The correlation between sunspot activity and climate during this period is striking (see Illustration 88). Other historical periods of low sunspot activity have been detected either directly, or by the analysis of carbon-14 isotope ratios from ice cores or tree rings. Other periods include the Sporer Minimum (1450-1540), and less dramatic Dalton Minimum (1790-1820). In total, 18 periods of sunspot minima have been found since the beginning of the Holocene. Studies indicate that the Sun spends up to a quarter of its time in these minima.292
The behavior of other Sun-like stars have been used to estimate solar irradiance during the Maunder Minimum. Lean, et al., estimated solar irradiance then was 0.15% to 0.35% lower than the present mean value.293 An independent estimate by Baliunas and Jastrow gave a range of 0.1% to 0.7% based purely on observations of solar-like stars. They concluded that a reduction in irradiance of 0.4% would be enough to explain the cold average temperatures of the Little Ice Age.294
There are good reasons for believing that changes in irradiance have been a significant factor in the rise in global temperature since the late 17th century. Reid pointed out the similarity between the overall level of solar activity, as expressed by the 11-year sunspot cycle, and globally-averaged sea-surface temperatures over the period from 1860 to the present.295 However, the Sun's contribution to more recent climate change remains controversial, partially because such links are based on correlations between the global temperature record and proxies for solar irradiance.
Illustration 89: Solar irradiance and observed sunspot activity. Source NASA.
The decadal variation in the number of sunspots suggests that they are an integral feature of the solar climate. Short term variations in the Sun's output have been tied to the sunspot cycle by satellite instruments. Comparing the two plots in Illustration 89, it is clear that total solar irradiance is directly proportional to sunspot activity. This relationship was shown empirically by NASA's Solar Maximum Mission/Active Cavity Radiometer Irradiance Monitors (SMM/ACRIM) experiment.
What troubles scientists about attributing decadal climate change to variation in the Sun's output, is that the actual measured differences in solar irradiance is not sufficient to cause the changes in earthly observed temperatures. Satellite measurements indicate the energy Earth receives from the Sun varies by a small amount. There is a measurable increasing trend, but it amounts to only 0.05% per decade. It has been estimated that increasing solar irradiance accounted for one quarter (0.25˚C) of last century's temperature increase.296
In conjunction with the sunspot cycle, the magnetic poles of the Sun reverse their polarity. These reversals influence the Sun's interaction with Earth's magnetic field. In addition to the 11-year solar cycle, the sunspot record shows century-scale modulation and periods of low activity. These changes in the Sun have led to speculation that solar variability is a trigger for other mechanisms that regulate Earth's climate. Influence on ozone production, thunderstorms and the influx of cosmic rays, have all been proposed as amplifiers for solar variation.
In contrast, the human-caused global warming scenario, as presented by the IPCC and its adherents, states that the Sun doesn't need to increase the energy it showers on our planet. The IPCC's models indicate that raising the amount of CO2 in the atmosphere is sufficient to account for global warming. To see if this is possible, we need to examine the greenhouse effect more closely, and determine if basic physics supports the greenhouse gas hypothesis.
Solar Spectrum and Greenhouse Absorption
The light coming from the Sun is spread over a wide spectrum of wavelengths. Wavelength determines the color of a photon, or packet, of light energy. Shorter wavelengths have higher frequencies and more energy in each photon. Light toward the higher frequency, blue and ultra-violet end of the spectrum, is more energetic than light from the longer wavelength, red and infrared end of the spectrum.
The hotter an object, the higher the average frequency of the light it emits. This is why large hot stars appear blue in color while the cooler red giants appear orange or red in color. By examining the color of light a hot object emits, it is possible to measure its temperature. This is how the temperature of red-hot molten lava in volcanoes, and steel in steel mills is measured. But, hot objects do not just produce a single frequency of light, they emit a range of colors fitting a characteristic curve called the black-body radiation curve.
Discovering how to calculate this curve was a major challenge to theoretical physicists during the late 19th century. The problem was finally solved in 1901, by German physicist Max Planck,297 and is formulated in Planck's law of black-body radiation.298 This is why the black-body radiation curve is also called the Plank curve.
