In the Amazon, or in the parts of the Amazon that people have mowed down and converted into grazing pasture, the average abattoir-bound cow has nearly three acres to himself. Nice for the cow, perhaps, but senseless and dangerous in every other way. Every year, on average, tropical deforestation accounts for fifteen per cent of global greenhouse emissions. About half of the contributing deforestation occurs in South America; deforestation in the Amazon recently increased. If the trend continues, scientists have found, it could lengthen the forest’s dry season, triggering even greater warming and drying, killing trees in the nearby (still intact) forest, and eventually causing mass tree mortality and an entire ecosystem shift—from rainforest to savannah. The tipping point for such a collapse in the Amazon is between twenty and twenty-five per cent deforestation—fifteen to seventeen per cent is already gone. “If you exceed the threshold,” Carlos Nobre, a Brazilian climate and tropical-forest expert, told me, “fifty to sixty per cent of the forest could be gone over three to five decades.”

An urgent new report from the United Nations’ Intergovernmental Panel on Climate Change (I.P.C.C.), published this morning in Geneva, Switzerland, highlights “what has been true all along,” Deborah Lawrence, an environmental-sciences professor and tropical-ecosystem expert at the University of Virginia, said. “We cannot make our climate goals without stopping deforestation and better managing agriculture.” In the past, I.P.C.C. reports focussed on what various energy futures would mean for the atmosphere. How much might we reduce the use of fossil fuels, by when, and what might that look like for all of earth’s systems? In this case, however, the panel focussed exclusively on land—and specifically on how people’s unsustainable use of land is dramatically contributing to climate change. According to the report’s findings, land use is responsible for twenty-three per cent of global greenhouse-gas emissions—half from carbon dioxide emitted through deforestation, half from agriculture. (If pre- and post-production activities in the global food system are included, the emissions are estimated to account for as much as thirty-seven per cent of total human-caused greenhouse-gas emissions.) In turn, climate change is hastening land degradation, destabilizing the food supply, and harming the livelihoods of hundreds of millions of people. “Climate change creates additional stresses on land, exacerbating existing risks to livelihoods, biodiversity, human and ecosystem health, infrastructure, and food systems,” the report’s summary for policymakers states. “Some regions will face higher risks, while some regions will face risks previously not anticipated.”

Since the industrial revolution, the overland air temperature has risen by about 1.53 degrees Celsius, or nearly three degrees Fahrenheit—almost double the average increase over the land and sea combined. Around the world, there have been more frequent, more intense, and longer-lasting heat waves, along with changing precipitation patterns—heavier bouts of rain over all, and more frequent and intense droughts in some regions, including the Mediterranean, many parts of South America, much of Africa, and northeastern and western parts of Asia. Climate change has increased risks from water scarcity, soil erosion, vegetation loss, wildfire damage, permafrost thaw, coastal dissolution, and tropical crop decline. In the Global South, climate change has caused the yields of some crops, such as maize and wheat, to decline, and has lowered animal-growth rates and productivity in African pastoral systems. Interviews and community surveys with indigenous groups in the African drylands and high-alpine regions of Asia and South America also detail the ways that changes in regional climates have further challenged their food security. As of 2015, five hundred million people lived in areas that were affected by desertification.

That’s the present moment. The report states that, even if we dramatically reduce greenhouse-gas emissions in the next decade, achieve carbon neutrality by 2050, and thereby limit warming to between 1.5 and 2 degrees Celsius—something the world is nowhere near on track to accomplishing—food security is at risk in the course of the twenty-first century, especially for the most vulnerable people. Extreme-weather events will get worse and more frequent, disrupting the supply chain. Higher atmospheric levels of carbon dioxide will lower the nutritional quality of crops. In a middle-of-the-road scenario—where technological progress, production, and consumption patterns continue as they have in the past—cereal prices in 2050 will see a median increase of 7.6 per cent, with the possibility of a twenty-three-per-cent increase over current levels. “Exceeding the limits of adaptation can trigger escalating losses or result in undesirable transformational changes,” the report added, including forced migration, conflicts, and extreme poverty.

People today use seventy-two per cent of the earth’s ice-free land for food, animal feed, fibre, timber, and energy. A quarter of that land is subject to what the report called “human-induced degradation.” If you share the responsibility equally across the planet, that is roughly three acres for every person—about equal to a cow in the Amazon—but, if you scale it by a fraction of the global economy, Lawrence pointed out, “everyone in the United States is responsible for deforesting, degrading, or somehow altering ten acres of land.” That is significantly more than any person needs to thrive. Lawrence went on, “You can almost excuse us on the atmosphere—we thought it was so big. . . . But, with the land, we know it is finite. We don’t need scientists to tell us that.”

Fortunately, science has been advancing particularly rapidly in terms of land-based natural climate solutions. The conservation of carbon-rich land—forests, wetlands, mangroves, peatlands—improved forest and grazing-land management, and systemic changes in food production and consumption could provide as much or more mitigation as eliminating the emissions from all global transportation, or all the electricity and heat used in buildings globally. Plant-based diets, featuring coarse grains, legumes, fruits and vegetables, nuts, and seeds, present opportunities, according to the report, “for reducing GHG emissions from food systems” by as much as 3.4 gigatons of carbon dioxide (or its equivalent) per year, by 2050. Food loss and waste represented a third of global food production and, from 2010 to 2016, equalled eight to ten per cent of total greenhouse-gas emissions from food systems, costing roughly a trillion dollars per year. Such waste could be minimized through improved harvesting techniques, on-farm storage, infrastructure, and packaging. Changing agricultural methods to reduce soil erosion—which, in some cases, when conventional tillage is practiced, is occurring at a rate as much as a hundred times higher than soil is forming—could easily keep more carbon in the ground. Other farming programs, like agroforestry and perennial crops, can also sequester meaningful amounts of carbon from the atmosphere.

The longer the world waits, the less capacity such solutions have for making a difference, as we are on the verge of surpassing tipping points in many places besides the Amazon. Other land-based tipping points involve the world’s permafrost, which holds an estimated 1.5 trillion tons of carbon, and is already rapidly melting. As wet, boggy, and peaty soil thaws and releases methane, it speeds up warming, which speeds up melting. As boggy soils drain to rivers and oceans in the Arctic, peaty soils will dry out and turn into tinder. Even if those soils don’t burn, natural decomposition will send the peat’s carbon into the atmosphere. That’s why, with natural climate solutions, it is important to remember that it’s “a ‘yes, and’ relationship,” Katharine Mach, a senior research scientist in earth systems at Stanford University, said. “Carbon stored in the biosphere is at risk if we don’t simultaneously get energy, industry, and transport emissions in check.” At that point, it won’t matter if everyone becomes a vegetarian.