Landfills are the third largest source of atmospheric methane (https://www.epa.gov/land-research/quantifying-methane-emissi...) from human activities, a greenhouse gas worse than CO2.

Methane has a GWP of 27-30 over 100 years (https://www.epa.gov/ghgemissions/understanding-global-warmin...), meaning it is 27-30 times more effective at trapping heat than CO2.