I tried calculating it two ways to see. Firstly, based off two full fuel tanks for a 777. Apparently a 777-200 has a 117350 L fuel tank, and the density of jet

I tried calculating it two ways to see. Firstly, based off two full fuel tanks for a 777. Apparently a 777-200 has a 117350 L fuel tank, and the density of jet fuel is approximately 800 g / L. The emitted carbon dioxide combines oxygen from the air with carbon from the jet fuel. Apparently the average chain length is 12, so it would be C_12H_10 for a straight hydrocarbon. Factoring in the molecular masses, the amount of CO2 from two full tanks would be: (12 * 12.011 + 212.01115.999) / (1.0080 * 10 + 12 * 12.011) * 117350 * 800 * 2/ 1E6 = 643 tonnes. Now, they might not use the full tank. Another estimate is that long haul flights use about 0.1 g / passenger / km. Apparently it is 4308 km in a straight line from SF to London, and a 777 seats about 350 people depending on the configuration, so multiplying by 2 for a return trip comes out at 302 tonnes of CO2 (that's probably an underestimate since it is a straight line). So I think 4000 tonnes is an overestimate (and they said tons, which is even more than a metric tonne, at 4480), but the emissions of such a flight is still significant (and the global warming impact is higher because combustion also disperses water, which also has a warming effect at that altitude).