Lying in wait for us is a marine timebomb. 29 per cent of world fisheries are in a state of collapse according to Canadian scientist Boris Worm and colleagues (2007). The majority could be gone by the 2040s they warn. Plagues of jellyfish in the world's oceans signal the impact of overfishing and nutrient pollution, while carbon emissions are turning them acidic, imperilling the entire marine food chain.

FAO (2008) says "the maximum wild capture fishery potential from the world's oceans has probably been reached" and the same applies to freshwater.

If we cannot double fish production as food demand doubles, then we will have to get the additional 100 million tonnes of meat from land animals. This will require a billion tonnes more grain and 1000 cubic kms of extra fresh water.

FAO's projected increase in world meat demand by 2050 is 185 million tonnes. Add this to the fish deficit and we would need to discover three more North Americas to grow sufficient grain to feed all these animals. This gives some impression of the scale of the challenge of meeting global protein demand by the mid-century.

Julian Cribb is an award winning science writer with over 7000 published articles. He is a Fellow of the Australian Academy of Technological Sciences and Engineering (ATSE) and principal of Julian Cribb & Associates, consultants in science communication.

His forthcoming book The Coming Famine is about the global food crisis.