The UK's chief scientific officer has warned that climate change will result in more flooding, droughts and storms and add to "massive problems" facing the planet.
Sir John Beddington said that the effects of climate change were already apparent in the UK and would get worse over the next 25 years.
He told BBC's Today programme: "The variation we are seeing in temperature or rainfall is double the average.
"That suggests that we are going to have more droughts, we are going to have more floods, we are going to have more sea surges and we are going to have more storms.
"These are the sort of changes that are going to affect us in quite a short timescale. You only have to look at the last few years to see how that is starting to manifest itself even in the UK.
"In 12 years there will be another billion people on the planet. We have big issues of food security, water security and energy security and many, many people will start living in cities.
"These are massive problems. Climate change is just going to make it worse."
Beddington also attacked climate change sceptics, who argue that rising carbon emissions are not causing the planet to warm.
He said: "Those uncertainties are completely outweighed by the enormous body of evidence that shows it is happening and is happening in the sort of ways climate models would expect.
"For example, the Arctic is heating up vastly faster than other parts of the world - this is exactly what the climate scientists are predicting."
Governments around the world are trying to stop global temperatures from rising above 2C by curbing carbon dioxide emissions.
But some experts say this target is unrealistic because agreement and implementation progress too slowly to make a difference.
Researchers at the Neils Bohr Institute in Copenhagen recently said that climate change would lead to more storms such as Hurricane Katrina, which devastated parts of the southern US in 2005.
They said a 2C increase in global temperatures would result in a tenfold increase in Katrina-like storms.