Where do greenhouse gas emissions come from?
Which countries are most responsible for causing human-induced climate change?
And have governments pledged tough enough cuts so far to keep the global average temperature rise within "safe limits"?
As the UN summit in Copenhagen approaches, we look at the past, present and possible futures of climate change.
Global emissions have risen steadily in recent decades.
But when trying to assign "responsibility" for causing climate change, how should they be measured?
Populous developing countries such as China and India have relatively high overall emissions - comparable with many developed countries.
But each of their citizens produces a much smaller amount than counterparts in regions such as North America or Western Europe.
Countries that industrialised early and grew rich early because of that industrialisation, such as the UK, Germany and the US, have a higher "historical footprint".
In some peoples' eyes, this gives them a higher responsibility for curbing the problem.
A number of academic teams have calculated how emissions are likely to rise in the next few decades, and what that is likely to mean in terms of rising temperatures.
Their projections are not exact because there are many sources of uncertainty in the calculations, including the exact relationship between greenhouse gas levels and temperature rise.
A number of developed countries and blocs have set targets for cutting their emissions, some of which depend on what other countries do.
The EU, for example, will cut emissions by 20% from 1990 levels - but if there is a global deal, that will rise to 30%.
Some developing nations have also pledged to reduce the rate at which their emissions are growing.
If implemented, are these curbs enough to keep the global average temperature rise below 2C - the target adopted by G8, the EU and a number of major developing countries?
According to the European Climate Foundation analysis - and others - commitments made so far are probably not enough to meet the G8 target.