The greenhouse effect is accelerating the global water cycle almost twice the rate predicted by climate change models, say researchers.
Oceanographer Dr Susan Wijffels of the CSIRO and colleagues report their findings today in the journal Science.
"The models predict a 4 to 5 per cent amplification of the global water cycle per degree of warming, instead of 8 per cent, " says Wijffels.
"It's a significant underestimation. That's a cause for concern."
The transport of water through the atmosphere from the mid-latitudes of Earth to the poles and tropics is called the global water cycle.
How much water evaporates in dry areas and falls as rain in wet areas is vital to society, says Wijffels.
Global climate models predict that as the globe warms, this will heat the lower atmosphere and enable it to hold more moisture, thus speeding up the global water cycle.
"It's like the rich get richer scenario where the wet places will get wetter and the dry places will get a lot drier because the conveyor belt is speeding up between those two places, " says Wijffel. Verification difficult
But attempts to verify the predictions of climate change models have been fraught, with actual measurements of rainfall giving a mixed and confusing picture.
"Rain is most horrible thing to measure because it happens so locally and is so spotty in space and time, " says Wijffel.
To make matters worse, measurements have been short-term and are usually taken on land, whereas 71 per cent of the Earth's surface is covered by oceans.
Instead Wijffels and colleagues have looked to the oceans to measure changes in the global water cycle over the past 50 years. Salinity pattern changes
The more rain that falls in a particular part of the ocean, the more fresh water dilutes out salinity. The more evaporation there is, the higher the ocean's salinity.
Wijffels and colleagues have found that the difference between the saltier and fresher areas have become more marked in the past 50 years, indicating that more water is being pumped through the global cycle.
"We've been able to pick up a very strong and clear fingerprint of the accelerating water cycle in the ocean salinity field, " says Wijffels.
She and colleagues have found the same fingerprint across the globe including in the North and South Atlantic, the North and South Pacific and the South Indian ocean basins.
"The fact that we see it independently across the ocean basins gives us some confidence that it's a real phenomenon and we're not just seeing a whole bunch of statistical noise, " says Wijffels. Underestimating models
All climate change models show a relationship between the changing salinity patterns and the water cycle speed.
The researchers used this to calculate that the water cycle accelerates by 8 per cent per degree of surface warming.
But, says Wijffels, this rate of acceleration is only reflected in models that include a high degree of warming.
On the whole, the models underestimate the acceleration at 4 to 5 per cent per degree of warming, she says.
Wijffels says the new data will be combined with other observations to help improve global climate change models.
She emphasises that the findings have implications for long-term average rainfall trends, which should not be confused with shorter-term trends that lead to phenomena such as La Ni?a.
"Variability will always be there but the question is whether, on average, a place will get drier or wetter, " says Wijffels.
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greenhouse ['ɡri:nhaus]n. 温室
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statistical [st?'tistik?l]adj. 统计的；统计学的
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conveyor [k?n'vei?]n. 输送机，[机] 传送机；传送带；运送者，传...
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implication [,impli'kei??n]n. 含义；暗示；牵连，卷入
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scenario [si'nɑ:ri?u, -'n?-, -'nε?-]n. 方案；情节；剧本
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calculate ['k?lkjuleit]vi. 计算；以为；作打算; vt. 计算；预测...
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evaporate [i'v?p?reit]vt. 使……蒸发；使……脱水；使……消失; v...
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accelerate [?k'sel?reit]vt. 使……加快；使……增速; vi. 加速；...
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prediction [pri'dik??n]n. 预报；预言
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latitude ['l?titju:d]n. 纬度；界限；活动范围
气候变化加速水循环
据研究人员称温室效应正加速全球水循环，它的速度几乎是气候变化模型预测速率的两倍。
来自联邦科工研究组织的海洋学家Susan Wijffels 博士和他的同事们今天在科学杂志上发表了他们的研究成果。
Wijffels博士说：“这个模型预测每升温一度全球水循环增幅为4%至5%，而不是实际上的8%。”
“这是严重的低估，是值得关注的。”
水通过大气从地球的中纬度运送到两极和热带的过程称之为全球水循环。
Wijffels博士表示，有多少水在干旱地带蒸发在潮湿地区形成下雨对于社会来说是重要的。
全球气候模型预测，随着全球变暖，这将加热低层大气并使它能够容纳更多的水分，从而加速全球水循环。
Wijffel博士解释道：“这现象就像富者更富，潮湿地区会更潮湿，干旱地方会更干燥，因为两地之间的运输带正在加快。”要验证此观点很难。
但是为了证明气候变化模型的预测，用了对于降雨的实际测量，给出了一幅混合的令人困惑的图片。
Wijffel博士说：“降雨是最难测量的，因为它是局部性的并且在时间和空间上层次不齐。”
更糟的是，测量是短期行为并且通常在陆地上测量，而海洋覆盖了地球表面的71%。
因此 Wijffels博士及其同事已经在海洋上测量在过去的50年中全球水循环的变化及盐度分布的变化。
在海洋中的某一区域，雨下的越多，海水的盐度就被稀释的越低。蒸发的越多，海水的盐度就越高。
Wijffels及其同事发现咸水和淡水区域的差异在这50年中变得更为明显，这表示有更多的水正在进入水循环。
Wijffels说：“我们已经找到了在海洋盐度地区加速水循环的强有力的证据。”
她和她的同事在全球包括南北大西洋，南北太平洋及南印度盆地中找到了相同的证据。
Wijffels说：“我们独立地在海洋盆地中发现的这些给了我们一些信心，这是一个真实的现象，我们不仅仅是得到了一大堆数据的噪声。”我们低估了模型。
所有的气候变化模型都显示了盐度分布变化和水循环加速之间的关系。
研究者运用这个模型来计算出地表每变暖一度水循环加快8%。
但是，Wijffels说，这个加快的速度仅仅反映了模型，其中包括一种高度的变暖。
总而言之，她说，模型低估了加速的速率，约为变暖每度4%至5%的速率。
Wijffels说新数据将结合其他观测结果以帮助改进全球气候模型。
她强调这个结果暗示了长期平均降雨趋势，不应与短期现象造成的趋势混淆，例如拉尼娜现象。
Wijffels说：“变化永远都存在，但问题是平均来说一个地区是否会变得更干燥或更潮湿。”
标签：气候变化，海洋珊瑚礁，盐度，水，水供应
