Only One Planet


GLOBAL WARMING: The "Greenhouse Effect"
The Earth is heated by the Sun. The atmosphere that blankets the Earth is made up, for the most part, of Oxygen and Nitrogen (99%). However, there are traces of many other gasses, including water vapour, carbon dioxide (CO2), ozone, methane and nitrous oxide. These gasses are responsible, to a very great extent, for regulating our planet's surface temperature by trapping infra-red radiation which is re-radiated by the planet's surface. This is known as the "Greenhouse Effect". The diagram below shows a somewhat simplified explanation of how this works.

Solar energy (that is energy from our Sun) passes through our atmosphere and reaches the Earth's surface, where it heats both the land and the oceans. Of course, some of this energy is reflected from light coloured areas such as glaciers. Some of the energy that is absorbed by the Earth is re-radiated from the surface as infra-red (IR) radiation. When this IR radiation reaches the greenhouse gas layer some passes straight through and is lost to space, but some is absorbed by the greenhouse gasses and re-radiated as IR, with some of this being directed back down to the Earth.

This greenhouse effect is actually very important to life-as-we-know-it on our planet because the energy reflected/re-radiated back down from the greenhouse layer warms the Earth's surface by about 32C; that means that, without the greenhouse effect, our mean global temperature would be around -18C. BRRRRRRRRR!

Certain activities of industrial-age Man, mainly the burning fossil fuels like coal, oil and gas and tropical deforestation (see Rainforests), are causing these "greenhouse gasses" to increase, resulting in a slow, but inevitable, increase in the surface temperature of our planet.

Over the past hundred and twenty-odd years this temperature rise has been measurable and has been recorded at about 0.6C. Seven of the ten warmest years in the 20th century occurred in the 1990s. Over the next hundred years it is projected that the surface temperature will rise by another 2 to 3C (some estimates come out as high as a 5.8C rise). Now, that may not seem like much. Perhaps you think that it's just the difference between a mild and a slightly warmer day. However, the startling fact is that the average surface temperature during the last ice age was only 5C lower than we now experience. So, I hope that you can see that a 2 to 3C rise will result in an extremely dramatic change in climate conditions. (It's more than interesting to note that ice core samples show that glacial periods are associated with low CO2 concentrations, and interglacial periods with high CO2 concentrations.)

Here's the information on Global Mean Temperatures, since they began to be recorded in 1880:

The mean global temperature over the entire period has been 13.9C (that's the horizontal zero line on the graph above). I've included the eight biggest volcanic events (magnitude 5 or greater) of this period, numbered in blue, to determine any effects these might or might not have on global temperature. The biggest three were 1,3 and 5. For the most part there doesn't seem to be any correlation with an expected cooling due to volcanic dust in the atmosphere, except, perhaps, a subtle steepening of the initial decline after volcano 1 (Krakatua).

From other data (such as tree ring records, borehole temperature measurements, permafrost, ice sheets,  glaciers and ice cap measurements) it is widely believed that the current increases in global temperature are the most extreme for at least 500 years and possibly 1500 years or more. The difference between then and now is that those periods did not have the problems of six and a quarter billion people. 

The next thing to look at is how greenhouse gasses have been increasing in the atmosphere. The observatory at Mauna Loa, in the Pacific, has been recording atmospheric CO2 since 1958. The data shows categorically that there has been significant and steady increases over the past 44 years. A 17.5% increase in fact. The pre-Industrial Revolution levels of CO2 were around 280 ppmv (parts per million by volume) or 75% of the present values.

You'll notice firstly that the curve (known as the "Keeling Curve") is steepening, indicating that CO2 levels are increasing faster as time goes on. This is in line with increasing industrial activity and also tropical deforestation.

You'll also notice that, for any year, there is a variation of 5 to 7 ppmv. That's because there is a very distinct seasonal cycle, as you can see from the next graph, which shows CO2 levels, month-by-month, for the past three years:

Is there really a valid correlation between temperature changes and fluctuations of the CO2 level? Well, an ice-core sample from Vostok in Russia provides data going back almost 450,000 years on temperatures and CO2 levels. I've taken the data from both results and plotted them on the same graph below. I believe that anyone would agree that there is an excellent match that higher CO2 levels mean elevated temperatures and vice versa.

There are other associated effects with the warming which will add to the effect. For instance, the World's glaciers and ice sheets will melt faster. Now, ice is a good reflector of the Sun's radiation, but the much darker oceans are better absorbers. So, as the ice melts and joins the oceans it's role as a reflector will change to an absorber and cause even more warming effect.

Such a rapid "global warming" will have dramatic effects on the World's eco-systems. Many species of animals and plants will not be able to adapt to these changes. The sea levels around the world will rise (by between 50 and 100 centimetres) as the warming oceans expand and (to a lesser extent) the polar ice sheets melt, flooding many coastal areas and low islands. Weather systems will become more extreme with more storms, droughts and floods. Agriculture will have to change, as traditional crops become harder to grow under the changing climatic conditions. The nightmare is that many of the World's poorest areas will find the most difficulty in coping with these changes. We will undoubtedly see increasing famines, pestilence and human tragedy on a most dramatic and terrible scale. In 1996 The London School of Hygiene and Tropical Medicine warned that nine of the world's ten most dangerous vector-borne diseases (including malaria, dengue fever, and yellow fever) are likely to expand or shift their ranges due to climate change.

Now go here to find out WHAT CAN, IS AND IS NOT BEING DONE

The BBC has an excellent resource linking to their own latest Climate Change News items HERE.



| home | sitemap | state of the planet | sustainable living | newsfeed | glossary | links | email |

This page last updated: 01 September 2002