The invention of net zero: Best ideas of the century

In 2005, physicists David Frame and Myles Allen were on their way to a scientific conference in Exeter, UK, and had been, in Frame’s words, “fiddling about” with a climate model to prepare for their presentation.

At the time, most research focused on stabilising the number of greenhouse gas molecules in the atmosphere to prevent dangerous climate change. But scientists were struggling to figure out how much the world would warm at a fixed level of greenhouse gas concentration.

Frame and Allen turned the problem on its head. Rather than focusing on atmospheric concentrations, they turned to emissions. What if humanity shut off human-caused carbon dioxide emissions? The duo tested the idea out on their climate model right there on the train. The result? Global temperatures remained stable at their new level. In short, the world would stop warming once humanity reached “net-zero” carbon emissions. “It was quite cool sitting on the train looking at these figures for the first time and thinking: ‘Wow, that’s a big deal,’”

That presentation – and the subsequent Nature paper published in 2009 detailing their findings – kicked off a new way of thinking in the climate community. Before the advent of net zero, it was widely believed humans could still emit a fair chunk of emissions, around 2.5 gigatonnes per year – about 6 per cent of annual global emissions today – while holding global temperatures stable. But now, it was clear emissions would have to reach net zero to stabilise the climate, with any human-made emissions balanced by equivalent removals from the atmosphere.

The idea of a global need to eventually reach net-zero CO2 emissions quickly caught on, becoming a headline conclusion of an Intergovernmental Panel on Climate Change report in 2014. The next question was timing: when would we have to get to net zero? With parallel research uncovering the dangers of surpassing 1.5°C of warming, the world decided in Paris in 2015 to aim to keep warming as close as possible to that threshold. That meant achieving net-zero emissions by roughly mid-century.

Almost immediately, governments around the world came under intense pressure to set net-zero goals. Hundreds did so, along with thousands of companies and financial institutions, which spotted the economic opportunities a clean-energy transition promised. This net-zero fever has led to some dubious pledges that rely far too heavily on using the world’s forests and swamps as sponges for human pollution, but it has also changed the trajectory of the century. Three-quarters of global emissions are now covered under a net-zero pledge, and projections for warming this century have fallen from around 3.7°C-4.8°C before Paris to 2.4°C-2.6°C under current climate promises.