The UK's most powerful computer was installed with much fanfare in the Met Office's headquarters in Exeter, Devon earlier this year.
The machine is capable of 1,000 billion calculations every second and is a key part of helping scientists predict the weather and climate change over the longer term.
However the "supercomputer" is also one of the country's least green machines. It uses enough energy to power a small town - producing 12,000 tonnes of carbon dioxide a year - and has made the Met Office one of the worst public buildings in Britain for pollution in a new green league table.
It is the latest embarrassment for the Met Office after the machine famously helped predict a "BBQ summer" which turned out to be another wash-out.
Maurice Spurway, of Friends of the Earth, said it is "ironic" that a computer designed to help stave-off climate change is responsible for such high levels of pollution.
"Life is full of ironies and I think this is one of those situations," he said.
But Barrry Grommett, spokesman for the Met Office, said the computer was "vital" to British meteorology and to help predict weather and environmental change.
"We recognise that it is big but it is also necessary. We couldn't do what we do without it," he said.
"We would be throwing ourselves back into the dark ages of weather forecasting if we withdrew our reliance on supercomputing, it's as simple as that."
Despite its lack of green credentials Mr Grommett said the rating process was "fundamentally flawed" and claimed its carbon footprint was more than offset by the benefits it delivers.
These included saving lives through severe weather warnings while its forecasting for the global aviation industry saved an estimated 20 million tonnes of carbon dioxide every year.
He also pointed out that the Met Office has always had a big carbon foot
The green league table of public buildings was published by the Department of Communities and Local Government.
The Met Office's HQ came 103 in a list of 28,259 public buildings which are responsible for the highest carbon footprint in the UK.
Top of the list was Manchester University's Oxford Road campus, second was the Royal London Hospital and the Scarborough Sports Centre came third.
The supercomputer - more powerful than 100,000 standard PCs - was installed in the Met Office's new £80 million headquarters in May.
It processes information from satellite images and was hailed as capable of predicting disastrous weather events that were previously unforeseeable such as the infamous hurricane of 1987.
