US cities are significantly undercounting their greenhouse gas emissions, according to a new study published this week in Nature Communications.

The researchers compared self-reported emissions data from 48 US cities to independent estimates based on federal data about factories, power plants, and roads, among other sources.

They found cities are under-reporting by nearly 20 percent. When adjusted to include all US cities, that figure exceeds the total emissions of the state of California by almost 24 percent.

Mitigating the emissions of greenhouse gases, which include carbon dioxide and methane gas, is an essential step in addressing climate change.

But, the researchers say, there is currently no universal way that cities measure emissions, and inconsistent and inaccurate data make mitigation much more difficult.

"We haven't had a systematic regulatory approach to controlling greenhouse gas emissions in the US," Kevin Gurney, the paper's lead author and a Northern Arizona University professor, told The New York Times.

Instead, cities all use different methods for reporting their emissions, and many are leaving out specific fuels or sources, resulting in the inconsistent data.

On average, the cities studied under-reported emissions by 18.3 percent, with some discrepancies that were significantly higher. For instance, the researchers found self-reported emissions in Cleveland, Ohio, were 90 percent lower than the researchers' estimates.

But cities contribute three-quarters of all carbon dioxide emissions from fossil fuels, and as urban populations continue to grow, projections show urban areas tripling by 2030, according to the study.

The researchers said many cities have pledged to curb emissions, but in order to successfully mitigate greenhouse gases, there needs to be a standard method for measuring and reporting.

The city of Indianapolis, Indiana, for instance, is aiming to reduce emissions by 20 percent from 2016 to 2025. But because the study found the city underestimating its emissions by nearly 27 percent, "it will be difficult to know when and if this target is truly achieved or track progress towards it," the paper says.

President Joe Biden has pledged to combat climate change and greenhouse gas emissions. In one of his first actions in office, Biden rejoined the international Paris climate agreement, a commitment made in 2015 to curb global warming by cutting greenhouse gas emissions.

Biden also revoked a permit for the controversial Keystone XL oil pipeline and temporarily halted new oil and gas leases on Alaska's Arctic National Wildlife Refuge.

He also worked with other Democrat lawmakers to develop a $US2 trillion climate plan that calls for 100 per cent clean energy by 2035.

This article was originally published by Business Insider.

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