Most people who have heard of the Paris Agreement will know of it in terms of its aim to limit global warming to 2°C, and to work towards limiting warming to just 1.5°C above pre-industrial temperatures. What is less well known is the text in the agreement that states the need for ‘balance between anthropogenic emissions by sources and removals by sinks of greenhouse gases’ to achieve the temperature goal.
The observant reader will notice that this sentence is open to interpretation. What quantity is being balanced? Is it the net amount of greenhouse gas emissions? If so, how do we account for all the different gases, which each affect the climate in a different way, and which gases are included? We could convert all emissions to “CO2-equivalent” emissions, for example. Or should we balance the radiative forcing generated by the emissions, as the temperature responds to changes in the radiative forcing?
A new paper by Fuglestvedt et al. discusses the options and what the consequences would be for different interpretations of balance. The article considers scenarios that could achieve the Paris Agreement, and explores what impact assumptions make on the temperature outcome.
Even in the most ambitious scenarios, there are always some greenhouse gas emissions that cannot be eradicated. For example, methane (CH4) and nitrous oxide (N2O) emissions from agriculture. In these scenarios, we assume that an “equivalent” amount of CO2 must be removed from the atmosphere to compensate for these persistent emissions, which are shown in the left-hand panel below. This concept is uncontroversial. The question of how to work out how much CO2 to remove to balance the persistent methane and nitrous oxide emissions is less clear-cut.
The right-hand panel shows the “CO2-equivalent” emissions required to be removed from the air to achieve net-zero emissions, based on 5 commonly used emissions metrics. The N2O is similar using all the different metrics, however methane shows wide variation. This shows that the metric used to work out how much CO2 to pull out of the atmosphere really matters, as it varies by a factor of 5.
What would the implications be for temperature? The graph below shows an example of the temperature change if emissions are held at net-zero over 100 years, based on the metrics in the figure above, and one new application of GWP (based on Allen et al., 2016) labelled GWP*. The thick lines are based on balance defined by offsetting any remaining emissions at 2100 in a high ambition scenario from the IPCC’s 5th Assessment Report (RCP2.6). The thin lines are for a range of other scenarios from the report.
The brown lines show balance as calculated using Global Warming Potential over 100 years (GWP100), which is the metric used in the Kyoto Protocol and most widely in both policy and scientific reports. The graph shows that if balance is achieved by using GWP100 for a century, the global mean surface temperature will have steadily declined over that period. Using other metrics changes the amount of cooling, but they do all produce cooling. This shows that these metrics give too much weight to methane when considered in terms of temperature. Another recent paper, by Tanaka and O’Neill, showed a similar result, though framed differently: unless we exceed the temperature targets and need to cool to later achieve them, net-zero emissions are not required to limit warming to 1.5°C or 2°C.
The green lines show the temperature response when achieving balance using the GWP* metric. This is a metric which uses GWP100 to equate a pulse of CO2 with a change to the emission rate of short-lived gases like methane, designed to better reflect the response of the climate system to these different types of gases.
These definitions of balance do limit warming, so in that sense are not in direct contradiction with the Paris Agreement temperature goal. However, if policies are to be determined based on the “best available science” (a phrase also used in the Paris Agreement), then the implication that a balance of sources and sinks leads to cooling, and not temperature stabilisation, should be understood.
In this paper, we have set out some of the implications for using a metric like GWP100 to define balance. The next question is whether policies and activities related to the Paris Agreement should use GWP100, despite its shortcomings, or whether a metric that links emissions to temperatures (and is therefore more consistent with these two aspects of the agreement) would be more useful. It is unlikely that policy makers or scientists would be well placed to make this decision alone, and so ongoing dialogue between scientists and policy makers will be valuable for informing the ongoing policy process in the wake of the Paris Agreement.
Figures taken from:
Implications of possible interpretations of ‘greenhouse gas balance’ in the Paris Agreement
J. Fuglestvedt, J. Rogelj, R. J. Millar, M. Allen, O. Boucher, M. Cain, P. M. Forster, E. Kriegler, D. Shindell
Phil. Trans. R. Soc. A 2018 376 20160445; DOI: 10.1098/rsta.2016.0445. Published 2 April 2018