Updated carbon footprint calculation factors

Scientific insight is continuously improving, providing us with information that helps our impact assessment calculations be more accurate and better able to predict environmental impacts and how much our products contribute to climate change.

In January this year, the Intergovernmental Panel for Climate Change of the United Nations (IPCC) published an updated assessment report (AR5) with new Global Warming Potential (GWP) factors. Since LCA software makers want their customers to benefit from these latest insights as soon as possible, we expect an update for the carbon footprint methods and multiple impact methods that include climate change in LCA tools soon.

From a producer’s or point of view, changing the factors every 6 years can be an enormous hassle: all previous results and conclusions need to be revised. From a scientist’s point of view, on the other hand, the hassle is worth it: new data are available from measurements and experiments, which makes it possible to improve the simulation models and better estimate the potential impact of greenhouse gas emissions on climate change.

Each updated GWP assessment report brings in new and specific model improvements. For instance, in AR5, IPCC have included “models [that] reproduce observed continental scale surface temperature patterns and trends over many decades, including the more rapid warming since the mid-20th century and the cooling immediately following large volcanic eruptions.” As a result of these model improvements, we can now have more confidence that in the future high sea levels will vary much more strongly. In other words: the old factors weren’t wrong, but the new ones are better!

What Has Changed In The New GWP Factors?

How will the updated GWP factors change carbon footprint calculations? GWP factors express how much heat a greenhouse gas traps compared to an equivalent amount of CO2 within a certain period of time. To illustrate the changes, we will look at impacts over a period of 100 years, the default for the Kyoto protocol and for carbon footprint studies. Alternatives for sensitivity analysis are 20 and 500-year timeframes. Looking at these different periods of time can yield interesting results – greenhouse gases that are removed from the atmosphere quickly, such as methane, have more impact on climate change in the 20-year timeframe than in the 100-year timeframe.

The following table shows the GWP factors for the accumulated impact over 100 years of the ten most prominent greenhouse gases in each of the five assessment reports. There you can see that the factor for methane – the second most important greenhouse gas – has increased more than ever and that the factor for dinitrogen monoxide (N2O) – the third most important greenhouse gas – has decreased more than ever. With the exception of HFC-134a and HCFC-142b, the factors of the other top ten gases have not changed that much since the third assessment report.

Note: The IPCC gives additional factors for fossil methane, but these factors should be ignored. The factor for biogenic methane in the table above was calculated by subtracting 2.75 kg of CO2 per kg of methane from the methane factors. We took the molar mass of CO2 divided by the molar mass of CH4 for the correction factor of 2.75. The changes of the GWP factors for 20 year accumulated impact are similar: fossil methane changed from 72 to 84 kg CO2 equivalents per kg in the latest reports and N2O changed from 289 to 264 kg CO2 equivalents per kg. The GWP 500 factors for methane and N2O were not published in the latest report.

What Is The Source Of These Greenhouse Gases?

Methane emissions mainly take place in the life cycle of fossil resources and in livestock systems. Cows, for example, release large amounts of methane because feed ferments in their stomachs. Methane is also released in large amounts from manure. N2O emissions mainly arise from animal manure, from burning of fossil fuels – especially in vehicles – and during the use and production of artificial fertilizers for growing crops.

How Does Recalculating Results Change Carbon Footprints?

What is the effect of the updated GWP factors on the carbon footprint of different products? The table below shows the carbon footprints of several products in kg CO2 equivalents per given unit, both as published with the AR4 (2007) GWP100 factors and recalculated with AR5 (2013) factors.

Sources: FAO: Tackling climate change through livestock; ecoinvent 3.01 allocation, default.

The results show that the carbon footprints of milk, beef and rice increase by about 3% when calculated with the latest GWP factors. The latest results more accurately reflect the fact that methane, which contributes heavily to the carbon footprint of these products, has more effect on climate change than we realized before. The carbon footprints for chicken products and wheat, however, decrease when based on the latest GWP factors. N2O is more dominant in the carbon footprints of these products than methane. Products for which the carbon footprint is dominated by fossil resource use – electricity and car transport – have only slightly larger carbon footprints in the updated assessment report. Carbon dioxide emissions dominate in the life cycle of these products, and methane emissions are only slightly more prominent than N2O emissions.

Conclusions: What Is The Impact Of The Changed Factors?

The IPCC has updated their GWP factors for calculating carbon footprints for the fourth time, because of new scientific insight. This means that updates in LCA software can soon be expected.

Although the factors for the two most prominent greenhouse gases after carbon dioxide – methane and N2O – have changed more than ever, the carbon footprints of most products are not affected that much. Agricultural products are the exception. The carbon footprints of rice, beef, milk and dairy have grown with the new factors, due to large methane emissions in the production process. The carbon footprints of other crop and meat products, however, are more influenced by N2O and have therefore decreased slightly. Nevertheless, the updates in the assessment report may have consequences for studies comparing products, such as conventional and organic food and beverage products.

Note: The new GWP factors will become available in SimaPro with the coming database update.

Tommie Ponsioen

Tommie worked for PRé as a Technical Consultant from 2012 until 2015. As a part of the Consultancy Team, he worked with databases and methods. Tommie collaborated in projects such as Prosuite and improving the ReCiPe method.

Stay up to date with our newsletter

Newsletter Registration



Newsletters *
Conditions *