Almost 200 nations have united in a global agreement on climate change. But the efforts of a few can still undermine the hopes of the many.

After the 1997 Kyoto Protocol, it took almost a quarter of a century to achieve the recent Paris accord. For the first time, it obligates both advanced nations as well as emerging and developing economies to reducing greenhouse-gas emissions.

However, the Paris consensus remains fragile. Advanced nations want less pollution but are not eager to pay penalties for their historical pollution. Emerging economies want change but hope to industrialize as well. Island nations seek the toughest targets to avoid sinking under the sea level.

Second, the Paris accord requires governments to hold the rise in average global temperatures to well below 2 degrees Celsius relative to pre-industrial levels, while nations must subsequently work on limiting the increase to 1.5 degrees. Still, current national plans would still allow temperatures to rise by 3 degrees by the end of the century. That may have far more severe consequences than anticipated.

Third, the accord compels governments to reassess and resubmit their emission targets every half a decade, starting in 2023. To defuse global warming to 1.5 degrees, human emissions of carbon dioxide — primarily from coal, oil and gas — must go down to zero by 2050. The real question is whether such promises can cope with the ultimate costs of containing global warming.

Finally, the body of the Paris accord is said to be binding under international law. However, due to US objections, key elements of the deal, including the national plans to limit emissions and specific commitments to provide financial aid, are released separately or mentioned only in legal decisions accompanying the deal.