Fresh after Auckland declared a climate change emergency, cities have been warned their emissions could nearly double by 2050 without urgent action.

A major report just released by C40 Cities – an international network that includes Auckland and represents around a quarter of the global economy – found that "consumption-based" emissions from nearly 100 of the world's big cities already represent 10 per cent of global greenhouse gases.

Consumption-based emissions covered what city businesses and residents used, ate, and wore, and how these items were made and moved.

On the bright side, the report revealed an incredible opportunity for cities to cut these back.

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But the clock was ticking: they had to be at least halved by 2030 to maintain the possibility of keeping global temperature rise below 1.5C.

When combined with firm city efforts to reduce local emissions, this would allow cities to deliver 35 per cent of the emission savings needed to put them on a path to 1.5C.

High income areas, which generated the bulk of emissions, needed to cut their emissions much faster – two-thirds by 2030.

But the research found that if nations, business, cities and citizens took ambitious climate action over the next 10 years, cities could be on track to reduce their emissions in line with a 1.5C world.

The report explored six sectors where leaders, businesses, and citizens in the world's cities could do this: food, construction, clothing, vehicles, aviation, and electronics.

Those measures ranged from cutting the amount of flights between cities and "meat free Mondays" at schools and public buildings, to switching to lower-impact materials such as sustainable timber, and running community workshops to repair electronics and appliances.

Mayors were already leading the response to the climate emergency by setting science-based targets compatible with keeping the global temperature rise to 1.5C, and taking action to slash local emissions from buildings, energy, transport and waste.

But much more needed to be done, C40 Cities executive director Mark Watts said.

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"This is a wake-up call for all leaders, business, and citizens to consider both the local and global climate impact of the things they consume, and an opportunity to better engage citizens and businesses in solving the climate emergency."

The call came just days after Auckland councillors voted to formally declare climate change an "emergency" in the region – a gesture that, while not committing the council to any code or plan, aimed to symbolise urgency.

Auckland had already set a target of slashing regional emissions by 40 per cent below 1990 levels by 2040, and was also working toward a broader goal of net zero emissions by mid-century.

Yet, the most recent data showed that, between 2009 and 2016, overall emissions increased by 5.6 per cent.

"If we continue this trend, Auckland's emissions will increase by 27.7 per cent by 2050," the council's chief sustainability officer John Mauro said.

"We need rapid changes in transport, energy use in industry and buildings and industrial processes to dramatically reduce emissions."

Mauro said looking at emissions in terms of consumption was an alternative to considering their source of production.

The global targets in the report didn't necessarily line up with those Auckland had designed – nor did they need to, he said.

With NZTA, Auckland Council is investing $900m in cycleway upgrades, including a shared path over the Harbour Bridge. Photo / Supplied
With NZTA, Auckland Council is investing $900m in cycleway upgrades, including a shared path over the Harbour Bridge. Photo / Supplied

"Cities and societies like Auckland will likely see a greater relative consumption-based footprint because we consume more.

"It should give us pause for thought for at least two reasons – first, because dealing with emissions requires some consideration of choices we make, from how much carbon is in the concrete we use in our infrastructure to what we eat for breakfast.

"Second, it should help us think more like a global system – one in which our impacts and decisions are interconnected, just like the climate itself.

"Ideally, a consumption-based inventory should sit alongside the sector-based inventory and will help us better think through solutions so our actions become more robust and have greater and wider impact."

As a signatory to the C40: Fossil Free Streets declaration, Auckland had pledged to significantly slash greenhouse gas emissions generated by transport and traffic.

Some of the work under way in the city included a $900 million upgrade of safe cycling infrastructure, the gradual electrification of diesel trains and the council vehicle fleet, and a hydrogen pilot plant at the Ports of Auckland that could fuel buses and ferries.

Elsewhere, the council was changing wastewater treatment processes to capture greenhouse gases and using biogas to generate electricity, and requiring "Homestar" as a sustainability standard for new homes in re-development locations.

But Mauro acknowledged these efforts alone wouldn't be enough.

"We need to do much more."