The extraction of coal from Australia's biggest mine would not directly affect the Great Barrier Reef, but burning it would help push the climate to a dangerous state, a Queensland court has heard.
Conservation group Coast and Country is objecting to Adani's plans to build the $16.5 billion Carmichael Mine in the Galilee Basin.
The Indian mining giant intends to export at least 50 million tonnes of coal a year from the Abbot Point terminal, north of Bowen.
Coast and Country claims the project would contribute to climate change when the coal is burnt overseas, and carbon emissions would damage the Great Barrier Reef through ocean acidification.
The Queensland Land Court heard on Tuesday a United Nations agreement across 200 countries dictates global warming should be kept under 2C.
That threshold would be reached after the emission of roughly 850 gigatonnes of C02, University of Queensland's Ove Hoegh Guldberg told the court.
The marine sciences professor said the Carmichael project could contribute 4.5 gigatonnes of emissions.
"We're talking about 0.5 per cent of the total emissions left ... before we push the climate into a very dangerous state," he said.
"That's an enormous amount of C02 over the life of the mine."
Adani argues the need for coal is driven by demand, not supply, and even if the project does not go ahead, coal-fired power stations would simply find other sources.
Lawyer Peter Ambrose, for Adani, said the situation was similar to when a motorist drove a car - emitting C02 - but the car manufacturer was not directly responsible for the subsequent damage.
"It's not possible that the mere extraction could damage the Great Barrier Reef," he put to Prof Guldberg.
"The mere extraction, digging up the coal and putting it into a boat, doesn't have an impact," Prof Guldberg conceded.
The case, before Land Court President Carmel MacDonald, is expected to run until the end of April.