Lawyers have told the Court of Appeal that Government climate advisors failed to follow the law.

In a hearing Wednesday, Lawyers for Climate Action’s arguments were rejected by the Climate Change Commission, which provides advice to the Climate Minister on how fast emissions should be reduced.

The advocacy group wants the commission to recommend more ambitious cuts to greenhouse gases. To achieve that, the lawyers sued the commission in the High Court last year, arguing that the body’s first advice breached the provisions of the Zero Carbon Act.

Justice Jill Mallon dismissed all the grounds of review, concluding “the commission knew what it was doing and had reasons for its approach”.

Lawyers for Climate Action – represented by James Every-Palmer KC and Jenny Cooper KC – now hope to convince three Court of Appeal justices of the merits of the case.

The Climate Change Commission’s lawyer Victoria Casey KC said the body unveiled an “ambitious” plan. Relative to 2019 emissions, it would contribute “genuine, real reductions”.

Cooper noted the Zero Carbon Act required New Zealand to meet specific targets plus to contribute to international efforts to limit global warming to 1.5C. She proposed that analysis determining what 1.5C required should have been one of the commission’s first tasks.

But it wasn’t, she argued – late in the process, the commission conducted a “cross-check” relying on questionable choices to justify its original advice.

In response, Casey disagreed that the 1.5C exercise proposed by the lawyers was appropriate. But while the need to move fast and contribute to the temperature goal was balanced by other concerns, it was not “an afterthought”, she said.

Although not an excuse, Casey said, the commission “grounded its advice in the real world”.

Noting the commission’s statement that New Zealand “should contribute more than the global average required”, Every-Palmer outlined how it then made decisions meaning the country will do less.

The commission chose to use a landmark scientific report from leading experts on the global action required for a decent shot at limiting Earth’s warming to 1.5C. In doing so, Every-Palmer argued, the body needed to apply it in a “scientifically valid” way.

The scientific report, released in 2018, concluded that the world’s net carbon emissions in 2030 must be 40 to 58% lower than net emissions in 2010, to achieve 1.5C. Notably, both the 2010 and 2030 figures include the carbon dioxide absorbed by trees.

The commission proposed a starting point for a greenhouse-cutting target that could use international carbon credits (a “Nationally Determined Contribution”). It also suggested domestic “carbon budgets” to 2025, 2030 and 2035 – and concluded that if these were followed, Aotearoa’s carbon emissions would fall 55% by 2030.

To reach this conclusion, the commission compared the gross figure for 2010 (excluding carbon-sucking trees) against a net figure for 2030 (including vegetation). In its final advice, the body warned its domestic pathway was “not directly comparable” with the major climate report.

Pointing to evidence from pre-eminent climate scientists, Every-Palmer said the analysis was “incoherent”.

If countries chose to make inconsistent calculations, the “global figures would not add up” to the effort required for 1.5C, he said.

Using net 2010 emissions as the starting point – as he argued should have been done – would require New Zealand to take more ambitious action.

For the commission, Casey argued the scientific report on 1.5C was not intended to be used in the way Lawyers for Climate Action proposed. The scientists state the estimates relate to global action and “do not indicate requirements”.

It was common for countries to use gross-net targets, she added.

Cooper argued that the net emissions target in 2030 as guided by the commission will be higher than actual net emissions in 2010. Therefore, New Zealand would not make a contribution to 1.5C, she suggested.

Casey rejected that claim, presenting a contrasting estimate that Aotearoa’s net carbon emissions should drop to the 40 to 58% threshold in the “early 2030s” and fall quickly from there.

Covering similar ground to the first hearing, Every-Palmer disputed the commission was free to choose a “carbon accounting” system, because a specific one is referenced in the legislation. The method reflects “what the atmosphere sees”.

Yet the commission selected another that smooths out the peaks and troughs caused by tree planting and harvesting. The year the smoothing effect is introduced – just before lots of felling, which releases carbon – is problematic, Every-Palmer said.

Referencing expert witnesses, Casey said the chosen accounting system is “established practice”. She argued there was little evidence that lawmakers intended a change. Officials must smooth out the peaks and troughs of forestry emissions, she suggested, otherwise plantation forests would essentially drown out the carbon-cutting action the country introduces.

Every-Palmer also argued it was inconsistent to choose a 2005 baseline for the international target, while also counting the emissions absorbed by forests planted between 1990 and 2005.

Crown Lawyers representing the office of the Climate Minister are due to appear Thursday.

Casey suggested the justices consider the skills and experience of the body’s commissioners and experts, demonstrating they “probably did know what they were up to”.

Lawyers for Climate Action asked the court to order the commission to restart its advice process for upcoming targets.

The panel of three judges – Court of Appeal president Mark Cooper, Justice Murray Gilbert and Justice David Goddard – regularly challenged the lawyers’ arguments, noting that the commission was an advisory body rather than a decision-maker.

The justices discussed the broad meaning of “contributing” to 1.5C, a key point in the original case.

The court cannot intervene if a body makes a debatable choice, only if it has breached legislation and the provisions lawmakers laid out.

The Zero Carbon Act states the commission must consider multiple relevant issues before providing its reports including public feedback, international obligations, economic feasibility and “a broad range” of scientific advice.