C

Few dishes taste better than a juicy cut of beef. One survey in 2014 found that steak was Americans' favorite food. Unfortunately, by cooking so many cows, humans are cooking themselves, too.

The influence of food on greenhouse-gas(GHG)emissions(排放)can slip under the radar. In a survey in Britain last year, the share of answerers saying that "producing plants and meat on farms" was a "significant contributor" to climate change was the lowest among ten listed activities. Yet two papers published this year in Nature Food find that food, especially beef, creates more GHGS than previously thought.

This March researchers from the European Commission and the UN's Food and Agriculture Office released a study estimating that the global food system was responsible for 34% of GHG in 2015. The paper assigns the full impact of deforestation to the agriculture that results from it; includes emissions after food is sold(such as from waste and cooking); and counts non-food crops like cotton. But even when the authors took away emissions from sources like transport and packaging, they still found that agriculture generated 24% of GHGS.

Another recent paper, by Xiaoming Xu of the University of Illinois and eight co-authors, allocates(分配)this impact among 171 crops and 16 animal products. It finds that animal-based foods account for 57% of agricultural GHGS, versus 29% for food from plants. Beef and cow's milk alone made up 34%. Combined with the earlier study's results, this implies that cattle produce 12% of GHG emissions.

Relative to other food sources, beef is uniquely carbon-intensive. Because cattle emit methane(甲烷)and need large grasslands that are often created by cutting more forests, they produce seven times as many GHGS per calorie of meat as pigs do. This makes beef a bigger share among foods than coal is among sources of electricity.

The simplest way to cut beef output is for people to eat other animals instead, or become vegetarians. But convincing people to give up their burgers is a tall order.

Fortunately, lab-grown meats are moving from Petri dishes(培养皿)to high-end restaurants. Doing without beef from live cattle is hard to imagine, but the same was true of coal 100 years ago. Lab-grown meat could play an essential role in slowing a climate disaster.