The memorandum of understanding between California and Mexico, similar to one the state struck last year with China, doesn't set binding targets for reducing greenhouse-gas emissions. But Brown called it an important step toward addressing one of the world's most potent problems.
The difficulty, he said, is persuading people to take action now when the worst damage from climate change remains some years away.
"Can we imagine the future ... and do something about it in the present?" Brown said.
The agreement calls on Mexico and California to share strategies on addressing climate change. Although California has already established an ambitious cap-and-trade program to limit emissions, Mexico's efforts are more nascent.