As climate summit approaches, UN reports existing 1.5C warming limit is no longer achievable

A UN climate report released Wednesday finds the globe will unavoidably slip past the Paris Climate Agreement’s 1.5-degree Celsius warming target in the next few years. The task now, the report states, is to make the period of warming “overshoot” as limited as possible, eventually bringing global average temperatures back down.

The annual UN Emissions Gap report is designed to inform decision makers headed into the next round of global climate talks in Turkey, known as COP31, next month. While past gap reports have expressed skepticism about meeting the 1.5-degree target, this one marks the first time that its authors have pronounced that missing the mark will be “imminent and unavoidable.”

“We are now saying this window has closed, but we can open it again by making the overshoot as brief as possible,” UN Environment Program Executive Director Inger Andersen said on a call with reporters. “This is about how to find a way back as shortly and as quickly as possible with as limited an overshoot as possible.”

Past reports have examined the distance between countries’ emissions reduction commitments and what it would take to limit global warming to 1.5 C above preindustrial levels.

However, this year’s report redefines the gap. Instead, it examines new emissions overshoot scenarios, and what it would take to limit peak warming to just 1.7 to 1.8 degrees C before bringing temperatures back down to 1.5 C through emissions cuts and the use of carbon removal technologies.

The report makes clear that every additional fraction of a degree of warming above 1.5 C “raises the risk of irreversible changes including in ice sheets, permafrost carbon, ocean circulation, and coral systems, along with devastating human impacts.”

Each added fraction of a degree of warming can have an “immense” impact “as measured in terms of damage, cost and loss of lives,” said climate scientist Michael Mann of the University of Pennsylvania. Mann was not involved in producing the UNEP report.

According to the UNEP, which is based in Nairobi, the lowest overshoot scenario would keep peak warming to between 1.7 and 1.8 degrees C and return global average temperatures to at or below 1.5 C by 2100.

However, to follow such a pathway, global greenhouse gas emissions would need to be slashed by far more than anything currently on the table at the upcoming COP31 talks. This would require an 18% cut by 2030, 45% by 2035, and 69% by 2040, relative to 2025 levels.

Meeting this lowest-possible overshoot scenario would require a tripling of renewable electricity capacity by 2035, and dramatic cuts to fossil fuels, including the phaseout of coal by 2040, the report states.

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In addition, there would need to be a major scaling up of carbon removal technologies, which so far have not been proven to work at such a large scale.

In stark contrast to these figures, the fulfillment of existing emissions reduction pledges would only yield a 5-to-8% emissions cut by 2030 and an 11-to-15% reduction by 2035, the report found. This would be associated with a peak warming of about 2.6 degrees C.

If countries simply follow these current policies, then warming could reach a peak of about 2.8 C. That estimate has increased since last year, in part because of the withdrawal of the United States from the Paris Agreement and retraction of its emissions reduction commitments, the report states.

Even the scenario that assumes countries meet their existing reduction commitments, and nations with net-zero pledges succeed in fulfilling them, still brings warming to a peak of 2.1 C.

Gaby Oraa/Reuters/File

Clearly, overshooting 1.5 degrees C by the most limited amount possible and then coaxing temperatures back down over time is a tall order compared to what countries have pledged to do so far.

“Without urgent action, the lowest possible 1.5 C overshoot pathway will quickly fade,” the report warns. “Every five years of CO2 emissions at current levels pushes peak warming up by around 0.1 C.”

Mann said an additional 0.1 C of global warming “Permanently cuts global GDP by roughly 1.2% (over $1.2 trillion annually) and pushes approximately 140 million more people into life-threatening, dangerously hot climates.”

Journey to the disappearing edge of the Arctic

Overshooting the 1.5-degree target can be viewed as a milestone, but it doesn’t mean an absolute failure of climate science and policy, according to climate scientist Kate Marvel of Project Drawdown, a climate research and solutions nonprofit. Marvel was not involved in the UNEP report.

“There’s nothing special about any given temperature threshold,” she said. “The right unit isn’t degrees, it’s tons — of carbon dioxide, methane and other greenhouse gases. If we want to avoid the worst impacts, our task is and has always been to reduce these emissions as quickly as possible.”

“Passing 1.5 degrees is a sign that we’re not moving fast enough, but it doesn’t mean we’re doomed,” she said .

