Mexican President Enrique Pena Nieto could use some good news. Tumbling oil prices have hurt his vaunted energy reform. Top aides, the first lady and Pena Nieto himself are answering to charges of corruption, even as the authorities have managed to find the remains of just one of 43 college students who were reported kidnapped and disappeared on their way to a protest last September.

So the Mexican government's pledge to slash greenhouse gas emissions brought some welcome applause. Green groups hailed Pena Nieto's plan as bold and pioneering, and the White House praised Mexico for "setting an example for the rest of the world" with a "landmark" initiative it called "timely, clear, ambitious, and supported by robust, unconditional policy commitments."

By promising to halt the rise of climate-warming gases by 2026, and then drive them down 22 percent by 2030, Mexico became the first developing nation to meet the United Nations' challenge to publish a road map for combating climate change.