There were cows a decade ago as well. The herd size dictates a level in the atmosphere it does not constantly increase the amount of methane in the atmosphere, there is an equilibrium.

Increasing herd sizes increases that equilibrium level and that is partially responsible for the overall increase.

But there are mitigations that can be applied there are feed additives that reduce the cow related methane emissions to 1%.

Overall point is cow farts can be a drop in the bucket in the overall scheme of things, all we have to do is modify their diet slightly. Should we do it yes. Is it a huge issue no.

Industrial methane leaks are a bigger problem, that methane when it breaks down will turn into co2 that was not there to begin with contributing to global warming permanently.

Look up the methane cycle. Methane is a flow gas. CO2 is a millennial gas that can stick around for millennia.