There are many units by which to measure the impact of climate change: degrees of increasing temperature, feet of rising sea level, dollars needed to adapt to a warming world. But a group of scientists in California have put forth an intriguing new unit of measurement: kilometers per year.

    Writing in a paper published Wednesday in Nature, scientists describe what they call the velocity of climate change, or more specifically, the speed of Earth's shifting climatic zones. As global temperature rises over the next century, the scientists argue, Earth's habitable climatic zones will start moving too, generally away from the Equator and toward the poles. That means many species of plants and animals will also have to move in order to survive. Whether or not they do will depend on several factors, but two of the most important are how fast a species can adjust its habitat range, and how quickly that range is moving out from under it.

    Until now, ecologists have mostly focused on these factors as they affect individual species, but the new paper takes a more global view. By combining temperature projections on a very fine scale with global topographic maps, researchers have predicted change not for specific species, but for the climatic zones they need to keep up with.

    Indeed, because global temperature is rising now, ecosystems are already on the move. "Once you explain it to people, it makes intuitive sense," says co-author David Ackerly, a University of California, Berkeley, biologist. "We know what it's like to drive north to escape the heat. It's concrete, rather than the abstractness of rising average temperatures."

    More than intuitive, this new index could also prove very useful, especially to conservationists who work to keep species from extinction. While the average velocity of climate change may be a bit less than a half-kilometer per year worldwide, according to the paper, it can be significantly faster or slower depending on the local topography. In deserts and other flat areas, such as the Amazon basin, climatic zones will move faster, while hilly or mountainous terrain will slow things up. "In the Northern Hemisphere, for example," explains lead author Scott Loarie, "north-facing slopes tend to be cooler and wetter than south-facing slopes."

    In short, opposite sides of a mountain may have different climates, even though they're close to each other. In areas with varied terrain including lots of hills, therefore, hospitable conditions might be available relatively nearby. "That was the unexpected message," says Loarie, an ecologist at the Carnegie Institution for Science at Stanford University. "There's lots of buffering capacity in heterogeneous landscapes."

    According to the velocity maps that Loarie and his colleagues put together, only 8% of the world's national parks and other preserves will retain their current climate over the next century, compounding the problem of how to keep species from going extinct. One way to do that is simply to move them. But that's not only extraordinarily difficult, it can also backfire - just ask anyone in the southeastern U.S. about the inexorable advance of the imported invasive species the kudzu vine. "For some species on the brink of extinction, physically moving them might be our only option," says Loarie, "but setting aside connected, heterogeneous landscapes that allow natural movement will almost certainly be an better use of conservation dollars."

    He and the other co-authors, including scientists at Climate Central in Palo Alto and the California Academy of Sciences in San Francisco, emphasize that their velocity maps are oversimplifications - at least so far. For one thing, they do not account for the unique characteristics of various species within a given ecosystem. Some species may have more tolerance for climate changes than others, and may not need to move as quickly; some species may be intolerant of change but unable to move. Other species may be sensitive to changes in rainfall, while still others responsive only to temperature - and changes in these weather patterns may not happen at the same rate. "The complexity is daunting," says Ackerly.

    Nevertheless, while the climate-velocity concept is still crude, it's promising enough that Ackerly is collaborating with an organization called the Bay Area Open Space Council on habitat conservation strategies in central California. The new research informs one of the key challenges conservationists face: having only limited funds to buy up land, and, thus, having to spend wisely. "What we bring," says Ackerly, "is the ability to think about how topography might affect those decisions."

