The new global warming potential (GWP) measurement, created by researchers at Oxford University, is known as GWP* and provides a more accurate indication of the net impact of all pollutants on climate change.
Pastoral Greenhouse Gas Research Consortium manager Mark Aspin says GWP* does not alter New Zealand’s greenhouse inventory but splits the methane reduction target from nitrous oxide and carbon dioxide.
NZ’s greenhouse gas volumes remained the same in absolute terms but GWP* changes how they are dealt with and is reflected in the Zero Carbon Bill.
The Bill, now before Parliament, sets of goal of a 10% reduction in methane below 2017 levels by 2030 and a 24-47% reduction by 2050 along with net zero emissions of all other greenhouse gases by 2050.
Previously emissions were termed as carbon dioxide equivalent but an Oxford briefing paper said in the case of methane, that does not reflect its relatively short 10-year life span in the atmosphere.
Under the original carbon dioxide equivalent measure, 1kg of methane was equivalent to 28kg of carbon dioxide but did not contribute 28 times the warming of carbon dioxide because of to its short lifespan.
GWP* adds a one-off permanent increase in methane to the greenhouse gas inventory when emissions are rising along with a one-off pulse emission of carbon dioxide. Conversely, when emissions are reducing it provides a one-off permanent decrease in methane and a one-off removal of carbon dioxide.
The paper says the warming effect of methane is determined more by the rate of emissions in any given decade while the warming effect of carbon dioxide is determined by its accumulation.
Temperatures continue to rise in response to falling carbon dioxide emissions so long as they remain above zero but temperatures fall when methane emissions decline.
“In contrast, constant methane emissions hold temperature at an elevated level but cause no further warming.”
Aspin described the science of soil carbon as a developing field being helped by a research grant in which 500 different sites from around NZ will be sampled.
The research will provide a snapshot of NZ’s soil carbon levels. Resampling those sites will show whether NZ’s carbon stocks are increasing or decreasing.
Aspin says the fact most of NZ was covered in trees up to 150 years ago indicates carbon levels could be still relatively high.
