Rare, medium or well done?
Official statistics currently predict that our planet will have heated by 1.5C by 2050 - and by at least double that amount by this time next century. But there are many scientists around the world who are sceptical about the accuracy of these forecasts.
Remember, in the 70s, we were being warned of an approaching Ice Age ... whatever happened to that?
How can we ever be sure who to believe - and how to plan for the future?
A potted guide to the Greenhouse Effect
The Greenhouse Effect gets a very bad press nowadays, but it's a natural phenomenon. Without the Greenhouse Effect, there would be no life on our planet. Because even with the sun's rays shining on the planet, the Earth's temperature would be on -5 C, much too cold.
What the Earth's got going for it, is that it's surrounded by a layer of 'greenhouse gases' such as CO2, methane and water vapour. These let sunlight through to heat the planet, but then trap much of the resulting heat - or infra-red radiation - from the warmed Earth so that it does not escape into space. Making Earth inhabitable.
You won't find any scientists arguing about this. But what they do argue about is the so-called Enhanced Greenhouse Effect - or "global warming". This asserts that human activities are causing the Earth's atmosphere to warm up above and beyond what would have occurred naturally.
But despite what's been reported in the press, there is no scientific consensus that this is the case.
But, it's getting hotter, right?
Certainly over the last few years we've experienced some record-breaking temperatures - but then accurate records don't go back that far. What we do know though, is that for the past 100,000 years, the temperature of the earth's surface has fluctuated widely.
Even in the shorter term, the climate has varied. If you'd lived in Chaucer's time, for example, you'd remember gorgeous weather, with copious vineyards in production in the South of England, and temperatures around 2 C higher than today. And if you'd lived around the time of Elizabeth I, you might have enjoyed frost-fairs on the Thames - when the average temperature was 1 C less than it is now.
Models and supermodels
The reason that scientists are concerned about the current increase in temperature, is because human activities on Earth are without doubt increasing the amount of greenhouse gases in the atmosphere.
Since the Industrial Revolution, the amount of CO2 in the atmosphere has risen by about a third. And that's just one greenhouse gas.
But if it's us who are causing this latest increase in temperature, then we will have hijacked the earth's natural climate cycles and who knows what the end result might be...?
To try to monitor and predict climate change, meteorologists depend on vast computers. The idea is: you feed in all the relevant variables, and let the technology do what no human could manage.
But it's not as easy as it sounds. For a start, you would ideally have to monitor all weather variables, not just on the Earth's surface but from the deepest oceans to high in the atmosphere - at every conceivable point, all the time.... And you need all this information from as far back in time as possible.
Also, it has to be over a long time period so that the chaotic relationships that exist between all these variables under a wide variety of conditions can be assessed.
In the UK, it's the Hadley Centre that compiles the statistics used by the government and other official bodies. They consider that without intervention, the global temperature could rise by over 3C in the next hundred years. Many other computer models agree - roughly - with these figures. But researchers acknowledge that there is not only uncertainty as to exactly how warm it will get, but also on how separate areas of the world will be affected.
The computer models have their critics however:
- how can you confidently predict a scenario that's never happened before?
- how can you be sure you're measuring enough variables?
- how can you take account of things that might occur in the near future which could have a dramatic effect, eg. large volcanic eruptions, or unforeseen effects of new technologies.
Other lines of attack
Because there are so many strands of evidence relating to global climate change over time, there are correspondingly many branches of science involved in assessing the evidence. For example there are ecologists, biologists and atmospheric scientists studying the question.
And, surprise, surprise, they don't all agree with one another!
Fossil specialists with an interest in climate change - or 'palaeoclimatologists' as they like to be called - believe that there's another way to predict what might happen in the future. And that's to look at what's happened in the past, when global warming occurred before.
Take the humble leaf:
A fossilised leaf can speak volumes. Even if it's 100 million years old. And it's very simple in principle.
- Its area represents a rain-gauge - it tells you the precipitation levels at the time it was alive. Large leaves are typical of rainforests while small leaves (or even no leaves in the case of cacti) are found in deserts.
- Its 'toothiness' is like a thermometer - it tells you the temperature at the time the leaf was alive. Leaves with smooth edges are most common in tropical plants, while toothed (jagged) edged leaves are found in cooler climates like those of Britain today. It's believed that this technique can spot the average temperature to within ONE degree at the time and place the leaf was alive.
Palaeoclimatologists have compiled comprehensive records of the Earth's conditions in the past and believe they have a valuable contribution to make to the global warming debate - and one which disagrees on some points with the computerised climate models. But the climate modellers aren't interested in statistics from the past. They say conditions were different then, even down to the shape and size of the continents.
This article is an edited transcript of a Background Brief programme originally transmitted in 1999. Climate science has moved on since then, as the 2006 Climate Chaos series demonstrates.
First broadcast: Friday 15 Oct 1999 on BBC TWO