropospheric temperatures than had been expected.
The second piece of work looked at satellite measurements of tropospheric
temperatures. For the past two decades, microwave detectors, placed on a series of satellites flying in orbits that take them
over both poles, have been used to calculate the troposphere's temperature. (Microwaves radiated from the atmosphere contain
a host of information about its temperature and humidity.) Here, too, the data are problematic. Because the satellites are
looking down through the whole atmosphere, measuring the temperature of the troposphere requires subtracting the effects of
the stratosphere—the atmospheric layer above it. But when this has been done, the result suggests, like the over-corrected
data from the radiosondes, that the troposphere is cooling down relative to the surface.
However, Carl Mears and Frank Wentz of Remote Sensing Systems, a firm based
in Santa Rosa, California, think that this trend, too, is an artefact. It is caused, they believe, because the orbital period
of a satellite changes slowly over that satellite's lifetime, as its orbit decays due to friction with the outer reaches of
the atmosphere. If due allowance is not made for such changes, spurious long-term trends can appear in the data. Dr Mears
and Dr Wentz plugged this observation into a model, and the model suggested that the apparent cooling the satellites had observed
is indeed such a spurious trend. Correct for orbital decay and you see not cooling, but warming.
The third paper, by Ben Santer of the Lawrence Livermore National Laboratory
in California and his colleagues, argues that it is, indeed, errors in the data that are to blame for disagreements between
the predictions of computer models about how the troposphere should behave and what the weather balloons and satellites actually
detect. Dr Santer's team compared 19 different computer models. All agreed that the troposphere should be getting warmer.
Individual models have their individual faults, of course. But unless all contain some huge, false underlying assumption that
is invisible to the world's climatologists, the fact that all of them trend in the same direction reinforces the idea that
it is the data which are spurious rather than the models' predictions.
It is, nevertheless, doubtful that these papers will end the matter. Studying
the climate is a hard problem for three reasons. The system itself is incredibly complex. There is only one such system, so
comparative studies are impossible. And controlled experiments are equally impossible. So there will always be uncertainty
and therefore room for dissent. How policymakers treat that dissent is a political question, not a scientific one.