The International Energy Agency predicts renewables will become the world’s second-largest source of power generation by 2015 and close in on coal as the primary source by 2035.

But according to the 2012 edition of its flagship publication, the World Energy Outlook, the agency warns this rapid increase is critically dependent on continued subsidies.

It says in 2011, these subsidies (including for biofuels) amounted to $88 billion, but over the period to 2035 need to amount to $4.8 trillion; over half of this has already been committed to existing projects or is needed to meet 2020 targets.