Assessment of urgent impacts of greenhouse gas emissions-the climate tipping potential (CTP)

The impact of anthropogenic greenhouse gas (GHG) emissions on climate change receives much focus today. This impact is however often considered only in terms of global warming potential (GWP), which does not take into account the need for staying below climatic target levels, in order to avoid passing critical climate tipping points. Some suggestions to include a target level in climate change impact assessment have been made, but with the consequence of disregarding impacts beyond that target level. The aim of this paper is to introduce the climate tipping impact category, which represents the climate tipping potential (CTP) of GHG emissions relative to a climatic target level. The climate tipping impact category should be seen as complementary to the global warming impact category. The CTP of a GHG emission is expressed as the emission's impact divided by the 'capacity' of the atmosphere for absorbing the impact without exceeding the target level. The GHG emission impact is determined as its cumulative contribution to increase the total atmospheric GHG concentration (expressed in CO2 equivalents) from the emission time to the point in time where the target level is expected to be reached, the target time. The CTP of all the assessed GHGs increases as the emission time approaches the target time, reflecting the rapid decrease in remaining atmospheric capacity and thus the increasing potential impact of the GHG emission. The CTP of a GHG depends on the properties of the GHG as well as on the chosen climatic target level and background scenario for atmospheric GHG concentration development. In order to enable direct application in life cycle assessment (LCA), CTP characterisation factors are presented for the three main anthropogenic GHGs, CO2, CH4 and N2O. The CTP metric distinguishes different GHG emission impacts in terms of their contribution to exceeding a short-term target and highlights their increasing importance when approaching a climatic target level, reflecting the increasing urgency of avoiding further GHG emissions in order to stay below the target level. Inclusion of the climate tipping impact category for assessing climate change impacts in LCA, complimentary to the global warming impact category which shall still represent the long-term climate change impacts, is considered to improve the value of LCA as a tool for decision support for climate change mitigation.