The city of San Diego is rumbling ahead on its Chargers stadium plan and it’s going to have to spend more money to make it more environmentally friendly – a requirement if it wants to get on a much speedier track in order to please the NFL.

Last week, the city formally asked Gov. Jerry Brown to allow the project to receive an expedited review for any potential CEQA lawsuits filed against the Mission Valley stadium plan. Major projects in California often face lengthy court opposition on environmental grounds, something that’s almost certain to happen here. The Chargers have said they don’t believe the city’s speedy environmental review for the project will pass legal muster. Brown’s approval, for which the city’s been angling for some time, would allow the city to find out if it’s project meets state standards much quicker than it would otherwise.

To get the governor’s signature, city leaders have to prove the project is good for the earth. The stadium is going to be LEED certified, use solar energy and the like. One problem is that in order to get the OK for the speedy review, the project can’t increase greenhouse gas emissions over the existing stadium. City leaders have determined this would be impossible. Here’s where the money comes in.

The city will have to purchase carbon credits to offset the greenhouse gas increase, which it estimates will be nearly 200,000 metric tons between now and 2035. That’s the equivalent of the annual greenhouse gas emissions from almost 40,000 passenger vehicles.

This will be expensive, though the exact costs depends on what kind of carbon credits the city buys and when it makes the purchase. Emilie Mazzacurati, CEO of climate change consultancy Four Twenty Seven, said there’s wide variety in carbon credit pricing, depending on whether the city buys through the state’s cap-and-trade program or from other markets.

“In any case, they would probably be wise to budget at least $1 million,” Mazzacurati said.

For now, the city doesn’t know how much this will cost. Craig Gustafson, a spokesman for Mayor Kevin Faulconer, said the city is talking with state agencies to figure that out.

Gustafson said the money would come from the tens of millions of dollars the city has set aside for permitting or contingency funds in the stadium budget.

“There would be no add-on costs for GHG offsets on top of what we already proposed for the project,” Gustafson said.

He said that the city wouldn’t need to purchase the carbon credits unless a stadium deal actually happens. The stadium project is open for public review through Sept. 29; the governor’s decision on whether to grant the expedited process would come after that.