The county council needs to reassess the controversial Norwich Western Link road for its impact on climate change, a group fighting the plans has said.
Stop the Wensum Link (SWL) made the call following a report by the Intergovernmental Panel on Climate Change (IPCC), which said human activity is impacting the climate.
"The IPCC Report is a code red for humanity: climate change is indisputably human-driven and happening fast," said David Pett, lawyer for campaign group Stop the Wensum Link.
"It is hugely concerning that temperature targets set under the Paris agreement may be broken within the next 20 years, causing irreversible environmental damage and more extreme weather events.
“We are shocked by the lack of comment from Norfolk County Council, a council which has still yet to declare a climate emergency.
"We question how much longer the council can keep its head buried in the sand.
"Surely the time has now arrived to give this issue some urgent attention."
Mr Pett called on the council to reassess all ongoing infrastructure projects for their environmental impact, including the Western Link, and for the council to declare a climate emergency.
He added: "This is important when transport accounts for around a third of total emissions in Britain and toxic fumes from cars, vans and lorries cause thousands of premature deaths every year."
The NWL is expected to cost £198m and would link the Northern Distributor Road to the A47 west of Norwich.
In May, new Green councillors to County Hall called on NCC to declare a climate emergency.
Responding to the calls to reassess the route for its climate impact, Martin Wilby, cabinet member for Highways, Infrastructure and Transport, said: “The county council has pledged to achieve net-zero carbon by 2030.
"To support that ambitious policy our new transport plan, that cabinet endorsed last week has a carbon-cutting commitment at its heart.
"Our aim is to support Norfolk’s economy, ensure our communities are set up to cope with planned growth, and crucially, to limit impacts on the environment.
"Key challenges that the plan seeks to address include carbon reduction and air quality.”
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