the past century, solar irradiance has increased, which in itself would account for a 0.2ºC rise in surface temperature if no other mechanism existed to affect the transfer of this energy from the sun to the Earth. However, various factors in the atmosphere affect this process.
The most obvious is the effect of the clouds.
The importance of water
Clouds are composed of water vapour and droplets; the higher the concentration of water droplets, the darker and more thunderous are the clouds. By and large the lower the clouds, the higher the concentration of water droplets. When a cloud appears to obscure the sun it does so by reflecting the sun’s energy, including that in the visible spectrum, back into space, so that its light fails to reach us on Earth. This reflective action depends largely on the concentration of water as droplets. Because molecules of water also absorb the warming, shorter-wave, infrared energy, we also lose much of the sun’s heat when it is cloudy.
The water molecules in the clouds also affect the surface temperature of the Earth in their role as a greenhouse gas. They blanket over the Earth and prevent the escape of infrared energy from its surface, stopping it cooling. That is why cloudy nights are much warmer and balmier than clear nights. On cloudless nights the temperatures tend to drop rapidly once the sun sets as the atmosphere lacks the greenhouse effect of the water in the clouds. It is evident on these occasions that our comfort depends to a much greater extent on the water in the atmosphere than it does on CO2.
We are coming to realise that clouds themselves, especially their disposition and composition, are also affected by solar activity, although the contribution this makes to the temperature of our planet is difficult to quantify.
The concentration of water in the atmosphere varies. Even on a ‘dry day’ the air we breathe is moist and the air we exhale is saturated. If one looks at the amount of ice deposited in the freezer compartment of a refrigerator it is evident that the air in the refrigerator, which may have appeared to have been dry, in fact contained a lot of water, some of which was deposited as ice when it was trapped inside the refrigerator when the temperature fell.
Greenhouse gases, like carbon dioxide and water, merely store part of the energy that originated in the sun. They absorb some of the energy that radiates from the sun when it shines and from the Earth after it has been warmed by the sun. At night CO2 and water vapour act together as a blanket over the Earth, minimising the loss of heat as the atmospheric temperature begins to fall.
The effects of the various components involved in determining the Earth’s temperature are difficult to separate quantitatively. The overall effect of the clouds is especially difficult to measure, as it varies enormously from time to time and from place to place. Its effect on the sun’s energy depends upon whether it is present as a vapour or as droplets. If all the water in the atmosphere acted effectively as a greenhouse gas it would contribute a massive 96 per cent to this effect, dwarfing any contribution from CO2.
It is generally agreed that the water vapour and droplets in the clouds contribute, at the very least, 50 per cent of the total global warming effect, although many others suggest that the figure should be much nearer 93 per cent. The trouble is that the amount of water vapour in the atmosphere varies from time to time, and it is impossible to predict.
A year without summer
A lot of polluting particles in the atmosphere will enhance the effect of the clouds in insulating Earth from the effect of the sun. It is believed to be responsible for the absence of significant global warming in southern China and parts of India over the past 15 years (2008 saw their coldest winter for 50 years). Volcanic eruptions spew out polluting particles (as well as CO2 and other greenhouse gases), which influence the climate. The Tambora volcanic eruption of 1815 produced a year without a summer due to the water vapour and the particles released when it erupted. They formed a cloud over large parts of the planet. The Krakatoa volcanic explosion of 1883, which released huge amounts of debris, CO2 and sulphur dioxide into the atmosphere, affected the temperature of the world for two to three years. Although CO2 and sulphur dioxide are greenhouse gases, the world cooled.
As a result of the reduction in the concentration of particle pollutants that used to hang in the air over London, before the Clean Air Acts of the 1950s, the sun’s rays are now better able to penetrate the atmosphere. This has led to an average increase in temperature in the city of about 2ºC. This is as great as the increase in temperature predicted in the next hundred years by some of the models of global warming! Together with urban warming, this has resulted in the temperature in central London being up to 4ºC warmer than the neighbouring countryside.
Scientific controversy centres on the relative importance of the enormous, but almost impossible-to-measure, effect of water vapour and clouds, the effect of changes in the sun’s activity and the effect of the tiny but increasing 0.038 per cent of CO2 in the atmosphere, in the production of global warming. It is because these effects are impossible to separate and measure that it so difficult to make reliable mathematical models to predict the behaviour of Earth’s temperature in the future.
STANLEY FELDMAN
DOGMA
We know that mankind causes global warming.
AS THE MOLTEN MASS that was to form our planet solidified, the gases from volcanic eruptions and constant asteroid bombardment came to determine the composition of its atmosphere. It is thought to have resulted in an atmosphere largely made up of water vapour, with around 15–20 per cent of carbon dioxide (planets such as Venus and Mars contain about 95 per cent CO2 in their atmosphere). Other gases that were present, as minor players, included sulphur dioxide, carbon monoxide and nitrogen. Even before the advent of plant life and photosynthesis, the concentration of CO2 was decreasing as more and more of it became converted to chalk and sand under the very high temperatures that were present as the fiery Earth started to cool. The greenhouse-gas effect of the huge amount of CO2 in the atmosphere up to 50 million years ago did not prevent the molten mass of Earth from cooling, although it may have slowed the process.
Figure 3.1: Deep sea temperature – in spite of very high levels of CO2, Earth continued to cool. For the past 50 million years the level of CO2 has remained historically low.
Although the first evidence of life can be traced back some 2–3 billion (thousand million) years, it was not until about 400–500 million years ago that the first recognisable animal life forms appeared. By this time it is calculated that the CO2 concentration in the atmosphere had decreased from 15–20 per cent to 4–5 per cent. By the time the first marine organisms emerged on to the swampy land, many millions of years later, the CO2 levels had fallen still further, to about 1.5 per cent in the atmosphere, many times higher than today.
In spite of the enormously high levels of CO2 in the planet’s atmosphere when it was born, the Earth cooled down. This is not surprising, since it was originally very much hotter than the surrounding atmosphere. There is evidence that the cooling continued, even when the surface became much colder, causing large parts of the planet to be covered with ice. It is believed
