Rob Wiblin of 80,000 hours did the following analysis, which doesn't take into account the secular trends detailed here:
Some people can't decide which is worse: the harm to animal welfare caused by eating chickens, or the harm from climate change caused by eating cows.
One kilogram of cow meat produces 35kg of CO2e compared with 5kg for chicken meat. Each broiler chicken produces some 1.5kg of meat and lives in a farm for ~2 months. So the trade-off is that sparing one year of chicken-life in a factory farm - and twelve chicken deaths - by instead eating cow meat instead will lead to an extra ~0.3 tonnes of CO2e.
For context, the global emissions of CO2e are ~8 tonnes per person - ~20 tonnes per person in the developed world - for a total of ~50 billion tonnes.
The World Health Organisation speculates that 5,000 tonnes of CO2e will cause the loss of one year of healthy life.
If this number is to be believed, avoiding the loss of 1 year of healthy human life from climate change would would come at the cost of 17,000 years of chickens living in factory farms (100,000 chicken lives in total).
Even if climate change is significantly worse than that number suggests, that seems like an enormous amount of animal misery.