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兩位頂尖的美國科學家在一篇旁徵博引的論文中指出，毫無疑問的，人類的活動的確對全球的氣候持續造成可預測得的衝擊。
卡爾（Thomas Karl）是國家海洋暨大氣總署（NOAA）國家氣候資料中心的主任，崔伯斯（Kevin
Trenberth）則是國家大氣研究中心氣象分析部門的主任，兩人共同指出，工業排放是過去50年來造成氣候變遷的主要因素，且遠超過自然本身的力量。
他們的文章刊登在2003年12月3日發行的《科學》期刊，為該期「地球現況」系列專題中的一篇。
他們在文章中指出：「無庸置疑，大氣的組成因為人類活動而改變，而目前人類對於全球氣候最大的影像在於製造溫室氣體。可能的影響包括更頻繁的熱浪、乾旱與大量的降雨與其他相關的影響，另外包括森林大火、高溫、植被改變與海平面上升等，都，則是區域情況而定。」
卡爾和崔伯斯利用氣象觀測資料以及多重電腦模式，仔細描繪了地球未來數十年氣候可能出現的變化。
文章中引述的研究結果指出，從1990年到2100年間，由於人類對氣候的影響，全球氣溫有90%的機率會升高攝氏1.7～4.9度（華氏的3.1～8.9度）。
這樣的增溫對於社會與環境都有廣泛的影響，包括冰河與格陵蘭的的冰原持續消融，將導致世界各地的海岸地區被淹沒。作者們的推論資料來自氣象學家的電腦模擬實驗、氣候變化觀測資料，以及過去百年來的氣象變化資料。
不過卡爾指出，對於氣候變化以及變化的方式依然有不確定性。如果氣溫只升高攝氏1.7度，那麼預期中的變化將會比較小；如果升高了攝氏4.9度，那麼就會帶來巨大的衝擊，其結果可能難以預見。
從工業時代以來，大氣中的二氧化碳濃度已經提高了31%，由原來的280ppmv到目前的370ppmv（編按：1ppmv係指同溫同壓下，二氧化碳體積佔空氣體積的比例為百萬分之一，也就是二氧化碳與空氣分子數目比為百萬分之一）。
作者指出，其他的人類活動，包括硫化物與煤灰顆粒排放，以及都市地區的發展，都對氣候造成顯著且更具區域性的衝擊。這類活動會可能會強化溫室氣體造成的大尺度暖化效應，也可能掩蓋住（但並非抵銷）大尺度的暖化效應。
卡爾和崔伯斯又指出，如果人類能夠成功的減少溫室氣體排放，而大氣中的二氧化碳濃度保持穩定，那麼未來數十年，全球平均氣溫估計只會升高攝氏0.5度。
這是因為溫室氣體雖會藉由循環作用從大氣中消散，但其過程是緩慢的。作者們說：「依據現在與將來預期會有的狀況，將來必定會有重大的氣候變化。」
如果目前溫室氣體的排放情形繼續下去，那麼地球將會出現過去一萬年來最快速的氣候變遷，這種變化可能會影響到目前的海洋循環以及氣候型態。
此外，有些自然現象也會加速暖化，例如積雪融化的結果，深色土壤與海洋表面會吸收更多的太陽輻射能量，進一步使得氣溫升高。
卡爾和崔伯斯說，需要更多的研究才能確定雲量增加對溫度的衝擊，以及大氣變化對聖嬰現象的影響。聖嬰現象是是太平洋海水週期性的變化，會牽動全球氣候模式。
作者們呼籲使用多重電腦模示研究，以確定氣候與天氣中各複雜層面的變化，電腦模示必須能整合地球天氣系統中的所有化學、物理與生物因子。因此，有必要進行國際合作、建立全球的氣候觀測系統，以收集與分析資料。
作者們的結論是：「氣候變遷是不折不扣的全球議題，也可能是人類未來最大的挑戰之一。假若國際間沒有更多、更密切的合作與行動方案，則氣候變遷的問題將可能無法順利克服。」
全文詳見：http://www.ens-newswire.com/ens/dec2003/2003-12-05-09.asp#anchor2
版權歸屬Environment News Service (ENS)，台灣環境資訊協會(龐中培
譯，劉彥蘭 蔡麗伶審校)
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BOULDER, Colorado, December 5, 2003 (ENS) - There is no longer any doubt
that human activities are having measurable, and increasing, impacts on
the global climate, concludes an extensive review of climate research by
two top U.S. scientists.
