Delaying action on global warming will only increase the costs and reduce the options for dealing with its worst effects, according to a draft report by UN experts.

The final draft of the report by the Intergovernmental Panel on Climate Change says global warming will continue to increase unless countries shift quickly to clean energy and cut emissions.

A leaked version circulating with media outlets and news agencies says that despite national policies and international efforts emissions of carbon dioxide and other greenhouse gases that are warming the planet grew 2.2% per year on average between 2000 and 2010, compared with 1.3% per year from 1970 to 2000.

The two main drivers were a sharp rise in economic growth and a steady growth in the world's population, the report said. The largest contributor to global emissions was the burning of oil and coal and the draft report says its contribution is expected to rise. Unless "explicit efforts" are made to reduce emissions, the experts warn, increased conservation and efficiency will not be enough.

With increasing demand for energy and the growing use of coal to generate electricity, the experts say emissions from the sector are projected to double or triple by 2050 from the level in 2010 unless improvements in clean energy are "significantly accelerated".

International climate negotiators agreed at the 2009 UN climate change conference in Copenhagen that global warming this century must increase by less than 2C to avoid the worst impacts of climate change.

Scientists say that target requires atmospheric concentration of carbon dioxide, the main greenhouse gas, to stay below 530 parts per million (PPM). The level recently surpassed 400PPM.

The report said the majority of scenarios to stay below 530PPM throughout the 21st century would require reducing greenhouse gas emissions by 40-70% of 2010 levels by 2050. The experts call for new patterns of investment and a transformation into a low-carbon economy.

The global total annual investment in the energy system is presently about US$1.2tn. The experts estimate that in order to stabilise the atmospheric concentration of CO2 between 430 and 530ppm investment in fossil fuels would have to decline by $30bn a year between 2010 and 2029, while investment in non-carbon producing energy sources would have to rise by $147bn a year.

The report argues many renewable energy technologies are increasingly efficient and cost-effective but need support if their market share is to increase.