Classical physics failed to explain the observed behavior of glowing hot objects at high frequencies. Classical theory predicted an impossible rise of the energy density towards infinity, dubbed the ultraviolet catastrophe. It was Plank's new approach, treating light energy as small, individual packets, that solved the problem. In doing so, he created a new quantum theory of physics. Plank's work was extended by Einstein, in 1905, when he published his paper on the photoelectric effect, for which Einstein received the Nobel Prize.
Illustration 90: Plank's black-body radiation curve for several temperatures.
Several Plank curves for different temperatures are shown in Illustration 90, along with the ultraviolet catastrophe curve predicted by classical theory. Notice that the temperatures here are given in degrees Kelvin. A single degree in temperature Kelvin is the same as a single degree Celsius, but the Kelvin scale is an absolute temperature scale. Unlike the Celsius scale, which places zero at the freezing point of water, the Kelvin scale takes zero (0˚K) to be the coldest possible temperature, equivalent to -273.15˚C. On the Fahrenheit scale, 0˚K is equivalent to -459.67 degrees. All objects that are warmer than absolute zero emit radiation in accordance with the Plank curve for their temperature.
Visible light is a small portion of the electromagnetic spectrum, and only a portion of the light emitted by the Sun. The curve of light frequencies given off by the Sun is shown in Illustration 91. It is not surprising that the light frequencies we see with our eyes are the frequencies of peak output by the Sun.
In order to properly understand the greenhouse effect, we must take into account the nonlinearity of the effect of increasing the concentration of atmospheric greenhouse gases. This is caused by different greenhouse gases having different spectra for the absorption of thermal radiation.
Illustration 91: The Sun's energy spectrum, Source: the COMET Program.
Each greenhouse gas has a range, or spectrum, of radiation frequencies it will absorb and re-radiate. For individual atoms, these frequencies are related to the energy levels of electrons orbiting each atom, which only come in discrete steps. These energy levels correspond to the energy of different photons and hence, to specific frequencies of electromagnetic radiation. If molecules only absorbed radiation at precisely those frequencies, very little interaction of molecules and radiation would take place. This is due to the low probability of radiation of exactly the right frequencies striking the gas molecules. However, there are factors which result in the absorption of radiation at frequencies near those in its spectrum. One such factor is the Doppler effect, resulting from the motion of the molecules.
Gas molecules, made up of multiple atoms, can also absorb radiation due to natural vibration modes of their chemical bonds and rotation of the molecule as a whole. Absorption of a photon results in a change in electronic energy accompanied by changes in the vibrational and rotational energies. The molecule's electrons get excited, its bonds vibrate like springs, and the whole molecule spins like a top.
Illustration 92: Absorption spectra of the major greenhouse gases.
This causes each greenhouse gas to have a characteristic profile, called its absorption spectrum, which describes its potential for absorbing light of different frequencies. The combined vibrational-rotational spectra of a gas can contain te
ns of thousands to millions of absorption lines. Illustration 92 shows the absorption spectra for the main greenhouse gases, plotted against the spectrum of incoming light from the Sun. Also shown is the outgoing infrared energy radiating from Earth back into space.299
The two peaks in the upper part of Illustration 92 represent two “windows” of almost clear radiation transmission. These windows are a consequence of the total atmospheric absorption spectrum shown at the bottom of the diagram, which sums up the affects of all atmospheric gases on light transmission. The one labeled “solar radiation” represents the transmission of visible sunlight, while the peak labeled “terrestrial radiation” represents the major band of long wavelength, infrared energy radiated back into space.
As mentioned in Chapter 7, only 51% of incoming solar radiation is absorbed by Earth's surface. This light energy is absorbed by the ocean and land, as well as living plants. Plants convert a small portion of this energy into chemical energy by photosynthesis, but most is translated into heat. When all the various factors are considered, only an annual average of ~235 W/m2 of light energy is absorbed at a typical earthly location, out of the 1366 W/m2 available at the top of the atmosphere. Earth's atmosphere recycles heat coming from the surface and delivers an additional 324 W/m2, resulting in the habitable temperatures at the surface.300 This all forms part of what scientists call Earth's Energy Budget, shown in Illustration 93.301 The portion of Earth's re-radiated infrared energy, which gets trapped by greenhouse gases in the atmosphere, depends on the frequencies of the outgoing radiation and the radiative efficiency of the gases involved.
