Indian Prime Minister Narendra Modi, left, and U.S. President Barack Obama, right, watch the Republic Day parade in New Delhi, India, Monday, Jan. 26, 2015. (Stephen Crowley/AP)

THE U.S.-India relationship has often seemed more attractive in theory than in practice. The world’s two largest democracies share concerns about Chinese expansionism; throw in big economic potential and an active Indian immigrant population in the United States, and success seems guaranteed. Yet both sides have often felt disappointed in the results.

So President Obama and Prime Minister Narendra Modi deserve credit for redoubling their efforts. In his second term, President George W. Bush made a strong effort to improve ties. It took Mr. Obama a while to pick up the baton, particularly as the previous Indian government flagged in energy. But with the energetic Mr. Modi in charge, Mr. Obama made his second state visit to New Delhi this week. There were some modest accomplishments — and, as usual, some hints of how much more might be done.

One obvious area of potential cooperation was suggested by the sooty air in and around the capital while Mr. Obama was in town for lavish Republic Day celebrations. In some very back-of-the-envelope calculations, Bloomberg News estimated that exposure to air pollution during Mr. Obama’s short stay would shave six hours off his life. Unlike China, India doesn’t go to great lengths to conceal the extent of its pollution problem from its people. But as in China, India’s air pollution is harming health and contributing to climate change. The country recently became the third-largest emitter of greenhouse gases, behind China and the United States.

Mr. Obama and Mr. Modi made a bit of progress on this front. They agreed to proceed with an international treaty to phase out hydrofluorocarbons, short-lived but potent greenhouse gases in appliances such as refrigerators and air conditioning units. The leaders announced U.S. financing for Indian solar projects and new cooperation on air quality. They also trumpeted a “breakthrough” on a nuclear power deal Mr. Bush struck nearly a decade ago with a previous Indian government, ostensibly putting it back on track. However, it remains unclear whether the latest agreement will be enough to attract U.S. nuclear firms into the Indian market — or whether the Indian Parliament will have to act to convince them. U.S. companies reacted with caution.

All of this falls far short of the clear emissions commitment and timetable that Mr. Obama struck with Chinese leaders last year. India has resisted calls to formally cap its carbon dioxide emissions, and that’s not a sustainable position for the world’s third-largest emitter. It needs to undertake a large-scale transformation of its chaotic energy sector. As new infrastructure goes in, it must be more sensitive to long-term environmental costs. But the small steps taken this week may lead to larger progress. As in the trade and security realms, the United States and India seem to have accepted that progress will be hard-won but worth the effort.