Warmest January-April on record; new record Arctic sea ice minimum

April 2007 was the third warmest April for the globe on record, and the first four months of 2007 were the warmest ever, according to statistics released this week by the National Climatic Data Center. The global average temperature for April was 1.19�F/0.66�C above the 20th century mean. Over land, April global temperatures were the warmest ever measured. Ocean temperatures were a bit cooler (seventh warmest on record), thanks to the cooling associated with the disappearance of the winter El Ni�o event.

April temperatures were particularly warm across portions of Europe and Siberia, where readings of 5�C (9�F) above average were common (Figure 1). The UK recorded its warmest and driest April on record. In the U.S., April ranked near normal in temperature, but a record cold snap hit the eastern half of the country April 4-10, setting 900 daily low temperature records. The long duration of the cold outbreak, combined with the large number of hours that remained below freezing, and strong winds that occurred in many areas, contributed to crop losses that could reach into the billions of dollars. The damaging effects of the record cold were made worse by near-record warmth in March that helped induce an earlier spring blossom--in some cases two weeks prior to crop development in 2006. More than 2,500 daily record-high temperatures were set in the contiguous U.S. in March, and it was our second warmest March on record.

April's cold snap in the U.S. shows that although the globe as a whole may be warming, there is still plenty of natural variability capable of bringing very cold weather to local regions.


Figure 1. Temperature departure from average for April 2007. Image credit: National Climatic Data Center.

Arctio sea ice extent
After three months of not recording a record monthly minimum, sea ice extent in the Arctic recorded a new record low extent in April. The record warmth of the first four months of 2007, combined with the 8th lowest snow cover on record observed in winter 2006/2007, combined to produce the record low ice extent (Figure 2).


Figure 2. Arctic sea ice extent for April, for the years 1979-2007. April 2007 set a new record for the lowest Arctic sea ice extent measured. April sea ice coverage has declined about 9% since 1979. Image credit: National Snow and Ice Data Center.

Atlantic storms
The computer models have been forecasting development of several low pressure systems along the East Coast over the next few days. All of these lows will be non-tropical in nature, due to the high levels of wind shear. The models are beginning to hint that wind shear could relax over the ocean waters north of Panama next week, and we may have to watch that area for tropical development if the shear does indeed relax as forecast.

Jeff Masters