Taking climate overshoot seriously

The global average temperature will likely soon cross an important guardrail agreed to by the international community, causing escalating risks. Discussing the implications of, and developing effective responses to, climate overshoot is increasingly urgent. This applies globally and locally.

The topic should be front-stage at our forthcoming general election. Currently it barely registers. Instead, household living costs are the top priority. Yet humanity has only one habitable home. And failing to reduce global greenhouse gas emissions urgently will impose immense long-term costs on every household.

In late 2015, the international community agreed at the Paris climate change conference to keep the increase in the global average temperature to “well below 2C above pre-industrial levels” and to pursue efforts to limit the temperature increase to 1.5C above pre-industrial levels. These commitments reflected mounting scientific evidence that risks and costs globally escalate rapidly as the temperature exceeds the 1.5C increase.

Regrettably, the global average temperature is likely to exceed 1.5C on a multi-year basis by about 2030. Equally, despite massive worldwide investments recently in renewable energy technologies, the even more critical 2C limit risks being breached by 2050, if not before.

Yet the ecological and societal dangers of exceeding 1.5C are immense, especially if such overshooting is significant and sustained. In particular, it could precipitate the crossing of several critical tipping points in the Earth system, inflicting profound, persistent and irreparable damage. This includes the irreversible accelerated melting of the Greenland and West Antarctic ice sheets, generating multi-metre sea-level rise and an eventual shut-down of key ocean currents. For such reasons, minimising the extent and duration of any such overshooting is imperative.

Responding to temperature overshoot

This will require two complementary responses: first, rapid acceleration of mitigation efforts to limit the magnitude of overshooting 1.5C; and second, putting mechanisms in place to bring the temperature back to that level.

Pursuing this latter goal, however, will be difficult technically and politically. Specifically, it will necessitate global net-zero emissions by mid-century, particularly for carbon dioxide, followed by a sustained period of net-negative emissions. This means going well beyond the point where carbon dioxide removals merely balance any ongoing residual carbon dioxide and non-carbon dioxide emissions. Globally, in other words, more carbon dioxide must be removed from the atmosphere than the emissions being generated. And the greater the temperature exceeds 1.5C, the greater the required removals.

Given the longevity of carbon dioxide in the atmosphere and the long lags in the carbon cycle, reversing the warming since pre-industrial times will be complex, costly and slow. For instance, overshooting 1.5C by, say, 0.3C could take many decades to reverse, while returning the temperature to near the pre-industrial level might take centuries.

Moreover, the required carbon dioxide removals will be substantial. Reducing the temperature by just under 0.1C, for instance, would likely require cumulative net-negative carbon dioxide emissions of at least 200 gigatonnes, assuming net-zero carbon dioxide emissions had already been achieved. By comparison, current annual global carbon dioxide removals from afforestation are about 2 gigatonnes, and annual global carbon dioxide emissions are about 40 gigatonnes.

Methods of carbon dioxideremoval

The methods for removing carbon dioxide from the atmosphere include various nature-based approaches, such as afforestation, better forest management, soil carbon sequestration, and peatland and wetland restoration, along with several technological and engineering approaches, such as direct air carbon capture and storage and bioenergy with carbon capture and storage.

All such methods, however, raise multiple scientific, analytical, ethical, funding and regulatory issues. One of these is ensuring any carbon sequestration is permanent or, if not, that prompt substitution occurs in the event of a reversal. Another is determining how the high and ongoing costs of large-scale, long-term sequestration should be shared.

If overshooting the 1.5C guardrail is to be limited and temporary, the question is not whether carbon sequestration will be needed, but rather which methods should be deployed, to what extent, in what ways, by whom and for how long. Answering such questions will pose formidable challenges for the international community and individual governments.

Ideally, an evidence-based, coordinated and durable global strategy should be negotiated, with robust accounting standards, rigorous monitoring, reporting and verification arrangements, agreed burden-sharing principles, and effective compliance mechanisms. Humanity’s experience of climate negotiations since the 1990s, however, highlights the multiple geopolitical and other obstacles that could imperil any progress.

Additional policy risks

Yet without an effective global strategy for sustained net-negative emissions, other serious policy challenges will arise. Above all, the growing costs of climate-related impacts and adaptation will prompt pressure to cool the planet through solar radiation modification. This entails large-scale interventions to reflect more sunlight back into space. The main options include stratospheric aerosol injection, cloud brightening, cirrus cloud thinning and surface brightening.

But all such approaches are deeply problematic. Their costs and impacts are uncertain. They entail significant risks. They fail to address ocean acidification and the disruption of terrestrial ecosystems. And at best, they merely slow the pace of warming without confronting the root cause of the problem. Worse, if suddenly halted, perhaps because of major global conflicts, they could generate a termination shock – that is, a rapid period of warming with vastly different regional impacts.

Whither NZ’s long-term climate strategy?

Achieving net-zero greenhouse gas emissions globally, if not net-negative emissions, and minimising the extent and duration of exceeding the 1.5C guardrail, will require an increasing number of countries to adopt net-negative greenhouse gas targets. Higher-income countries with large cumulative emissions per capita, such as New Zealand, will be expected to take the lead with more stringent domestic targets. For New Zealand, at least three main policy issues will arise: how much to contribute to the global effort and thus what targets to set; which methods to employ; and how to share their inevitable costs fairly across the community.

Thus far such matters have barely registered in the public domain. To be sure, the Climate Change Commission discussed and officially recommended adopting a net-negative emissions target at least for long-lived gases by 2050. In its report on the 2050 target review released in December 2024, the commission proposed replacing the current net-zero target that covers all gases other than biogenic methane with a net-negative target.

Specifically, it recommended that “net accounting emissions of greenhouse gases other than biogenic methane” should be “at least negative 20 MtCO2e by the calendar year beginning on 1 January 2050”. Additionally, it proposed including international shipping and aviation emissions within the net-negative 2050 target.

The commission also recommended that emissions of biogenic methane be “at least 35-47 percent less than 2017 emissions by the calendar year beginning on 1 January 2050” as this would significantly reduce the total amount of warming caused by New Zealand. At the same time, it highlighted the need for “further reductions and removals of greenhouse gases” beyond 2050 to lower greenhouse gas concentrations in the atmosphere. In other words, net-negative targets and a credible pathway for achieving them will be required for many generations.

The commission’s advice was rejected by the coalition Government in late 2025. Ministers claimed, with minimal evidence and contrary to the commission’s analysis, that any strengthening of our existing 2050 targets would be too costly, cause greater implementation challenges, and increase uncertainty.

Moving forward

Though regrettable, this decision will inevitably be revisited. After all, for economic and diplomatic reasons, New Zealand will wish to contribute to the expected global negotiations on the implications of, and responses to, climate overshoot. Ultimately, it will lack integrity without a credible long-term net-negative greenhouse gas target.

Also, given the unanimous advisory opinion of the International Court of Justice last year on the legal obligations of states to take action to address climate change, New Zealand could face legal challenges if it fails to comply with its obligations under international law. Equally, without credible targets, some exports might be put at risk (eg by carbon border adjustment mechanisms).

Aside from this, while prevarication and delay may be politically convenient, they do not constitute a prudent, morally responsible or cost-effective long-term climate strategy.

As the general election approaches, an honest, evidence-informed public debate is needed about the increasing risks of climate overshoot and how best to respond, globally and locally. Civil society organisations, responsible businesses and scientific experts need to mobilise to contribute to such a debate. Far-sighted political leadership is equally essential. Too much is at stake for narrow, myopic, partisan politics.