With the Paris Agreement in 2015, nearly 200 nations submitted plans to cut greenhouse gas emissions. Because it was clear, however, that these measures would not be nearly enough to meet the Paris goals, the agreement stipulated that states would submit commitments to more radical cuts five years later.
In the intervening years, storms, floods, drought and forest fires have made climate change a terrifying reality for increasing numbers of people. And last year, the Intergovernmental Panel on Climate Change (IPCC) warned of much worse to come if we exceed 1.5 degrees of warming.
To meet this goal, the IPCC said we must cut emissions to net-zero by 2050. Last November, the European Commission published a "strategic vision" to get there, which it's hoped the European Union will adopt at a summit in Brussels on June 20.
So what does "net-zero" mean?
Net-zero means a radical change across the entire economy, doing away with fossil fuels and other sources of emissions wherever possible. For the rest, every ton of CO2 we do emit must be matched by a ton that we remove from the atmosphere.
Which sectors can and can't we "decarbonize?"
So far, experts say we've done the easy bit: Renewable electricity generated has plummeted in price and is already overtaking fossil fuels in some countries. But then things get more complicated, because power that comes and goes with the weather needs to be backed up with something more constant like biofuels — or nuclear, which the EU strategy expects to generate 15% of our power at net-zero.
Read more: Is the IEA underestimating renewables?
We will probably also need intelligent grid systems and ways to store electricity, such as giant battery plants. This technology isn't all there yet, but the power sector is still seen as relatively low-hanging fruit.
Harder to decarbonize, but to some degree still possible — with massive deployment of heat pumps and by converting renwable power to gas — are heating, shipping and industrial processes. But then there are planes and cows. Renewable-powered flying isn't on the cards yet, and our appetite for beef implies climate-killer methane.
What are negative emissions?
That's where negative emissions come in. To meet the net-zero target, we will literally have to suck CO2 out of the atmosphere. The obvious way is with trees and plants, which do it anyway.
That means turning over vast areas of land to forest and wetlands, as well as looking at other natural forms of carbon sequestration. One idea is bio-energy with carbon capture and storage (BECCS): Energy crops, such as maize, absorb carbon as they grow. We then burn them (generating energy we can use), capturing the carbon they emit, so it can be buried or recycled rather than going back into the atmosphere. But this would require so much agricultural land it could threaten food security.
Carbon capture and storage (CCS) technology hasn't progressed much beyond the pilot-project stage, and has mostly been used with fossil fuel power plants or to capture industrial emissions.
And then there is direct air capture (DACCS), which splits CO2 from the air around us. But it's in even earlier stages of development than CCS.
What about carbon offsetting?
Leaving carbon capture aside, there is another way to reach the big zero: carbon credits. Rather than a country offsetting emissions it can't — or won't — get rid of against carbon sequestered on its own soil, offsetting schemes allow it to pay for things such as tree-planting or forest conservation abroad.
But the United Nations Framework Convention on Climate Change's REDD+ offset scheme has been criticized for lacking "environmental integrity" and violating indigenous land rights. And it obviously doesn't work if every country wants to emit more than their share. Ultimately, offsetting means some countries will have a negative balance — absorbing more carbon than they emit.
How much traction does net-zero have in EU states?
In Europe, Denmark and Norway have enshrined net-zero by 2050 in law, while Sweden's climate legislation commits it to meeting the target by 2045. Still, all three Nordic countries either allow or don't explicitly rule out international carbon offsetting to reach the goal. Countries including France, Spain, the United Kingdom, Germany and the Netherlands are debating similar steps.
Read more: Sweden to end net carbon emissions by 2045
The target year of 2050 is a negotiation between the necessary and the possible. The IPCC says if we cut global emissions to net-zero, we can still keep warming below 1.5 degrees. If we end up with net-zero by 2070, we can still hope for no more than 2 degrees.
Read more: How to make the world affordably carbon-free
Europe's share of the global target
Environmentalists say Europe, which is responsible for a disproportionate share of the CO2 that's been warming the planet since the industrial revolution began, must do better. Climate Action Network Europe, made up of 160 environmental and development organizations is calling for a 2040 deadline.
Read more: Have we already blown our carbon budget?
The European Commission's vision says the 2050 goal will make the EU "one of the first to achieve net-zero greenhouse gas emissions and lead the way the worldwide," implying that other countries will be less ambitious, and therefore that the IPCC's global net-zero target will be missed.
So, we still have a few decades then?
A zero-carbon economy won't be built in a day. The deadline is just over 30 years away. By then, our streets must be cleared of all combustion-engines. The Climate Change Commission advising the UK government says no new petrol and diesel cars should be sold from 2035 at the absolute latest. Meanwhile, several EU countries have new fossil-fueled power plants, which have an average lifespan of 50 years, either in planning or under construction right now.
Most of what net zero requires is already technically possible. But the longer we take to change our systems and behavior, the heavier we will rely on the more dubious business of extracting carbon from the atmosphere.
Are there more important targets to shoot for?
Committing to net-zero makes it clear that cutting emissions isn't enough. It means there's no point in building natural gas infrastructure, for example, even if it would save a lot of emissions from coal. But talking about 2050 might give the impression we can leave the toughest jobs for last, betting on technologies that haven't been invented yet and future generations being able to stomach even more drastic change than we're facing now.
Read more: Europe's shocking failure to act on climate
Scientists say in terms of action, countries have to focus on targets for 2030, which under the Paris Agreement have to be made tougher and resubmitted by 2020. So far, Europe isn't even on track to reach the original goals it set back in 2015.