Very few recognised scientists deny that GHG emissions are rising, as are global temperatures. Do the sceptics have a good argument? Read on and join the debate...

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Sceptics can be broken into two camps; those that note weaknesses in the data and those that believe global warming is down to other factors.

Is the data reliable?

As we have seen there are a number of different historical sources measuring temperatures across the world. The further back these records stretch the less control scientists have over their accuracy. With small numbers, such as a rise in average global temperatures of 0.75°C over a century, room for manoeuvre is slim.

Don't scientists build in a margin of error?

Yes. For the figure above it is +/- 0.2°C. The statement that the 1990s were the hottest decade in 1000 years is now made with over 90% certainty.

Isn't there evidence for other warm periods over the past 1000 years?

Sceptics have referred to a Medieval Warm Period from 1000 to 1300 AD. This does seem to have occurred, though temperatures are likely to have been generally lower than in the twentieth century and it may well have been a localised event in the northern hemisphere rather than global warming.

Can anything have skewed temperature recordings?

Some have argued that much of the recorded warming is due to the world's increasing urbanisation. This is countered by evidence showing that temperatures have risen evenly on both still and windy nights, wind being likely to disperse local warming of cities and towns. Furthermore, increases on land have echoed those observed over oceans, again undermining the theory of 'urban heat islands'.

Are there unanswered anomalies?

A few. One of the main observations is that global warming has not been consistent across the globe, and that some areas, especially in the southern hemisphere and parts of Antarctica, have actually cooled.

One study noted that the natural variation of temperatures over a century may be closer to 1°C rather than 0.5°C as previously thought. That would place the rise occuring in the 20th century within 'natural' boundaries. However, most still agree that the rate of warming since 1990 is unprecedented in the last 1000 years.

Since the late 1970s, satellites have measured temperatures rising more slowly in the upper troposphere than on the surface, evidence that would undermine the case for man-made global warming. However, this data has recently been re-evaluated and shows the discrepancy as too small to be relevant.

Is the IPCC a reliable voice?

Some argue that, because of its international nature, the IPCC is a politicised body. However, its work has been backed by a joint statement of support by 16 leading GHG emitters - including the UK, China and India - and by the US's leading scientific body. Following its recent report in 2007, it was even accused of being too conservative in its statements.

OTHER THEORIES

What else might be causing global warming?

The sun. Solar activity (broadly, radiation and sunspots) is not constant over time; sometimes it flares up, sometimes it cools off, and the position of the Earth in its orbit also makes a contribution. Historically, there has been a pretty close fit between solar activity and global temperatures and it is likely that solar activity has been high over the past 70 years.

Understanding of solar activity impact is limited. Some argue that it is under-estimated, but the scientific consensus states that it cannot be held responsible for the increased rate of global warming seen since the 1990s.

What about cosmic rays?

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Even less is understood about this topic and research is ongoing. Cosmic rays are high-energy particles spat out by exploding stars elsewhere in the universe. If the frequency of cosmic rays drops, due to the changing behaviour of the sun, it has been suggested that cloud formation would decline. Clouds can help reflect solar radiation, so fewer clouds could cause the earth to warm.

Is there anything that could lead to global cooling?

Particles in the atmosphere reflect solar radiation. Man-made particles have led to 'global dimming'. Between 1960, when records began, and 1990, when action was taken against CFCs, it is thought the world saw a 4% reduction in direct radiation from the sun. This probably masked the early warning signs for global warming and contributed to the period of global cooling seen in the mid-twentieth century.

Would anything else have led to cooling?

Land use might also have a cooling effect. Deforestation can increase the reflectivity of land, so more of the sun's heat would be bounced back into space. This is especially marked in mountainous regions, where tree-less snow fields would prove a powerful 'reflector'.

On balance, would natural factors have led to global warming or cooling? Overall, the IPCC feels that looking at natural factors alone, the world should have been looking at a period of cooling.

REASONS FOR QUESTIONING THE EVIDENCE

What is the motivation of those who question global warming?

With so many uncertainties, there is legitimate scientific debate about the relative contributions to global warming. Some scientists feel that the scientific consensus is being used to beat down those who wish to take a different standpoint.

However, the role of 'interested parties', such as fossil fuel producers, has sullied the overall reputation of the alternative viewpoint. Recently, the Royal Society publicly accused Exxon Mobil of funding a scientific think tank that was sceptical of man-made global warming.

Why would an oil company wish to deny global warming?

Producers of fossil fuels are some of the richest companies in the world. If their product is found to cause harm then it puts them in a difficult position, like tobacco companies. Therefore, it could be in their interests to weaken the link between global warming and fossil fuel greenhouse gases. In their defence, these companies would argue that it is sensible 'due diligence' to fully understand the problem before making costly long-term commitments.

What is the argument about cost?

A prominent group of sceptics feel, given the uncertainties, that it would be premature to act now. Money could be better spent on more immediate problems, for example alleviating poverty in Africa. In the future, better technology could also help us battle the issue of warming.

One risk with this standpoint is that by the time the evidence becomes clear, the moment for effective action may have passed.

Furthermore, the cost argument ignores the potential new economic opportunities arising from a low-carbon future and the capacity to deal with more than one problem concurrently.

Could scientists be rushing to the wrong conclusions? Some view global warming in the context of similar scares that turned out not to be true. For example, cooling in the early part of the 1970s led many, especially in the media, to stoke fears of a looming ice age. This however did not have anywhere near the scientific consensus that supports global warming.

SCIENTIFIC DISPUTE - THE ISSUES

- Few scientists deny that the world is warming and GHG emissions are rising.
- Debate focuses on limitations in the data and those who believe GHGs are not the culprits.
- Long-term trends are difficult to draw from the data. However, the rate of warming since the 1970s is alarming.
- Main alternative 'warmers' include the sun's activity and cosmic rays. Both are long shots.
- Some fossil fuel companies have had a role in funding climate change sceptics.

Extract taken from Pocket Issue, Global Warming www.pocketissue.com