    用以衡量氣候變化影響的單位有很多：氣溫上升的度數，海平面上升的尺數，為適應這個在變暖的世界所需支付的金錢數。然而在美國加州，一些科學家們新發現了一個有趣的衡量單位：千米數每年。

    在周三版《自然》雜志上的一篇文章中，科學家描述了他們所謂的氣候變化速率，更確切地說，是地球變更其氣候帶的速度。科學家表示，在下一世紀中，隨著全球氣溫不斷上升，地球上適宜居住的氣候帶也會有所變動，逐漸遠離赤道向兩極發展。這就意味著許多動植物物種若要繼續存活就必須遷移。它們是否能夠存活與諸多因素密切相關，其中最重要的兩點是：該物種調整其棲息地范圍的速度，以及該范圍自身變化的速度。

    在這以前，生態學家大多關注氣候變化的各種因素對個體物種的影響，但是這篇文章更加綜合地從整體的視角看問題。通過將氣溫變化預測很好地和全球地形圖相結合，研究人員預言變化不會是個別物種的，氣候帶也將產生變化，這也是他們需要跟進的。

    事實上，由于全球氣溫正在不斷上升，生態系統早已經開始變化了。"一旦你向人們解釋這個現象，它就有了直觀的意義。"合著者戴維·阿克利，來自加州柏克萊大學的一位生物學家說道，"我們知道向北發展以躲避氣溫的上升是怎么一回事。它是具體的，而非抽象的平均氣溫上升。"

    除了直觀以外，這項新的指數也被證實是非常有用的，尤其是對那些致力于保護物種免于滅絕的環保主義者們來說。世界范圍內氣候變化所引起的氣候帶平均變化速率是略小于0.5千米每年，然而文章還指出，由于各地地形不同，該數字很可能會有較大幅度的浮動。在沙漠地區以及其他平原地區，如亞馬遜盆地，氣候帶的變化會更快，相對而言，山區、丘陵地區比較緩慢。"比方說，在北半球，"主筆斯科特·勞瑞解釋到，"朝北的山坡往往比朝南的陰涼潮濕。"

    簡言之，即便彼此靠得很近，一座山峰的兩側也可能會有不同的氣候。因此，地形多變的地區，包括丘陵地帶，在相對其比較近的地方仍然可能找到宜人的環境。斯坦福大學卡耐基科學院的生態學家勞瑞說："我們沒有預料到是。在這多樣化的環境中其實還有很多的緩沖能力存在。"

    勞瑞和他的同事們繪制的變化速率地圖顯示，世界上僅有8%的國家公園以及其他自然保護區能在下個世紀保持其現有的氣候，這也加劇了如何防止物種滅絕這一問題的嚴重性。解決方案之一就是遷移那些面臨瀕危的物種。然而那樣做不僅異乎尋常地困難，而且可能產生事與愿違的結果--比如美國東南部人盡皆知的葛藤，該植物在被引進后攻擊性地大肆蔓延。"對一些瀕臨滅絕邊緣的物種進行物理遷移可能是我們唯一的選擇，"勞瑞說，"但是那些允許自然遷移的環境多樣性環境相互連接，如果撇開這些，那幾乎肯定是節省資金的更好的方法。"

    他以及其他的合著者們，包括帕洛阿爾托氣候研究中心和舊金山加利福尼亞科學院的科學家們，他們強調指出，該變化速率圖尚有些過度單純化，至少目前是這樣的。首先，他們沒有對給定生態系統中各物種的自身特征做出說明。有一些物種可能比其他的物種對氣候變化具有更強的適應性，就不需要迅速轉移；還有一些物種雖然適應性也強，但可能沒法遷移。一些物種可能在雨季會對變化非常敏感，而另一些只會對溫度敏感--并且氣候的變化形式很可能不會總是以相同的比率發生。阿克利說："其復雜性令人望而生畏。"

    盡管氣候變化速率的概念尚未成熟，但是目前阿克利正于一個叫做海灣地區綠地委員會的合作，他們致力于研究加利福尼亞中部地區居住地的環境保護戰略，這是非常振奮人心的。新的研究指出環保主義者面臨的主要挑戰在于：能用來盡量收購土地的資金非常有限，因此必須理智地花錢。阿克利說："我們能夠提供的是有能力去思考地形會如何影響到這些決策。"