The Whole Truth: How our farms contribute to climate change
This reporting is part of Stuff s fact-checking project, The Whole Truth Te Tikanga Katoa. You can read the rest of our fact-checks here . Read this story in te reo Maori and English here . / Panuitia tenei i te reo Maori me te reo Pakeha ki konei . There is an ongoing debate about New Zealands methane emissions, and to what extent they need to fall to meet the countrys climate change obligations. Many argue they need to reduce by only a modest amount relative to CO2 to achieve a form of net zero warming. This is reflected in the Climate Change Response Act 2002, which set a lower target for methane (24-47% by 2050) than CO2 (net zero by 2050). Others have gone further and said biogenic methane from New Zealands farm animals is not contributing to global warming. That includes the lobby group Groundswell NZ , which has said the Government could not produce any evidence that [agricultural] emissions are warming the climate. Another group, FARM, has claimed that farmers are already at net zero. Whether the targets are fair - and whether methane reductions, in particular, should be higher or lower - is a political decision that science alone cannot answer. But science can inform the debate. The climate is warming due to emissions of greenhouse gases (GHG) such as carbon dioxide (CO2), methane, and nitrous oxide. These gases all warm the climate, but do so in different ways. Methane is potent but short-lived; CO2 is weaker but lasts for centuries. To stop global warming, GHG emissions need to reduce. But because the gases have different impacts, which ones to reduce - and by how much - are subject to debate . This is particularly true in New Zealand, which has a GHG footprint dominated by biogenic methane (from farm animals), not CO2, which is the gas of primary concern for most of the developed world. Picture a small farm. It started with 500 cattle in 1970, and the herd size has not changed since. This farm is next to a factory that burns coal. It was also set up in 1970. The farms cattle produce methane, and the factorys coal emits CO2. Both are warming the atmosphere. At first, the farm had a larger warming impact because methane has a more potent heat-trapping effect than CO2. This has since changed. Thats because most of the methane breaks down after 12 years . When methane emissions are stable for a long enough period, the amount breaking down is close to - but not exactly - the amount of methane being added (one estimate indicated a reduction of around 10-22% would be required in New Zealand). Its like a river. The factory, meanwhile, is pumping out CO2 that will not break down for centuries. Its like a bathtub. The CO2 will keep accumulating until the tap - the coal-burning - is turned off. This analogy, adapted from this explanation from Dr Michelle Cain, shows the difference between a flow gas (methane) and a stock gas (CO2). Even when the factory closes, it will continue to have a similar impact on the atmosphere as a farm that has stabilised its emissions. It does not mean the farm has a neutral impact on the climate. The cattle are keeping the atmosphere warmer than if they did not exist. This is called marginal warming. The cattle are not causing additional warming, like a factory that burns coal. To better explain this, we can borrow another metaphor, this time from the Parliamentary Commissioner for the Environment . You get up one morning at 8am, and its 10C. It will stay at this temperature all day, so you turn on a heater in a room. At midday, the rooms temperature is 20C. The heater is causing marginal warming of 10C. At 2pm, the temperature has risen further, to 24C. The heaters marginal warming is 14C, but the additional warming since midday is 4C. You turn the heater off, and by 6pm, the temperature has dropped to 18C. The marginal warming is 8C, and the additional warming since midday is -2C. The heater, like a stable herd of cattle, is keeping the room warmer than it otherwise would be, even if the amount of warming it is adding has dropped. All livestock contribute to marginal warming and collectively do so to a significant extent. The warming caused by New Zealand's livestock to date is significantly greater than the warming contribution from all of the fossil carbon dioxide emitted since 1850, the Parliamentary Commissioner for the Environment said in a recent report. It is unclear how much additional warming they are causing. A recent study funded by Beef and Lamb estimated sheep meat was not causing additional warming, but beef meat was. Emissions from dairy cows more than doubled between 1990 and 2014, and have since stabilised, meaning they are likely still causing additional warming, too. From a global perspective, the kind of warming you focus on can be political. Developing countries that are still growing their herds to feed their populations look worse through the lens of marginal warming, because every cow or sheep added makes a potent difference. A developed dairy-exporting nation that built its herds up long ago, however, looks better using marginal warming, because that only counts the impact of new cows or sheep being added and not existing herds. Long-term, (mostly) stable sources of biogenic methane do not cause additional warming to the climate but still cause marginal warming. The climate is warmer than it would be without New Zealands farm animals, and reducing their methane emissions would reduce New Zealands contribution to climate change. A source of CO2 emissions causes additional warming until those emissions are stopped. This difference in the gases is already reflected in the law and its split-basket reduction targets.