Illustration 93: Earth's Global Mean Energy Budget. Source Kiehl and Trenberth, 1997.
The radiative efficiency of a gas is not based solely on the narrow frequencies of light in its absorption spectrum. It does depend upon the line spectrum of the substance, but that spectrum can be broadened by both atmospheric pressure and temperature. Increased absorption, due to broadening, can cause a positive feedback loop. On Mars the atmosphere is carbon dioxide, but at a low pressure and temperature. Therefore, the absorption spectrum of carbon dioxide on Mars is not significantly broadened and the greenhouse effect is even less than accounted for by the low density.
On Venus, on the other hand, the high pressure and temperature broadens the absorption spectrum of carbon dioxide so it is a more effective greenhouse gas than on Earth. But remember, the atmosphere of Venus is 90 times thicker than Earth's and is 96% carbon dioxide, making the atmospheric carbon dioxide concentration on Venus 300,000 times higher than on Earth. Even so, the high temperatures on Venus are only partially caused by carbon dioxide; a major contributor is the thick bank of clouds containing sulfuric acid.302
The conclusion drawn from physics is that there is a limited amount of outgoing heat energy available for greenhouse gases to absorb. Also, the amount of radiation absorbed depends upon the spectrum of the radiation impinging upon the gas. A small increase in a greenhouse gas, under conditions of low concentration, can have more of an impact than a much larger increase under conditions of high concentration. This results in a nonlinear response to increasing atmospheric concentrations of CO2.
The source of the nonlinearity may be thought of in terms of a saturation of the absorption capacity of the atmosphere in particular frequency bands. The concentration of greenhouse gases can make the atmosphere essentially opaque in a particular band. If the atmosphere absorbs 100% of the radiation in a frequency band, no amount of additional greenhouse gas will increase heat absorption. Under these conditions, the atmosphere is said to be saturated in that particular frequency band.
At lesser concentrations, the absorption of radiation is described by Beer's Law.303 Beer's Law relates the amount of radiation absorbed to a gas's absorption coefficient, a frequency-dependent molecular property, the length of the path radiation must travel through the atmosphere, and the concentration of the gas. This relationship produces what is known mathematically as a logarithmic curve, which results in a decreasing warming effect for increasing gas concentration.
Illustration 94: Nonlinear increase in radiative absorption with increasing greenhouse gas concentration.
As seen in Illustration 94, the increase from A to B produces a much bigger impact on the proportion of radiation energy absorbed than the increase from C to D even though the magnitude of the increase from C to D is larger than the increase from A to B. In fact, from point C, no increase in concentration, no matter how large, will produce as much of an impact as the increase from A to B.
Complicating factors include the partial overlap of absorption bands between CO2 and H2O, which limits to how much warming an increase in carbon dioxide can cause. The 13,000 Gt of water in the atmosphere (~0.33% by weight) are responsible for about 70% of all atmospheric absorption of radiation.304 If the CO2 and H2O absorption spectra completely overlapped, there would be no significant role for atmospheric carbon dioxide to play in greenhouse warming.
