President Barack Obama says reaching a legally binding global climate treaty will be very hard and take some time.
The president made his assessment after announcing an agreement at the U.N. climate conference in Copenhagen that includes a mechanism for nations to verify reductions of heat-trapping gases.
Obama says simply waiting for a treaty, however, would have resulted in no progress at all.
He says the nations of the world will have to take more aggressive steps to combat global warming in the future. But he said the first step is to build trust between developed and developing countries.