With questions still hanging over crucial issues around climate change, scientists are adamant about one thing: the debate is far from finished. Few now dispute that the climate is warming, but there remains a split over whether or not human beings are the cause.

THE CASE FOR MAN-MADE CLIMATE CHANGE

According to most climate scientists, the current period of global warming is being driven by man-made carbon dioxide (CO2) emissions. A greenhouse gas, CO2 in the atmosphere reflects heat given off by the Earth back to surface. In simple terms, scientists believe human activities, such as burning fossil fuels and deforestation, have disrupted the natural balance of greenhouse gases, causing temperatures to rise above normal levels.

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Prof Piers Forster, a lead author on the Intergovernmental Panel on Climate Change's (IPCC) fourth report and a climate-change researcher at the University of Leeds, admits that the picture is complicated. However, he insists that greenhouse-gas emissions are the only way to explain the temperature rises being experienced.

"Certainly the story is quite complex," he said. "Since 1970, however, we really start to see a big temperature rise and an increase in greenhouse gases - it is hard to ignore."

Experts also claim that improved understanding of the effects of other pollutants, such as aerosols, help to explain anomalies in the data, such as a period of climate cooling between 1940 and 1970. They believe that soot particles and aerosols masked the overall warming effect of carbon dioxide by providing a protective blanket that acted like a sunscreen to reflect the Suns rays.

But they also admit that there are many questions that still need to be answered before the Earth's climate and the impact of global-warming can be fully understood.

Recent research has also focused on natural systems that may be worsening or dampening the effects of climate change. The role of water vapour, a greenhouse gas, is one of the most poorly understood. Others include the effect of the ocean's ability to absorb heat and carbon dioxide.

THE CASE AGAINST MAN-MADE GLOBAL-WARMING

Scientists on the opposite side of the argument insist that the warming of the planet is due to natural cycles that have been repeated throughout history.

They seize upon one of the most compelling uncertainties in the climate-change debate: the role of the Sun. It is widely accepted that historic ice ages and subsequent warm periods were due to shifts in the Earth's orbit in relation to the Sun, or to the Sun's activity.

Sceptics claim that we are in a period of high solar activity and that warming will end when that activity falls. In the recent report by the IPCC, solar activity had the largest error margins of all the influences on climate. Volcanic activity is also believed to have played a major role in historic shifts.

Prof Richard Lindzen, a prominent climate-change sceptic and meteorologist at the Massachusetts Institute of Technology, said: "[The influence of] volcanoes and solar variability is essentially unknown. The issue is reduced to essentially religious faith."

Another key objection of sceptics is the apparent inability of climate-change computer models to accurately explain the level of global warming that is occurring. Evidence from ice and seabed cores also points to the cyclic nature of the warming and cooling of the Earth.

Prof Bob Carter, a marine geophysicist at James Cook University in Queensland, Australia, said: "That climate changes frequently, rapidly and sometimes unpredictably has been conventional knowledge among Earth environmental scientists since the early days of ocean drilling in the 1970s."

Prof Don Easterbrook, a geologist at Western Washington University, believes the Earth is in the grip of a 30-year warming cycle. He said: "Past climatic trends indicate that global climates should begin to cool sometime between now and 2010."