Scientific Aspects of New Zealand's 2050 Emission Targets
This note seeks to clarify the scientific basis and climate outcomes of emission reduction targets for New Zealand proposed in the Zero Carbon Bill and of alternative targets for methane emissions.
Read the full report: Scientific aspects of 2050 methane targets.pdf (3.42MB)
New Zealand’s total gross greenhouse gas emissions to date (fossil carbon dioxide, nitrous oxide, and biogenic methane) are estimated to have contributed a little over 0.0028°C to the observed global warming of about 1°C above pre-industrial levels. While small in absolute terms, New Zealand’s share in global warming to date is more than 4 times greater than its share of the global population and about 1.5 times greater than its share of the global land area.
New Zealand’s biogenic methane emissions currently make a bigger estimated contribution to global warming than cumulative emissions of fossil carbon dioxide and nitrous oxide combined. If gross emissions of those three gases continued at current rates, biogenic methane would remain New Zealand’s largest single contributor to global warming for the next six decades despite its relatively short lifetime in the atmosphere compared to carbon dioxide and nitrous oxide.
Reducing net emissions of long-lived greenhouse gases to or below zero as quickly as possible is essential to support the temperature goal of the Paris Agreement. The net-zero target proposed in the Zero Carbon Bill could be achieved in different ways, such as reducing all gases individually to zero, or offsetting nitrous oxide emissions with additional carbon dioxide removals. The climate outcomes under different approaches would be very similar if the Global Warming Potential is used to compare nitrous oxide and carbon dioxide emissions.
Reducing New Zealand’s biogenic methane emissions creates unambiguous and substantial benefits to the climate, in addition to the benefits of reducing long-lived gases. However, methane reductions should occur only in addition, not as a substitute to reducing emissions of long-lived greenhouse gases to net zero. Otherwise the cumulative warming from long-lived gases could eventually outweigh any benefit from methane reductions. The Zero Carbon Bill’s provision of a separate target for biogenic methane emission helps avoid perverse outcomes that could occur from trade-offs between those gases under an all-gases target.
Climate science cannot tell us how much New Zealand should reduce its emissions: the lower all emissions including methane can go, the better for the climate. The question for agriculture is what methane emission reductions are possible while still helping to sustain and support New Zealand’s economy and maintaining viable and vibrant rural communities and businesses.
The IPCC identified a range of 24-47% global agricultural methane emission reductions by 2050, relative to 2010, in emission pathways that keep warming to 1.5°C. This wide range reflects different scenarios, strategic choices, and economic assumptions to achieve the temperature limit at the least cost globally. While this range can serve as reference point, it does not in itself prescribe a specific target for methane emissions reduction by any individual country. A national target necessarily depends on national value judgements around what is an appropriate contribution by New Zealand and the economic cost of reducing emissions in New Zealand.
Some stakeholders have advocated an alternative methane target, with reductions set such that future methane emissions do not create additional warming above current levels. For this goal to be met, New Zealand’s biogenic methane would need to be reduced by 10-22% below current levels by 2050, depending on future changes in global methane emissions. Whether this approach is more equitable depends on whether equity is defined as causing the same additional warming or as making the same effort to reduce future emissions. The two are not the same. For short-lived gases like methane, a target based on ‘not causing additional warming’ amounts to a grand-parenting approach, i.e. an entitlement to continue to emit methane in future at a level that is determined solely by past emissions regardless of abatement potential or cost. Like all grand-parenting approaches, this raises equity issues that cannot be resolved by climate science.