A climate change project that involves people finding an alternative to using a car one day a week could become a "game-changer" amid warnings from an international report.
The Sixth Assessment Report, released this week by the UN's Intergovernmental Panel on Climate Change, found New Zealand had just over 10 years before global warming reached a "critical" threshold likely to impact agriculture and health.
The report - authored by 200 scientists and drawing on more than 14,000 studies - found human-induced climate change was now influencing weather and climate extremes across the world.
Global temperatures have risen to 1.1C above pre-industrial times and if nations couldn't keep the warming within 1.5C of pre-industrial levels, the planet would experience more heatwaves, longer warm seasons and shorter cold seasons, the report stated.
It also stated that at 2C of global warming, heat extremes would more often reach critical tolerance thresholds. Nations can only pump another 400 gigatonnes of carbon dioxide before hitting that threshold which - at current global emission rates - would happen within just 11 years.
The authors warned unless there were immediate, rapid, and large-scale emissions reductions, limiting warming to close to 1.5C - or even 2C – would prove beyond reach.
However, a Wednesday Challenge project is hoped to make "real change" and potentially reverse Tauranga's ever-increasing growth of carbon emissions.
A city carbon footprint study held in 2015/16 found Tauranga's transport emissions made up 61 per cent of all emissions with 97 per cent coming from road transport from use of petrol, diesel and LPG.
Tauranga's transport emissions were higher than Wellington's and Dunedin's and this was considered due to lower records of commuter trips using public transport. They were also expected to have grown due to the city's rapid population growth.
Project director Heidi Hughes said the Wednesday Challenge aimed to get 20 per cent of the city's population using an alternative to a car one day a week.
"If we reach our target of 20 per cent mode share, we will drop 8500 tonnes of carbon emissions which we will save from going into the atmosphere a year," she said.
"That's a 1 per cent drop in our transport emissions in Tauranga. That might not seem like a lot but it's us reducing our carbon emissions which are going up. So it's breaking that cycle.
"This has the potential to be a real game-changer."
The programme is still awaiting final sign-off but it has already received support from Bay of Plenty Regional Council which has included it in the council's Climate Change Action Plan.
Hughes, who resigned as a Tauranga City councillor last year, referenced the report's warning of needing swift action to help curb the impact of climate change.
"We see this as a perfect opportunity to do this."
Hughes said she expected the biggest challenge would be changing some people's mindset of finding reasons to justify their car use to one of finding ways to make it work.
"People have all the excuses in the world such as 'I need to pick up things on the way home and I need to take my car'. If it's only one day a week, maybe they can do this any other day of the week."
Hughes acknowledged there were plenty of people the Wednesday Challenge simply would not suit "but there are a whole lot of people in the category of 'if you had a think about it, you could make it work' ".
"If people have some incentive, we really think there's a good 20 per cent in that category, and if they stepped up and did it on a Wednesday, we would have a measurable drop in carbon emissions and a drop in congestion."
Already the carless Wednesdays challenge has attracted buy-in from local businesses keen to get employees on board. An app, prizes and support to help people get started were all expected as part of the project.
Hughes said she hoped the project empowered people "to be part of the solution".