BECAUSE things are the way they are, things will not stay the way they are, German poet, playwright, and theatre director Bertolt Brecht famously stated.
It’s a quote that is particularly apt for planet Earth in the opening decades of the 21st century.
Sceptics who question or deny human-induced global warming rightly point out that Earth’s climate has changed fundamentally, and changed many times, during the planet’s 4.5 billion-year existence.
However, they tend to gloss over the fact that Homo sapiens, as a species, evolved less than half a million years ago and that fully modern humans like ourselves only emerged during the last 200 000 years or less, in the proverbial blink of a geological eye. So our ability to experience, and survive, significant climate change has been limited.
It’s crucial to appreciate just how precarious the foundation is on which all life on Earth depends.
Our climate is controlled by the planet’s atmosphere, an almost unbelievably thin band of mixed gases that extends just 100km above the surface of the Earth, kept in place by gravity. The troposphere, the most crucial part of the atmosphere that supports most life, is a mere 18km or so thick at the equator and as little as 10km at the poles – not even as far as the city centre to Claremont.
Although Earth’s temperatures vary from between a maximum of about 57°C to a lowest recorded minimum of –89.2°C, the average global temperature during the past century has been around a relatively balmy 14°C or so, ideal for life, but definitely getting warmer, which is bad news.
In contrast, the surface temperature of Venus is 467°C, while that of Mars varies between about –143°C and 27°C.
On Earth, it is the apparently unique combination of atmospheric chemistry and the physical phenomenon of the so-called greenhouse effect that forms the planet’s life- support system, creating a natural thermostat that protects and nourishes life.
The atmosphere contains about 78.08 percent nitrogen, 20.95 percent oxygen, a variable amount of water vapour that averages around 1.247 percent, 0.93 percent argon, 0.038 percent carbon dioxide, and traces of hydrogen, helium and other “noble” gases.
This particular combination allows us to breathe – just slightly more oxygen or methane and we’d spontaneously combust. But crucially, it also maintains an even, moderate climate regime by absorbing solar radiation that naturally warms the surface of the Earth through heat retention to just the right temperature, maintained by the greenhouse effect. Without this, the Earth would be much colder, with an average global temperature probably as low as –18°C.
The greenhouse effect works like this: Earth derives its energy as heat from the Sun and it arrives in the form of short wavelength ultraviolet radiation. Some 30 percent of this UV radiation is reflected away by the atmosphere – if it all arrived, Earth would be scorchingly hot. Part of the 70 percent that gets through is absorbed by the atmosphere, and only about 50 percent reaches the surface of the Earth. In turn, some of this heat energy is reflected back upwards from the planet’s surface in the form of long-wavelength infrared radiation.
If this reflected radiation, or heat, all immediately escaped back through the atmosphere into space, Earth would be much too cold to support life.
But the chemical structure of the so-called “greenhouse gases” in the atmosphere allows them to absorb much of this and reflect some back towards Earth again to help it maintain its temperature equilibrium, while the rest is trapped and slowly released into space – the original space blanket, as it were.
Water vapour and carbon dioxide (CO2) are the two most important greenhouse gases, while methane, nitrous oxide, ozone and several other gases also contribute. Nitrogen and oxygen, by contrast, have almost no greenhouse effect. This means over 99 percent of the gases in the atmosphere are not greenhouse gases, so warming is due to just a tiny fraction of the total gases. This is why our actions are important – we can double that greenhouse gas total quite easily, even though the atmosphere is so vast.
We’ve known about the greenhouse effect for a long time. It was discovered as far back as 1824 by the French mathematician Joseph Fourier. However, it was the prominent Irish physicist John Tyndall whose 1858 experiment first proved the phenomenon.
The Swedish scientist Svante August Arrhenius, originally a physicist who helped found the science of physical chemistry, developed a theory to explain Earth’s ice ages. Then, in 1896, he became the first scientist to speculate that, because of CO2’s capacity to absorb radiation and retain heat, changes in the CO2 concentration in the atmosphere could substantially alter the global surface temperature through the greenhouse effect. He predicted that CO2 emissions from the burning of fossil fuels like coal would cause global warming.
Other activities that release huge quantities of CO2 and other greenhouse gases into the atmosphere are industrial processes ike cement making, and – particularly – the clearing of forests and areas of other natural vegetation for agriculture, because all plants, and especially trees, absorb and hold (or “sequester”) carbon from the atmosphere.
The proportion of CO2 in the atmosphere has varied widely and is believed to be probably the most fundamental factor (but not the only one, obviously) affecting global climate. Arrhenius expected the CO2 concentration to double in about 3 000 years – but most climate models now suggest this will take only about 100 years at current rates.
From tiny bubbles trapped deep in the ice and retrieved from cores reflecting more than 800 000 years of climate history, scientists have shown that CO2 values were as low as 180 parts per million (ppm) deep in the past, and that during the pre-industrial level before the large-scale burning of fossil fuels, they were 270ppm.
Measurements at the Mauna Loa Observatory in Hawaii show that CO2 concentrations have increased from about 313ppm in 1960 to about 389ppm in 2010, exceeding the 650 000-year geological record maximum of 300ppm.
Global emissions of all greenhouse gases from human activities increased 79 percent between 1970 and 2004. As of July this year, the CO2 concentration had already reached 392ppm. Last month, it had fallen slightly to 389.53ppm.
The Intergovernmental Panel on Climate Change, the most authoritative scientific body on climate change, set up jointly by the UN Environment Programme and the World Meteorological Society in 1988, believes that if the CO2 concentration can be restricted to between 350ppm and 400ppm, peaking between now and 2015, global average temperature can be kept to between 2°C and 2.4°C higher than the pre-industrial temperature.
The critical 2°C warming from pre-industrial levels is a figure derived from the second IPCC report, and is the highest increase that can be afforded if the world is to have just a 50 percent chance of avoiding the worst effects of climate change.
In contrast, allowing the CO2 concentration to climb to between 570ppm and 660ppm, peaking between 2050 and 2080, could see the global average temperature climb to somewhere between 4°C and nearly 5°C above the pre-industrial level, it has warned.
That may not sound like a lot to the uninitiated, but it’s extremely significant and would in fact it mean an entirely different Earth of the kind last experienced millions of years ago before any human species existed. For example, the last time the planet was just four degrees warmer on average, there were no polar ice caps.
One of the major hopes for COP17 is that an agreement will be forged to keep the global temperature increase to below 2°C, although many nations – including the Africa bloc – believe that is still too high and they want to try and limit it to just 1.5°C.
l John Yeld is the Cape Argus’s Environment & Science Writer.