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Methane absorbs much more heat than carbon dioxide, but once it enters the atmosphere, it lasts on average about 12 years before being broken down. A small proportion lasts for decades. In contrast, carbon dioxide remains in the atmosphere for many centuries.
The metric used for reporting under the United Nations Framework Convention
on Climate Change is called the Global Warming Potential (GWP).
Over 100 years, the current GWP of methane is 25. This means that emitting one tonne of methane has the same warming effect, averaged over the next 100 years, as emitting 25 tonnes of carbon dioxide.
Source of methane
Methane is produced naturally as well as through human activities. Methane
concentrations in the atmosphere have been relatively stable over the past two thousand years but have more than doubled during the last two centuries as a result of human activities, with agriculture as the largest source of emissions worldwide.
Most of the methane from agriculture comes from ruminant animals and, to a lesser degree, the flooding of rice paddies. Landfills and biomass burning also contribute to global methane emissions. Methane is also a fossil fuel and, in some industrialised countries, a significant amount of the gas leaks into the atmosphere during the extraction of oil, gas and coal. Globally, the amount of methane from such fossil sources is roughly three quarters of that generated from agriculture.
Wetlands are the largest natural source of methane. The gas is produced when microbes break down organic matter in the absence of oxygen. Additional smaller contributions come from deposits of frozen methane (clathrates) on the seafloor, wild fires and the digestive tracts of some plant eating animals, such as termites.
New Zealand is unusual among developed countries with its strong base in primary
production and a high proportion of electricity generated from renewable sources. As a result, almost half of the country’s greenhouse gas emissions come from agriculture, in the form of methane and nitrous oxide.
The agriculture sector contributed 48 percent of New Zealand’s gross emissions in 2014. Methane accounts for 43 percent of all emissions (from all sources). More than 80 percent of New Zealand’s total methane comes from ruminant farm animals – cattle, sheep, goats and deer – mainly as a result of enteric fermentation. The great majority of that comes from the rumen, the enlarged modified foregut of ruminant animals. Only about three percent comes from animal manure.
In recognition of the fact that methane from ruminant livestock makes up most
of New Zealand’s agricultural greenhouse gas emissions, the Government and the
pastoral industry are jointly investing in research to lower methane production in
the rumen. Read more
Most of New Zealand's agricultural animals spend all their time outside grazing on pasture. This poses a stiff challenge for the measurement and quantification of emissions. New Zealand is investing significantly in the search for cost effective measures to reduce agricultural greenhouse gas emissions, and this has required New Zealand scientists to develop best practice techniques for quantifying these emissions. Read Why do we need to measure?
SF6 tracer technique
Portable accumulation chambers
GreenFeedTM and other ‘hoods’
Paddock-scale micro-meteorological techniques