Useful metrics for ethical decision making: a reply

Thank you for the opportunity to respond to the letter by Kalmar et al. and their endorsement of the significant scientific limitations of global warming potential (GWP) with respect to short-lived climate pollutants [1]. There are political reasons why the United Nations Framework Convention on Climate Change (UNFCCC) continues to use GWP100 as the common metric for reporting major, regulated greenhouse gases. Despite the clear scientific issues with GWP100 for short-lived climate pollutants [2], its simplicity allows political trading between different regulated greenhouse gases in terms of carbon dioxide equivalence (CO2e) – the fungible carbon dioxide market. But volatile anaesthetics are not regulated, so we have latitude to balance medical and environmental needs. This is important because it means that we should be looking at them through robust climate/environment science rather than the GWP lens. Global warming potential is an international policy metric set by the UNFCCC as a negotiating tool; it was never intended for micromanaging low concentration of short-lived climate pollutants. In that context, we agree with Kalmar et al. However, their statement that “reducing desflurane emissions will not lead to increased carbon dioxide emissions from other sources” should be challenged. Statistics from the NHS show that the use of desflurane has indeed diminished substantially, yet the use of sevoflurane has remained unchanged [3]. Conversely, total intravenous anaesthesia (TIVA) has increased to 26% of all general anaesthetics in the UK [4] – with one possible explanation given that TIVA has been widely promoted as more ‘environmentally friendly’. Full cradle-to-grave life-cycle analyses of TIVA vs. volatile anaesthesia are fraught with uncertainty, not least because they rely on CO2e values from GWP100. If one focuses on the point of use (i.e. the delivery of a general anaesthetic), it is undoubtedly true that TIVA requires substantially more single-use plastic [5], which must be manufactured and incinerated, releasing real carbon dioxide in the process. Conversely, the putative climate impacts of the volatile agents have been calculated incorrectly using GWP100 to convert volatile emissions to CO2e [6]. This grossly inflates their actual contribution and leads to perverse responses that do not stand up scientifically. There is a real risk that the UK and other international healthcare policies are essentially mitigating/trading sub-part per trillion atmospheric concentrations of short-lived volatiles and inadvertently increasing the emissions of real carbon dioxide that will accumulate in the atmosphere for decades to come, continuing to drive up global warming. Kalmar et al. question the relevance of radiative forcing as a metric. From the climate scientist's perspective, radiative forcing is the fundamental measure of how far the planet is from equilibrium (see Figure 2.10 from [7]) and reflects very closely the actual temperature change (see Figure 7.8 from [7]). The time evolution of the effective radiative forcing (and thus temperature change) from all agents since 1750 demonstrates the inexorable rise in forcing from carbon dioxide as the latter accumulates in the atmosphere over time. In contrast, the radiative forcings from short-lived climate pollutants tend to plateau after the initial rise, as expected for gases that do not live long enough to accumulate. Only those short-lived climate pollutants with rapidly rising emission rates and high atmospheric concentrations (such as methane and, to a lesser extent, HFC-134a) offer significant abatement opportunities – and only if no additional carbon dioxide is emitted as a consequence of mitigation. Global warming potential cannot possibly give us the same information because it knows nothing about atmospheric concentrations, climate sensitivity, the circulations of the atmosphere and oceans, and so on – all the elements we identified in Figure 1 of our paper [8] that lead to climate impact. This is why the Intergovernmental Panel on Climate Change (IPCC) and many leading climate scientists have persistently underlined the serious limitations of GWP and noted its unsuitability for estimating the cumulative effects of short-lived climate pollutants – see the references in our paper [8]. We are surprised, therefore that Kalmar et al. state “the Intergovernmental Panel on Climate Change does confirm that all known factors are considered in the reported GWP values, ensuring its reliability as a metric to describe the actual effects”; the cited reports do not support this statement. Large numbers and their misinterpretation have been the bane of this debate, but numbers are meaningless unless they are placed in context. Of course, desflurane is many times more radiatively efficient than carbon dioxide for the reasons we discuss in our paper [8]. Radiative transfer theory (Schwarzchild equation) essentially says that radiative efficiency decreases as the baseline concentration increases. Hence, gases present at low concentrations, such as many hydrofluorocarbons, tend to have radiative efficiencies that are orders of magnitude higher than carbon dioxide and, consequently, much higher GWP values (e.g. 2540 for desflurane). But their very low concentrations mean that their contribution to actual radiative forcing and global warming is minute compared with carbon dioxide. This is fundamental physics. If one took GWP in isolation then it is easy to see how a mitigation strategy could develop that focused on these high-efficiency trace gases – especially when accompanied by misleading carbon dioxide ‘equivalence’. Kalmar et al. suggest that cumulative heat absorption over variable time horizons would be a suitable measure for judging the climate effects of desflurane. But as we have already emphasised [8], natural variability will utterly swamp the contributions from volatile agents, whether we use heat accumulation or radiative forcing. Satellite measurements over the last 20 years of Earth's energy imbalance (i.e. planetary heat gain or loss) show large variations in monthly to decadal timescales that exceed ±1 Wm−2 [9]. This demonstrates that the Earth is not a simple closed system, that heat accumulation is not irreversible, as stated by Kalmar et al., and that natural weather and climate variability can induce large excursions in the Earth's energy imbalance and heat accumulation. This is not surprising because we are dealing with complex fluid flows in the atmosphere and oceans as well as complex latent heat exchanges by the water cycle [8]. We further note that 1 Wm-2 is equivalent to heating of 510,000 gigajoules per second compared with a few hundred gigajoules over 20 or 100 years from one bottle of desflurane. This emphasises yet again that the impact of volatiles is vanishingly small, whichever measure and over whatever time period we choose; it will be lost in the ‘turbulence’ of the oceans and atmosphere. It also emphasises, as we have stated before, that the forcing from greenhouse gases needs to be both sustained and substantial in order to overcome Earth's natural variability – and explains why it is carbon dioxide, not the volatiles, that is playing a critical role in driving climate change. So, how should anaesthetists approach their choice of anaesthetic? We would argue that good, scientific common sense should prevail. There is no doubt that anaesthesia, and healthcare in general, contributes to climate change – via the emissions of carbon dioxide from travel, from buildings and infrastructure, and from the vast amount of equipment and consumables used every day. Nitrous oxide, too, is an important greenhouse gas that is commonly used in anaesthesia and which offers obvious mitigation opportunities. In contrast, with whatever lens we look through, whether it be radiative forcing or heat accumulation, the contribution from volatile agents is vanishingly small and inconsequential. Anaesthetists should be able to use their personal judgement on what is best for their patients without the impedance of the putative climate impacts of volatile gases. Whether TIVA is a wise alternative remains to be seen once a proper audit of its real carbon dioxide emissions in a hospital setting has been conducted. Putting these debates around ‘small gains’ and ‘quick wins’ to one side will enable the worldwide anaesthetic community to step back and take a long look at their personal and professional emissions of major greenhouse gases. There is no doubt that urgent action is needed to limit global warming, and that means focusing our efforts on bringing real carbon dioxide and other major greenhouse gas emissions to zero. We must all seek to do this in both our professional and personal lives if we are to avoid a dangerous long-term future for all life on Earth. These are the ethical decisions we should be making.