According to data from the Global Historical Climatology Network of land temperatures (GHCN) and the Extended Reconstructed Sea Surface Temperature (ERSST) data sets, the period from 1910 through mid-1940s had a global warming trend of 0.23°F (0.13°C) per decade for a net warming of 0.8°F (0.45°C)—leaving only 0.63°F ± 0.36°F (0.35°C ± 0.2°C) net warming potential for CO2 emissions from fossil fuel use during the post-WWII period.305 Ignoring the possible contributions of all other natural drivers of planetary temperature change, a 30% increment in atmospheric carbon dioxide during the later part of the 20th century caused, at most, about 1°F in temperature increase. Based on these data, most empirical calculations yield a projected temperature increase of less than 1.8°F (1°C) for a doubling in CO2.306
Why does carbon dioxide figure so prominently in the IPCC's predictions? The warming levels quoted by the IPCC are generated by GCM computer models, models that have been written based on the assumption that CO2 is the primary driver of Earth's climate. Climate model results vary widely when the underlying assumptions are changed. In a study of 108 different models, based on doubling CO2 levels, the temperature predictions ranged from a low of 0.29°F to a high of 15.6°F (0.16- 8.7°C).307 In order to get their computer models to come close to matching the past century of climate data, a number of assumptions about linkage among climate mechanisms and positive feedback loops have been made by the modelers. These assumptions are not justified by the physics of the greenhouse effect or empirical data from past climate variation.
The scientific evidence is clear: there is a significant effect from increasing CO2 at low concentrations, with decreasing impact as concentrations rise. But, for CO2 to play a dominant role at high concentrations, the level of atmospheric CO2 must rise to levels not seen since the PETM (page 63) or possibly the end of the Precambrian Snowball Earth period, more than 550 million years ago. The net effect of all these factors is that doubling atmospheric carbon dioxide levels today would not double the amount of global warming. The IPCC's models wildly overestimate the impact of CO2.
If the variation in irradiance is too small to cause the recent observed changes in temperature directly, and greenhouse gases cannot account for the warming, then what does? Other causes of warming must be investigated. Because the statistical fit of historical data, linking the sunspot cycle and climate variation, is so good, other mechanisms coupled to variation in the Sun's activity level have been proposed.
Other Possible Links to the Sun
Since ancient times, the Nile River has been the life blood of Egypt. Beginning as two separate rivers, deep in the heart of Africa, the mighty river flows for some 4,000 miles before it empties into the Mediterranean Sea. The Blue Nile originates in the Abyssinian Mountains of Equatorial Africa, while the White Nile emerges from Lake Victoria. The two merge into one at what is now Khartoum, Sudan, and flow north through the ancient lands of Nubia and Egypt.
Illustration 95: Statue of Hapi.
The Nile was so central to the lives of Egyptians that th
ey simply called it Iteru, meaning River. Without the River and its annual inundation, Ancient Egypt would never have come into being. Its fertile valley was renewed every year by the annual flood, which deposited rich silt along the riverbanks. The River filled all areas of life with religious symbolism: the creator sun-god Ra was believed to be ferried across the sky daily in a boat, while hymns to Hapi, the river god personifying the Nile, praised his bounty. Egyptian creation myths revolve around a primordial mound rising from the River's flood waters.308 Amazingly, the linkage between the Sun and River gods in Egyptian mythology are echoed in modern day scientific findings.
Knowledge of the water level of the Nile River was critically important for agriculture in Egypt throughout its history. Measurements of these levels have been carried out since the times of the pharaohs.309 Shakespeare mentioned a tower used in the measurement of water levels in Antony and Cleopatra (Act II Scene VII). These special towers, called Nilometers, are used to gauge the rise and fall of the Nile's waters to this day.
Scientists, investigating ways that solar variability influences Earth’s climate, have compared the Sun's activity level with the level of water in the Nile. Annual records of the Nile's water level are uninterrupted for the years 622-1470 AD.310 Using a technique called Empirical Mode Decomposition (EMD), which is designed to deal with non-stationary, nonlinear time series, researchers have identified two time scales in the water level data that can be linked to solar variability: an 88 year period and a longer period of about 200 years. According to Alexander Ruzmaikin, Joan Feynman and Yuk Yung, “This suggests a physical link between solar variability and the low-frequency variations of the Nile water level. This involves the influence of solar variability on the North Annual Mode of atmospheric variability and the North Atlantic and Indian Oceans patterns that affect rainfall over Equatorial Africa, where the Nile originates.”311 It seems that the deeper scientists look into the relationship between Earth's climate and the Sun, the more connections they find.
The Resilient Earth: Science, Global Warming and the Fate of Humanity Page 19