Falling greenhouse gas emissions?
A Defra press release in 2008 UK on track to meet Kyoto targets as emissions continue to fall said:
Provisional statistics published today for total UK Greenhouse Gas emissions for 2007 showed a drop of two per cent over the previous year, with 639.4 million tonnes carbon dioxide equivalent, down from 652.3 million tonnes in 2006.
The decrease in CO2 emissions resulted from fuel switching from coal to natural gas for electricity generation, combined with lower fossil fuel consumption by households and industry.
The ‘dash-for-gas’ was reducing our carbon emissions by 2% (but now we appear to be on the rise again).
Greenhouse gasses embodied in international trade
Interesting research commissioned by Defra, estimated the embodied carbon in the goods embodied in UK trade flows: Devleopment of an Embedded Carbon Emissions Indicator (2008) by Wiedmann et. al.. Appendix 13 gave estimates of greenhouse gas emissions due to imports and exports. The difference increased at an average of 8.2 million tonnes of carbon dioxide equivalent a year in the decade to 2004. The key message we can draw from this is that the UK was increasingly “exporting our greenhouse gas pollution”. Added to the calculations in the Defra press release, they cancel out much of the reductions of the “dash for gas” years.
The Wiedmann et. al. research presents an important analysis – even if a key message is has to be teased out. That message may not have been welcome to the funding department, Defra, who before they were relieved of the responsibility were charged with telling the good news about the UK’s “reduction” in carbon emissions”.
The assessment of non CO2 greenhouse gasses
Different man-made climate forcing agents reside in the atmosphere for different times. Black carbon is typically removed in a matter of days but has such a strong impact it is an increasing cause for concern. Methane lasts in the atmosphere about a decade before natural processes remove most of it. Methane is an interesting case for carbon accounting. How should this gas with a relatively short residence in the atmosphere be compared with carbon dioxide?
The answer has been to use global warming potential (GWP) which spreads the impact of a greenhouse gas over a specific number of years, commonly 20, 100 or 500 years. The Kyoto protocol set an international standard by choosing the 100 year measure for GWPs (GWP100) and in the case of methane used the value from the IPCC Second Assessment Report. That gave a value for methane of 21 times carbon dioxide. It is likely that this value is used in Wiedmann et. al.
This value is at odds with current science. For example, the paper by Shindell et al., Improved Attribution of Climate Forcing to Emissions gives a value of GWP100 for methane of 33. This is due to the interaction with black carbon. Measured over 20 years this paper gives methane a GWP of 105 times carbon dioxide. The GWP20 of methane is thus five times the Kyoto figure.
It is hard to find a historic justification for choosing a 100 year time span over a 20 year one. There are those today who argue that the correct response to climate change is to tackle the increase in temperature quickly by curbing short-term forcing agents, such as black carbon and methane, while the world gets to grips with the longer term carbon-dioxide problem. This is an argument for giving methane the higher rating of GWP20.
Hansen et. al. Global warming in the twenty-first century: An alternative scenario suggests the following strategy:
We suggest equal emphasis on an alternative, more optimistic, scenario. This scenario focuses on reducing non-CO2 GHGs and black carbon during the next 50 years. Our estimates of global climate forcings indicate that it is the non-CO2 GHGs that have caused most observed global warming. This interpretation does not alter the desirability of limiting CO2 emissions, because the future balance of forcings is likely to shift toward dominance of CO2 over aerosols. However, we suggest that it is more practical to slow global warming than is sometimes assumed.
If carbon accounting is to be meaningful to strategies such as this then it should reflect the importance of methane with a current science and a better choice of time frame for GWP than 100 years.
One difficulty with Wiedmann et. al. and some other forms of carbon accounting is that the measures used in the assessment of non-CO2 greenhouse gasses is not easily ascertained and do not – at the publication stage – lend themselves to alternative strategies. Another problem is their measures are not updated to represent current science.
An example: What is the emboded carbon in imported beef?
According to Defra funded research by Adrian Williams, the carbon footprint of beef is 14 times deadweight when methane is measured using GWP100. This becomes 25 times deadweight when GWP20 is used. The model that calculated this pre-dates the uprating of methane in Schindel et. al. so it can be reasonably assumed that the impact of beef can double when an uprated GWP20 for methane is used. Sheep meat has slightly larger footprint.
The UK imports about 333,000 tonnes of beef and sheep meat which gives a carbon footprint of about 4.9 million tonnes of carbon-dioxide equivalent using a GWP100 for methane. This becomes 9.8 million tonnes of carbon dioxide equivalent using a GWP20.
The Wiedmann et. al. Report actually gives the embodied carbon-dioxide equivalent for all agricultural imports as 3.4 million tonnes for 2004.
There does seem to be a question to answer here.
Both reports are buried somewhere on the Defra website.
Does this mean that an answer from Defra cannot be expected soon?
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