Miner's Canary Is Still Singing
By Sallie Baliunas,
Ph.D., and Willie Soon, Ph.D.
Harvard-Smithsonian Center for Astrophysics
bird of the air shall carry the voice, and that which hath wings shall tell the matter.
Dec. 12, 1901, Guglielmo Marconi (18741937) sent radio waves across the Atlantic
Ocean. But radio waves, like light waves, travel in straight lines. So how was it possible
that the radio waves followed the curve of the earth? Since the waves were received over
the horizon, out of direct line of sight, they must have "bounced" off something
in the sky.
something was the ionosphere, the layer of charged particles, that reflects certain
wavelengths of radio waves, discovered in 1924 by Edward Appleton (18921965).
their discoveries, these two scientists were awarded the Nobel Prize in
PhysicsMarconi in 1909 and Appleton in 1947. Why all the excitement about the
the human impact on global warming requires knowing what that impact looks like. Since the
atmospheric concentrations of minor greenhouse gases such as carbon dioxide and methane
are increasing, the climate is expected to change.
scenarios of climate change make specific forecasts based on projected increases in these
minor gases. But, as WCR readers know, detecting human impact on climate change is
trickytake, for example, the complexity of the natural, underlying variations in the
climate, the backdrop against which the human effect must be seen.
the expected human effect from natural climate changes is difficult near the earths
surface. One reason why is that the predicted change is not very largearound 1°C in
the global average temperature during the 20th century. Given spotty surface coverage,
uncertain corrections for urban heat island effect, and so on, the instrumental surface
record does not yield a confident detection of the human-caused global climate warming.
is there somewhere in the climate system, a place where the global signal is supposed to
be bigger, and perhaps easier, to see? Yes, according to the modelsin the upper
reaches of the atmosphere.
do the different layers of the atmosphere look like? Starting at the surface is the
troposphere where the temperature falls with altitude until the tropopause, at roughly 10
to 15 km (Figure 1). From there, the temperature warms with increasing altitude through
the stratosphere, up to the stratopause near 50 km. Above the stratosphere, the
temperature cools with increasing altitude, in a layer called the mesosphere.
1. The temperature of the upper atmosphere with height at sunspot maximum and sunspot
about 100 km, the mesopause marks the final reversal in the temperature trend with
heightit now rises rapidly through the thermosphere, the penultimate layer before
"space" is encountered.
the thermosphere is the ionosphere, extending roughly from 100 to 400 km. As its name
suggests, this layer consists of positively and negatively charged particles, or ions, in
surprisingly high proportions (about 1 particle in 2,000 is an ion, and the rest are
neutral atoms or molecules). The chief reason for the presence of ions is the suns
X-ray-ultraviolet (XUV) light. The suns energetic radiation at those wavelengths
dissociates the oxygen (O2) and nitrogen (N2) molecules into
individual atoms. The radiation also ionizes some of the atoms by stripping an electron.
This leaves free, negatively charged electrons and the positively charged remainder of the
high energy in the XUV part of the suns spectrum heats the ionospheric particles so
much that they are moving quite swiftly. In addition, the suns XUV light (in fact,
the entire spectrum of the suns radiation) varies with the 11-year sunspot cycle,
causing a large swing in the temperature of the ionosphere every 11 years. As a result,
the temperature of the ionosphere is high and varies from about 500°C at sunspot minimum
to about 1500°C at maximum (Figure 1).
in a coal mine
forecast large temperature changes in the upper atmosphere when CO2 is doubled.
The region near the stratopause cools by 10°C to 12°C; near the mesopause temperature
drops 6°C to 12°C, with larger cooling above.
the ionosphere cools even moreas much as 50°C. So the ionosphere seems a good place
to look for the arrival of the enhanced greenhouse signal from human actionsthe
equivalent of the death of the miners canary.
may seem puzzling that the upper layers would cool for increased CO2 levels,
which is opposite the warming trend for the low atmosphere. Added CO2 can cause
warming at the surface because CO2 absorbs and re-radiates the earths
CO2 means more absorption and re-emission. Because the density of air near the
surface is high, the re-radiated energy cannot easily escape to space. Hence, the low
layer of air warms.
dragged upward by currents in the air, has a different impact in the upper layers. There,
CO2 also absorbs energy and re-emits it. But compared with the surface, the
ionosphere is one trillion times less dense99 percent of the mass of the atmosphere
sits below an altitude of 30 km.
the upper layers of air are so rarefied, the radiation can easily escape to spaceand
cool those layers.
do the model calculations of cooling in the ionospheredifficult as they are to
makecompare with the data?
data mostly begin near the International Geophysical Year, 1957. Four decades of records
seem long enough to check for cooling trends as large as those forecast.
temperature of the ionosphere is not directly measured, other related properties, like the
electron density and the height of a layer in the ionosphere that reflects radio waves
(called the F2 layer), are measured. Some results for individual sites suggest
trends consistent with anthropogenic cooling from greenhouse gas increases, such as
Germany, Finland, and two Southern Hemisphere stations.
since the anthropogenic signal should be global, Upadhyay and Mahajan gathered a more
global data base in order to make a more definitive test of greenhouse gas cooling in the
ionosphere. Using 31 stations scattered worldwide, they saw that individual station
records differ widely in the long-term patterns that they showsome indicate heating,
some cooling and others no trend. Considered together, no global, long-term cooling trend
is seen among the records (Figure 2).
2. Differing trends (km/year) of the changing height of the peak of the F2
layer in the ionosphere from stations worldwide. No global cooling trend is evident.
of the difficulties in seeing a cooling trend, as large as it is expected to be, is hinted
at in Figure 1: the effect of the 11-year sunspot cycle on the temperature and other
properties of the ionosphere. In the case of temperature, the forecasted signal for
doubling CO2 is around 50°C. But the change in temperature in the ionosphere
due to the sunspot cycle is 1000°C! Were looking for a very small residual, so the
accurate removal of the suns signal is critical.
makes the expected signal of anthropogenic climate change in the ionosphere extremely
difficult to see against an 11-year solar effect that is 40 times larger (for going
halfway to a doubling of CO2).
and precise records may reveal anthropogenic cooling in the ionosphere. But for now, it
may be best to return to the surface of the earth.
Bremer, 1992, Ionospheric trends in mid-latitudes as a possible indicator of the
atmospheric greenhouse effect, Journal of Atmospheric and Terrestrial Physics, 54,
Portmann, et al., 1995, The importance of dynamical feedbacks on doubled CO2-induced
changes in the thermal structure of the mesosphere, Geophysical Research Letters, 22,
H., and R.G. Roble, 1992, Cooling of the upper atmosphere by enhanced greenhouse
gasesModelling of thermospheric and ionospheric effects, Planet. Space Sci., 40,
H.O., and K.K. Mahajan, 1998, Atmospheric greenhouse effect and ionospheric trends, Geophysical
Research Letters, 25, 33753378.