高温死亡人数激增，学会看酷热指数、湿球温度和露点才能更好保命｜科学60秒

2024-07-04 15:07

高温拯救计划

夏天正一年比一年热，发生热浪的频率也一年比一年高，高温持续的时间更长、更强烈。短时间内，我们无法战胜高温天，那应该如何在极热的日子里保证自己的安全？

中国版《2023 柳叶刀人群健康与气候变化倒计时报告》的数据表明，2022 年中国人均热浪天数达到了 21 天，与热浪相关的死亡人数达到破纪录的 5.09 万人。2024 年 6 月中旬，山河四省不少地区气温都超过了 40℃，高温不仅连续多日不断，一天中高温的持续时间也可能达到 8 小时甚至 10 小时以上。

不只是中国，从加沙到印度，从孟加拉国到菲律宾，酷热难耐已成家常便饭。自 5 月初以来，墨西哥遭受了创纪录的热浪袭击，上百只吼猴因脱水和中暑而从树上坠落致死，鸟儿被救助并送入空调房。

我们不断向大气中排放着热量，这也推动着我们向创纪录的高温不断迈进。从绝对意义上来说，并非每一个夏天都会比上一年更热，这因地而异，但总体来说，现在我们所经历的破纪录的夏天，在二三十年后看来将是一个普通而凉爽的夏天。

高温之所以危险，是因为它会给我们的身体带来压力。如果你暴露在高温环境下却没有及时补充水分，你的身体就会开始脱水：你的血液会变稠，你的心脏必须更加努力地泵血；你的身体无法正常排汗，也就是进行自我冷却。

高温对某些群体（例如老年人和幼儿）尤其危险的原因之一是，他们的身体不太善于调节体温，不太善于利用自然冷却系统，因此很难适应高温。患有某些疾病的人，尤其是心脏或肺部疾病的人，通常风险更高。如果你正在服用某些药物，你也可能面临更高的风险，因为有些药物也会阻碍出汗过程，而你可能不会意识到这一点。

缺乏意识实际上是高温带来的更大的问题之一，因为它是一种看不见的威胁。提到暴雨，你可能会想到路面积水、房屋被淹，提到地震，你可能会想到地动山摇，建筑倒塌，但高温，是一种很难具象化的极端天气。

另一方面，我们通常能在事后很快知道其他极端灾害造成的死亡人数，但高温的后果可能会有延迟，可能须要等到几天、几周甚至几个月后才能知道热浪造成的死亡人数。

我们现在遇到的热浪与我们的父母或祖父母成长过程中遇到的热浪并不相同，人们倾向于根据过去的经验来评估风险，如果未来，甚至是现在与过去不符，那就意味着人们可能不会根据他们真正需要的信息做出决定。

对目前使用的许多热量测量方法，很多人也缺乏认识和理解，例如酷热指数（heat index）和湿球温度（wet bulb）。

酷热指数是一种综合空气温度和相对湿度的热指标。湿度更高时，水分的蒸发率就会降低，人体通过排汗带走热量的过程就会变得更慢，真正感受到的温度也就更高。

体感温度、表观温度则包括温度、湿度和风寒效应（wind chill），这里的风寒是气象学中的一个概念，指的是有风吹过时，能让你更好地凉快下来，因为它有助于身体的蒸发冷却过程，也就是出汗。

湿球温度包括以上提到的所有因素，再加上云量和暴露在阳光下的程度。湿球温度的问题在于，它给出的数字很难理解，当湿球温度为 27℃ 左右时，你会开始感到非常不舒服，而 32℃ 左右则令人难以忍受。但对应到温度，你不会觉得“32℃ 也太危险了”。

有些人还会关注露点（dew point），也就是空气中所含气态水达到饱和而凝结成液态水所需降至的温度。如果露点低于 13℃，你就会感觉很舒服；但如果在 13~18℃ 之间，你就会开始感到黏糊糊的；如果超过 18℃，那感觉就像在空气中游泳一样；如果露点上升到二三十摄氏度，天气会闷热得让人难以忍受。有纪录以来最高的露点是 35℃，于 2003 年 7 月 8 日下午在沙特阿拉伯宰赫兰录得，当时气温为 42℃，酷热指数高达 81℃。

对 65 岁以上的老年人群体，他们知道自己所在年龄段的人具有更高的风险，但当被问及他们个人是否认为自己有风险时，他们通常会回答“没有”，因为他们仍然觉得自己很健康，他们会说“我还能独立生活，什么事都能自己做，我并不虚弱”。他们中的很多人并不理解酷热指数，不知道应该把空气湿度也考虑进去，当面对 40℃ 的酷热指数时，他们很难理解其中的信息。

未来的夏天有多热，其实掌握在我们自己手中。《科学》（Science）上的一项研究表明，对于 2020 年左右出生的孩子来说，根据政府目前承诺的减少排放，他们一生中经历的热浪次数将是 1960 年出生的人的 7 倍；如果我们将排放量控制在比工业化前高出约 2℃ 或 1.5℃ 的范围内，那么这一数据将降至 6 倍或 4 倍。

我们越努力控制温室气体排放，未来的夏天就越不会热到难以忍受。

除了继续等待人类逐渐摆脱化石燃料、减少碳排，我们还采取哪些预防措施来保证自己和身边人的安全？

就今天而言……[查看全文]

How We Can Keep Ourselves and Our Communities Cool in the Summer Heat

Rachel Feltman: It’s not just your imagination: heat waves are actually getting hotter. They’re also happening more often, lasting longer and starting earlier in the year.