联合国周三发布的一份气候报告指出，未来几年全球气温将不可避免地超过《巴黎气候协定》设定的1.5摄氏度升温目标。报告称，当务之急是尽可能缩短升温“超调”时期，最终使全球平均气温回落。

联合国年度《排放差距报告》旨在为即将于下月在土耳其举行的下一轮全球气候谈判（即COP31）的决策者提供信息。尽管以往的差距报告对实现1.5摄氏度温控目标持怀疑态度，但这份报告首次明确指出，未能实现目标“迫在眉睫且不可避免”。

联合国环境规划署执行主任英格·安德森在与记者的电话会议上表示：“我们现在认为这个窗口已经关闭，但我们可以通过尽可能缩短过度消耗的时间来重新开启它。关键在于如何尽快找到一条回归正轨，并尽可能减少过度消耗。”

过去的报告研究了各国减排承诺与将全球变暖限制在比工业化前水平高 1.5 摄氏度以内所需措施之间的差距。

然而，今年的报告重新定义了差距。它着眼于新的排放超标情景，以及如何将升温峰值限制在1.7至1.8摄氏度，然后再通过减排和使用碳移除技术将气温降至1.5摄氏度以内。

该报告明确指出，气温每升高不到 1.5 摄氏度，“都会增加冰盖、永久冻土碳、海洋环流和珊瑚系统等发生不可逆转变化的风险，并给人类带来毁灭性的影响。”

宾夕法尼亚大学气候科学家迈克尔·曼恩表示，气温每升高不到一度，都会产生“巨大”影响，“无论从破坏、成本还是生命损失来看都是如此”。曼恩并未参与联合国环境规划署这份报告的撰写。

根据总部位于内罗毕的联合国环境规划署的说法，最低的超调情景将使升温峰值保持在 1.7 至 1.8 摄氏度之间，并在 2100 年使全球平均气温恢复到 1.5 摄氏度或以下。

然而，要实现这样的目标，全球温室气体排放量需要大幅削减，远超即将召开的COP31气候大会上提出的任何方案。这意味着到2030年，温室气体排放量需要比2025年的水平降低18%，到2035年降低45%，到2040年降低69%。

报告指出，要实现最低限度的超支目标，到 2035 年可再生能源发电能力需要增加两倍，并大幅削减化石燃料的使用，包括在 2040 年前逐步淘汰煤炭。

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此外，还需要大幅扩大碳去除技术的规模，而迄今为止，这些技术尚未被证明能在如此大规模下发挥作用。

与这些数据形成鲜明对比的是，报告指出，即使现有减排承诺得以兑现，到2030年也只能实现5%至8%的减排，到2035年也只能实现11%至15%的减排。这将导致全球气温峰值上升约2.6摄氏度。

报告指出，如果各国继续沿用目前的政策，那么全球变暖的峰值可能达到约 2.8 摄氏度。这一估计值较去年有所上升，部分原因是美国退出了《巴黎协定》并撤回了其减排承诺。

即使假设各国履行其现有的减排承诺，并且有净零排放承诺的国家成功履行承诺，全球变暖的峰值仍将达到 2.1 摄氏度。

Gaby Oraa/路透社/档案照片

显然，将气温超标幅度控制在尽可能小的范围内（超过 1.5 摄氏度），然后随着时间的推移逐步降低气温，与各国迄今为止所承诺的目标相比，是一项艰巨的任务。

报告警告说：“如果不采取紧急行动，最低限度的1.5摄氏度升温路径将很快消失。按照目前的二氧化碳排放量，每五年就会使全球升温峰值升高约0.1摄氏度。”

曼恩表示，全球气温再升高 0.1 摄氏度“将永久性地使全球 GDP 下降约 1.2%（每年超过 1.2 万亿美元），并将大约 1.4 亿人推入危及生命、酷热难耐的气候中。”

前往北极消失的边缘地带

气候科学家、气候研究与解决方案非营利组织“减缓气候变化项目”（Project Drawdown）的凯特·马维尔（Kate Marvel）表示，虽然超过1.5摄氏度的目标可以被视为一个里程碑，但这并不意味着气候科学和政策的彻底失败。马维尔并未参与联合国环境规划署的这份报告。

“任何特定的温度阈值都没有什么特别之处，”她说。“正确的单位不是摄氏度，而是吨——二氧化碳、甲烷和其他温室气体的排放量。如果我们想避免最严重的后果，我们的任务始终是尽快减少这些排放。”

“升温超过 1.5 度表明我们行动不够快，但这并不意味着我们注定要失败，”她说。