And changing they are, for the extent of the Arctic sea ice in summer has declined by 30% in the past 30 years, and that loss is accelerating.The Arctic is also a driver of climate change, though, because the whiteness of ice means it reflects sunlight back into space, thus cooling Earth,whereas the darkness of open water means it absorbs that light.The less of the former that is happening, and the more of the latter, the faster global temperatures will rise.Start, then, with the ice. At the moment this is monitored mainly by satellite. Measuring the extent of the Arctic's ice from space is easy.Measuring its thickness is trickier. From orbit, this is done by a mixture of radar and laser beam.Icesat 2, an American craft, provides laser-altimeter data that record the height above sea level of the top of the snow that overlies the ice.

译文
它们正在发生变化，因为在过去的30年里，北极海冰的面积在夏季减少了30%，而且减少的速度还在加快。北极区也是气候变化的推动者，冰的白度意味着它将太阳光反射回太空，从而给地球降温，而无冰水面的暗度意味着它吸收了这个光。前一种情况发生得越少，后一种情况就越常见，全球气温上升的速度就会越快。首先从冰开始。目前这主要是由卫星监测的。从太空测量北极冰的范围很容易。测量冰的厚度则更加棘手。在轨道上，这是由雷达和激光束共同完成的。美国飞船Icesat 2提供激光高度计数据，该数据记录了覆盖在冰层的雪顶的海拔标高。

Cryosat 2, a European one, uses radar to penetrate the snow and measure the height of the top of the ice itself.The thickness of the ice in a particular place can then be calculated by applying Archimedes' principle of floating bodies to the mixture of ice and snow,and subtracting the thickness of the snow. However, Julienne Stroeve of University College London, now safely returned from her leg of the mission,believes that the data collected by these two satellites may be inaccurate, leading to an overestimation of the ice's thickness.When all is working perfectly, the return signal for Cryosat 2 comes exactly from the boundary between the ice and any overlying snow.Dr Stroeve thinks, though, that this is not always what happens.Dr Stroeve thinks, though, that this is not always what happens.Variables such as layering within the snow, along with its temperature and salinity,might affect the returning radar signal by changing the snow's structure and density.

译文
欧洲飞船Cryosat 2使用雷达穿透积雪并测量冰顶部本身的高度。将阿基米德的浮体原理应用于冰雪混合中，然后减去雪的厚度，就可以计算出某个地方冰的厚度。但英国伦敦大学学院的Julienne Stroeve（现在她已经从任务中安全返回）认为这两个卫星所收集的数据可能不准确，从而导致对冰厚度评估过高。当一切正常运行时，Cryosat 2的返回信号就会准确地从冰和覆盖积雪的边界传回来。但Stroeve博士认为情况并非总是如此。但Stroeve博士认为情况并非总是如此。雪中的分层、温度和盐度等变量，可能会改变雪的结构和密度，从而影响返回的雷达信号。

This could cause the signal to be reflected from inside the snow layer, rather than from the boundary where it meets the ice.If that were happening, it would create the illusion that the ice beneath the snow is thicker than is actually the case.To investigate this possibility Dr Stroeve took a purpose-built radar on board Polarstern.Each week, she and a colleague mounted this 170kg instrument on a sled and dragged it to a new site, to sample different snow conditions.As they towed it, they sent radar pulses on the frequency bands used by the satellites downwards into the snow and measured the amount of backscatter.

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这可能会导致信号从雪层内部反射，而不是从它与冰接触的边界反射。如果这种情况真的发生了，就会产生一种错觉，即雪下的冰比实际情况要厚。为了调查这种可能性，Stroeve博士将一个特制的雷达带上了极星号。每周，她和一名同事将这个170公斤重的仪器安装在一个雪橇上，拖到一个新的地点，以采样不同的雪况。当他们拖着雪橇的时候，他们在卫星使用的频段向雪中发射雷达脉冲，并测量反向散射的数量。

The deflection of the signals in this backscatter gives a picture of howparticular snow conditions might be changing the way the satellite's radar is returned.Dr Stroeve's radar died on January 31st— one of many of the expedition's machines that fell victim to the Arctic winter.But by the time that had happened she had managed to gather a fairly good set of data.Her conclusion is that the reflection does indeed sometimes come from the interface between snow and ice, as it is supposed to. But not always.The discrepancy is important. Her measurements already show that the ice is "definitely thinner than the satellites suggested".She has yet to analyse the data fully, but preliminary investigation indicates that both snow depth and temperature influence backscatter.It therefore looks likely that the amount of Arctic sea ice around has been overestimated.

译文
散射中信号的偏差说明了特定的雪况可能会如何改变卫星雷达返回的方式。Stroeve博士的雷达损坏于1月31日——这是牺牲于北极冬季的众多探险设备中的一个。但当这一切发生时，她已经设法收集到了一组相当好的数据。她的结论是反射有时确实是来自雪和冰的接触面，应该如此，但不常发生。这种差异很重要。她的测量结果已经表明，冰层“肯定比卫星预测的要薄”。她还没有充分分析数据，但初步调查表明，雪的深度和温度都会影响后向散射。因此，周围北极海冰的数量似乎被高估了。

注释
1.illusion 错觉

But this premise turns out to be an illusion.

但是这一假设原来只是一种错觉。

2.accelerate 加快

Catalysts are something used to accelerate chemical action.

催化剂用来加快化学反应。

3.tricky 棘手的

Shifting winds have made the operation trickier on already difficult terrain.

变化的风向再加上复杂的地形，使得灭火工作更加棘手。

4.overestimate 评价过高

In fact, we frequently overestimate our own abilities.

事实上，我们经常高估自己的能力。