A failed deal in the Copenhagen climate conference will call for an extra investment of 500 billion U.S. dollars to cope with global warming, the chief of the Paris-based International Energy Agency (IEA) said in Paris on Tuesday.

"If there are no immediate measures to reduce the emission of carbon dioxide, 500 billion U.S. dollars should be additionally invested to redeem the delay and to retrieve the trajectory" that limits the increase of global temperature around two degrees Celsius, Nobuo Tanaka said at a press conference.

Actually, Tanaka doesn't expect an international treaty to be signed at the United Nations climate conference. But a "strong commitment" would be announced, he said.

"A very firm message about the environment, which Copenhagen is dealing with, needs to be issued to investors. Without a clear message, it is very difficult for private sector to take part in the investment," he noted.

The IEA has proposed a plan to limit the concentration of greenhouse gas emissions at 450 parts per million (ppm) of CO2 equivalent, and believes this scale can hold a hike of two degrees Celsius in average temperature.