Origin Of The Greenhouse Effect Primary And Secondary Effects

The earth is a planet in dynamic equilibrium, in that it continually absorbs and emits electromagnetic radiation. It receives ultra-violet and visible radiation from the sun, it emits infra-red radiation and energy balance says that 'energy in' must equal 'energy out' for the temperature of the planet to be constant. This equality can be used to determine what the average temperature of the planet should be. Both the sun and the earth are black-body emitters of electromagnetic radiation. That is, they are masses capable of emitting and absorbing all frequencies (or wavelengths) of electromagnetic radiation uniformly. The distribution curve of emitted energy per unit time per unit area versus wavelength for a black body was worked out by Planck in the first part of the twentieth century, and is shown pictorially in Fig. 1. Without mathematical detail, two points are relevant. First, the total energy emitted per unit time integrated over all wavelengths is proportional to (T/K)4. Second, the wavelength of the maximum in the emission distribution curve varies inversely with (T/K), that is, lmax a (T/K) 1. These are Stefan's and Wien's

Wavelength/ |m

FIGURE 1 Black body emission curves from the sun (T ~ 5780 K) and the earth (T ~ 290 K), showing the operation of Wien's Law that 1max a (1/T). The two graphs are not to scale.

Wavelength/ |m

FIGURE 1 Black body emission curves from the sun (T ~ 5780 K) and the earth (T ~ 290 K), showing the operation of Wien's Law that 1max a (1/T). The two graphs are not to scale.

Laws, respectively. Comparing the black-body curves of the sun and the earth, the sun emits UV/visible radiation with a peak at ca. 500 nm characteristic of Tsun = 5780 K. The temperature of the earth is a factor of 20 lower, so the earth's black-body emission curve peaks at a wavelength which is 20 times longer or ca. 10 mm. Thus the earth emits infra-red radiation with a range of wavelengths spanning ca. 4 50 mm, with the majority of the emission being in the range 5 25 mm (or 400 2000 cm 1).

The solar flux energy intercepted per second by the earth's surface from the sun's emission can be written as Fs(1 — A)pRe2, where Fs is the solar flux constant outside the Earth's atmosphere (1368 Js 1m 2), Re is the radius of the Earth (6.38 x 106 m), and A is the earth's albedo, corresponding to the reduction of incoming solar flux by absorption and scattering of radiation by aerosol particles (average value 0.28). The infra-red energy emitted per second from the earth's surface is 4pRe2sTe4, where s is Stefan's constant (5.67 x 10 8 J s 1m 2K 4) and 4pRe2 is the surface area of the earth. At equilibrium, the temperature of the earth, Te, can be written as:

Using the data above yields a value for Te of ca. 256 K. Mercifully, the average temperature of the earth is not a Siberian —17 °C, otherwise life would be a very unpleasant experience for the majority of humans on this planet. The reason why our planet has a hospitable higher average value of ca. 290 K is the greenhouse effect. For thousands of years, absorption of some of the emitted infra-red radiation by molecules in the earth's atmosphere (mostly CO2, O3 and H2O) has trapped this radiation from escaping out of the earth's atmosphere (just as a garden greenhouse operates), some is re-radiated back towards the earth's surface, thereby causing an elevation in the temperature of the surface of the earth. Thus, it is the greenhouse effect that has maintained our planet at this average temperature, and for this fact we should all be very grateful! This phenomenon is often called the 'primary' greenhouse effect. It is, therefore, a myth to portray all aspects of the greenhouse effect as bad news; it is the reverse that is true.

Evidence for the presence of greenhouse gases absorbing infra-red radiation in the atmosphere comes from satellite data. Figure 2 shows data collected by the Nimbus 4 satellite circum-navigating the earth at an altitude outside the earth's troposphere (0 < altitude, h < 10 km) and stratosphere (10 < h < 50 km). The infra-red emission spectrum in the range 6 25 mm escaping from earth represents a black-body emitter with a temperature of ca. 290 K, with absorptions (i.e., dips) between 12 and 17 mm, around 9.6 mm, and l < 8 mm. These wavelengths correspond to infra-red absorption bands of CO2, O3 and H2O, respectively, three atmospheric gases that have contributed to the primary greenhouse effect.

