气候变化并非导致哈维飓风的原因，但却让它变得更严重

雨仍然在下，休斯顿的生灵仍然面临着危险。现在讨论气候变化在得克萨斯州这次可怕灾害中所扮演的角色还为时尚早。不过此刻——当全球目光汇聚，而美国总统和国会中的大多数人还否认全球变暖这个事实的此刻——我们认为是时候重申现代科学告诉我们的情况了。

我们发现随着气候变暖，全球各地强降水事件的频率和强度都有所增加，美国也不例外——包括休斯顿，近几十年来，这里反复遭遇洪水，其级别在历史上都属罕见。气候科学告诉我们，这是大气变暖，水蒸气含量增加所致。水蒸气增加是气候变暖最易于理解、证据最多的方面。这一点通过观察已经证实，也符合我们对大气的物理认知，任何可信度高的计算机模型给出的预测也印证了它。

与其他暴雨活动类似，热带气旋可以高效地将水蒸气转化为水。尽管热带气旋与全球变暖的关系还有很多不确定之处，但它们会产生更多雨，可能是现代科学在这个话题上最有信心的推测。唯一的争议在于，温度每上升一度，雨量会增加多少，这种效应又在如今观测到的风暴中起到了多大影响。

（也许更确定的事情是随着海平面上升，风暴潮引发的洪水也增加了，因为水的起始点更高。风暴潮不是这次威胁得克萨斯州生灵的洪水的元凶，但如果哈维飓风的登陆点不同，可能就会是这种情况。）

如今，科学家有能力对特殊事件进行归因研究，从而对温室气体排放对该事件的影响进行定量的表述，这次哈维飓风事件也是这样。研究的成果好坏，根据事件的类型而有所不同。飓风属于较难的一类，不过归因研究已经足够给出一些有用的信息。我们认为针对哈维飓风会有一些研究。在研究完成之前，我们不应对结果进行过于详细的推断。

然而，根据对其他降水事件的研究——包括去年在路易斯安那州发生的洪灾——我们有理由推测，研究的结果会是气候变暖对哈维飓风导致的降雨产生一定的影响，而且，这个影响未必微不足道。归因研究——至少是优秀的归因研究——绝不会说全球变暖“导致”了某一事件，绝不。另一些因素会是直接原因，但是变暖改变了背景条件，让事件的规模多多少少出现了改变。

在这次事件中，造成灾难的最大原因之一，在于哈维的缓慢移动。这是由于特殊的气候条件导致的，风暴陷入了两个高气压系统中，而高速气流在北边太远的地方，无法把风暴拉出去。换句话说，哈维在同一地区待了好几天时间，倾泻了大量雨水，而不是像普通的风暴一样走出很长一段距离并把这些雨水沿途洒下。

科学文献中有一些想法认为，全球变暖可能会让发生这种情况的可能性增加。然而，它们仍然只是猜测，并未得到科学家的广泛认同。我们默认的假定是这些因素都是自然随机的事件，气候变暖所扮演的角色，只是带来了更多的水蒸气。

综上所述，考虑到这个拥有650万人口的大都市位于泛滥平原上，支流、小溪、江河纵横其中，即便人类对气候没有任何影响，在同样的气象状况下，这依旧会是一场巨大的灾难。不过气候变暖带来的额外降水，则加剧了灾害带来的人员伤亡和经济损失。当人们讨论采取措施减少碳排放以及在开发中更多考虑洪水风险的代价是否过于昂贵时，需要衡量的是如果什么都不做，代价又是什么。任何因为气候变暖带来的额外降水量都应该在计算中予以考虑。（财富中文网）

作者苏珊娜·卡玛戈是哥伦比亚大学海洋与气候物理学教授。亚当·索贝尔是哥伦比亚大学应用物理学与应用数学教授。

译者：严匡正

The rain is still falling, and lives are still very much at risk in Houston. It’s early to be talking about the role of climate change in the awful events unfolding in Texas. But at this moment—when the world is watching and the U.S. president and majority party in Congress are in deep denial about the reality of global warming—we think it is appropriate to restate what current science tells us.

As the climate warms, we have seen heavy precipitation events increasing in intensity and frequency around the world, including the U.S.—and including Houston itself, where recent decades have seen repeated floods with magnitudes that would have historically been rare. Climate science tells us to expect this as a consequence of the increased water vapor content of a warmer atmosphere. The increase in water vapor, in turn, is one of the most well understood and verified aspects of warming. It is in the observations, and is consistent with our physical understanding of the atmosphere, as well as with the predictions of virtually every plausible computer model.

Tropical cyclones are similar to other truly heavy rain events in that they are very efficient in turning that water vapor into rain. While there is much uncertainty about how tropical cyclones should change with global warming, that they should produce more rain is perhaps one of the most confident predictions of current science on this topic. The only debate is how much heavier the rains should get with each degree of warming, and to what extent this effect is already present in observed storms today.

(Perhaps the only greater certainty is that with higher sea level, storm surge-induced flooding is increasing due to the water’s higher starting point. Storm surge is not the cause of the current life-threatening flooding in Texas, but it could have been if Harvey had made landfall in a different location.)

In the case of Harvey, as with other specific events, scientists now have the capability to perform attribution studies which make more quantitative statements about how greenhouse gas emissions influenced that event. How well such studies can be done differs from one event type to another. Hurricanes are among the more difficult ones, but attribution studies on them can be done well enough to produce useful information. We expect that such studies will be done on Harvey. We should not make detailed statements presupposing their outcomes before they are done.

Based on many other studies of precipitation events, however—including a recent one on the Louisiana floods of last year—it is reasonable to speculate that they will find that warming amplified Harvey's rainfall by a modest, but not necessarily insignificant, percentage. Attribution studies—at least good ones—will never say that global warming "caused" an event, full stop. Other factors are always the proximate causes. But the warming changes the background conditions in a way that can make an event more or less likely or change its amplitude.

In this case, one of the most important causes of the disaster is Harvey's slow motion. That in turn is a consequence of the specific weather situation, in which the storm is caught between two high-pressure systems and the jet stream is too far away to the north to pull it away. In other words, it has just been sitting and dumping its huge quantities of rain in the same area for days, rather than spreading it out over a long track as a typical storm would.

There are some ideas in the scientific literature that suggest that global warming may make this situation more probable. However, these ideas are still speculative and not widely agreed upon by scientists. Our default assumption is that these factors are natural, random accidents, and that the role of warming is just to add more water vapor to the situation.

All this said, given a metro area of 6.5 million people that basically sits in a paved floodplain, crossed by bayous, creeks, and rivers, and the same meteorological situation, we would be seeing a very major disaster even without any human influence on climate. But any extra rainfall due to warming just increases the human and financial toll of the disaster. When people argue that it is too expensive to take action to reduce carbon emissions or to enforce development practices that take flood risk into greater consideration, that needs to be measured against the cost of not doing anything. Any extra inches of rain due to warming should be part of that calculation.

Suzana Camargo is a professor of ocean and climate physics at Columbia University. Adam Sobel is a professor of applied physics and applied mathematics at Columbia University.