As the U.K. prepares to host the next round of major global climate talks in November, Prime Minister Boris Johnson’s government has adopted the most ambitious emissions-reduction goal among large economies. It’s hoping that leading by example will encourage other countries to put forth stronger targets to arrest global warming. That’s why the U.K. government’s backing of a new coal mine has baffled climate experts. Endorsing the extraction and use of the dirtiest fossil fuel directly contradicts its green agenda.

“It’s a disastrous decision for the U.K.’s claim to climate leadership,” said Rebecca Willis, a professor at the Lancaster Environment Centre. “It’s confusing to people that, as the country leading international climate negotiations, the U.K. is telling other countries what it expects of them, but consenting to a new development digging out the most polluting fossil fuel.”

The U.K. government’s own adviser on these issues, the Climate Change Committee, declared last week that the decision to open up a new coal mine “gives a negative impression of the U.K.’s climate priorities.”

First, the facts. The new mine, located in the West Cumbria region, is expected to receive permission to keep digging till 2049—just one year before the U.K.’s legally mandated deadline to reach net-zero greenhouse gas emissions. The country will use some of the coal for making steel, but 85% of it is marked for export to Europe.

In that period, the CCC expects the project to add about 400,000 metric tons of carbon dioxide equivalent of emissions each year. Willis’ analysis, which accounts for all emissions from burning the coal from the mine, whether added to the U.K.’s ledger or not, comes to a staggering 420 million tons. That’s approximately how much the U.K.