For Scientific American’s Science Quickly, I’m Rachel Feltman. Today I’m talking to associate editor Andrea Thompson, who covers the environment, energy and earth sciences for SciAm. She’s here to tell us why we’re not likely to beat this heat anytime soon and to give us some tips on how we can keep each other safe—even on the most sweltering days.

So it feels like summer has gotten a lot worse since I was a kid—honestly since I first moved to the New York City area, like, a little more than a decade ago. Is there data to back that up?

Andrea Thompson: There absolutely is, and I know exactly what you mean. I grew up in Atlanta and moved to New York 15 or so years ago, and summers now feel more like the summers I remember in Atlanta growing up, which is not, as far as I’m concerned, not a good thing, but there is a ton of data backing up that summers really do that—they are as bad as they feel like they are.

Temperatures are actually some of the most robust trends we have in terms of climate change. We have much longer data. It’s very good-quality data that’s taken, especially in the U.S., from all over, and it’s very clear that summers are getting hotter sort of at every scale—so from the scale of the globe down to the local level. You know, you see more year-to-year variation from the weather at the local level because it has a bigger impact, whereas at the global level, that sort of gets ironed out a little bit. But, you know, we can tell that summers are hotter, that the heat waves that happen in those summers are happening more often, are lasting longer, are more intense, they start earlier in the year, and they hang around longer.

And there were several particularly big heat waves last summer that helped lead it to be the [Northern Hemisphere’s] hottest summer in the last 2,000 years.

Feltman: Wow, yeah, and it doesn’t seem like we’re off to a great start this summer either.

Thompson: No, we’re not. And even before summer there were major heat waves across a large swath of Asia starting in April...

Feltman: Hmm.

Thompson: From Gaza to India, Bangladesh and the Philippines, where it was just really brutal heat that would be much more typical of summer.

We have seen really terrible heat in Mexico since the beginning of May. There were howler monkeys dropping out of trees from dehydration and heatstroke, birds having to be rescued and put into air-conditioning, obviously people being very affected. And we’ve started to see that more in the mainland U.S. now. There is a recent heat wave across the West, and now it’s hitting us in the East.

Feltman: I feel like I’m probably not going to love the answer to this, but what is the outlook for future summers?

Thompson: There was a really great quote from the commerce secretary, Gina Raimondo, a couple years ago, when she was referring to some of the heat waves that year, and she said something like, “This [hot] summer is [going] to be one of the coolest ... of the rest of our lives.”

And that really struck me—and I actually did a story about it—but essentially what is record-breaking now is going to be an average summer in 20, 30 years, and the nature of climate change, because we’re continually adding heat to the atmosphere, we are stacking the deck towards more record heat and away from record cold.

And of course, that doesn’t mean that every summer is hotter than the last because you do still have variation, and it depends on where you are, but in general.

Feltman: So what’s so dangerous about this increase in heat?

Thompson: Heat, in general, is dangerous because it stresses out our bodies.

So if you’re subjected to heat, especially if you’re not hydrating, your body starts to dehydrate: your blood gets thicker, and your heart has to work a lot harder to pump it.

Feltman: You know, even understanding that when we drink water, it ends up in our blood, I don’t think about my blood getting thicker, yeah.

Thompson: Yeah, exactly. And if you’re not hydrating, your body doesn’t have, you know, those reserves to sweat, which is our body’s natural cooling system. That’s what sweat is. Even though it makes us feel gross, it’s actually really helpful to us.

Feltman: I try to remind myself when I’m feeling disgusting that it would be very bad if it just stopped.

Thompson: Yes, exactly. And one of the reasons heat is particularly dangerous for certain groups—like those older individuals, very young children—is that their bodies aren’t as good at regulating their temperature, at taking, you know, advantage of these sort of natural systems, so they have a much harder time adjusting to the heat. And there’s also people with certain health conditions, especially related to the heart or lungs, are generally at higher risk. If you have—are on certain medications, some of those can impede the sweating process, and a lot of people aren’t aware of that.

And it’s sort of lack of awareness that can actually be one of the bigger problems with heat because it’s sort of this invisible threat. You know, when we think about tornadoes, we envision the funnel cloud. With hurricanes you can, you can show people what storm surge coming in looks like. But it’s really hard to visualize heat.

