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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.
它們正在發(fā)生變化，因為在過去的30年里，北極海冰的面積在夏季減少了30%，而且減少的速度還在加快。
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,
北極區(qū)也是氣候變化的推動者，冰的白度意味著它將太陽光反射回太空，從而給地球降溫，
whereas the darkness of open water means it absorbs that light.
而無冰水面的暗度意味著它吸收了這個光。
The less of the former that is happening, and the more of the latter, the faster global temperatures will rise.
前一種情況發(fā)生得越少，后一種情況就越常見，全球氣溫上升的速度就會越快。
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.
首先從冰開始。目前這主要是由衛(wèi)星監(jiān)測的。從太空測量北極冰的范圍很容易。
Measuring its thickness is trickier. From orbit, this is done by a mixture of radar and laser beam.
測量冰的厚度則更加棘手。在軌道上，這是由雷達(dá)和激光束共同完成的。
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.
美國飛船Icesat 2提供激光高度計數(shù)據(jù)，該數(shù)據(jù)記錄了覆蓋在冰層的雪頂?shù)暮０螛?biāo)高。
Cryosat 2, a European one, uses radar to penetrate the snow and measure the height of the top of the ice itself.
歐洲飛船Cryosat 2使用雷達(dá)穿透積雪并測量冰頂部本身的高度。
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,
將阿基米德的浮體原理應(yīng)用于冰雪混合中，然后減去雪的厚度，
and subtracting the thickness of the snow. However, Julienne Stroeve of University College London, now safely returned from her leg of the mission,
就可以計算出某個地方冰的厚度。但英國倫敦大學(xué)學(xué)院的Julienne Stroeve（現(xiàn)在她已經(jīng)從任務(wù)中安全返回）
believes that the data collected by these two satellites may be inaccurate, leading to an overestimation of the ice's thickness.
認(rèn)為這兩個衛(wèi)星所收集的數(shù)據(jù)可能不準(zhǔn)確，從而導(dǎo)致對冰厚度評估過高。
When all is working perfectly, the return signal for Cryosat 2 comes exactly from the boundary between the ice and any overlying snow.
當(dāng)一切正常運行時，Cryosat 2的返回信號就會準(zhǔn)確地從冰和覆蓋積雪的邊界傳回來。
Dr Stroeve thinks, though, that this is not always what happens.
但Stroeve博士認(rèn)為情況并非總是如此。
Variables such as layering within the snow, along with its temperature and salinity,
雪中的分層、溫度和鹽度等變量，
might affect the returning radar signal by changing the snow's structure and density.
可能會改變雪的結(jié)構(gòu)和密度，從而影響返回的雷達(dá)信號。
This could cause the signal to be reflected from inside the snow layer, rather than from the boundary where it meets the ice.
這可能會導(dǎo)致信號從雪層內(nèi)部反射，而不是從它與冰接觸的邊界反射。
If that were happening, it would create the illusion that the ice beneath the snow is thicker than is actually the case.
如果這種情況真的發(fā)生了，就會產(chǎn)生一種錯覺，即雪下的冰比實際情況要厚。
To investigate this possibility Dr Stroeve took a purpose-built radar on board Polarstern.
為了調(diào)查這種可能性，Stroeve博士將一個特制的雷達(dá)帶上了極星號。
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.
每周，她和一名同事將這個170公斤重的儀器安裝在一個雪橇上，拖到一個新的地點，以采樣不同的雪況。
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.
當(dāng)他們拖著雪橇的時候，他們在衛(wèi)星使用的頻段向雪中發(fā)射雷達(dá)脈沖，并測量反向散射的數(shù)量。
The deflection of the signals in this backscatter gives a picture of how particular snow conditions might be changing the way the satellite's radar is returned.
散射中信號的偏差說明了特定的雪況可能會如何改變衛(wèi)星雷達(dá)返回的方式。
Dr Stroeve's radar died on January 31st— one of many of the expedition's machines that fell victim to the Arctic winter.
Stroeve博士的雷達(dá)損壞于1月31日——這是犧牲于北極冬季的眾多探險設(shè)備中的一個。
But by the time that had happened she had managed to gather a fairly good set of data.
但當(dāng)這一切發(fā)生時，她已經(jīng)設(shè)法收集到了一組相當(dāng)好的數(shù)據(jù)。
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.
她的結(jié)論是反射有時確實是來自雪和冰的接觸面，應(yīng)該如此，但不常發(fā)生。
The discrepancy is important. Her measurements already show that the ice is "definitely thinner than the satellites suggested".
這種差異很重要。她的測量結(jié)果已經(jīng)表明，冰層“肯定比衛(wèi)星預(yù)測的要薄”。
She has yet to analyse the data fully, but preliminary investigation indicates that both snow depth and temperature influence backscatter.
她還沒有充分分析數(shù)據(jù)，但初步調(diào)查表明，雪的深度和溫度都會影響后向散射。
It therefore looks likely that the amount of Arctic sea ice around has been overestimated.
因此，周圍北極海冰的數(shù)量似乎被高估了。
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