William and Kate must have been exhausted when they touched down yesterday at London’s Heathrow Airport after 11 days of glad-handing their way across Canada, and down to California. Prince William landed a helicopter in the waters off Prince Edward Island; Kate helped plant a tree in Ottawa; they visited underprivileged kids in Los Angeles.

Maybe the only thing more exhausted than Will and Kate—all the fuel burned during their trip.

Even though the royal couple flew commercial back to Heathrow—and made time toward the end of their trip to celebrate the 20^th anniversary gala event for Tusk Trust, a sub-Saharan environmental conservancy group—carbon-dioxide emissions from the trip racked up with each stop of the private jet. The final tally: Some 250 tons of CO₂, or more than a dozen energy-loving Americans use during an entire year, according to the United Nations.

“For dignitaries with means, there is no excuse not to make all their travel—flights, hotels, car transport—carbon neutral while supporting new technologies, such as wind and solar energy and reforestation projects,” says Eric Carlson, founder of the CarbonFund.org Foundation.

Calculating the carbon footprint of Will and Kate’s North American tour was relatively simple. Except for the commercial flight home, the royals flew by private jet—usually an Airbus A130, which uses nearly 4 gallons of fuel for each air mile, though they also traveled short distances by helicopter. No matter the number of people in tow on each private flight, Will and Kate are ultimately on the hook for total emissions—without the royals, the entourage wouldn’t exist. Motorcades, though often in excess of 20 vehicles, also traveled only short distances and would not have had a significant impact on Will and Kate’s total carbon footprint.

To get the 250-ton figure, we used the widely accepted (PDF) standard of about 21 pounds of carbon dioxide for every gallon of jet fuel burned. By the time Will and Kate landed back in Heathrow, they had traveled nearly 10,000 air miles.

Of course, flying around the world is one of the crucial parts of the job William was born into, the one Kate recently entered. Alternatives—trans-Atlantic Zeppelin flight? Riding steerage class on a steamship? Skyping?—might be earth-friendly, but are unrealistic. Carbon offsets are one (albeit imperfect), better-than-nothing way for the royals to make up for the carbon dioxide created by their travels. If the young royals were so inclined to offset their carbon emissions from this recent trip it would cost roughly $2,500.

“It's extremely important for people in the public eye, like William and Kate, to lead environmentally sustainable lives,” Carlson adds. “People look to celebrities and dignitaries for leadership—their actions are reported on far more than the people working on an issue day in and day out—and what they say and do can make a big difference.”