The mass of the Earth's atmosphere is 5.2867 x 10^18 kg.

After a tasty lunch at the Burger KIng of chilli and onion rings with a chocolate milk shake, I probably blow about 10 grams of methane.

According to Google there are 6,379,157,361 people in the world.

Assume on a daily basis that everyone on Earth is as offensive as I am.

6.4 x 10^9 x 10^-2 = 64 million kilograms of methane per day.

Methane concentation in the Earth's atmosphere is 1.7 parts per million.

Assume (incorrectly) that methane simply accumulates in the atmosphere and is not continually destroyed by natural processes.

How long would it take for post-digested chilli and onion rings to double current methane levels?

1.7x10^-6 / (6.4x10^7 / 5.2867x10^18) = 140428 days = 384 years

Gary