The GWP is a way of summarising thses factors. For example methane has a 100 year GWP of 21, indicating that a given weight of methane released now would have 21 times as much effect on global warming over the next 100 years as would the same weight of carbon dioxide. 100 year Global Warming Potentials, from IPCC 1996 Table 2 has been derived by multiplying the estimated New Zealand emissions of each of the major greenhouse gases in 1990 and 1995, by their 100 year GWPs. Methane emission estimates are better than those for N 2 O, but they also still contain some uncertainties. NIWA, Agresearch and Landcare are collaborating to better define methane emissions from New Zealand agriculture. .
All amounts are in gigagrams.
The reported emissions of HFCs are potential rather than actual emissions - they are based on the quantity imported, and will probably be released to the atmosphere over a number of years rather than just in the year imported. It comes from the same numbers used to develop Table 2. The dominant effect of methane is clear. Climate Change 1995 - The Science of Climate Change. Summary for Policymakers, and Technical Summary of the Working Group I Report.