Questioning global warming

Drought is a problem across the world

By BBC News Online's Jonathan Amos

There is nothing so hot as the debate that has surrounded global warming.

Most mainstream scientists believe that human activity - notably emissions of greenhouse gases - has contributed to a significant increase in the average surface temperature of the planet.

However, there is still a sizeable group of researchers who dissent from this consensus.

They question much of the science which underpins the global warming hypothesis:

Urban heat islands

Data from weather stations on land and at sea have been used to reconstruct variations in the Earth's annual-mean surface temperature over the past century.

These show a warming in the range 0.3-0.60C over the period. But the sceptics doubt whether much, or any, of the warming can be linked to increases in C02.

They make the point that much of the data comes from weather stations close to towns and cities. The warming may simply reflect the heat associated with the growth of those towns and cities.

Any "real" warming that may exist once this bias has been properly extracted falls well within the "noise" of natural climate variability.

• See also: Little evidence

Satellites and balloons

Sceptics emphasise the inconsistencies between the surface temperature records and the data produced by satellite and balloon studies.

The latter show little if any warming in the last two decades of the low to mid-troposphere - the atmospheric layer extending up to about 8km from the Earth's surface.

Climate models generally predict that temperatures should increase in the upper air as well as at the surface if increased concentrations of greenhouse gases are causing the warming recorded at ground level.

• See also: Looking for the greenhouse signal

Questionable computers

Sceptics say the scenarios of future climate change that are produced by computer models are deeply flawed. They believe the task of simulating the complexities of our climate system is beyond the capabilities of even the fastest supercomputers.

Certain phenomena, such as cloud formation, oceanic heat transport and the mixing of the air, are still so poorly understood that certain assumptions have to be made about the way the atmosphere behaves.

The sceptics challenge these assumptions and argue that any modelling is fatally undermined as a result.

Fashionable Sun

There is a growing movement that argues that the Sun is a more significant factor in climate change than the rising load of man-made heat-trapping gasses in the atmosphere.

It has been suggested that the solar wind and the Sun's magnetic field can limit the number of cosmic rays (high-energy particles) that enter the Earth's atmosphere. The cosmic rays are said to collide with air molecules to produce secondary particles that seed the cloud types that act to cool the Earth.

In other words, increased solar activity means fewer cosmic rays, fewer clouds, and more warming.

Some greenhouse sceptics argue that this correlation between Earth temperature and solar activity is better and smoother than for any correlation with CO2.

• See also: The role of the Sun