Executive summary
This report provides information on the global warming that might occur from future emissions of methane from livestock in New Zealand under different assumptions and scenarios. Specifically, the report seeks to answer three main questions, namely:

Given our past emissions, what would be the warming contribution of future methane emissions in New Zealand if they were held steady at current levels?
Given the most recent projected emissions of methane in New Zealand what additional warming might be expected?
What annual reductions in methane emissions would be required to avoid any additional warming contribution from New Zealand methane emissions by 2030 or by 2050?

The answers to these questions are highly relevant as New Zealand is considering what emission target(s) it might set for different greenhouse gases as part of its contribution to the objectives of the Paris Agreement. These questions are answered by developing estimates of historical emissions of methane from livestock husbandry in New Zealand back to the 19th century, and combining such estimates with more recent data from the national greenhouse gas emissions inventory and projections by the Ministry for Primary Industries. These past and future emission estimates, which have contributed to the increase in global methane concentrations, were then fed into a simple climate model that has been used widely internationally to simulate the climate change resulting from different emission scenarios. The indirect climate effects of methane emissions are included in the simulations, and the effect of uncertainties in key parameters are considered. The climate model, called MAGICC, is based on the physics and chemistry of greenhouse gases and their fate in the atmosphere, including the relatively short lifetime of methane. The model can be calibrated to simulate results from a range of much more complex climate models and is thus also able to provide insight into model-based uncertainties of future projections.
The main finding from this study is that if New Zealand were to hold its livestock methane emissions constant at 2016 levels, the amount of methane in the atmosphere due to those emissions would level out within a decade, but warming from this methane would still increase for well over a century, albeit at a gradually declining rate. This ongoing additional warming is largely due to the inertia of the climate system, which is still responding to the historical increase in methane emissions from New Zealand since the 19th century and results in a long-lasting component of warming from methane despite the relatively short lifetime of the gas itself. It would take several hundred years of constant methane emissions before warming due to those emissions ceases to increase entirely.
If New Zealand’s methane emissions were held constant from today onwards, the additional warming in 2050 would be about 10-20% above the warming that has been caused by New Zealand’s methane emissions to date. By 2100, the additional warming would increase to about 25-40%, and to about 40-55% by 2200. The ranges reflect the fact that the warming effect exerted by New Zealand’s methane emissions depends on global methane concentrations. Hence the actions taken by other countries during this period to address climate change, including through reducing their methane emissions, will influence the warming caused by on-going methane emissions from New Zealand.
If New Zealand wished to ensure that its livestock methane emissions cause no additional future warming relative to the warming caused by those emissions to date, it would have to reduce those methane emissions by about 10-22% below current levels by the year 2050, and 20-27% by 2100. The ranges in the emission reductions are due to different assumptions regarding actions taken by other countries that will influence future concentrations of greenhouse gases, and represent best estimates based on a range of different climate models. As a general rule, the greater the assumed action globally to reduce methane from the extraction, transport and use of fossil fuels (coal, oil and gas), the lower global methane concentrations and hence the more effective the warming caused by remaining sources of methane becomes.
Even though this study demonstrates that constant methane emissions result in some additional future warming, this does not mean that methane has the same long-lasting impact on climate as carbon dioxide. Continued emissions of carbon dioxide by New Zealand at today’s level would eventually result in greater additional warming than continued emissions of methane, given the long lifetime of carbon dioxide and its resulting accumulation in the atmosphere. Warming due to longlived greenhouse gases will continue to increase until their net emissions reach zero. Reducing net emissions of long-lived greenhouse gases to zero is therefore a physical necessity if New Zealand wishes to halt its further contribution to climate change.
By contrast, reducing New Zealand’s livestock methane emissions by 22% (as a best estimate) below today’s levels would avoid additional warming from those emissions in 2050 relative to today, if the world as a whole undertakes actions consistent with the Paris Agreement. Additional reductions would be needed beyond 2050 to maintain this level of warming into the more distant future. Reducing New Zealand’s emissions by more than 22% below today’s levels by 2050 would reduce the warming caused by New Zealand’s livestock methane emissions below today’s levels. However, how much New Zealand can and wants to reduce its emissions to reduce its overall contribution to warming will depend on complex economic, social and other environmental considerations that physical metrics can inform but not answer.
Full report available at https://www.pce.parliament.nz/publications/a-note-on-new-zealand-s-methane-emissions-from-livestock including the Parliamentary Commissioner for the Environment's A note on New Zealand’s methane emissions from livestock.