In recent months, a cascade of countries, cities, states and provinces, and businesses have set “net zero” targets. These pledges aim to cut carbon emissions, while mopping up, or otherwise paying to offset, the remainder.

Can we believe them? As Greta Thunberg said at Davos earlier this year: “Forget about net zero, we need real zero.”

Like a pithy quote, net zero has gone viral. Amazon, Ikea, Delta Airlines, and even oil and mining firms like BP and Rio Tinto have announced their intention to get there. Remarkably, a new report from the London think tank ECIU shows that 49 percent of global GDP is already covered by cities, states, provinces and countries that either have net zero targets or are working toward them.

That is welcome news from a scientific perspective. The landmark 2018 report from the Intergovernmental Panel on Climate Change (IPCC) showed that we need to bring emissions of CO2 to net zero by mid-century in order to limit climate change to 1.5°C.

The groundswell of net zero targets is also good news for the Paris Agreement, which calls for a balance of sources and sinks of greenhouse gases in the second half of the century. The announcements will give countries a much-needed boost this year to “ratchet” their pledges – the first new commitments since 2015 – when they meet in Glasgow in November.

Ultimately, however, the atmosphere responds to outcomes, not announcements. The truth is that significant uncertainty surrounds the outcomes of these targets. Anyone setting a net zero target – and the citizens, activists, consumers, and investors holding them to account – therefore needs to ask the following questions.

First, what are you covering? For countries, the focus is usually on territorial emissions. But for many sub-national jurisdictions – cities in particular – a large chunk of emissions may be driven by decisions or products made outside their territory. Similarly, companies can set net zero commitments that cover only their direct activities, or they can include emissions across their entire value chain, including use of their products. Finally, some targets aim just at CO2 – which the IPCC singled out, after all – while others include greenhouse gases such as methane.

Second, when is your deadline? The IPCC’s benchmark of net zero carbon by mid-century is the global average. Some will need to get there sooner. From an equity perspective, a “first in, first out” policy makes sense. Those that have benefited most from carbon emissions in the past should end them as soon as possible.

Third, how will you get there? If you plan to use novel options that are not yet mass-market, what steps are you taking to develop them today, and what will you do if they don’t succeed? A critical question is what is “net” and what is “zero?” Many companies and even some countries have staked at least part of their target on buying “offsets” from others. Most current removal options are “nature-based solutions”, such as planting trees to remove carbon, which can certainly help and can have many other benefits for health and biodiversity.

But as more targets aim for net zero, we need to make sure that we are not all counting on using the same trees, soils, and other “natural sinks,” which are increasingly threatened. Importantly, even with the best conservation efforts, the world simply does not have enough natural solutions to offset all future emissions. If you are planning on offsetting, you need to be thinking about “engineered” techniques that remove CO2 from the air and store it away permanently – such techniques are currently expensive and are not yet at scale.

Fourth, what are you doing today? Mid-century targets that do not come with immediate emissions reductions and investments in transformation are not credible.

Fifth and finally, how will you stay on track over 30 years of disruptive change? Is the target just an aspiration in a press release, or have you enshrined it in law and policy? How will you track and report on progress? Who will hold you to account? More broadly, CEOs, mayors, governors, and heads of state come and go. For net zero targets to be durable they must be grounded in viable business models and long-term social consensus. This means making sure that workers and communities who could most affected climate change or left behind by decarbonisation are protected and brought along.

The surge of net zero targets shows that we are increasingly aware of just how much we need to do to stop climate change. But getting there requires immediate action and answers to some hard questions.

Myles Allen is Professor of Geosystem Science and is a Lead Author on the Intergovernmental Panel on Climate Change (IPCC) Special Report on 1.5C

Thomas Hale is Associate Professor of Global Public Policy, Blavatnik School of Government

Tim Kruger is Programme Leader, Climate Research Programme, Oxford Martin School

Stephen Smith is project manager, Greenhouse Gas Removal Hub