Many people have asked how a country’s carbon footprint compares to the territorial emissions reported to the United Nations Framework Convention on Climate Change (UNFCCC)?

In hindsight, it was perhaps remiss of us not to include this data in the paper. However, it was not our motivation.

Focusing on whether the emissions are bigger or smaller misses a large part of the story. The main focus of the paper is on what consumption categories cause emissions and how this varies across countries with different incomes. The current approach in the UNFCCC reveal what the emissions are from electricity, transportation, and so on. This is vital information, however, only a carbon footprint can reveal what the emissions are to produce food, car, television, or to get a hair cut. This information is needed by consumers if they want to reduce their own personal footprint (http://www.springerlink.com/content/1976p75463860416/). The real value of a carbon footprint is the information it reveals, not necessarily if the footprint is bigger or smaller than the territorial emissions.

In a broader global perspective, if a country’s carbon footprint grows over time relative to territorial emissions then it means that the country is increasingly importing products at the expensive of domestic production. If that country has an emission limitation in the Kyoto Protocol and the imports are coming from countries without emission limitations then global emissions may increase while it “appears” emissions are decreasing. This is known as carbon leakage (http://www.nature.com/climate/2008/0804/full/climate.2008.25.html).

In fact, this is what is happening now. Since 1990 the emissions in countries with an emission limitations have largely stabilized, while the emissions in countries without emission limitations have increased about 80%. The main reason for this is the explosive growth of China, however, recent research has shown that about 50% of the growth in Chinese emissions since 2000 is due to the production of products exported from China and consumed in other countries (http://www.agu.org/pubs/crossref/2009/2008GL036540.shtml).

An analysis of the carbon footprint is useful for an analysis of carbon leakage, but the footprint better measured another way (http://dx.doi.org/10.1016/j.ecolecon.2007.10.014). The carbon footprint in our article only directly considers trade to final consumers (such as households) and trade to industry is calculated internally in the model. Thus, the footprint of the USA includes emissions that occur to extract iron-ore in Australia, to produce steel in China, to produce a car in Japan, which is consumed in the USA. While this is perfectly logically, it means that the imported emissions are not directly comparable to the import statistics which only consider the first trading partner (the trade from Australia to China is not included in the US trade statistics). To make trade statistics and imported emissions consistent it is more intuitive to consider only the first link in the supply chain, but include imports to both industry and final consumers. This method was used in another of our papers (http://pubs.acs.org/doi/abs/10.1021/es072023k) and we call a Consumption-Based Inventory.

In the two different approaches the imported emissions differ, but the exported emissions need not differ if the arithmetic is done correctly. In the Carbon Footprint of Nations paper, the imported emissions cover the global emissions required to produce the final consumption in each country, while in a Consumption-Based Inventory the imported emissions cover the domestic emissions in each country to produce total (final and industry) consumption in each country. If trade to industry and final consumers is include, then only domestic emissions are included to avoid double counting.

Both approaches do lead to some confusion, but they serve a different purpose. A Carbon Footprint tells us what consumption categories are important and how we may reduce emissions by consuming differently. The Consumption-Based Inventory focuses on trade relationships and how the trade relationships redistributed emissions between countries. Depending on the question being asked, one method is usually preferred.