In September, California governor Jerry Brown signed a bill mandating that by 2045, all of California’s electricity will come from clean power sources. Technological breakthroughs in producing electricity from sun and wind, as well as lowering the cost of battery storage, have played a major role in persuading Californian legislators that this goal is realistic.

James Robo, the CEO of the Fortune 200 company NextEra Energy, has predicted that by the early 2020s, electricity from solar farms and giant wind turbines will be cheaper than the operating costs of coal-fired power plants, even when the cost of storage is included.

Can we therefore all breathe a sigh of relief, because technology will save us from catastrophic climate change? Not yet. Even if the world were to move to an entirely clean power supply, and use that clean power to charge up an all-electric fleet of cars, buses and trucks, one major source of greenhouse gas emissions would continue to grow: meat.

The livestock industry now accounts for about 15 percent of global greenhouse gas emissions, roughly the same as the emissions from the tailpipes of all the world’s vehicles. But whereas vehicle emissions can be expected to decline as hybrids and electric vehicles proliferate, global meat consumption is forecast to be 76 percent greater in 2050 than it has been in recent years. Most of that growth will come from Asia, especially China, where increasing prosperity has led to an increasing demand for meat.

Changing Climate, Changing Diets, a report from the London-based Royal Institute of International Affairs, indicates the threat posed by meat production. At the UN climate change conference held in Cancun in 2010, the participating countries agreed that to allow global temperatures to rise more than 2°C above pre-industrial levels would be to run an unacceptable risk of catastrophe. Beyond that limit, feedback loops will take effect, causing still more warming. For example, the thawing Siberian permafrost will release large quantities of methane, causing yet more warming and releasing yet more methane. Methane is a greenhouse gas that, ton for ton, warms the planet 30 times as much as carbon dioxide.

The quantity of greenhouse gases we can put into the atmosphere between now and mid-century without heating up the planet beyond 2°C – known as the “carbon budget” — is shrinking steadily. The growing demand for meat means, however, that emissions from the livestock industry will continue to rise, and will absorb an increasing share of this remaining carbon budget. This will, according to Changing Climate, Changing Diets, make it “extremely difficult” to limit the temperature rise to 2°C.

One reason why eating meat produces more greenhouse gases than getting the same food value from plants is that we use fossil fuels to grow grains and soybeans and feed them to animals. The animals use most of the energy in the plant food for themselves, moving, breathing, and keeping their bodies warm. That leaves only a small fraction for us to eat, and so we have to grow several times the quantity of grains and soybeans that we would need if we ate plant foods ourselves. The other important factor is the methane produced by ruminants – mainly cattle and sheep – as part of their digestive process. Surprisingly, that makes grass-fed beef even worse for our climate than beef from animals fattened in a feedlot. Cattle fed on grass put on weight more slowly than cattle fed on corn and soybeans, and therefore do burp and fart more methane, per kilogram of flesh they produce.