What is climate change?
When people talk about climate change they are usually talking about the “greenhouse effect”.
When the sun’s rays heat the earth’s surface, that surface then reflects heat back into space. The greenhouse effect happens when some of this radiation is trapped by so-called “greenhouse gases”.
Carbon dioxide (CO2) and water vapour, both by-products of life, act as greenhouse gases by absorbing the rays and stopping their warmth from leaving our planet.
Under normal circumstances the greenhouse effect is a good thing – without it the earth would not be warm enough to support life as we know it.
The problem is that the amount of carbon in the atmosphere is dramatically increasing, and now the warming is spiralling out of control.
What does it have to do with fossil fuels like petrol?
The burning of fossil fuels – coal, oil and gas – is polluting the atmosphere by causing the rise in carbon emissions.
If nothing is done, the amount is predicted to rise to more than double pre‑industrial levels by the end of the century.
Other gases can also act as greenhouse gases, methane being the most important. Methane is given off by cows, landfills and rotting vegetation, among other things.
But carbon dioxide is the main problem, because there is so much of it.
The increases in greenhouse gases are leading to a steady rise in the earth’s temperature, with potentially catastrophic consequences.
But aren’t changes to the earth’s temperature natural?
Natural fluctuations do occur in the earth’s climate. In medieval times Europe basked in unusually warm weather due to a pressure difference that developed over the north Atlantic and lasted for centuries.
And in Shakespeare’s time, a change in the Gulf Stream – the Atlantic current that brings water from the tropics to warm northern European countries – triggered the “Little Ice Age”, which led to the Thames freezing over.
Another factor that can influence the earth’s temperature is the activity of the sun, which can increase or decrease the amount of solar rays reaching our planet.
People who deny that human activity is responsible for global warming claim that such natural effects are responsible.
But the facts do not fit. The temperature is rising at a far more rapid rate than can be explained by any natural effects.
There is no evidence that the pressure difference that warmed medieval Europe is operating today.
And a recent study of the sun’s activity found that, while the amount of solar rays reaching earth has fluctuated since 1975, the rise in temperatures has been constant. This means the two are not linked.
So how warm will the planet get?
A Nasa study shows the earth’s surface has warmed by an average of 0.2 degrees celsius every decade for the past 30 years.
The same study says the planet is already as warm as at any time in the last 10,000 years – and is within one degree of being its hottest for a million years.
The relentless rise in the amount of carbon in the atmosphere, and the accompanying recent rise in global temperatures, are now accepted as fact by every serious climatologist.
The generally conservative Intergovernmental Panel on Climate Change recently predicted a rise of between two and six degrees by the end of this century – but others argue that it could be as high as eight degrees.
A major reason for the uncertainty is what are known as “feedback effects”.
The disappearance of the polar ice caps seems to be accelerating temperature increases in those regions. As ice melts and reveals the darker ocean below, less of the sun’s rays are reflected back into space.
And as Siberia, the world’s largest peat bog, begins to melt, there are fears that it could unleash billions of tons of methane, further accelerating global warming.
What will the warming mean for us?
A four degree rise in global temperatures by the end of the century will have catastrophic effects for humanity.
Many populated areas of the world such as southern Europe and the lower half of the US could become uninhabitable deserts.
Low-lying countries such as Bangladesh and Holland and cities such as New Orleans, Venice and Bombay could disappear altogether, flooded by rising sea levels as the ice caps melt.
As oceans warm it could unleash more devastating and more frequent hurricanes – some scientists believe this is already happening.
The effects on animal and plant life might result in many of our vital food sources running out.
Some have predicted that of the approximately seven billion people on our planet, only a billion might survive such catastrophic changes.
The question now is how humanity responds. While world leaders stall, we need to be more radical than ever.
John Parrington is a lecturer in cellular and molecular pharmacology at the University of Oxford