Net zero emissions: What is it and why does it matter so much? 

A U.N-backed global campaign to slash climate-changing emissions has added new high-profile members to its ranks, including social media giant Facebook and Ford, the first U.S. car company to join.

The "Race to Zero", launched in June, brings together businesses, cities and other organisations that aim by around mid-century to cut their planet-heating emissions to net zero - meaning they produce no more emissions than they can offset through measures such as planting trees.

As New York's annual Climate Week started on Monday, the British government - host of the next U.N. climate summit - said 22 regions, 452 cities, 1,128 businesses, 549 universities and 45 major investors had now signed up for the "Race to Zero".

It may be the latest buzzword in the world of climate action but it's key to keeping us safe from harm, scientists say. The U.N. climate science panel has said that man-made carbon dioxide emissions need to fall by about 45% by 2030, from 2010 levels, and reach "net zero" by mid-century to give the world a good chance of limiting warming to 1.5C and avoiding the worst impacts of climate change,

Under the 2015 Paris Agreement, nearly 200 countries said they would act to limit the rise in global average temperatures to "well below" 2 degrees Celsius above pre-industrial times and strive to keep it to a ceiling of 1.5C.

But the world has already heated up by about 1.1C and is currently on track for warming of at least 3C this century as emissions continue to rise. Scientists say that would bring ever-worsening extreme weather and potentially catastrophic sea level rise, making some parts of the planet uninhabitable and fuelling hunger and migration.

That - and mounting public pressure - is why a growing number of countries, companies and others are promising to cut their planet-warming emissions to net zero by 2050 or before.

Achieving net zero emissions isn't the same as eliminating all emissions.

It means ensuring any human-produced carbon dioxide or other climate-changing emissions that can't be done away with are removed from the atmosphere some other way. This can be done naturally, such as by restoring forests that suck CO2 out of the air. Or it can be done using technology that can capture and store emissions from power plants and factories or directly pull CO2 from the atmosphere.

Planting more trees worldwide is a popular way to absorb and store more carbon, but technologies that do the same job are still expensive and have yet to be deployed on a large-scale. Scientists say carbon "removals", in any form, cannot substitute for cutting planet-heating emissions as fast as possible.

According to the World Resources Institute (WRI), a Washington-based think-tank, 19 countries and the European Union have adopted a net zero target, with deadlines ranging from 2030 to 2050. The tiny Himalayan kingdom of Bhutan has already achieved its 2030 goal, though measures such as adopting renewable energy, promoting electric cars and restoring or planting forests.

Some countries, including Denmark, France, New Zealand and Britain, have enshrined their targets in law. Others are less binding.

More than 100 additional nations are considering putting in place net-zero targets, WRI says - but the fraction of global emissions covered by some form of nationally adopted net-zero targets still hovers around 10%.

Several hundred cities and more than 1,000 companies also have committed to cut their emissions to net zero in the coming decades.

WRI and the 2050 Pathways Platform - which work with governments and others on their climate commitments - say cutting emissions within national boundaries should be the first priority, with efforts to offset what remains only considered after that. Right now, countries vary in whether their net zero targets can include offsetting emissions internationally, such as by paying to protect forests in the Amazon.

To be credible, net-zero targets should cover all greenhouse gases, including methane, and all economic sectors, as well as international aviation and shipping, WRI says. Those trying to achieve net zero emissions should do so by 2050 or earlier, with the highest-emitting countries doing the most, fastest. Plans also need to be reached in consultation with those they will affect and clearly communicated, WRI said.

When it comes to companies, net zero targets vary widely in terms of which parts of supply chains - and sources of emissions - they cover, and are difficult to compare, says the Science Based Targets initiative (SBTi), which has released guidelines to help remedy that.

U.N. climate chief Patricia Espinosa says companies, cities and others that join the "Race to Zero" campaign will be held to their promises, although it is unclear how that will be done.

She and other officials have called for governments to stick to an international deadline for submitting stronger climate action plans by the end of 2020.

But she has also admitted that less than half are likely to do so, as the coronavirus pandemic distracts politicians and delays U.N. climate talks.

Getting net zero targets into national plans for this decade and into day-to-day decision making is crucial, according to WRI researcher Kelly Levin, to avoid investments going into high-carbon technologies or infrastructure.

How can net-zero targets be translate into short-term practical tools? One way is by using carbon budgets, as in Britain, which place a cap on the total amount of greenhouse gases the country can emit over a five-year period, Levin said.

Britain, however, has yet to set updated carbon budgets - which began in 2008 - since adopting its net zero emissions goal last year.