The planet is being consumed by humans

For Western consumers, giving up steak and lamb is arguably the single most important personal contribution to tackling both the climate and biodiversity crises.

Mark Lynas is a writer on climate change, and visiting fellow at the Alliance for Science at Cornell University. The opinions in this article belong to the author.

(CNN)Humanity is on a collision course with nature. Already 72% of the global ice-free land surface is dedicated to supporting our species, and between a quarter and a third of the entire 'net primary production' of the planet is consumed by humans.

Net primary production is a measure of the combined photosynthetic output of all the world's plants. Because we grab so much for ourselves, smaller and smaller amounts are left in the food chain for the rest of life on Earth.
No wonder so many other species are going extinct, displaced to the margins of existence in disappearing forests and degraded ecosystems. According to the latest International Union for Conservation Nature's (IUCN) Red List, 40% of amphibians, 25% of mammals, 14% birds and 33% of corals are threatened with extinction.
    And it's only expected to get worse. Within another 30 years the global human population will exceed 9 billion people, meaning we will have to increase food production by at least another 70% in order to stave off mass famine.

    Living in a climate of change

    Superimposed on all of this is the climate crisis, which will expand deserts, shift rainfall patterns beyond recognition and make large areas of the world's current continental breadbaskets too hot or dry to grow crops.
    These challenges are outlined in the latest special report of the UN's Intergovernmental Panel on Climate Change (IPCC), this time focusing on the issue of land use.
    Addressing the challenge of land use requires dramatic changes in how we farm food.
    The implications are profound. Just as the climate crisis demands that we transform our energy use, abandoning fossil fuels and switching to clean energy sources worldwide, addressing the challenge of land use requires dramatic changes in how we grow and consume food.
    The two crises are also intimately linked. The scientists make clear that without addressing land it will be impossible to fully tackle global heating, because 22% of greenhouse gas emissions arise from agriculture, forestry and other land use.
    Although the IPCC experts are cautious in their use of language and do not make specific recommendations, it is clear from the report that a massive priority is to shift diets in developed countries away from the current heavy use of meat and dairy products.

    The problem with meat

    The majority of the world's land is used not to feed humans directly but to support livestock. Over-consumption of meat is unhealthy, and also an environmental disaster: rainforests are cleared in Brazil both to provide pasture for beef cattle, but also to grow soya crops for export to markets like Europe where they are mostly used in animal feed.
    Ruminants like sheep and cattle do not only degrade land directly through over-grazing and pollution from manure, they also release enormous quantities of methane, a global warming gas thirty times as powerful as carbon dioxide.
    For Western consumers, giving up steak and lamb is arguably the single most important personal contribution to tackling both the climate and biodiversity crises. A largely vegetarian -- or better still, vegan -- planet would be able to dramatically reduce agriculture, sparing more land for nature.
    However, the farming lobbies are powerful. In Europe, farmers are supported by subsidies, without which much livestock production would already be uneconomic. Europeans pay through taxes to support unnecessary land destruction by agriculture -- hardly a sensible policy.