Developers working for Google have significantly misstated how much carbon two proposed AI datacentres will contribute to the UK’s total emissions in planning documents reviewed by the Guardian.

The tech company wants to build two huge datacentres – one 52-hectare (130 acre) project in Thurrock and another at an airfield in North Weald, both in Essex. To do so, developers are required to submit planning documents calculating how much carbon these projects will emit as a proportion of the UK’s total carbon footprint.

In both cases, they appear to have compared one year of the proposed datacentre’s emissions with the UK’s entire five-year carbon budget, understating the significance of their emissions by a factor of five, according to experts at the tech justice nonprofit Foxglove.

Greystoke, a company planning to build another datacentre in north Lincolnshire, one of the largest in the UK, also appears to have misstated the emissions of its project in the same way. Taken together, the three developments will account for more than 1% of the UK’s carbon budget in 2033. This is the equivalent to the emissions of a mid-sized city such as Bristol.

“Google has serious questions to answer about its dubious datacentre pollution figures,” said Tim Squirrell, the head of strategy for Foxglove, which discovered the errors. “By comparing one year of datacentre emissions with five years of UK emissions, they’re making the environmental impact look five times smaller than it really is.”

He added: “Unless they can explain themselves, it looks like they are seriously misleading the council and the public over the climate pollution their facility will cause.”

These apparent misstatements are another example of a pile-up of faulty calculations surrounding AI development and its environmental footprint in the UK. Last month, the Guardian reported on a large gap between the government’s plan to decarbonise the economy and its roadmap for AI computing.

The two government departments behind these plans appeared to differ on their estimates for how much UK energy datacentres will use – by a factor of 10.

Google’s Thurrock datacentre, to be built on “grey belt” land, has claimed its emissions will amount to 0.033% of the UK’s budgeted carbon footprint between 2028 and 2032. In fact, its emissions during that period will be 0.165% of the total.

The North Weald datacentre, to be built on an airfield near Epping Forest, has said it will emit 0.043% of the UK’s total carbon budget from 2033 to 2037, but it will actually emit 0.215% of the total.

Steven Heather, a local councillor, said this datacentre had been given outline planning permission only and the planning department would research what Google was proposing.

“If there is a gross error, they’ll obviously pick up on it. When it goes to the submissions stage, the developers will have to come back with the proper figure.”

Greystoke’s datacentre in north Lincolnshire, Elsham Tech Park, has stated in planning documents that its emissions will be 0.1043% of the UK’s carbon budget in 2033, when in fact they will be 0.5215%.

Google and Greystoke have publicly argued that their developments will have a tolerable, in some cases insignificant, impact on the UK’s climate roadmap. Elsham Tech Park’s developers have said it will improve biodiversity on its campus by having bird and bat boxes, and wildflower grassland.

All three developments are classed as having a “minor adverse” impact on the UK’s overall climate goals.

However, the Thurrock datacentre will produce more emissions than an international airport, and Elsham Tech Park’s peak emissions will reach 1m tonnes of CO2 equivalent in 2033-34 – just short of the 1.2m tonnes of carbon emissions of all the UK’s domestic flights.

Google representatives did not respond to a request for comment.

In a comment to the Guardian, Greystoke appeared to acknowledge the discrepancy, saying: “We expect to submit revised figures to the local planning authority as part of the planning process.”

Elsham Tech Park said its development would “see £10bn of private investment, generating thousands of well-paid operational and construction jobs in the region, and supporting local supply chains”.