Sarah Thomson, a New Zealand law student, filed a Statement of Claim in 2015 against New Zealand’s Minister of Climate Change Issues alleging that the Minister had failed in several respects regarding the setting of greenhouse gas emissions reduction targets required by New Zealand’s Climate Change Response Act of 2002. That Act implements New Zealand’s responsibilities as a ratifying Annex I member of the United Nations Framework Convention on Climate Change (UNFCCC). It requires the Minister to set an emissions reduction target in keeping with the statements of the Intergovernmental Panel on Climate Change (IPCC), and to consider whether to revise that target as the IPCC issues updated findings. In March 2011, pursuant to the 2002 Act, the Minister had set a target of 50% reduction from 1990 greenhouse gas (GHG) emissions levels by 2050. The Minister did not review or revise that target following the 2014 issuance of the IPCC’s Fifth Assessment Report. In July 2015, in advance of the 21st Conference of the Parties to the UNFCCC in Paris, the Minister submitted New Zealand’s intended nationally determined contribution (INDC)--and then subsequently a nationally determined contribution (NDC)--consistent with a “provisional target” of only 30% reduction from 2005 levels by 2030. As Thomson's Statement of Claim noted, “this equates to a reduction of 11% below New Zealand’s 1990 emission levels by 2030,” and thus “will not, if adopted by other developed countries in combination with appropriate targets set by developing countries, stabilize greenhouse gas concentrations in the atmosphere at a level that would prevent dangerous anthropogenic interference with the climate system.” Thus Thompson challenged both the target set pursuant to the 2002 law and the target set as part of New Zealand's NDC pursuant to the United National Convention on Climate Change and the Paris Agreement, both ratified by New Zealand.
The High Court of New Zealand issued its decision after the 2017 election had ousted the prior government, replacing it with a government whose members had campaigned on a commitment to eliminate all GHG emissions by 2050. Nonetheless, the court reviewed both the 2030 and 2050 targets set by the prior government's Minister and ruled on their legality. With respect to the 2050 target, the court determined that although the Minister had discretion under the 2002 act to review and determine New Zealand's emissions reduction target for 2050, that discretion was limited by the act's purpose and by the evidence contained in IPCC Fifth Assessment Report (AR5), both of which argued strongly for a lower national emissions target than the one set based on the IPCC's Fourth Assessment Report (AR4). However, because it was not clear that a review of the 2050 target in light of AR5 would certainly have led to the target's revision, and because the question was largely mooted by the recent election ("[s]uch a declaration would now be of historic interest only"), the court concluded merely that the Minister should have reviewed the target, but not that a decision to maintain the existing target after such a review would necessarily have been unlawful. With respect to the 2030 target in New Zealand's INDC and NDC, the court determined that it had authority to review the Minister's setting of that target but no grounds for invalidating it because the Minister had not made “any reviewable error for which the Court may intervene.”
Regarding the authority of the courts to review climate change policy, the decision noted:
"It may be appropriate for domestic courts to play a role in Government decision making about climate change policy . . . The courts have not considered the entire subject matter is a “no go” area, whether because the state had entered into international obligations, or because the problem is a global one and one country’s efforts alone cannot prevent harm to that country’s people and their environment, or because the Government’s response involves the weighing of social, economic and political factors, or because of the complexity of the science. The courts have recognised the significance of the issue for the planet and its inhabitants and that those within the court’s jurisdiction are necessarily amongst all who are affected by inadequate efforts to respond to climate change. The various domestic courts have held they have a proper role to play in Government decision making on this topic, while emphasising that there are constitutional limits in how far that role may extend. The IPCC reports provide a factual basis on which decisions can be made. Remedies are fashioned to ensure appropriate action is taken while leaving the policy choices about the content of that action to the appropriate state body."