China and the US are to be the clear focus of the next year of climate change negotiations, following a hard-fought climate conference that ended in Doha on Saturday night.

The world's two biggest emitters of greenhouse gases hold the key to forging a new global agreement on climate change, that for the first time would bind both developed and developing countries to cut their emissions. But both face severe political problems that will make the talks for the next few years extremely difficult.

Young people from all over Europe are working with broadcasters to create films about improving their environment.

What are the small things they can do to make a big change? How can they all do something to make their planet more sustainable? Simple. They each need to do a little bit. That's the idea of Energy Bits.

Global warming is caused by a whole host of gases and particles. In addition to the chief villain – carbon dioxide from fossil fuel use – two of the most important are methane and nitrous oxide, both of which are generated in large quantities by agriculture and fossil fuel extraction, among other sources. Then there are all the refrigerant gases used in the world's air conditioners, fridges and freezers; the soot generated by cars, industrial plants and cooking fires the world over; and even the vapour trails left by in the sky by aircraft.

With so many warming agents at work, companies and governments are often faced with complex and even controversial decisions about which ones to prioritise. The debate stems from the fact that the various gases and particles operate at very different timescales. Carbon dioxide, nitrous oxide and some refrigerants stay in the air for centuries or even millennia, locking in warming for all the time they are there. The others – collectively know as short-lived climate pollutants or SLCPs – create a burst of warming that is powerful but brief. Soot's impact is gone within a few weeks. Methane stays in the air for an average of around 12 years (confusingly, it then becomes CO2) and hydroflurocarbons, used in refrigeration and insulation foam, typically last around 15 years.

Fresh water is crucial to human society – not just for drinking, but also for farming, washing and many other activities. It is expected to become increasingly scarce in the future, and this is partly due to climate change.

Understanding the problem of fresh water scarcity begins by considering the distribution of water on the planet. Approximately 98% of our water is salty and only 2% is fresh. Of that 2%, almost 70% is snow and ice, 30% is groundwater, less than 0.5% is surface water (lakes, rivers, etc) and less than 0.05% is in the atmosphere. Climate change has several effects on these proportions on a global scale. The main one is that warming causes polar ice to melt into the sea, which turns fresh water into sea water, although this has little direct effect on water supply.