The Net-Zero Imperative

Many numbers have been tossed around at the UN Climate Change Conference in Paris: targets for emissions reductions, temperature goals, budgets for mitigation and adaptation, and so on. But the only number that truly matters is the net amount of carbon dioxide we can emit if we are to stabilize the planet’s temperature at any level.

OXFORD – The world has reached an historic agreement on climate change. The deal concluded at the United Nations Climate Change Conference in Paris commits countries to take steps to limit warming to “well below” 2º Celsius relative to pre-industrial levels and to pursue “efforts” to limit warming to 1.5ºC. It also obliges developed countries to provide $100 billion per year in assistance to developing countries. But, unfortunately, the final negotiations dropped the one number that truly matters for the future of our planet: zero.

That is the net amount of carbon dioxide we can emit if we are ever to stabilize the planet’s temperature at any level. Zero, none, nada. The Earth’s atmosphere-ocean system is like a bathtub filling up with CO2 and other greenhouse gases: The higher the level, the warmer the planet will be.

The emissions tap must be turned off once the bathtub reaches a level associated with a certain level of warming – say, 2ºC, above which, scientists nearly unanimously agree, the risks become severe, tipping points become possible, and civilization’s ability to adapt is not guaranteed. Otherwise, the atmospheric bathtub will keep being filled, warming the planet 3º, 4º, 5º, and so on, until emissions eventually stop – or we go extinct. The sooner we turn off the tap, the lower the temperature at which the climate stabilizes, the less risk we will face, and the lower the cost we will incur in adapting to a warmer planet.

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