Why it’s not so easy to calculate global greenhouse gas emissions

Who is respons­ible for world green­house gas (GHG) emis­sions? Which coun­tries are exem­plary, which cit­izens are major con­trib­ut­ors? The answer is far from simple. And for good reas­on: there is no dir­ect meas­ure­ment of GHG emis­sions at a nation­al level. The meas­ure­ment of GHG emis­sions from human activ­it­ies is based on indir­ect estim­ates. For example, fuel sales data can be cross-ref­er­enced with their emis­sion factor (i.e. the amount of GHG emit­ted per unit of energy) to estim­ate trans­port-related emis­sions. This can be done for each of the GHG emit­ting or cap­tur­ing sec­tors: energy, indus­tri­al pro­cesses, agri­cul­ture, land use and waste.

China leads with 11.2 Gt CO2e emit­ted in 2014, fol­lowed by the United States (5.7 Gt CO2e in 2019), the European Uni­on (3.3 Gt CO2e in 2019) and India (2.5 Gt CO2e in 2016). These fig­ures are those of the nation­al green­house gas invent­or­ies, reg­u­lated by the Kyoto Pro­tocol since 2005. They account for sev­en GHGs (CO2, CH4, N2O, HFCs, PFCs, SF6 and NF3) using a meth­od defined by the Inter­gov­ern­ment­al Pan­el on Cli­mate Change (IPCC). Each coun­try in Annex I of the United Nations Frame­work Con­ven­tion on Cli­mate Change (i.e. 43 States, includ­ing the European Uni­on¹) is required to sub­mit its nation­al GHG invent­ory each year. This oblig­a­tion will be exten­ded to all mem­ber coun­tries from 2024.

The levels used vary from one sec­tor to anoth­er, from one coun­try to another.

Is this cal­cu­la­tion meth­od the right one to use? “This indic­at­or is inten­ded for polit­ic­al pur­poses and is very use­ful for defin­ing the tools for imple­ment­ing nation­al strategies to reduce GHG emis­sions,” says Etienne Math­i­as, a land sec­tor expert at Citepa, the organ­isa­tion respons­ible for cal­cu­lat­ing the invent­ory in France. “How­ever, it has sev­er­al short­com­ings for an inter­na­tion­al com­par­is­on. The IPCC defines guidelines with dif­fer­ent levels of pre­ci­sion,” explains Juli­en Vin­cent, head of invent­ory meth­od­o­logy at Citepa. 

Emis­sions can be cal­cu­lated on the basis of default para­met­ers (level 1), rep­res­ent­at­ive of the nation­al level (level 2), or even refined at the scale of a GHG emis­sion site (level 3). The levels used vary from one sec­tor to anoth­er, from one coun­try to anoth­er. While this has little influ­ence on the cal­cu­la­tion of energy-related CO2 emis­sions, oth­er sec­tors can show great vari­ab­il­ity between states. “Fugit­ive emis­sions from oil and gas extrac­tion (e.g. meth­ane leaks) have very high levels of uncer­tainty, even for developed coun­tries,” says Juli­en Vin­cent. Etienne Math­i­as adds: “Releases from agri­cul­ture and espe­cially the land sec­tor present even great­er uncer­tain­ties, which can be as high as sev­er­al mil­lion tonnes of GHGs, par­tic­u­larly as many undeveloped coun­tries have little activ­ity data and emis­sion factors.” Anoth­er lim­it­a­tion is that only 48 coun­tries have sub­mit­ted at least one invent­ory to date.

To fill these gaps, let us look at one of the pro­jects provid­ing har­mon­ised GHG emis­sion maps across the globe: the Cli­mate­Watch indic­at­or² from the World Resources Insti­tute. It com­piles sev­er­al inter­na­tion­al data­bases. While the rank­ing remains unchanged, this time it shows that the 10 highest emit­ting coun­tries togeth­er emit more GHGs than the rest of the world. For 2019, Chin­a’s total is 12 Gt CO2e (up sharply since the early 2000s), com­pared with 19.7 Gt CO2e for the rest of the world.

So why do coun­tries like China have such high emis­sions? Partly because of their pop­u­la­tion size. Based on the Cli­mate­Watch indic­at­or for 2019, the highest emit­ting cit­izens are those of the Solomon Islands (69.2 t CO2e/capita/year), Qatar (40.5 t CO2e/capita/year) and Bahrain (33.1 t CO2e/capita/year). Each Chinese cit­izen con­trib­utes 8.41 t CO2e each year. In India, the world’s fourth largest emit­ter of GHGs, emis­sions amount to only 2.4 tonnes of CO2e/inhab/yr.

Anoth­er inter­est­ing aspect is car­bon foot­print­ing. Until now, the indic­at­ors men­tioned only reflect the emis­sions of cit­izens with­in their own coun­try. Some coun­tries, such as China, are major export­ers of goods and ser­vices. The car­bon foot­print, on the oth­er hand, takes into account the emis­sions linked to the con­sump­tion of cit­izens. It adds up the emis­sions of house­holds, domest­ic pro­duc­tion, and imports, minus the emis­sions asso­ci­ated with exports. For example, in the case of France, while ter­rit­ori­al emis­sions amount to 5.4 t CO2e/inhabitant/year, the car­bon foot­print will rise to 8.9 t CO2e/inhabitant/year in 2021 accord­ing to the Ser­vice des don­nées et études stat­istiques³. Indeed, more than half of France’s car­bon foot­print comes from impor­ted goods and ser­vices and impor­ted raw mater­i­als or semi-fin­ished products.

There is no stand­ard­ized meth­od for cal­cu­lat­ing car­bon foot­prints on a glob­al level. Accord­ing to the Exiobase data­base⁴, a sig­ni­fic­ant part of Chin­a’s emis­sions is due to the pro­duc­tion of goods and ser­vices for Europe or the US. While China accounts for 24.1% of glob­al GHG emis­sions, this fig­ure drops to 19.2% when con­sid­er­ing its car­bon foot­print, behind Europe (20.2% of the glob­al car­bon foot­print) and the US (19.8% of the glob­al car­bon footprint).

There is no stand­ard­ised meth­od for cal­cu­lat­ing car­bon foot­prints on a glob­al level.

“You have to keep in mind what each of the indic­at­ors illus­trates, they all have a dif­fer­ent pur­pose,” says Juli­en Vin­cent. “For example, per cap­ita emis­sions are aver­ages and do not rep­res­ent income levels and oth­er social inequal­it­ies.” In an art­icle pub­lished in Nature Sus­tain­ab­il­ity in 2022⁵, Lucas Chancel estim­ates that in 2019, half the pop­u­la­tion was respons­ible for 12% of glob­al GHG emis­sions. The richest 10% were respons­ible for 48% of glob­al GHG emis­sions in the same year.

Anaïs Marechal