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To report net emissions from sectors, it is necessary to convert emissions of each individual gas into a common unit (commonly called &quot;CO2-equivalent emissions&quot;. The approach adopted in the UNFCCC is to use the so-called 100-year Global Warming Potential, which multiplies the actual emission of each gas by a weighting factor to give the CO2-equivalent emissions. This weighting factor is based on the warming effect the emission of one kg of this gas makes over 100 years, relative to the warming effect from an emission of one kg of CO2. 
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The Intergovernmental Panel on Climate Change (IPCC) regularly updates this weighting factor based on new scientific insights and the rising background concentrations of greenhouse gases in the atmosphere. 
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Up until now, the weighting factors used in national inventories were those based on a report by the IPCC in 1995, which indicated the 100-year Global Warming Potential of methane to be 21, and that of nitrous oxide to be 310. For emissions from 2013 onwards (that is, starting with the next emissions inventory to be submitted in April 2015), updated weighting factors will be used, based on a re-assessment of the science behind those factors by the IPCC in 2007. The new weighting factors will be 25 for methane and 298 for nitrous oxide (the new values will be backcast for future calculations of time series from 1990, thus avoiding any artificial step-change between 2012 and 2013 emissions). 
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Note that the IPCC issued a further revision of those weighting factors in 2014, with a factor of 28 for methane and 265 for nitrous oxide, but these new factors will not be used in reporting and accounting under the UNFCCC until 2020. Weighting factors for the Global Warming Potential and other possible metrics to compare greenhouse gases change naturally over time as the climate warms and background concentrations of greenhouse gases continue to rise, but some of those revisions also reflect new scientific insights into the ways in which different gases contribute to warming. 
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<a href="newsletter,listing,147,new-zealand-greenhouse-gas-inventory-2014-estimates.html" target="_blank" title="http://www.nzagrc.org.nz/newsletter,listing,147,ne... - link will open in a new window">See more about the latest NZ GHG inventory figures</a> 
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<a href="fact-sheets.html">See our factsheet on GHG metrics</a><br />
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