Scientists have identified a staggering 5.5 billion ton gap between greenhouse gas emissions acknowledged each year by the world’s nations and the emissions calculated by independent models, an accounting discrepancy that threatens to complicate the already difficult task of resetting the world’s climate trajectory. The reasons for the gap — which is roughly as large as annual emissions by the United States, the world’s second-largest emitter — are highly technical. But the largest involves countries claiming major reductions to their annual emissions due to forests sucking carbon dioxide out of the air. This includes carbon that humans have unleashed by burning fossil fuels, meaning that large-emitting countries that happen to have large forests are arguably getting an offset for their, and the world’s, pollution.

“There is a gap of 5.5 gigatons of CO2, which is a huge gap,” said Giacomo Grassi, a forest expert at the European Commission’s Joint Research Centre, and lead author of the study published Monday in Nature Climate Change. “This gap is quite new.”

As part of the international Paris climate accord and prior agreements, nations are supposed to report detailed information about their emissions to the United Nations, including those related to their forests and other land use. But between 2005 and 2015, Grassi and fellow scientists found, some countries have claimed to take so much additional carbon out of the air that they have created uncertainty about how to evaluate whether they were meeting their individual climate goals.

“It’s like if the navigation system provides information in miles, and the car dashboard in kilometers,” said Grassi, one of 22 separate authors from countries around the world, including Italy, Japan and the United States.

If countries are using one accounting method but independent models are using another, the results will make it difficult to determine where the world actually stands in its emissions-cutting goals.

It is not that 5.5 billion tons of emissions per year — or rather their opposite, greenhouse gases absorbed by the Earth — are being missed entirely. Rather, the issue is how they are being categorized, who gets the credit and whether that is altering the goal posts for how aggressively individual nations need to cut their fossil fuel emissions.