How does carbon dioxide warm the planet?
Energy from the sun arrives as visible and ultraviolet light, which is absorbed by the Earth. Some of this energy is released from Earth again as heat. Greenhouse gases in the atmosphere trap this heat and bounce it back towards the Earth’s surface.
Laboratory experiments show that carbon dioxide can absorb and then emit heat, while satellite observations reveal that the amount of heat being released into the atmosphere is changing.
Some greenhouse gases, such as methane and nitrous oxide, are more potent than carbon dioxide, but they are removed from the atmosphere within decades.
In contrast CO2 emissions cause the greatest concern because they stay in the atmosphere for hundreds of years, meaning that the planet is expected to continue warming long after carbon emissions are cut.
How do we know humans are responsible?
Throughout the Earth's history it has been through natural cycles of cooling and heating – that's why ice ages happen. But the strongest evidence for the current warming being man-made is the rate at which it is occurring.
In the distant past it has taken thousands of years for the Earth to slowly swing from an ice age to a very warm state, but over the past century the change has been unprecedentedly fast.
Scientists have searched in vain for natural factors which could explain this rapid increase, such as variations in the Sun's radiation and volcanic eruptions, but none show any long-term changes which can account for the sudden uptick in warming.
Carbon dioxide is known to be a greenhouse gas, and is also known to be released into the atmosphere by the burning of fossil fuels. CO2 levels are currently higher than at any point in the past 800,000 years, offering convincing evidence that man is warming the planet.
What are the consequences?
The IPCC forecast in 2007 that global temperatures will rise by between 1C and 4C above pre-industrial levels by 2100. This has been refined, but not dramatically changed, in its latest assessment.
The general outcome of global warming across the planet is that on average there will be more hot days and nights, and fewer cold days and nights, but this does not apply equally to all parts of the world.
What we are likely to experience is an increase in extreme weather, with more droughts in hot regions and more rainfall in wet regions. Ice caps will melt, raising sea levels.
The consequences for mankind could be dramatic. Lord Stern, who authored a key 2007 report on the economics of climate change, believes that changes of 3C or 4C above pre-industrial levels could make parts of the world uninhabitable, leading to mass migration and potential conflict as a result.
Where are the areas of uncertainty?
Many areas that were previously viewed as uncertain have become clearer since the last IPCC report, meaning their latest findings will be heralded as the most accurate ever picture of climate change.
For example, the contribution of melting ice caps to sea level rise will be addressed in this year's report for the first time, in theory providing a more accurate picture of the future of our oceans.
However, some levels still remain highly controversial. For example, the models used to draw up the latest report do not take into account the effects of methane released from the sea beds or the Arctic permafrost as it melts, because it is still not possible to say exactly how strong its effect will be.
Questions also exist over how strongly, if at all, climate change is affected by variations in radiation given off by the Sun and by aerosols which are released by the burning of fossil fuels and have a cooling effect by deflecting solar radiation away from Earth.
What about the pause?
A favourite argument of sceptics is that no significant global warming has happened since the late 1990s, despite fifteen years of carbon dioxide being continually pumped into the atmosphere.
However, the term "global warming" does not mean that each year must necessarily be hotter than the last, and scientists insist temperatures will soon begin rising again.
Factors such as variations in the Sun's energy output, greater storage of heat by the deep oceans, and increased aerosols released by volcanic eruptions in recent years have been put forward to explain the current hiatus in warming.
Scientists argue that these factors are masking the overall warming effect, and that the continued rise of other warning signs, such as melting ice, show that climate change is continuing apace.
What can be done to tackle this?
That depends on how severe you believe the problem is.
Some people think that carbon capture and storage – removing carbon from the atmosphere and squirrelling it away underground – could allow us to halt the warming of the planet without fundamentally changing our way of life.
Others believe that the issue is more urgent, and that carbon emissions must be cut dramatically in order to save the planet. Even then it could be hundreds of years before the warming trend starts to reverse, they say.
The latest IPCC report is expected to claim that we must reduce our reliance on fossil fuels while finding ways to extract carbon from the atmosphere.
Another option could be "geoengineering" the planet to reverse the effects of climate change – for example "seeding" clouds to deflect more solar radiation away from Earth – but it is unclear how effective such strategies could be and whether they could cause more harm than good.