What's with the name?
The official title is the 'United Nations Conference on Sustainable Development'. The '+20' is because the conference is being held on the 20th anniversary of the 1992 Rio earth summit, a huge event which was attended by more than 100 world leaders, including then US president George Bush. The original Rio earth summit helped launch several significant environmental conventions, including ones to tackle climate change and species loss – and the organisers are hoping that this year's event will also lead to some breakthroughs.
One of the main aims is to figure out ways of building what the UN calls a 'green economy', or a pathway to eradicating poverty and helping countries achieve sustainable development, especially poorer ones.
What does 'sustainable development' mean, exactly?
The UN defines it as meeting the needs of the present without compromising the ability of future generations to meet their own requirements. It's regarded as a pressing issue because despite hundreds of goals, targets and global commitments, 925m people still do not get enough to eat every day, and the world's population is forecast to jump from today's 7bn to around 9bn by 2050.
That will add pressure to already strained resources, says the UN, which estimates that by 2050, at least one in four people will probably live in a country affected by chronic or recurring shortages of fresh water. The population rise is expected to exacerbate a long list of environmental problems that the Rio summit and many others have failed to solve, from decimated fish stocks to the loss of species and wetlands.
Reality check: what might actually be achieved?
So far, it's hard to say. Unlike the first Rio earth summit, no ambitious treaties have been forged for leaders to sign on arrival. That is partly because enough already exist that have not been properly implemented. So, one of the measures many countries want to see here is a way of better monitoring and implementing existing agreements.
But that initiative, like so many other proposals that officials have been negotiating for the last six months, has been bogged down by disagreements that have left the final outcome unclear. Big ticket ideas such as phasing out environmentally harmful fossil fuel subsidies have been watered down in the talks. So have plans to strengthen the UN Environment Programme, and draw up a new set of sustainable development goals to start after the current millennium development goals end in 2015.
Some progress is expected on ocean protection, but negotiators from European countries, traditionally among the strongest advocates at international green summits, are worried about what can be achieved.
Who's on the guest list?
Unfortunately, between the eurozone crisis and the US presidential election, the list of world leaders from major economies is not nearly as large as it was at the 1992 Rio summit. US president, Barack Obama, will not be here (perhaps he remembers that George Bush attended the 1992 conference, only to lose that year's election to Bill Clinton five months later). France's François Hollande is coming, as are some Scandinavian leaders, but Germany's Angela Merkel and the UK's David Cameron are staying home. Notably, the leaders of the big emerging economies of China, India, Brazil and Russia are all planning to be in Rio, but exactly whether they'll agree on any action when they get here remains to be seen.