The share of renewables in the global energy mix has increased over the past decade to more than 15 percent but doubts remain over whether a 2030 target of 30 percent is achievable, delegates to an international conference said on Wednesday.

Fossil fuels still account for 80 percent of world energy use, threatening efforts to reduce greenhouse gas emissions that are blamed for global warming, delegates said.

"The shift towards low carbon energy has started," the executive secretary of the UN Framework Convention on Climate Change, Christina Figueres, told the World Future Energy Summit.

"But it is not happening at the scale or speed required," she said.

The secretary general of the International Renewable Energy Agency (IRENA), Adnan Amin, told the conference that if nothing is done, "we will have a nine-percent gap" on the 2030 target.

"We have to bridge this gap and we can," said Amin, referring to an action plan released by the agency on Monday.

Based on the Global Futures Report 2013 by the Paris-based Renewable Energy Policy Network for the 21st Century, renewable energy currently accounts for about 17 to 18 percent of the world energy mix.

Fossil fuels -- oil, natural gas and coal -- still account for a massive 80 percent while nuclear energy accounts for around two to three percent.

The report, based on interviews with 170 specialists, gave a wide range of projections for future renewables use.

The most conservative was that the share of renewables would not exceed 20 percent by 2050. Other specialists predicted a share of between 30 and 45 percent, while the most optimistic said clean energy could make up anywhere between 50 and 95 percent.

At present, renewables account for at least 20 percent of the energy mix in at least 30 countries, and around 120 countries have various types of policy targets, the report said.

"I am not optimistic that the 2030 goals will be achieved. I have many question marks," said Fatih Birol, chief economist and director of global energy economics at the International Energy Agency.

He said the average price for oil in 2012 was $112 a barrel, an all-time high, adding that subsidies on fossil fuel are the "the public enemy number one for fighting climate change."

Subsidies on fossil fuels reached $523 billion in 2011, up more than 30 percent on the previous year, said Birol. This makes fossil fuels cheap and encourages rather than discourages their use.

"We definitely need renewables, but unless we have a reliable framework that makes investment in renewable energy profitable, we will not achieve the target," he said.

IRENA said in a report released on Wednesday that the cost of renewables was falling and that public debate on the issue was suffering from an outdated perception that renewable energy was not competitive.

"The cost of solar energy, for example, has dropped below the cost of diesel generation worldwide for communities living away from the electricity grid," the report said.

"2012 was the year when renewables came of age -- able to compete with other power generation technologies and increasingly without subsidies. It is time for the public debate to reflect this changing reality," Amin said.

Based on the latest report by Bloomberg New Energy Finance, investment in renewable energy last year dropped 11 percent to $268 billion. In 2011, it reached $302 billion, a rise of over 30 percent from the previous year.

French President Francois Hollande made an urgent appeal at the conference's opening session on Tuesday for greater investment in renewables.

"If we don't spend... we will have a catastrophe," he warned.