Simple climate change contribution measure ‘remains elusive’

The question of how to fairly quantify national contributions to global warming historically, and inform future policy on sharing the burden of reducing greenhouse gas emissions, has long been part of international discussions on global warming.

Researchers from the Center for International Climate and Environmental Research Oslo (CICERO) and the University of Oxford, UK, assessed if finding an answer to this question was possible or likely.

Writing this week in the journal Environmental Research Letters, the team argue there are fundamental value-related and ethical questions that cannot be answered through a single set of calculations.

Dr Steffen Kallbekken, from CICERO, said: “There are vast differences in income, population, resources, technologies and capacity across countries. Because of this, the idea of distributive fairness has played a crucial role in the international climate policy debate.

“However, trying to find a single common formula for policymakers to decide the distribution of national efforts in a climate agreement was never likely to succeed.

“Although it references equality principles, the United Nations Framework Convention on Climate Change 2016 Paris Agreement tacitly recognises this fact, as in practice the differentiation is left to each country to decide for itself.”

The team’s work presents a broad and systematic study of how the various scientific and policy-related choices influence the calculations of historical contributions for individual countries.

To calculate the relative contribution from countries/regions to global mean surface temperature (GMST) change, they constructed a time series of emissions of greenhouse gases (GHGs), aerosols and their precursors for each country/region, and used a simple climate model to calculate the temperature response.

The researchers identified six key choices in calculating historical contributions. First were the components to include in the calculations. Aside from CO2 from fossil‐fuel combustion and cement, other possibilities included other long‐lived GHGs, CO2 from land use change (LUC), changes in albedo, and short‐lived gases and aerosols not included in the Kyoto Protocol.

Dr Ragnhild Bieltvedt Skeie said: “Results on countries’ historical contributions vary widely depending on the components used. For example, considering only CO2 fossil fuel emissions between 1850 and 2012, the Annex 1 group of countries contributed 68 per cent of total warming. However, when methane and nitrous oxide emissions are taken into account, the contribution for the same group of countries drops to 54%.”

Other choices cover the start year from which emissions are included; which year the climate response is evaluated; whether variations in population size should influence the calculations; and the appropriate accounting basis for calculating historical contributions – either through emissions actually occurring in a territory; emissions that occur when fossil fuel extracted in a country is burned; or to allocate the emissions due to production to the country where the products are consumed.

Dr Skeie said: “Finally, there is the choice of climate change indicator. GMST change is the most commonly used indicator. However, other indicators such as sea level rise (or ocean heat content) and incidence of extreme events and damage costs could be more policy-relevant, albeit much harder to calculate and attribute.”

The team’s study showed that, although calculations of contributions to global warming can be useful as background information to justify and inform positions in negotiations, historical contributions are difficult to use directly in negotiations.

Dr Jan Fuglestvedt said: “Our results indicate there is no simple and single correct answer to the question of how much each country has contributed to global warming. There are too many fundamental value-related and ethical questions remaining, to which it is impossible to find a single answer.

“A more useful approach would be for scientific studies to present not just one set of calculated contributions – based on some of many choices – instead present a spectrum of results showing how the contributions vary according to a broad set of choices.”

An advance (pre-production) copy of the paper, for journalists’ use only, is available to download HERE

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