The Washington Post-How scientists test whether humans are causing our extreme weather
September 11, 2023 5 min 1052 words
这篇报道探讨了一门新兴的气候科学领域，即极端天气事件的归因研究，以及它如何证明全球范围内前所未有的热浪与人类引起的气候变化有关。通过将我们当前的世界与一个假想的世界进行比较，归因科学家运行模拟天气模型来检查人类对天气相关灾害的影响，这个假想的世界是一个从未发生过气候人为干预的世界。 气候模型通过数学方程式来描述能量和物质在海洋、大气和陆地不同部分之间的相互作用，帮助我们了解过去气候的变化以及未来可能的变化。通过归因研究，科学家可以在地球上回放过去几百年的气候，去除人类排放的所有温室气体。通过对比这个虚构的世界和我们现实的世界，他们可以看出极端事件（如洪水、干旱或寒冷天气）是否有所不同以及这些排放对我们的天气产生了什么影响。 最近的一项归因研究发现，北美和欧洲的热浪在没有气候变化的情况下几乎是“不可能的”。但在现实中，根据归因科学家的说法，我们可以预计它们将在北美每15年一次，欧洲每10年一次出现。这对于水资源管理者、城市规划者和政策制定者来说是非常重要的信息，涉及气候适应和韧性。 在过去，科学家们通常避免将任何单个事件与气候变化联系起来，因为天气本质上是不可预测的，没有单一的原因。但在2004年，被认为是第一个极端事件研究确定了气候变化“至少使”欧洲去年的热浪的风险翻倍，导致了7万多人死亡。自那时以来，科学家们已经研究了全球500多起与天气有关的灾害事件，其中71％被认为是由人类引起的气候变化使其更可能或更严重。 气象变异倡议（WWA）成立于2015年，由国际气候科学家组成，已经进行了50多项归因研究，大多数是在事件发生后或事件仍在发生时进行的。他们指出全球变暖是自2020年以来东非干旱、2022年南美热浪和2022年巴基斯坦洪水的主要驱动因素。然而，在2019年马达加斯加的干旱事件中，WWA发现降雨减少主要是由于自然气候变化，尽管联合国的声明与此不同。 总的来说，这篇报道强调了极端事件归因研究的重要性，它不仅有助于我们理解气候变化对日常生活的影响，还为决策者提供了有关气候适应和韧性的关键信息。这种研究通过科学方法将气候变化与人们的日常经验相连接，有助于提高人们对气候变化问题的认识和理解。此外，通过更快的计算机和更精确的气候模型，研究人员现在可以在几天内进行这些分析，而不是几个月。这使得归因研究变得更加实用和及时。 最后，这些研究结果对于气候政策的制定和应对极端天气事件的准备都具有重要的启示作用，强调了迫切需要采取行动来减少温室气体排放并应对气候变化的严重影响。
2023-07-31T20:03:08.142Z
An emerging field of climate science that analyzes extreme weather events is behind the assertion that the unprecedented heat waves sweeping the globe are a result of human-induced climate change.
Extreme event attribution examines the human fingerprint on weather-related disasters by comparing our current world — and its growing amount of weather anomalies — to an idealized one, where the human influence on climate never happened.
To do that, researchers run computer programs known as climate models that simulate weather patterns over time, not unlike those used for a local seven-day forecast. But they re-create the weather over decades or centuries, rather than hours or days.
“The really cool thing about climate models is that you have a world in a computer, and you can do experiments on it,” said Andrew Pershing, vice president of science at the research nonprofit Climate Central. “And you can literally do the experiment of what would this world look like if global warming never happened.”
Connecting a changing climate to human activity dates back to the work of two Nobel Prize winners for physics, Syukuro Manabe and Klaus Hasselmann, who pioneered the development of climate models starting in the 1960s. Climate models help us understand how the climate has changed in the past and may change in the future. They solve mathematical equations that describe how energy and matter interact in different parts of the ocean, atmosphere and land.
Attribution scientists use climate models to replay the past few hundred years on Earth, removing all greenhouse gas emissions made by humans. By contrasting this fictional world with our own, they can see whether extreme events such as floods, droughts or cold spells look any different and what effect those emissions have had on our weather.
For instance, an attribution study in July found that the heat waves in North America and Europe would have been “virtually impossible” in a world without climate change.
If humans had not warmed the planet by burning fossil fuels, these heat waves would still be considered rare. But in reality, we can expect them to come around every 15 years in North America and every 10 years in Europe, according to attribution scientists.
They also warn that, if humans continue to produce emissions at today’s rate, it will speed up to every two to five years starting in the mid-2030s.
“That is really important information for water resource managers, urban planners and policymakers regarding climate adaptation and resilience,” said Kevin A. Reed, professor of marine and atmospheric sciences at Stony Brook University.
Until recently, scientists largely avoided connecting any individual event with climate change, with the idea that weather is by its very nature unpredictable and has no single cause. But in 2004, what is considered the first extreme event study determined that climate change “at least doubled the risk” of the previous year’s heat wave in Europe that killed over 70,000 people.
Almost any weather event might occur by chance, but the authors argued that climate models could be used to tease out the role that humanity played in making such intense heat more likely. They simulated the climate with and without human emissions thousands of times, counting how many times a heat wave as extreme as the 2003 one popped up. While the event was rare in both cases, it occurred twice as often in the world with human emissions.
Since that pivotal first study, scientists have investigated more than 500 weather-related disasters across the globe, with 71 percent of them found to be made more likely or more severe based on human-caused climate change. With the help of faster computers and more precise climate models, researchers can now perform these analyses within days instead of months.
The World Weather Attribution (WWA) initiative, formed in 2015 by an international team of climate scientists, has carried out more than 50 attribution studies, most in the aftermath of events or while they are still happening. The WWA was responsible for the analysis of July’s extreme heat around the world, which took only five days to compile.
Many researchers see extreme event attribution as a communication tool that has the power to connect climate change with people’s everyday experiences. “Doing attribution in real time is what will be the most useful for people to get informed, while there is focus on an extreme event,” said WWA team member Robert Vautard, director of the Institut Pierre-Simon Laplace.
The WWA has pointed to global warming as the main driver for droughts in East Africa since 2020, a 2022 heat wave in South America and 2022 flooding in Pakistan.
But in the case of a 2019 drought in Madagascar, WWA found that reduced rainfall was mostly due to natural climate variation, despite the United Nations claiming otherwise.
Climate Central has adapted the methods developed by WWA to create the Climate Shift Index (CSI), a metric that reveals how much daily weather conditions have been altered by climate change. Pershing and his colleagues average results from 22 climate models, calculating the likelihood of local, daily temperatures with and without historical greenhouse gas emissions.
Their recent analysis that climate change made July hotter for more than 6.5 billion people, or 81 percent of Earth’s population, had looked at data for 4,700 cities and 200 countries. “Virtually no place on Earth escaped the influence of climate change” in July, Pershing said.
Reed’s laboratory specializes in event attribution studies that look specifically at the effect on hurricanes. For each study, he runs 40 simulations over the past 150 years, each with a slightly altered sea surface, atmospheric temperature and atmospheric humidity to add the element of chance in influencing weather conditions. He also includes a “preindustrial control run” that captures the climate of 1850, or a time before the rise in human emissions.
After zooming into a particular time and region to capture a target hurricane, Reed runs seven-day weather forecasts. To identify the human fingerprint, they compare hurricane features over that week, such as rain rates, accumulation amounts, intensity and size.
His study of the 2020 North Atlantic hurricane season, one of the most active on record, discovered that climate change increased rainfall rates by 11 percent and rainfall amounts by 8 percent.