Five scientific principles for operationalising net-zero pledges

The importance of net zero emissions became clear to scientists Myles Allen and Dave Frame on a train to Exeter. The year was 2005. Tony Blair, the UK Prime Minister at the time, had convened scientists for a conference in Exeter to ask what constitutes “dangerous climate change.” Allen and Frame were preparing their presentation and looking at results from climate models on a laptop.

Up until that time, the climate problem was almost universally framed in terms of the number of greenhouse gas molecules in the atmosphere. The first climate agreement, reached by governments under the United Nations back in 1992, had as its overarching aim the “stabilisation of greenhouse gas concentrations in the atmosphere that would prevent dangerous anthropogenic interference with the climate system”. For decades before that, scientists had been studying how much Earth’s temperature would change if CO₂ concentrations were suddenly doubled and then held constant.

As climate models developed, they incorporated more of Earth’s complexity. This led to more understanding but not necessarily less uncertainty. Not only did a range of warming possibilities emerge for a given concentration, but also a range of concentration outcomes for a given path of emissions.

By the 2000s, a troubling finding had emerged: it was remarkably hard to put an upper limit on how much the world will eventually warm for a fixed level of greenhouse gas concentrations. Climate models, as well as the record of observed warming, could not rule out that temperatures would keep creeping up over time.

But as scientists grappled with this uncertainty, they also questioned its relevance. It would be an odd future in which humanity held concentrations fixed indefinitely while the world warmed inexorably. If temperatures kept climbing, would our descendants not dial down emissions more, or even develop ways to remove CO₂ from the air?

What Allen and Frame discovered on the train – and published in the journal Nature in 2008 – was a better way to think about the problem. Rather than focussing on CO₂ concentrations and final temperature, they found a much clearer link between CO₂ emissions and peak temperature.

It just so happens that the timescales over which global temperatures rise in response to increased concentrations are very similar to the timescales over which the global carbon cycle draws down CO₂ from the atmosphere. As a result, if CO₂ emissions go to roughly zero, the two processes balance each other out: global average temperature will rest where it is at that time, neither rising nor falling further.

Essentially, every additional tonne of CO₂ put into the atmosphere by human activity will raise the maximum warming the world will reach. And, by extension, any limit to that warming – whether 1.5C, 2C, 4C or 1C – requires a limit to the total amount of CO₂ emitted. A global carbon budget, after which point CO₂ emissions must be “net-zero”.

Like many scientific discoveries, a few other researchers were already pulling together threads of the concept. But after the paper in Nature, net-zero went very rapidly from scientific finding to headline IPCC conclusion, to Paris Agreement, to mainstream climate target, all within little more than a decade.

Net-zero is now seen as an organisational imperative: nations, regions, cities, companies, even fossil fuel companies have committed. Taking stock of this shift, some of the scientists involved have recently reflected on the scientific origins of net-zero and its implications. With this in mind, here are five principles  for operationalising net zero pledges which flow from the science:

1. Cut faster

First, the most important factor determining peak global temperature is total CO₂ emissions over time. While this means net-zero CO₂ is a necessary point to reach in order to halt the temperature rise, it also means the pathway taken crucial. In particular, the faster emissions are cut now, the smaller the total emissions and the lower the peak warming. This is why it is so important that countries increase their pledges and actions to 2030, and that others set interim targets, in addition to their longer-term commitment to net zero.

2. Get into the mix

Second, the net-zero concept was developed for CO₂ emissions specifically. There are many other greenhouse gases, and scientists agree they will need strong reductions to keep temperature in check. But for gases such as methane, more potent but shorter-lived than CO₂, global temperature is determined more by the rate of emissions decline rather than the total over time. Climate action definitely should deal comprehensively with all planet-warming emissions. If that means your net-zero target is for all gases and not just CO₂, then it should involve CO₂ specifically going to at least net-zero.

3. Keep it real

Third, net-zero means that any remaining emissions at a given point in time are balanced by an equal amount of removals from the atmosphere. This is often not how net-zero is applied at the national or corporate level. Net-zero targets usually include the possibility of buying offset credits to balance emissions. More than 87% of current offsets available for regular customers to buy are not actually removals – they come from emission reduction projects. Emissions do need to come down, so high-quality emission reduction offsets can have a place. But if offsetting is to play a role in reaching real net zero, it will have to ultimately come from removal projects.

4. Stay down

Fourth, net-zero needs to be permanent. When CO₂ from fossil fuels is emitted into the air, the global temperature stays raised for multiple millennia – effectively permanently on timescales relevant to human beings. That means net-zero needs to be similarly durable. Any removal used to balance out residual emissions needs to be permanent. This is crucial because it is all too easy to imagine fossil fuel emitters claiming net-zero based on non-removal offsets, or tree-planting projects (which can literally go up in smoke without replacement), or leaky underground carbon storage (if not managed properly). If removals are not sustained over many decades, it won’t be enough.

5. Go global

Last but not least, what ultimately matters to the climate is the sum total of global action. Even more than net-zero organisations, we need organisations enabling a net-zero world. Individual net-zero targets may be great motivators for action, and global net-zero will require pretty much every activity to be 100% emission-free. But we need to avoid the fallacy of division. Some will actually need to go beyond net-zero, like Microsoft intends to. And perhaps, for some activities which are genuinely hard to go 100% emission-free, it may be better to be as ambitious as possible than to claim net-zero on the basis of dodgy accounting.

Net-zero targets are not without controversy. Much attention – all the way up to the top of the United Nations – is now focussed on improving their integrity. The scientific origins of net-zero point clearly to the importance of cutting emissions across the board now, complemented by the scaling up of durable carbon removal. While the science of net-zero is firmly in, the delivery is firmly not. From scientific beginnings, the net-zero train now needs to go from intent through integrity to implementation. We’re tracking the journey.

Dr Steve Smith, co-lead, The Net Zero Tracker and executive director, Oxford Net Zero.