Scientists know this when they state that over a 100 year period, methane traps 32 times more heat than CO2 [1]. This is the number widely cited. Over a 20 year period it is even worse: 104 times greater! Considering we are trying to drastically reduce the human contribution to global warming over the next 10 to 20 years, methane is a great thing to focus on.
Other thoughts:
It is interesting that the cow population only increased 35% since 1960 according to that source. But what about the methane per cow? Some quick searches suggests the slaughter age may have decreased from 3 years to 15 months during that time. They are growing much faster, fed grain that leads to the methane burps. At a minimum several times more emissions per cow.
CO2 only sticks around for 100 years, by your line of reasoning we should just shrug that off too?
Considering than methane is two orders of magnitude more potent a greenhouse gas than CO2 [0], removing it from the atmosphere will also do good alongside reducing CO2 emissions.
[0] https://www.scientificamerican.com/article/how-bad-of-a-gree...
It's a popular misconception that cows contribute greatly to the global warming. The general story is that recently a lot of people in emerging countries (and specifically China) have increased their beef consumption and you need more cows for that, and hence more methane emissions from their burps. And since methane is many times more potent than CO2 ... you know the rest.
What people don't take the time to check is if the story is true. The global inventory of cows was constant at very close to 1 billion for the last 45 years [1]. The half-life of methane in the atmosphere is about 7 years [2], so all the burps released by cows today simply replace burps that decayed from cows of decades past. Overall, cows don't contribute to an increase or decrease in the level of methane in the atmosphere.
[1] http://beef2live.com/story-world-cattle-inventory-1960-2014-...
[2] https://phys.org/tags/methane/