Cows, sheep, and other bovine animals have digestive systems that cause them to burp methane gas, which is one of the most powerful greenhouse gasses. A cow burps around 200 litres of methane into the atmosphere per day. Here's an example of the impact:
As a by-product of their digestion, New Zealand's forty-five million sheep and eight million cattle produce about 90 percent of that country's methane emissions, which equates to over 40 percent of the country's total production of greenhouse gases." source: The Suicidal Planet by Hayer Hillman
By reducing the amount of beef and lamb you consume, you can reduce your impact on global warming.