杏吧原创

Hundreds of people were arrested this week for taking part聽in protests organised by Extinction Rebellion, a group demanding that the UK set legally binding targets to reduce carbon emissions to net zero by 2025. The聽activists may not get their wish, but聽the Committee on Climate Change (CCC), which advises the UK government, is due聽to publish a report on 2 May that is expected to recommend a聽net-zero target for 2050.

This is, at least, an improvement on the UK鈥檚 current position, which is to cut carbon emissions by 80 per cent by the same date. When that goal was set a decade ago, the country was praised as聽a聽leader on battling climate change, but since then, many other countries have caught up. Lately, some have begun declaring plans to go further, faster.

The reason is simple: the science has moved on. A cut of聽80聽per cent was based on advice聽to limit global temperature rises to 2掳C, the line previously considered the threshold for dangerous global warming. But as聽the Intergovernmental Panel on Climate Change (IPCC) , limiting warming to the safer level of 1.5掳C requires the world to hit net-zero carbon emissions by about 2050.

Christiana Figueres, former United Nations climate chief, says聽it is possible that the UK could adopt such a target in time for a key UN climate summit in September. 鈥淎ll countries need to recalibrate their decarbonisation strategies. To go to zero net in the UK is absolutely key,鈥� she says.

The UK would not be the first to聽aim for net zero, but it could become one of the most credible. Observers think the country鈥檚 existing, legally binding framework and the oversight of聽the CCC mean it would be more聽likely to implement the target in law and meet it.

鈥淭here is much hope for the聽UK聽because, with the CCC, people聽would believe there is serious planning for net zero. Along with Sweden, it would be聽the most credible,鈥� says Oliver Geden of聽the聽German Institute for聽International and Security Affairs, which advises the German聽government.

Hitting the target

A group of other European countries, including France, Sweden, Norway and Portugal, as聽well as nations elsewhere, such as Costa Rica, the Marshall Islands and New Zealand, all have plans to聽hit the target, some by 2050, some earlier.

鈥淢ost of the interest is in Europe. The clear leader in all of聽this is obviously Sweden because they鈥檝e set a goal in law,鈥� says Richard Black, director of the Energy and Climate Intelligence Unit, a UK think tank.

The Swedish Climate Act came into force last year, demanding net-zero emissions through an 85聽per cent reduction in domestic greenhouse gas emissions by 2045. The remaining 15 per cent will need to come from other options 鈥� probably planting trees, removing carbon dioxide from the air or carbon offsets聽鈥� which a Swedish government commission is examining.

Many of the other countries talking about net-zero plans are still making political declarations rather than legislating for targets. 鈥淲e have more not-so-credible announcements than we have聽credible announcements,鈥� says Geden.

Net-zero targets are appealing because they are simple, says Corinne Le Qu茅r茅 at the University of East Anglia, UK, but there is no聽agreed definition of which emissions they actually mean. In聽particular, whether net zero means just CO₂ or all greenhouse gases (see 鈥�What about methane?鈥�, below), and whether it聽could be reached by buying carbon credits from other countries. Still,聽she says: 鈥淣ot聽all聽net zeroes are the same, but all net zeroes are聽good.鈥�

There is a good reason governments are talking about net zero, rather than about cutting emissions entirely. Solutions exist to cut emissions from energy and聽transport, such as through renewables and electrification of聽vehicles. But sectors such as farming will be difficult, if not impossible, to decarbonise.

Hence the emissions of some sectors will need to be balanced against one another to reach overall net-zero emissions, as Sweden is considering.

The option of carbon offsetting, where countries could choose to offset the final portion of their emissions, is appealing because cutting them could be too expensive or tricky at home. Offsetting might involve a reforestation scheme on the other side of the world, for example.

But that could end up just shifting, rather than solving, the problem. For one thing, planting trees is no guarantee of reduced emissions 鈥� they could be mismanaged, die or get cut down.

鈥淲hen we burn fossil fuels, we聽are guaranteed to emit carbon.聽When we plant a tree, we聽are not guaranteed it will absorb emissions,鈥� says Kevin Anderson of the Tyndall Centre for Climate Change Research in聽Manchester, UK.

And some say that the only way to be certain that emissions really are being reduced is to do it on your own territory, rather than having to rely on other countries.

鈥淲e have a moral responsibility to do as much as we can here [in the UK],鈥� says Josh Burke at the London School of Economics.

So, do we have the technology to get to zero? In theory, yes. But聽many of the key tools are untested at large scales, in particular capturing and storing carbon emitted from power stations and industrial processes, and ways of directly removing CO₂ from the atmosphere.

鈥淲e have all the tools on paper聽and in demonstrator form. What we don鈥檛 have is proven techniques for scale-up and financing them,鈥� says Myles Allen at the University of Oxford.

Without ways to actively capture CO₂ 鈥� effectively creating negative emissions to balance out the positive ones 鈥� it will be impossible to achieve net zero.

One of the most hopeful ideas is known as bioenergy with carbon capture and storage (BECCS), which involves growing plants to capture carbon, then burning them for energy and capturing the emissions. The聽overall effect should be a reduction in atmospheric CO₂. BECCS features in many of the scenarios for getting to net zero, but many people are sceptical about whether it can work.

Another idea is sucking CO₂聽from the atmosphere with machines, using fans that direct air into units where chemical processes trap the carbon ready for聽some form of storage.

鈥淲e don鈥檛 know if these things will work at global scale. Yet the language we use makes these things seem real, like they exist,鈥� says Anderson.

Then there are the sectors that聽are tricky. No one, not even Sweden, is talking yet about including emissions from international aviation in its net-zero targets. Flights account for about 2 per cent of global emissions today. But by 2050, the聽International Civil Aviation Organization by as much聽as 700 per cent.

One thing is clear: as the Extinction Rebellion protesters say, this an emergency. The longer that countries wait to set net-zero emission targets and implement the policies to get there, the harder it will be to get to zero. 鈥淭he sooner they are adopted, the聽easier it鈥檒l be,鈥� says Figueres.