The latest United Nations report on climate change warns that unless humans change their diet, efforts to curb greenhouse-gas emissions will fall short of earlier predictions. The special report released Thursday by the Intergovernmental Panel on Climate Change focuses on land use in raising cattle and other meat-producing livestock, and calls on people in wealthy nations to “reduce meat consumption” and change to plant-based diets as a “major opportunity for mitigating and adapting to climate change.” The report is meant to provide evidence ahead of climate negotiations around the world. “We don’t want to tell people what to eat,” said Hans-Otto Pörtner, an ecologist who worked on the report. “But it would indeed be beneficial, for both climate and human health, if people in many rich countries consumed less meat, and if politics would create appropriate incentives to that effect.”