Global warming – what is it?

Global warming is an increase in mean global temperature, which is an average of temperatures taken in various parts of the world at near surface level on land and sea. It’s now about 14.6º Celsius (about 58º in old money). On a mild December day in Britain it could be more or less that temperature outside. But of course this is a pure coincidence. In most other parts of the world the temperature today will have been quite different. That’s because it’s an average. Actually, the absolute figure is pretty meaningless, which is why commentators generally fix a base year and compare changes since that year.

More or less reliable statistics have existed only since 1880 and these show that the average global temperature in 2000 was 0.5ºC higher than in 1900. But this was not a continuous rise. It rose from the 1900s to 1940s, then fell in the 1950s and 1960s, and has been rising since the 1970s. The average temperature in the 70s was 14.01. Today it’s about 14.6, a rise of 0.6º. So, while it is not accurate to say (as some do) that temperatures have been rising since 1900 or since the industrial revolution, the world does seem to be currently warming – even though a century, let alone a few decades is the equivalent of a second in geological time over which changes in global temperatures (Ice Ages and Warm Periods) are measured.

Changes in the Earth’s temperature also mean changes in the Earth’s climate or, rather, since there’s no such thing as a single Earth climate, in the climates of the different parts of the world. When the Earth warms up this means, for instance, that the polar ice caps decrease in size and that glaciers everywhere retreat. Which is happening now.

So, it can be accepted that we are living in a period when the Earth is warming at least temporarily and that this is resulting in climate change.

The big question is: what is causing this? We know that in the past the Earth has warmed and cooled and that this has been due to natural phenomena such as volcanic activity, changes in the intensity of solar radiation or changes in the Earth’s tilt towards the Sun or its orbit round the Sun. Some scientists are suggesting that this is the case now, that the Earth is just warming up after the “Little Ice Age” that lasted from 1500 to 1850 and which may partly have been caused by a reduction in solar radiation.

But the majority of scientists take the view, to quote from a recent report of the Intergovernmental Panel on Climate Change (IPCC) which groups hundreds of scientists, specialists in their field, from all over the world:

“Most of the observed increase in globally averaged temperatures since the mid-20^th century is very likely due to the observed increase in anthropogenic greenhouse gas concentrations”.

Burning fossil fuels releases the greenhouse gas, carbon dioxide (CO₂), into the
atmosphere. CO₂ is called a greenhouse gas because, though it does not prevent heat from the Sun reaching the Earth, it prevents some of it from radiating back. Which is a good thing actually, since we need this. Without any greenhouse gases in the atmosphere the average world temperature would be minus 18ºC.

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