PICK a number and forecast global growth population by 2060.
You know that by 2050 it will be nine billon.
Exponentially that figure will increase to 10b by 2060, according to consultant Julian Cribb, author of The Coming Famine.
Mr Cribb told delegates at last week's Global Agribusiness Conference the consequences of global population growth were dire considering the confluence of peak oil, peak water, peak fish, peak land and peak phosphorous.
"Major lakes are vanishing, rivers are drying up and it is forecast we will have serious water shortages in the 2030s," he said.
"Each year more than 75 million tonnes of topsoil is being lost and the world will run out of good topsoil in 50 years.
"By 2050, 7.7b people will live in cities and the obvious question is who will feed them.
"Peak oil was in 2006 yet by 2030 there will be 1.2 billion vehicles on the planet and oil will be expensive.
"So what will fuel tractors?
"It will have to be alternative energy.
"We need to double food production with half the available land and no fossil fuels and with a downward trend in research and development spending.
"By the end of this century, grain won't be the mainstay of food production.
"We will have to reinvent farming and a food system of low input eco-farming that also reinvents the global diet.
"Cities will have to be re-designed to recycle water, nutrients and energy.
"On the future farm there will be a major focus on soil biology and nutrient soil recycling and more farms will be developed in urban areas.
"Already buildings in some parts of the world are housing farms."
According to Mr Cribb, fish farming, or aquaculture, will become bigger than the meat industry.
"Peak fish was reached in 2004 and what is being developed now are fish farms with fish feeding on algal blooms," he said.
"Algae will become a big crop by 2050 supplying food, liquid fuels, plastics and chemicals in a $25b industry.
"There will be massive opportunities for farmers and in the next 40 years, agriculture can become the backbone of the country.
"There is a need to get the public fired up and a need to project agriculture as a positive.
"The challenge is for us to rethink food itself, how we produce it and the systems we use to process, distribute and deliver it and to create diets and foods for the future that are safe, healthy, nutritious and tread less heavily on the planet."