Of course, the argument that the primary greenhouse gases have maintained our planet at a constant temperature of ca. 290 K pre-supposes that their


10.0 Wavelength/|m

FIGURE 2 Infra red emission spectrum escaping to space as observed by the Nimbus 4 satellite outside the earth's atmosphere. Absorptions due to CO2 between 12 and 7 mm, O3 (around 9.6 mm) and H2O (1 < 8 mm) are shown. (With permission from Dickinson and Clark (eds.), Carbon dioxide Review, OUP, 1982.)

concentrations have remained approximately constant over very long periods of time. This has not happened with CO2 and, to a lesser extent, with O3 over the 260 a (years) since the start of the Industrial Revolution, ca. 1750, and it is changes in the concentrations of these and newer greenhouse gases that have caused a 'secondary' greenhouse effect to occur over this time window, leading to the temperature rises that we are all experiencing today. That, at least, is the main argument of the proponents of the 'greenhouse gases, mostly CO2, equals global warming' school of thought. There is no doubt that the concentration of CO2 in our atmosphere has risen from ca. 280 parts per million by volume (ppmv) to current levels of ca. 380 ppmv over the last 260 a. (1 ppmv is equivalent to a number density of 2.46 x 1013 molecules cm 3 for a pressure of 1 bar and a temperature of 298 K.) It is also not in doubt that the average temperature of our planet has risen by ca. 0.5 0.8 K over this same time window (Fig. 3). What has not been proven is that there is a cause-and-effect correlation between these two facts, the main problem being that there is not sufficient structure or resolution with time in either the CO2 concentration or the temperature data. Even more recent data of the last 100 a (Fig. 4), where the correlation seems to be better established will not convince the sceptic. That said, as demonstrated most clearly by the recent IPCC2007 report [2], the consensus of world scientists, and certainly physical scientists, is that a strong correlation does exist.

By contrast, an excellent example in atmospheric science of sufficient resolution being present to confirm a correlation between two sets of data occurred in 1989; the concentrations of O3 and the ClO free radical in the stratosphere were shown to have a strong anti-correlation effect when data were collected by an aircraft as a function of latitude in the Antarctic (Fig. 5) [3].



is o


1998 Temperature Error limits

[95% confidence level]


1000 1200 1400 1600 1800 2000 Year

FIGURE 3 The average temperature of the earth and the concentration level of CO2 in the earth's atmosphere during the last 1000 a. (With permission from www.env.gov.bc.ca/ air/climate/indicat/images/appendnhtemp.gif and www.env.gov.bc.ca/air/climate/indicat/images/ appendCO2.gif)

320 310 300 290 280

Global Average Temperature and Carbon Dioxide Concentrations, 1880-2004

al b lo Gl

Year AD

Data Source Temperature: ftp://ftp.ncdc.noaa.gov/pub/data/anomalies/annual_land.and.ocean.ts Data Source CO2 (Siple Ice Cores): http://cdiac.esd.oml.gov/ftp/trends/co2/siple2.013 Data Source CO2 (Mauna Loa): http://cdiac.esd.oml.gov/ftp/trends/co2/maunaloa.co2

Graphic Design: Michael Ernst. The Woods Hole Research Center

FIGURE 4 The average temperature of the earth and the concentration level of CO2 in the earth's atmosphere during the 'recent' history of the last 100 a. (With permission from the web sites shown in the figure.)

Origin Greenhouse Effect
Latitude/degrees South

- CIO Mixing Ratio in ppt

- O3 Mixing Ratio in ppb

FIGURE 5 Clear anti correlation between the concentrations of ozone, O3, and the chlorine monoxide radical, ClO", in the stratosphere above the Antarctic during their Spring season of 1987. (With permission from Anderson et al., J. Geophys. Res. D. 94 (1989) 11465.)

There was not only the general observation that a decrease of O3 concentration correlated with an increase in ClO concentration, but also the resolution was sufficient to show that at certain latitudes dips in O3 concentration corresponded exactly with rises in ClO concentration. Even the most doubting scientist could accept that the decrease in O3 concentration in the Antarctic Spring was related somehow to the increase in ClO concentration, and this result led to an understanding over the next 10 15 a of the heterogeneous chemistry of chlorine-containing compounds on polar stratospheric clouds. Unfortunately, such good resolution is not present in the data (e.g., Figs. 3 and 4) for the 'CO2 versus T' global warming argument, leading to the multitude of theories that are now in the public domain.

I accept that it would be very surprising if there was not some relationship between such rapid increases in CO2 concentration and the temperature of the planet, nevertheless there are two aspects of Fig. 3 that remain unanswered by proponents of such a simple theory. First, the data suggest that the temperature of the earth actually decreased between 1750 and ca. 1920 whilst the CO2 concentration increased from 280 to ca. 310 ppm over this time window. Second, the drop in temperature around 1480 AD in the 'little ice age' is not mirrored by a similar drop in CO2 concentration. All that said, however, the apparent 'agreement' between rises of both CO2 levels and Te over the last 50 a is very striking. The most likely explanation surely is that there are a multitude of effects, one of which is the concentrations of greenhouse gases in the atmosphere, contributing to the temperature of the planet. At certain times of history, these effects are 'in phase' (as now), at other times they may have been in 'anti-phase' and working against each other.

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