With livestock production contributing to climate change, people need to drastically reduce how much meat they eat to help stave off ecological disaster, a group of scientists warn.

In a letter published in the Lancet Planetary Health, more than 50 scientists recommend setting 2030 as the peak year for meat consumption, after which it needs to drop dramatically. It's the same year that the United Nations' International Panel on Climate Change has set as a de facto deadline for the world to start sharply cutting carbon emissions.

Decreasing livestock production is important for two major reason, according to the letter. The first has to do with the carbon footprint of animal husbandry itself. Estimates vary, but a UN report found that raising animals for slaughter accounts for nearly 15% of global greenhouse gas emissions. That's five times as much as air travel and twice as much as the notoriously wasteful fashion industry. That comes from the fertilizer-intensive farming of grain to feed animals; the disposal of their manure; and the digestion process of cattle, sheep and goats.

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"If the livestock sector were to continue with business as usual, this sector alone would account for 49% of the emissions budget for 1.5°C by 2030, requiring other sectors to reduce emissions beyond a realistic or planned level," the scientists write.

The second reason is that much of the land now being used for livestock could be put to use repairing the climate. Several recent studies have shown that planting massive amounts of trees could absorb a significant portion of the extra carbon in the Earth's atmosphere. A widely discussed study in Science from July found that a massive worldwide program to plant forests could absorb as much as two-thirds of the carbon humans have emitted since preindustrial times.

"Restoring natural vegetation, such as forest, is currently the best option at scale for removing CO2 from the atmosphere," the scientists write.

However, much of the land that could accommodate trees is today used for livestock, the Lancet letter says. If that doesn't get re-forested, the "best option" for large-scale C02 removal disappears. That increases "the risk of temperatures rising high enough to tip various Earth systems into unstable states," the scientists write. "This instability could result in the loss of coral reefs and major ice sheets, and increases the uncertainty of maintaining life-supporting ecosystems."

While climate change is a global problem, the letter is aimed at richer countries, which consume most of the world's meat.

"The general trends are, high-income countries are nutritionally better off, are generally overconsuming calories, and are consuming large quantities of animal based foods, often at levels higher than what is recommended for their health," said Martin Heller, a research specialist at the Center for Sustainable Systems at the University of Michigan and a signatory to the letter.

By contrast, many poor countries eat very little meat. India eats the least, with an average consumption of seven pounds per person last year. The average resident in Tanzania, Ghana, Pakistan, Indonesia or Haiti consumed less than 25 pounds of meat in 2018, according to the Organization for Economic Cooperation and Development. In 2018, the typical American consumed nearly 10 times that amount of meat.

Americans eat 42% more meat, eggs and nuts than nutritionists recommend, according to the U.S. Department of Agriculture, while eating just three-quarters of the recommended amount of vegetables and less than half the recommended amount of fruit.

A UN study found that a sustainable level of meat consumption amounts to about half of current levels. For an individual, that works out to about one hamburger a week.

But a climate-conscious diet doesn't mean cutting out animal products altogether, Heller said. Not all land is suitable for farming, and ruminants — cattle, goats, camels or sheep — could be put to graze on lower-quality acreage that can't support crops for human consumption.

"In my own menu choices, I'm not a vegetarian — I do eat meat," Heller said. "I just make that a rare occasion, and even having a burger is kind of a special treat, not an everyday occurrence."