Our understanding of the influence of solar short-wavelength radiation on the Earth’s upper atmosphere is poorer than many realise. It is a significant area of research as some scientists believe that solar short-wave variations play a greater role in climatic variations than an initial analysis of the amplitude of those variations suggests, especially because of the fairly recent realisation that there are spectral variations as well as intensity variations in the short wavelength radiation coming from the sun.
At the very least it is commonly overlooked that just considering solar irradiance changes over the 11-year solar cycle when the cycle is rising and sunspot number increasing the climatic forcing effect of the slightly increased solar output is, over the 6-7 years of the cycle, the same as the climatic forcing effect of increasing greenhouse gas concentrations over the same period, and that is without any additional solar-terrestrial climatic coupling, such as the effects suggested below.
Ultra-Violet emission from the sun is modulated on an 11-year and 28-day period (solar rotation) with variability increasing as the wavelength decreases. In the Extreme UV the variation is between a factor of 2 – 10.
EUV is absorbed in the Earth’s Thermosphere at an altitude of between 90 – 500 km creating the ionosphere. In the thermosphere temperature increases with increasing altitude. At an attitude of 400 km the temperature is about 600K at solar minimum and about 1500K at solar maximum. The thermosphere at 400km also undergoes a fluctuation in density of a factor of 10 between solar min and max, this is important for the drag-limited lifetime of satellite orbits.
The thermosphere is thin and high and a trillionth of the density at sea level. Although it has nothing to do with climate directly it is still an important testbed for the testing of theories of increasing greenhouse gas concentrations in the troposphere and their possible effects on the thermosphere above it. Roble and Dickinson (1989) for example say this would decrease the temperature of the upper atmosphere due to CO2 radiational cooling.
Indeed, there is a at 400 km a 2 – 5 % decadal density change seen that is roughly in line with the models. But how can one really tell what changes are due to solar effects and what are due to AGW? The answer is surprising.
Solar Cycle 23 had a long, slow, decline and the minima has been extended so much that some solar scientists are speculating that the sun has undergone a significant change in behavior such as that seen in the Dalton minimum 200 years ago or possibly the far more prolonged Maunder Minimum of the 17th century, both of these changes in solar activity had important climatic repercussions on Earth. During the decline and subsequently solar EUV became anomalously low and the ionosphere was lower and cooler than normal.
The obvious question is, are solar EUV and terrestrial CO2 changes compatible with the thermospheric density changes?
An answer has come from the results of a study published by scientists at the National Center for Atmospheric Research (NCAR) in Boulder, Colo., and the University of Colorado at Boulder (CU). They looked at the solar EUV output between 1996 and 2008 running thermospheric response simulations with CO2 unchanged and with CO2 increasing over the same period.
The conclusion was that estimates of the effect of increasing CO2 explains only a small fraction of the observed density changes in the thermosphere. Solar EUV is the primary driver. However, looking at the results I would go further. The influence of increasing CO2 is in the statistical noise and is not, statistically speaking, needed to account for the thermospheric density changes at all.
Another interesting paper on solar influences has recently been published. Professors Jean-Louis Le Mouel, Vincent Courtillot and colleagues have been busy in recent years publishing papers considering how the Sun’s variable magnetic activity may influence terrestrial phenomena.
Figure 1 from their latest work displays some rather unexpected and surprising correlations between the long-term variation in the amplitude (A) of the length-of-day and two solar activity measures: sunspot number (SN) and neutron count (a proxy for incoming galactic cosmic rays), which were obtained from an observatory in Moscow, Russia. Top and bottom are raw data, the middle graph is detrended data. Click on image to enlarge.
Le Mouel explains the correlations in Figure 1 as being due to a plausible physical link of the 11-year solar activity cycle and a modulation of tropospheric zonal wind (note that winds above troposphere contribute less than 20% of Earth’s angular momentum). The researchers also make the important point that although the IPCC and many scientists generally rule out the role of solar irradiance impact on terrestrial climate because of the small changes in solar irradiance, such an viewpoint does not apply to the possibility of the large seasonal incoming solar radiation in modulating the length-of-day amplitude.
Consequently, Le Mouel et al. (2010a) say their paper “shows that the Sun can (directly or indirectly) influence tropospheric zonal mean-winds over decadal to multidecadal time scales.” And noting that “zonal mean-winds constitute an important element of global atmospheric circulation,” they go on to suggest that “if the solar cycle can influence zonal mean-winds, then it may affect other features of global climate as well, including oscillations such as the North Atlantic Oscillation and the Madden–Julian oscillation, (a coupling between atmospheric circulation and tropical convection.”)
Given the complexity of the atmosphere, and the ways the sun’s output can vary, it seems too sweeping to say that the observed small irradiance changes seen from the sun over a solar cycle can only account for a 0.1 deg C change in global annual average temperatures.