Thomas Karl, director of NOAA's National Climatic Data
Center, and Kevin Trenberth, head of the Climate Analysis Section at the
National Center for Atmospheric Research (NCAR), state that industrial
emissions have been the dominant influence on climate change for the
past 50 years, overwhelming natural forces.
Their study appears today in "Science" as part of the
journal's "State of the Planet" series.
"There is no doubt that the composition of the
atmosphere is changing because of human activities, and today greenhouse
gases are the largest human influence on global climate," they write.
"The likely result is more frequent heat waves, droughts, extreme
precipitation events, and related impacts, e.g., wildfires, heat stress,
vegetation changes, and sea level rise which will be regionally
dependent."
Karl and Trenberth cite atmospheric observations and
multiple computer models to paint a detailed picture of climate changes
likely to be felt on Earth in coming decades.
The article cites research indicating that, between 1990
and 2100, there is a 90 percent probability that global temperatures
will rise by 3.1 to 8.9 degrees Fahrenheit (1.7 to 4.9 degrees Celsius),
because of human influences on climate.
Such warming would have widespread impacts on society
and the environment, including continued melting of glaciers and the
great ice sheets of Greenland, inundating the world's coasts. The
authors base their estimate on computer model experiments by climate
scientists, observations of atmospheric changes, and recorded climate
changes over the past century.
But there is still uncertainty in understanding the
global climate and how it will change, says Karl. If temperatures rise
1.7 degrees, the expected changes would be relatively small, whereas a
4.9 degree increase could bring drastic impacts, some of which may be
unforeseen.
Carbon dioxide levels in the atmosphere have risen by 31
percent since preindustrial times, from 280 parts per million by volume
(ppmv) to over 370 ppmv today.
Other human activities, such as emissions of sulfate and
soot particles and the development of urban areas, have significant but
more localized climate impacts. Such activities may enhance or mask the
larger-scale warming from greenhouse gases, but not offset it, according
to the authors.
If societies could successfully cut emissions and
stabilize carbon dioxide levels in the atmosphere, temperatures would
still increase by an estimated 0.5 degree C (0.9 degree F) over a period
of decades, Karl and Trenberth warn.
This is because greenhouse gases are slow to cycle out
of the atmosphere. "Given what has happened to date and is projected in
the future, significant further climate change is guaranteed," the
authors state.
If current emissions continue, the world would face the
fastest rate of climate change in at least the last 10,000 years. This
could potentially alter ocean current circulations and existing climate
patterns.
Moreover, certain natural processes would tend to
accelerate the warming. For example, as snow cover melts away, the
darker land and water surface would absorb more solar radiation, further
increasing temperatures.
Karl and Trenberth say more research is needed to
determine the temperature impacts of increased cloud cover or how
changes in the atmosphere will influence El Nino, the periodic warming
of Pacific Ocean waters that affects weather patterns throughout much of
the world.
The authors call for multiple computer model studies to
address the complex aspects of weather and climate. The models must be
able to integrate all components of Earth's climate system - physical,
chemical, and biological. This, in turn, will require international
cooperation and the establishment of a global climate monitoring system
to collect and analyze data.
"Climate change is truly a global issue, one that may
prove to be humanity's greatest challenge," the authors conclude. "It is
very unlikely to be adequately addressed without greatly improved
international cooperation and action."
http://www.ens-newswire.com/ens/dec2003/2003-12-05-09.asp#anchor2
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