美國「憂思科學家聯盟」(Union of Concerned Scientists, UCS)表示，最近美國密西西比河沿岸豪雨及罕見洪災的導因是人為因素造成的氣候變遷。
反聖嬰現象就是赤道太平洋海平線的低氣溫，這個現象造成的降雨及旱災，影響美國各地噴射氣流以及風暴的移動。
反聖嬰現象轉移了降雨量，導致德州與奧克拉荷馬州的極度乾旱，但也同時讓南密蘇里州以及俄亥俄河谷面臨極端降雨量。
此外，一連串自然因素的總和以及北極海的融冰，為美國中部大部分區域帶來了一個寒冷的冬天，導致美國中西部的北方降雪量增加。而現在正值春季，降雪融化流入密西西比河，同時伴隨著豪雨肆虐。
根據美國國家海洋暨大氣總署(NOAA)的國家氣候資料中心(National Climatic Data Center)的資料顯示，上述這些因素都讓2011年的4月成為美國史上降雨量排名第10的紀錄，國家氣候資料中心保存了可追溯至1894年的氣象紀錄。數個降雨注入密西西比河的州也經歷了他們史上降雨量最多的4月。
除此之外，根據科學家觀察，人類因素引起的氣候變遷持續讓墨西哥灣暖化。
氣溫上升導致從墨西哥灣表面蒸發的水量增加，也讓通過墨灣上方的空氣溫度上升，增加了這些空氣夾帶水蒸氣的能力。
同時，自然洋流的轉移也導致墨灣內以及墨灣上空的溫度增加。
接著，自然的大氣循環模式將夾帶水蒸氣經過墨西哥灣上方，到達北美。根據國家環境衛星數據資訊局（National Environmental Satellite, Data, and Information Service）的資料顯示，墨西哥灣的氣溫比4月末平均氣溫高了約華氏2度，而美國中西部的部分地區此時正經歷強烈豪雨。
環保科學家聯盟指出，強烈暴風雨的數量大增是因為大氣中濕氣上升的緣故，而濕氣上升，則與全球暖化息息相關。就在今年的2月和4月間，有五個州的降雨量在這三個月的期間都破了史上最高的紀錄，這五個州分別是肯塔基州、印第安納州、紐約州、俄亥俄州，以及賓夕法尼亞州。
此刻，密西西比河的河面最高點正緩慢南移。美國陸軍工兵團持續監測到水位最高點向河流的南方下移，並且持續在密西西比河以及支流進行防洪的措施，尤其提高對路易斯安納州南部區域的警戒。
紐奧良的河面預計將在未來兩周維持接近目前的水位，密西西比河水位預計將在5月27日下降到低於洪水位。
在路易斯安納州，目前125個水閘中沿著莫根薩洩洪道(Morganza Floodway)有16個是打開的，美國陸軍工兵團表示，從莫根薩分洪道所放出來的水在阿查法拉亞河盆地(Atchafalaya River Basin)移動得比預期的還要慢。
邦卡萊分洪道(Bonnet Carre Spillway)上的350個水閘中，有335個水閘都已經打開，以將密西西比河的水導入龐恰特雷恩湖（Lake Pontchartrain），這些水閘預計還會持續打開2至4週。
當洪水蔓延過阿查法拉亞河盆地區，大約3900位居民可能需要避難所，現在已有22個避難所可供備用，共可收容8200人。大部份的居民預計將會與朋友及家人待在一起。
路易斯安納州的34個縣已經宣告進入縣內的非常時期。
為了從摩根薩分洪道下游撤離，已經進行計劃與準備。位於聖蘭帝縣(St. LandryParish)外圍堤防外的克羅茲泉以及梅爾維爾(Krotz Springs and Melville)社區已經進行強制性的撤離。聖蘭帝縣地勢較低的區域也已開始自願性的撤離。
聖馬汀縣也已宣布進行強制性撤離。
Human-induced climate change is contributing to the recent heavy rain and ongoing record flooding along the Mississippi River, according to the Union of Concerned Scientists.
The pattern of rains and drought is set up by the La Niña - cold tropical Pacific sea surface temperatures - which influences the jet stream and the movement of storms across the United States.
These conditions shift precipitation, helping to drive extreme drought in Texas and Oklahoma while also contributing to extremely wet conditions in southern Missouri and the Ohio River Valley.
Additionally, a combination of natural factors and Arctic sea ice melt brought a relatively cold winter to much of the central United States, causing more precipitation to fall as snow, rather than rain, on the upper Midwest. Now that it is spring, that snow is melting and feeding into the Mississippi at the same time heavy rainfalls have occurred.
All these factors helped make April 2011 the 10th wettest on record in the United States, according to the National Climatic Data Center at the National Oceanic and Atmospheric Administration, which maintains records going back to 1894. Several states where rainfall drains into the Mississippi also experienced their wettest April on record.
In addition, human-induced climate changes continue to warm the Gulf of Mexico, the scientists observe.
Higher temperatures increase the amount of water that evaporates from the gulf's surface as well as the temperature of air that moves over the gulf, increasing the amount of water vapor it can hold.
At the same time, shifts in natural ocean currents are also contributing to higher temperatures in and over the gulf.
Natural atmospheric circulation patterns then carry water vapor over the gulf to North America. According to the NOAA's National Environmental Satellite, Data and Information Service, the Gulf of Mexico was nearly two degrees Fahrenheit hotter than average in late April when parts of the Midwest experienced intense downpours.
An upsurge in the number of heavy rainstorms has been triggered by an increase in moisture in the atmosphere, which, in turn, is tied to global warming, the Union of Concerned Scientists points out. Five states: Kentucky, Indiana, New York, Ohio and Pennsylvania, broke precipitation records in the three-month period between February and April this year.
Today, the Mississippi River crest is slowly working south. The U.S. Army Corps of Engineers continues to monitor the crest southward down the Mississippi River and continues flood fighting measures throughout the Mississippi River and tributaries, with increased focus on conditions in southern Louisiana.
New Orleans is expected to remain at near the current level for the next two weeks. The river is forecast to fall below flood stage on May 27.
In Louisiana, there are 16 of 125 gates currently open along the Morganza Floodway. The Corps says the release of water from the Morganza Spillway is moving slower than anticipated through the Atchafalaya River Basin.
Three hundred and thirty-three of 350 gates are open on the Bonnet Carre Spillway that empties the Mississippi River into Lake Pontchartrain and is anticipated to remain open for two to four weeks.
As the floodwaters spread across the land, approximately 3,900 residents may require sheltering and 22 shelters are on standby with the capacity to house over 8,200 occupants. The majority of residents are expected to stay with friends and family.
Thirty-four parishes across Louisiana have declared a Parish State of Emergency.
Planning and preparations are underway for evacuations downstream from the Morganza Spillway. Mandatory evacuations are in place for Krotz Springs and Melville communities outside of the ring levee in St. Landry Parish. Voluntary evacuations are in place for the remainder of the low lying areas in St. Landry Parish.
Mandatory evacuations will be issued for St. Martin Parish beginning on Saturday, emergency officials warn.
全文及圖片詳見：ENS報導