And we actually—when we’re doing stories and trying to find photos, it’s really hard to find photos that telegraph heat really well. And so that makes it really hard to sort of get people to understand what the problem is. And another problem is: we know the death toll from other extreme hazards pretty quickly—in the aftermath, generally—but heat, there can be a disconnect because it can take days, weeks, even months to know what the full death toll from a heat wave is.

And, you know, these are what one expert told me are called mass casualty events, but we don’t really think of them like that because we don’t get those numbers until so far after the event has happened. So it’s, it’s sort of that and the fact that basically, you know, we have heat waves now that aren’t like the ones we grew up with or our parents grew up with or our grandparents grew up with. And we tend to base our assessments of risk on our past experience. So if the future, or even the present, isn’t a match for the past, that means people may not be making decisions based on the information they really need to, and there’s a lot of, also, lack of awareness and understanding of a lot of the heat measures that we use.

Feltman: Yeah, well I would love to talk a little bit more about those measures of heat because, we hear about heat index, we hear about wet bulb more and more now. So could you maybe demystify some of that?

Thompson: Heat index is the temperature plus humidity. You know, when it’s more humid, it makes bearing the heat a lot harder.

“Feels like” and/or apparent temperature are temperature, humidity and windchill—although it’s hard to think of windchill in the summer—but it’s basically when it’s breezy out, cools you down because it helps that sort of evaporative cooling process, that sweating is.

Global wet-bulb temperature is all of those things, plus looking at cloud cover and how much sun you’re exposed to because obviously it’s much cooler if you’re in the shade than if you’re in the direct sunlight. The problem with global wet bulb—even though it is a really good sort of all-encompassing measure—the numbers it gives you are very hard to sort of interpret because, you know, a global wet bulb in the 80s [Fahrenheit] is when you start feeling a lot of discomfort, and around 90 [degrees F] is just oppressive. But when you think of temperature, you don’t think, “Eighty-six [degrees F]—oh, that’s super dangerous.” So it’s, you know, it’s a little hard to give that information to people and have them be able to sort of reconcile that with temperatures.

From, like, a personal standpoint, when I’m trying to figure out what it’s going to be like, I look at the temperature, and I look at the dew point. And so the dew point is basically what the temperature would have to cool to to get 100 percent relative humidity—so to the point where you couldn’t have any more water vapor in the atmosphere. You don’t actually have to understand that to look at a dew point measure and know what it means. So if you have a dew point that is generally under, say, 55, you’re pretty comfortable. It’s really nice.

If you’re in the 55 to 65, that’s when you start to feel sticky, a little “ugh.” And then anything over 65 is when it starts getting just—like we used to joke when I grew up in Atlanta—like, you feel like you’re swimming through the air.

Feltman: Absolutely.

Thompson: And, you know, if we get up into the 70s, then it’s just, you know, really oppressive, and I think even in some recent heat waves, there was a dew point of 80, which I can’t even imagine.

Feltman: Wow, yeah—you’re literally swimming at that point.

Thompson: Yes, yes.

Feltman: You mentioned a couple of groups that are at particularly high risk—you know, children, the elderly. Do you think that the messaging about those risks has been really successful in, in reaching those people?

Thompson: From what I’ve heard from experts that study risk communication in heat health, people understand that, that there is risk to those groups when it is very hot out, but even in focus groups that were done all over the country, with people who were 65 and over, you know, they knew that that sort of age category was a risk, but when asked if they personally saw themselves at risk, they would often answer “no” because, you know, they still felt healthy.

They were like, “I’m still independent. I am able to, to do all these things.” So they didn’t—you know, they’re like, “I’m not frail.” And so understanding that that’s not necessarily the indicator of, of risk. And there was a lot of sort of not understanding that, say, heat index—they knew, you know, that that’s supposed to tell you information about how dangerous it is, but they didn’t know that heat index is temperature with humidity factored in and that when you factor in humidity, that really changes some of the risk with heat.

Thompson: And so, you know, if you don’t know that that’s what that measure means, you may not know what to do with the information that the heat index is 105 [degrees F] other than like, “Okay, I know it’s hot.” We think in a lot of cases that we’re giving people the information they need, but it’s not necessarily being relayed in the most useful way.

Feltman: What are precautions that people can take to stay safe when it is this hot out while we wait for governments to get rid of fossil fuels and cut our carbon?

Thompson: Yeah. I mean, that would—you know, the—one of the messages that I always try to give is that, you know, how hot those future summers are are very much in our hands.

Feltman: Right.

Thompson: The more we do to rein in greenhouse gas emissions, the less hot they will be. And there is a great study that showed, you know, for kids born around 2020, with current pledges from governments to reduce emissions, they will experience about seven times as many heat waves over the course of their life as people born in 1960.

If we keep emissions to where we limit warming to about 1.5 degrees Celsius [2.7 degrees F] above preindustrial, that will only be four times as many. So obviously that’s still more, but that’s still—you know, I, I would take that difference. But in terms of sort of today...[full transcript]

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