Thirty years ago, world leaders met at the United Nations Earth Summit in Rio and appeared to commit to action to tackle two of the world's greatest environmental threats. The Earth Summit launched the UN Climate Change Convention and the Convention on Biological Diversity. Science in Action assesses their success by talking to atmospheric scientist Sir Bob Watson, a former chair of the International Panel of Climate Change, and to Tom Oliver, professor of applied ecology at the University of Reading.

Arctic zoologist Kristin Laidre tells us about the identification of an unique population of polar bears in Southeast Greenland. The bears’ unusual habitat and means of survival may make them more resilient to the loss of sea ice as the Arctic region continues to warm. Finally, archaeo-geneticist Maria Spyrou talks about her team’s detective work which points to an area of Kyrgyzstan in Central Asia as the likely source of the 14th Century Black Death pandemic.

What is a quantum computer? Every year, new computers are being developed that are faster and smarter than ever before. But if you really want to take things to the next level, you have got to go quantum. CrowdScience listener Atikah in Hungary likes the sound of a quantum computer but wants to know what exactly is it, what can it do that a normal computer cannot, and how soon can he get hold of one?

The digital devices in our everyday lives - from laptop computers to smartphones - are all based on 0s and 1s: so-called ‘bits’. But quantum computers are based on ‘qubits’ - the quantum 0s and 1s that are altogether stranger, but also more powerful. CrowdScience presenter Alex Lathbridge picks the brains of quantum scientists to find out how these ‘qubits’ allow computers to perform calculations millions of times faster than normal - and discovers how much of the theory is being used in reality.

While quantum computers do exist, they are not yet big enough or stable enough to be really useful. Alex visits a working quantum computer to understand what they can do right now, and why it’s so incredibly difficult to scale them up. He hears from the engineers racing to overcome the obstacles and unlock the potential of these mega-powerful systems.

But once the engineering problems are solved, what then? What should we do when the first really powerful quantum computer comes online? We explore the exciting range of possible applications - from helping create new drugs, to making electric batteries much more efficient and maybe even helping farmers fertilize their crops for a fraction of the price.

Presenters: Roland Pease and Alex Lathbridge Producers: Andrew Luck-Baker and Cathy Edwards

(Photo: Earth Summit In Rio De Janeiro, Brazil, 2 June, 1992 Credit: Antonio Ribeiro/Gamma-Rapho/Getty Images)