The Miner's Canary Is Still Singing

By Sallie Baliunas, Ph.D., and Willie Soon, Ph.D.
Harvard-Smithsonian Center for Astrophysics

A bird of the air shall carry the voice, and that which hath wings shall tell the matter.

—Ecclesiastes, X, 20

On Dec. 12, 1901, Guglielmo Marconi (1874–1937) 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.

That something was the ionosphere, the layer of charged particles, that reflects certain wavelengths of radio waves, discovered in 1924 by Edward Appleton (1892–1965).

For their discoveries, these two scientists were awarded the Nobel Prize in Physics—Marconi in 1909 and Appleton in 1947. Why all the excitement about the ionosphere?

Detecting 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.

Computer-based 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 tricky—take, for example, the complexity of the natural, underlying variations in the climate, the backdrop against which the human effect must be seen.

Separating the expected human effect from natural climate changes is difficult near the earth’s surface. One reason why is that the predicted change is not very large—around 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.

Over the rainbow

But 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 models—in the upper reaches of the atmosphere.

What 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.

Figure 1. The temperature of the upper atmosphere with height at sunspot maximum and sunspot minimum.

At about 100 km, the mesopause marks the final reversal in the temperature trend with height—it now rises rapidly through the thermosphere, the penultimate layer before "space" is encountered.

Within 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 sun’s X-ray-ultraviolet (XUV) light. The sun’s 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 atom.

The high energy in the XUV part of the sun’s spectrum heats the ionospheric particles so much that they are moving quite swiftly. In addition, the sun’s XUV light (in fact, the entire spectrum of the sun’s 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).

Canary in a coal mine

Models 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.

But the ionosphere cools even more—as much as 50°C. So the ionosphere seems a good place to look for the arrival of the enhanced greenhouse signal from human actions—the equivalent of the death of the miner’s canary.

It 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 earth’s infrared radiation.

More 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.

CO₂, 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 dense—99 percent of the mass of the atmosphere sits below an altitude of 30 km.

Because the upper layers of air are so rarefied, the radiation can easily escape to space—and cool those layers.

Observe and learn

How do the model calculations of cooling in the ionosphere—difficult as they are to make—compare with the data?

The 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.

While 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.

But 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 show—some indicate heating, some cooling and others no trend. Considered together, no global, long-term cooling trend is seen among the records (Figure 2).

Figure 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.

One 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! We’re looking for a very small residual, so the accurate removal of the sun’s signal is critical.

This 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).

Long 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.

References:

J. Bremer, 1992, Ionospheric trends in mid-latitudes as a possible indicator of the atmospheric greenhouse effect, Journal of Atmospheric and Terrestrial Physics, 54, 1505–1511.

R.W. 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, 1733–1736.

Rishbeth, H., and R.G. Roble, 1992, Cooling of the upper atmosphere by enhanced greenhouse gases—Modelling of thermospheric and ionospheric effects, Planet. Space Sci., 40, 1011–1026.