Projected emissions based on the pledges made by individual countries to reduce their emissions by 2020 may be overly optimistic because of discrepancies in historical emissions calculations, according to researchers in Switzerland and Germany.

In a paper published in Environmental Research Letters, Joeri Rogelj of ETHZ in Switzerland warns that in 2020 the gap between emission levels in line with the 2°C benchmark (as recognized in the Copenhagen Accord and the Cancun Agreements) and aggregated national mitigation pledges may be larger than first thought.

Rogelj and his colleagues of the Potsdam Institute for Climate Impact Research in Germany have quantified the mismatch in emission levels that are officially reported by countries and the emission levels initially used by different research groups to construct 2020 emission benchmarks that are in line with 2°C.

“We looked at multiple methods for resolving this discrepancy, and in each case we found that the gap between pledges and benchmarks is larger than current estimates,” Rogelj told environmentalresearchweb. “For example, if we would apply this methodology to our earlier published estimates, our calculations show that the gap between the 44 Gt carbon dioxide-equivalent (CO2eq) benchmark and the forecasted 2020 emission levels would increase from 3.4–9.2 GtCO2eq to 5.4–12.5 GtCO2eq. For all options that we assessed, the relative increase of the gap would range from 22 to more than 100%.”

Rogelj believes these results are important because the benchmark approach is often used by policy advisers to check if we are “on track” with emission paths that can limit global warming to 2°C. “But in order for benchmarking to work, we need to make sure that all analyses begin from the same historical starting point,” explained Rogelj. “In many cases, this is not happening.”

“Differences between estimates from various sources will always exist,” he added. “Therefore we tried to quantify this mismatch and to find a consistent methodology to handle this”.

The ERL paper cites an example from the US which officially reported to the UNFCCC that carbon dioxide emissions from fossil fuel and mineral production in 2000 were 5.95 GtCO2. The US Energy Information Administration reported 5.89 GtCO2 for the same emission sectors. A lower estimate of 5.74 GtCO2 is reported in the Carbon Dioxide Information Analysis Center (CDIAC) database, and the International Energy Agency estimates 5.66 GtCO2.

“This shows that even a country like the US, which has ample resources to meticulously calculate its emissions inventory, has uncertainty in its results,” said Rogelj.

Rogelj was one of the lead authors of the United Nations Environment Programme’s (UNEP) Emissions Gap Report. “To ensure consistency and robustness, the analysis for that report already took into account the discrepancies in historical emissions with a method akin to those described in the ERL paper,” said Rogelj. “The findings of this paper will therefore not change the findings of the UNEP Emissions Gap Report.”