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.

感谢您有机会回复卡尔玛等人的来信。以及他们对全球变暖潜势 (GWP) 对于短期气候污染物的重大科学局限性的认可 [1]。

联合国气候变化框架公约 (UNFCCC) 继续使用 GWP ₁₀₀ 作为报告主要受监管温室气体的通用指标是有政治原因的。尽管短期气候污染物的 GWP ₁₀₀ 存在明显的科学问题 [2]，但其简单性允许不同受监管温室气体之间根据二氧化碳当量 (CO ₂ e ) – 可替代的二氧化碳市场。但挥发性麻醉剂不受监管，因此我们可以自由地平衡医疗和环境需求。这很重要，因为这意味着我们应该通过可靠的气候/环境科学而不是全球升温潜能值来看待它们。全球变暖潜势是联合国气候变化框架公约（UNFCCC）作为谈判工具制定的一项国际政策指标；它从来不是为了对低浓度的短期气候污染物进行微观管理而设计的。

在这方面，我们同意卡尔玛等人的观点。然而，他们关于“减少地氟烷排放量不会导致其他来源的二氧化碳排放量增加”的说法应该受到质疑。 NHS 的统计数据显示，地氟醚的使用量确实大幅减少，但七氟醚的使用量却保持不变[3]。相反，在英国，全静脉麻醉 (TIVA) 已增加到所有全身麻醉药的 26% [4]——考虑到 TIVA 被广泛宣传为更“环保”，一种可能的解释是。 TIVA 与挥发性麻醉的完整生命周期分析充满不确定性，尤其是因为它们依赖于 GWP ₁₀₀ 的 CO ₂ e 值。如果关注使用点（即全身麻醉剂的输送），毫无疑问 TIVA 需要更多的一次性塑料 [5]，这些塑料必须被制造和焚烧，在过程中释放出真正的二氧化碳。相反，使用 GWP ₁₀₀ 将挥发性排放物转换为 CO ₂ e 来计算挥发性物质的假定气候影响是错误的 [6]。这严重夸大了他们的实际贡献，并导致不科学的反常反应。确实存在这样的风险：英国和其他国际医疗保健政策本质上是在减轻/交易万亿分之一的大气中短期挥发物的浓度，并无意中增加了未来几十年将在大气中积累的实际二氧化碳的排放量，继续加剧全球变暖。

卡尔马等人。质疑辐射强迫作为衡量标准的相关性。从气候科学家的角度来看，辐射强迫是衡量地球偏离平衡状态的基本指标（参见[7]中的图2.10），并且非常密切地反映了实际的温度变化（参见[7]中的图7.8）。自 1750 年以来，所有物质的有效辐射强迫（以及温度变化）的时间演变表明，随着二氧化碳在大气中的积累，二氧化碳的强迫会不可避免地增加。相比之下，短期气候污染物的辐射强迫在最初上升后往往趋于稳定，正如对于寿命不够长而无法积累的气体所预期的那样。只有那些排放率迅速上升且大气浓度较高的短期气候污染物（例如甲烷，以及较小程度上的 HFC-134a）才能提供显着的减排机会——而且前提是缓解措施不会排放额外的二氧化碳。

全球变暖潜势不可能为我们提供相同的信息，因为它对大气浓度、气候敏感性、大气和海洋环流等一无所知——我们在论文 [8] 的图 1 中确定的所有导致气候影响。这就是为什么政府间气候变化专门委员会 (IPCC) 和许多领先的气候科学家一直强调 GWP 的严重局限性，并指出它不适合估计短期气候污染物的累积影响 - 请参阅我们论文中的参考文献 [8] 。我们很惊讶，因此卡尔马等人。声明“政府间气候变化专门委员会确实确认在报告的 GWP 值中考虑了所有已知因素，确保其作为描述实际影响的指标的可靠性”；引用的报告并不支持这一说法。

大量的数字及其误解一直是这场辩论的祸根，但除非将其放在上下文中，否则数字毫无意义。当然，地氟烷的辐射效率比二氧化碳高很多倍，原因我们在论文中讨论过 [8]。辐射传输理论（史瓦西方程）本质上是说，辐射效率随着基线浓度的增加而降低。因此，低浓度气体（例如许多氢氟碳化物）的辐射效率往往比二氧化碳高几个数量级，因此 GWP 值要高得多（例如地氟烷为 2540）。但它们的浓度非常低，这意味着与二氧化碳相比，它们对实际辐射强迫和全球变暖的贡献微乎其微。这是基础物理学。如果孤立地看待全球升温潜能值，那么很容易看出如何制定专注于这些高效微量气体的缓解策略——特别是当伴随着误导性的二氧化碳“当量”时。

卡尔马等人。表明不同时间范围内的累积热量吸收将是判断地氟烷对气候影响的合适措施。但正如我们已经强调的[8]，无论我们使用热量积累还是辐射强迫，自然变化都将完全淹没挥发性物质的贡献。过去 20 年地球能量不平衡（即行星热量增益或损失）的卫星测量显示，每月到十年的时间尺度存在巨大变化，超过 ±1 Wm ⁻² [9]。这表明地球不是一个简单的封闭系统，热量积累并非不可逆，正如卡尔马等人所说，自然天气和气候变化会导致地球能量不平衡和热量积累的大幅偏移。这并不奇怪，因为我们正在处理大气和海洋中复杂的流体流动以及水循环中复杂的潜热交换[8]。我们进一步注意到，1 Wm ^-2 相当于每秒加热 510,000 吉焦耳，而一瓶地氟烷在 20 或 100 年内加热几百吉焦耳。这再次强调，无论我们选择哪种衡量标准，无论我们选择什么时间段，波动性的影响都微乎其微；它会消失在海洋和大气的“湍流”中。正如我们之前所说，它还强调，为了克服地球的自然变率，温室气体的强迫需要持续且大量，并解释了为什么是二氧化碳，而不是挥发物，在气候变化中发挥着关键作用。推动气候变化。

那么，麻醉师应该如何选择麻醉剂呢？我们认为，良好的科学常识应该占上风。毫无疑问，麻醉和一般医疗保健会导致气候变化——通过旅行、建筑物和基础设施以及每天使用的大量设备和消耗品排放二氧化碳。一氧化二氮也是一种重要的温室气体，常用于麻醉，并提供明显的缓解机会。相比之下，无论我们从什么角度来看，无论是辐射强迫还是热量积累，挥发性物质的贡献都微乎其微且无关紧要。麻醉师应该能够根据自己的个人判断来判断什么对患者最有利，而不会受到挥发性气体假定的气候影响的影响。一旦对医院环境中的实际二氧化碳排放量进行了适当的审计，TIVA 是否是一个明智的选择还有待观察。

将这些围绕“小收益”和“速效”的争论放在一边将使全球麻醉界退后一步，长期审视他们个人和职业的主要温室气体排放。毫无疑问，需要采取紧急行动来限制全球变暖，这意味着我们要集中精力将二氧化碳和其他主要温室气体排放真正降至零。如果我们想避免地球上所有生命面临危险的长期未来，我们都必须在职业和个人生活中努力做到这一点。这些是我们应该做出的道德决定。