One reason that Iceland scored so well, beyond its energy policies, may be its economic tailspin, one of Western Europe’s worst, which began with a banking crisis in 2008, Mr. Esty said.

Another crucial caveat is that the researchers rely heavily on data that the countries themselves report to international groups like the United Nations and the World Bank. The researchers said that countries like Cuba, in ninth place, are thought to score artificially high because the data is either collected poorly or massaged to signal progress.

The lack of reliable environmental data is a major challenge, the researchers said. “There are so many countries that are not collecting even minimal data sets,” said Christine Kim, a researcher at Yale who is program manager of the project. “The state of the data hasn’t gotten much better in the last 10 years. We have better data on baseball than we do on climate change.”

Developing a system to quantify and track environmental performance would be essential to the success of any global climate treaty requiring industrialized countries to cut their emissions and emerging economies to reduce their emissions growth. Negotiators from around the globe failed to produce a binding agreement in Copenhagen last month, but plan to meet again in Mexico City in late November.

Discord over how to measure, report and verify climate data was one factor that stymied progress in Copenhagen. The Chinese, for example, use their own scales to measure factors like air pollution, and it is hard to translate their readings into accepted Western scientific scales.

Because a country’s final ranking is based on so many environmental factors, the devil is often in the details.

Extenuating circumstances may distort the data.

Countries like Slovakia, Serbia and Montenegro performed well in part because severe economic slumps in these places shut down polluting factories, Mr. Esty said.