For how long can we realistically expect to have oil? And which dwindling element is essential to plant growth?

With 7 billion people on the planet – theoretically from today – there will be an inevitable increase in the demand on the world's natural resources. Here are six already under severe pressure from current rates of consumption:

1. Water

Freshwater only makes 2.5% of the total volume of the world's water, which is about 35 million km3. But considering 70% of that freshwater is in the form of ice and permanent snow cover and that we only have access to 200,000km3 of freshwater overall, it isn't surprising that demand for water could soon exceed supply. The Food and Agriculture Organisation of the United Nations is predicting that by 2025, 1.8 billion people will be living in countries or regions with absolute water scarcity.

2. Oil

The fear of reaching peak oil continues to haunt the oil industry. The BP Statistical Review of World Energy in June measured total global oil at 188.8 million tonnes, from proved oil resources at the end of 2010. This is only enough to oil for the next 46.2 years, should global production remain at the current rate.

3. Natural gas

A similar picture to oil exists for natural gas, with enough gas in proven reserves to meet 58.6 years of global production at the end of 2010.

4. Phosphorus

Without this element, plants cannot grow. Essential for fertiliser, phosphate rock is only found in a handful of countries, including the US, China and Morocco. With the need to feed 7 billion people, scientists from the Global Phosphorus Research Initiative predict we could run out of phosphorus in 50 to 100 years unless new reserves of the element are found.

5. Coal

This has the largest reserves left of all the fossil fuels, but as China and other developing countries continue to increase their appetite for coal, demand could finally outstrip supply. As it is, we have enough coal to meet 188 years of global production.

6. Rare earth elements

Scandium and terbium are just two of the 17 rare earth minerals that are used in everything from the powerful magnets in wind turbines to the electronic circuits in smartphones. The elements are not as rare as their name suggests but currently 97% of the world's supply comes from China and they can restrict supplies at will. Exact reserves are not known.