Why is meat bad for the environment?

In 2019 a high-level report commissioned by the United Nations concluded that a huge reduction in meat-eating is ‘essential’ to avoid climate breakdown and keep global heating under 2C. According to the report the average world citizen needs to eat 75% less beef, 90% less pork and half the number of eggs, while tripling consumption of beans and pulses and quadrupling nuts and seeds. So why is meat so bad for the environment?