With Britain set to become the first major economy to commit in law to reaching a target of net-zero emissions by 2050, what is carbon neutrality, and how will nations reach it?
- Why net-zero? -
Nations are gathered this week in the German city of Bonn to discuss implementing the Paris climate deal -- a landmark accord that in 2015 committed countries to work to limit global temperature rises.
Paris aims to cap warming at two degrees celsius (3.6 Farenheit) and requires nations to submit individually defined plans to slash the greenhouse gas emissions that are driving the mercury up.
It also strives for a safer cap of 1.5C of warming which experts believe can stave off the worst social, economic and environmental effects of a hothouse Earth.
According to the Intergovernmental Panel for Climate Change (IPCC), the 1.5-C cap means most nations by mid-century need to be carbon neutral -- that is, either reduce emissions to zero or offset those that remain through ecological or technological solutions.
- Who's on board? -
Britain's net-zero law is expected to pass this month. France, another large historic emitter, has draft legislation for climate neutrality by 2050, as do Spain and New Zealand.
Sweden and Norway have laws holding them to carbon-neutral economies by 2045 and 2030, respectively, and a handful of other nations have laid out concrete net-zero timelines.
Richard Black, director of the Energy and Climate Intelligence Unit, which tracks where nations are on net-zero pledges, said the target was "probably the best single indicator of whether a nation is serious about delivering what it promised" in the Paris deal.
The European Union failed last week to agree to the 2050 goal, with some member states saying more discussion was needed.
Yvon Slingenberg, director of the European Commission's international policymaking division, told AFP she was confident that all EU states would back the 2050 goal by the end of the year.
"If Europe doesn't take the lead in the transition to climate neutrality then who can we realistically expect to really deliver climate neutrality in the world?"
- 'Economy-wide transformation' -
It remains to be seen whether the bloc will insist all members hit net-zero by mid-century, or whether an agreement can be struck whereby some nations offset the outlying emissions of others.
But the real emissions cuts will come from industry. Construction, transport, energy and agriculture all need sharp emissions reductions -- though national net-zero plans have so far fleshed out little in this regard.
The additional question of carbon-heavy aviation and maritime transport, which is by nature international, has yet to be successfully addressed.
Net-zero "will require a genuine transformation of the energy that we use, the way we're gonna heat our homes, the way we travel," said Archie Young, head of Britain's delegation in Bonn.
"It is a true economy-wide transformation."
- Journey matters -
The IPCC's climate report in October laid out several different scenarios -- or "pathways" -- that nations could take to reach net-zero.
By far the safest route to 1.5C is an immediate, drastic drawdown in fossil fuel usage, with emissions peaking in a few years and nearly halved by 2030.
Other 1.5C pathways involve mass deployment of carbon capture and storage (CCS) schemes, which would suck the excess CO2 from the atmosphere, eventually mitigating temperature rises.
Some experts fear that a 2050 goal, if not accompanied by interim emissions cuts targets, could see richer economies continue to burn fossil fuels for decades before turning to technology to drag emissions down.
Teresa Anderson, climate policy coordinator for ActionAid International, said the journey countries take in getting to net-zero would be crucial.
"A 2050 climate target will be almost irrelevant if it doesn't drive action in the next few years," she told AFP.
"National targets really need ambitious milestones for 2025 and 2030, so that we can know for sure that we are on track and driving the urgent transformation today."