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

Mark Henderson, Times Online 30 Oct 09;

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.
Exaggerated claims undermine drive to cut emissions, scientists warn
Mark Henderson, Times Online 30 Oct 09;

Exaggerated and inaccurate claims about the threat from global warming risk undermining efforts to cut greenhouse gas emissions and contain climate change, senior scientists have told The Times.

Environmental lobbyists, politicians, researchers and journalists who distort climate science to support an agenda erode public understanding and play into the hands of sceptics, according to experts including a former government chief scientist.

Excessive statements about the decline of Arctic sea ice, severe weather events and the probability of extreme warming in the next century detract from the credibility of robust findings about climate change, they said.

Such claims can easily be rebutted by critics of global warming science to cast doubt on the whole field. They also confuse the public about what has been established as fact, and what is conjecture.

The experts all believe that global warming is a real phenomenon with serious consequences, and that action to curb emissions is urgently needed.

They fear, however, that the contribution of natural climate variations towards events such as storms, melting ice and heatwaves is too often overlooked, and that possible scenarios about future warming are misleadingly presented as fact.

“I worry a lot that NGOs [non=governmental organisations] are very much in the habit of doing exactly that,” said Professor Sir David King, director of the Smith School for Enterprise and the Environment at the University of Oxford, and a former government chief scientific adviser.

“When people overstate happenings that aren’t necessarily climate change-related, or set up as almost certainties things that are difficult to establish scientifically, it distracts from the science we do understand. The danger is they can be accused of scaremongering. Also, we can all become described as kind of left-wing greens.”

Vicky Pope, head of climate change advice at the Met Office, said: “It isn’t helpful to anybody to exaggerate the situation. It’s scary enough as it is.”

She was particularly critical of claims made by scientists and environmental groups two years ago, when observations showed that Arctic sea ice had declined to the lowest extent on record, 39 per cent below the average between 1979 and 2001. This led Mark Serreze, of the US National Snow and Ice Data Centre, to say that Arctic ice was “in a downward spiral and may have passed the point of no return”.

Dr Pope said that while climate change was a factor, normal variations also played a part, and it was always likely that ice would recover a little in subsequent years, as had happened. It was the long-term downward trend that mattered, rather than the figures for any one year, she added.

“The problem with saying that we’ve reached a tipping point is that when the extent starts to increase again — as it has — the sceptics will come along and say, ‘Well, it’s stopped’,” she said. “This is why it’s important we’re as objective as we can be, and use all the available evidence to make clear what’s actually happening, because neither of those claims is right.”

Myles Allen, head of the Climate Dynamics Group at the University of Oxford, said: “Some claims that were made about the ice anomaly were misleading. A lot of people said this is the beginning of the end of Arctic ice, and of course it recovered the following year and everybody looked a bit silly.” Dr Allen said that predictions of how the world was likely to warm also needed to be framed carefully. While there was little doubt that the Earth would get hotter, there were still many uncertainties about the precise extent and regional impact.

“I think we need to be very careful about purporting to be able to supply very detailed and apparently accurate information about how the climate will be in 50 or 100 years’ time, when what we’re really giving is a possible future climate,” he added.

“We’re not in a position to say how likely it is and what the chances are of it being different. There’s an understandable tendency to want to make climate change real for people and tell them what’s going to happen in their postcode, and that’s very dangerous because it gets beyond the level on which current models can operate.”

Chris Huntingford, of the Centre for Ecology and Hydrology, said: “I think the research scientists in general are extremely cautious about making projections for the future, but that caution is vital. We don’t dispute that warming is happening, but it’s important that the NGOs and other people interested in the issue don’t always pick the high scenario and present it as fact.”

Temperature trends of the past two decades have also been widely mis-interpreted to support particular points of view, the scientists said. Rapid warming in the 1990s, culminating in the hottest year on record in 1998, was erroneously used to suggest that climate change was accelerating. Since then, temperatures have stabilised, prompting sceptics to claim that global warming has stopped.

“In 1998, people thought the world was going to end, temperatures were going up so much,” Dr Pope said. “People pick up whatever makes their argument, but this works both ways. It’s the long-term trend that counts, which is continuing and inexorable.”