Four years after the signing of the Paris climate accord, a major hitch remains: Countries have not been able to reach an agreement on an accounting trick that would allow them to count greenhouse gas reductions twice.

To be sure, the biggest issue remains national targets that are not sufficient to meet the goals of the landmark 2015 agreement signed by almost 200 countries — namely, limiting long-term warming to 1.5 degrees Celsius above pre-industrial levels.

But experts emphasize that poor accounting could compound the problem unless a solution is found. Imagine, for example, a wind plant in India that reduces carbon emissions by 1 ton a year. Starting from 2020, when the trading program linked to the agreement comes into effect, a country such as Germany could in theory buy this amount and count it under its reduction goals, while India could not.

But a group of countries led by Brazil argue that it should be counted for both, which would lead to overall global emissions greater than the sum of the amounts reported by each participating entity.

“It is a really strange position. Most countries don’t support it,” said Lambert Schneider of the Oeko-Institut in Berlin, who co-authored an article on the subject in the journal Science on Thursday.

The need to avoid double counting is spelled out in the accord, but countries failed to finalize the “rule book” on the subject last year in Poland, and it will come up for debate once more at a U.N. conference in December.

“It’s the main outstanding issue to operationalize the Paris agreement.”

The problem is similar for aviation.

Airlines are exempt from the Paris accord because historically states have been unable to agree on who takes ownership of the emissions — does it depend on the country of departure or arrival? Or even the nationality of the passengers?

But the sector committed in 2016, under the aegis of the International Civil Aviation Organization, to offset future emissions above 2020 levels. Airlines will therefore have to buy billions of tons of carbon credits from countries that are selling them. There, too, is a need to avoid double counting.

Aviation is a huge piece of the pie: If the sector were a country, it would fall right behind Germany. On this issue, a bloc of countries led by Saudi Arabia is leading the opposition to robust accounting.