The federal environment minister has argued in court that coal from Australia’s largest coalmine would have no “substantial” impact on climate change and as a result he did not need to consider whether it would affect the Great Barrier Reef.

The Australian Conservation Foundation challenged Greg Hunt’s approval of Adani’s Carmichael mine, alleging he failed to consider the impacts the burning of the coal from the mine would have on climate change and hence on the Great Barrier Reef.

Scientists have found the current mass bleaching event affecting 93% of the reef was made 175 times more likely by climate change and would become a biennial event within 20 years. After that point, the continued existence of the reef would be in doubt.

In federal court documents obtained by Guardian Australia, Hunt denied he failed to consider the impacts of coal on the reef.

In the outline of submissions filed on behalf of the minister, the Australian government solicitor explains that the minister did not think the burning of the coal “would be a substantial cause of climate change effects” and would have “no impact on matters of national environmental significance”.

The minister’s reasoning was that whether the burning of the coal would make climate change worse depended on whether it would increase the total amount of coal burned globally. But he notes there are a “raft of factors” that could affect how much coal was burned globally, including whether the coal from the mine displaced other coal and whether it was dealt with within various national emissions targets.

He concluded that there “was no requisite relationship between combustion emissions and increases in global temperature”.

Further, the minister argued that since the net impact was “difficult to identify”, there was no need to impose conditions on the mine, such as that climate impacts would be offset.

“Put simply, because any increase in net global greenhouse gas emissions was a matter of speculation, there was no need for or utility in the imposition of conditions.”

The Australian Conservation Foundation was represented by the Environment Defenders Office Queensland. The court case was heard in the federal court in Brisbane on Tuesday and Wednesday and a decision is expected within three to six months.