The national government has released a draft plan for reducing Japan's greenhouse gas emissions. The Mainichi Shimbun answers some common questions readers may have about how these emissions are calculated.
Question: How are carbon dioxide emissions measured?
Answer: They are not measured directly, but are calculated using statistics and common rules put together by the United Nations' Intergovernmental Panel on Climate Change (IPCC), which has scientists from around the world among its members.
Q: How are the calculations carried out?
A: In general, it is a multiplication of three factors: amount of fuel used, a coefficient of how much greenhouse gases come from that fuel, and a coefficient of how much those greenhouse gases contribute to global warming.
Japan's coefficients for how much greenhouse gas comes out of each type of fuel, such as gasoline or coal, are put together by a committee of the Ministry of the Environment. The committee calculates that for 1 kiloliter of gasoline, 2.3 metric tons of carbon dioxide is produced. Based on this, the government calculated that in 2013, vehicles in Japan produced 194 million metric tons of carbon dioxide emissions.
The coefficients for how much greenhouse gases contribute to global warming use carbon dioxide as a "1" and compare other gases to it. Methane, for example, gets a 25 ranking, meaning it contributes 25 times more to global warming than carbon dioxide does. The belches of cows contain lots of methane, and in 2013 the dairy cows of Japan produced the carbon dioxide equivalent of 7.4 million tons of greenhouse gases.
Q: Who calculates Japan's greenhouse gas emissions?
A: The Greenhouse Gas Inventory Office of Japan, situated within the National Institute for Environmental Studies. It uses around 100 statistics documents from within the nation, plus some dozens more not available to the general public. This data must be detailed and accurate, and in the new emissions-reduction plan, the government put forth a policy of preparing more detailed statistics and improving its calculations of emission coefficients. (Answers by Ai Oba, Science & Environment News Department)