Chinese President Hu Jintao captured headlines yesterday, promising the PRC would “endeavor to cut carbon-dioxide emissions per unit of GDP by a notable margin by 2020.” Reviews have ranged from an important step toward emissions caps to vague and disappointing.

For those serious about climate change, China is the whole ballgame. If 2000-2006 trends in Chinese and American emissions held in 2007, then the PRC’s emissions were nearly 15% larger than America’s and pulling away fast.

The financial crisis altered everyone’s carbon trajectory but lending-induced resilience in China’s heavy industries means their emissions kept rising. Looking ahead, nearly two-thirds of the global emission increase by 2020 could be due to the PRC.

Projecting forward is more discouraging. Say China achieves 8% average annual GDP growth to 2020 (or reports it has). Say it deftly cuts 2000-2007 annual emissions growth in half, to 6%. That would constitute a large drop in emissions per unit of GDP and a complete victory on that score.

At the same time, 6% annual growth to 2020 is the worst scenario to be found on China’s emissions. If you believe carbon emissions are extremely dangerous, 6% growth to 2020 is a complete disaster.

The issue is not whether specific commitments are forthcoming from Beijing but how much the freight train of Chinese emissions can be slowed, promises aside. To cut emissions growth in half is simultaneously a powerful accomplishment and an utter failure. That quandary, not Hu’s speech, is where attention should be focused.