Q&A: what do we know about climate change and the world's future?

Mark Henderson, Science Editor

Is the world warming?
Yes. Average global temperatures increased by 0.74C (1.33F) in the past century, and by 0.6C in the century before that.
Is there a parallel trend in greenhouse gas levels?
Yes. The concentration of carbon dioxide in the atmosphere has risen from 280 parts per million (ppm) in pre-industrial times to 387ppm today. Concentrations of methane have risen from 700 parts per billion (ppb) to 1,745 ppb. Today’s levels of both gases are the highest for at least 650,000 years.
Doesn’t the world’s climate always vary?
Yes. All sorts of factors affect the world’s climate, including changes in the Earth’s orbit, changes in the Sun’s intensity, volcanic eruptions, atmospheric pollution, and natural variations such as El Niño.
How do we know that man-made greenhouse gases, and not natural variation, are responsible for global warming?
Evidence of the past climate shows that rising greenhouse gas levels have been followed by warming. In the past decade, scientists have also established that it is impossible to account for recent observed changes in global temperatures unless human activities have had an impact.
Computer models of the Earth’s climate agree that natural variation can explain only a part of recent warming. Only if anthropogenic (man-made) greenhouse gases are included do the models replicate what has actually happened. The most recent report from the Intergovernmental Panel on Climate Change (IPCC), published in 2007, states that the evidence for global warming is unequivocal, and that human influence is “very likely”.
Has any more evidence emerged since the 2007 report?
Yes. A study published last year found the signature of human- induced global warming in Antarctica, the last continent on which it had not been detected. Other studies have attributed heavier rainfall, including wetter weather in Britain and increased saltiness of the sea, to anthropogenic greenhouse gases.
What has happened to temperatures over the past ten years?
The warmest year on record was 1998, in part due to a very strong El Niño, which has a heating effect. Since then, temperatures have stabilised, though at a very high level. The ten warmest years on record have all occurred since 1997.
Does this mean that global warming has stopped?
No. Natural climate variations mean that there will be periods of temperature stability even when the overall temperature trend points upwards. A study led by Jeff Knight and Peter Stott, of the Met Office, found that such hiatuses occur relatively often during periods of warming, and aren’t inconsistent with the upward trend.
Is it possible to link specific weather events to climate change?
Not usually: there have always been hurricanes, heat waves and hot and cold years, and while many of these may be consistent with global warming, it is generally very difficult to attribute responsibility. Dr Stott’s team has identified the signature of human-induced global warming in the 2003 European heat wave that caused 35,000 deaths.
What will happen to the climate in the future?
The IPCC has set out several scenarios, which predict global temperature increases of between 1.1C and 6.4C by 2100.
Why don’t these scenarios agree?
They depend on many different variables, not least the quantities of greenhouse gases that the world continues to emit. Even if greenhouse gas production were to cease today, the world would still warm by at least 1C, because carbon dioxide that has already been emitted stays in the atmosphere for 50 to 200 years.
Modelling the future climate also carries significant uncertainties, hence the range of possible outcomes.
What is happening to the Arctic sea ice?
It is in long-term decline, and global warming is having an effect. The record low was recorded in 2007, with an extent 39.2 per cent below the average for 1979-2001. It has since recovered, though not to its previous thickness. There is considerable natural variation in ice extent from year to year, but the overall trend is towards shrinkage. Models generally predict that the Arctic will be ice-free in the summer by 2060 to 2080, though some recent estimates have suggested this could happen more quickly.