Earth just experienced its warmest Northern Hemisphere winter on record, according to statistics released today by the National Climatic Data Center. The 3-month Northern Hemisphere winter period December 2006 through February 2007 had an average global temperature +0.72°C (+1.30°F) above normal, beating the previous record set in 2004 by a substantial +0.12°C. The Northern Hemisphere had its warmest winter ever measured, and the Southern Hemisphere (where it was summer) had its 4th warmest summer on record. Cooler than normal temperatures were observed over less than 15% of the globe, and nowhere did the cooling exceed 3° C (Figure 1). Record warmth was particularly noteworthy over land areas of the Northern Hemisphere poleward of 45° latitude, where temperatures a remarkable 5°C (9°F) above normal were common. Warming of almost the entire globe's land and ocean areas, with the greatest warming occurring in winter over the northern Northern Hemisphere's land areas, is a result that climate models have long been predicting would occur if human-emitted greenhouse gases were substantially affecting Earth's climate. This winter's pattern of record warmth closely matches the computer models projections, and adds additional support to the theory that human-emitted greenhouse gases are now causing a significant warming of the planet. We can expect further substantial warming in coming years as human-emitted greenhouse gases continue to increase at 2% per year.


Figure 1. Temperature departure from average for the winter of December 2006-February 2007. Image credit: National Climatic Data Center.

Influence of El Niño
The record warm winter was also partially due to the presence of a moderately strong El Niño event in the Eastern Pacific. December 2006 was the warmest December on record, January 2007 was the warmest January on record, and both of these record warmest months occurred when El Niño was at its peak strength. El Niño rapidly died out at the end of January, and global temperatures in February were not quite as warm, merely the 6th warmest on record. With El Niño gone and a possible La Niña event on the way later this year, we may not see any more record-breaking warmest months in 2007.

Sea ice extent and snow
The record winter warmth did not lead to record minimum sea ice coverage in the Arctic, which recorded its third lowest February coverage on record (Figure 2). Still, the sea ice coverage this month is very close to the minimum observed in 2005, and is 10% lower than it was 28 years ago. Winter 2006/2007 snow cover extent over the Northern Hemisphere was the 8th lowest extent in the historical record, and 3rd lowest on record over Eurasia. The low snow cover combined with the near record-low Arctic ice extent could lead to an early spring in the Arctic, and more record or near-record low sea ice coverage for the Arctic this year.


Figure 2. Arctic sea ice extent for February, for the years 1979-2007. The record lowest Arctic February sea ice extent occurred in 2005, and sea ice has increased slightly over that record low the past two years. Still, the February sea ice coverage has declined about 10% since 1979.