"I think there's a widening gap between where we are now and where the scientists say we ought to be," he told reporters.
The growing gap was not due to a slowing of political action but to increasingly dire warnings delivered by science, he said.
"The science has been consolidated in the past year in dramatic and stark terms," he said, without elaborating.
The European Union has a target to avoid a more than 2 degrees Celsius average global temperature rise over levels before the industrial revolution.
These temperatures can be avoided if concentrations of carbon dioxide and other greenhouse gases in the atmosphere are kept below about 450 parts per million, scientists say.
CO2 was rising at 2.5 parts per million per year, meaning greenhouse gases could be at 450 ppm in 10 years, Miliband said. "That (2 degrees) is a challenging thing to achieve. That's a stark challenge."
Miliband was previewing a meeting next week in Mexico, when energy and environment ministers in the world's top 20 greenhouse gas emitters will explore in an informal setting how to tackle climate change. "It's a chance for honest, open, safe dialogue... between the 20 biggest emitting countries."
Despite growing rhetoric about the dangers of climate change there is a policy vacuum globally. The Kyoto pact is the only game in town and its targets expire in 2012.
The pact does not set pollution caps for three out of the world's top four polluters, the United States -- which pulled out in 2001 -- China and India, and has only a very lax target for the other, Russia.
"We very strongly hope the United States will be part of a global climate change agreement for post-2012," Miliband said.