If global warming is occurring, why was the winter of 2007-2008 so cold and snowy?

The planet was much snowier and warmer than usual during the winter of 2007-2008, according to statistics released today by the National Climatic Data Center. Snow cover extent over the Northern Hemisphere during the period December 2007 - February 2008 was the fourth greatest on record, and was the greatest on record for January. Satellite-derived snow cover records extend back to 1967. Some regions of the Middle East, such as Baghdad, Iraq saw their first snow in living memory, and seasonal snowfall records were broken in Wisconsin and a few places in the Northeastern U.S. Surprisingly, the winter also ranked much above average in temperature--it was the 16th warmest December through February period in the 128-year global record. This puts the winter of 2007-2008 in the warmest 13% of all winters. Temperatures this winter were a bit cooler than recent winters because of an ongoing strong La Niña event in the Eastern Pacific Ocean, which has dramatically cooled the ocean surface waters. By one measure (the surface pressure difference between Darwin and Tahiti), February 2008 was the strongest February La Niña event on record. The last time we had a winter this cool was during 2000-2001, which also happened to be the last time we had a major winter La Niña event.


Figure 1. Departure of temperature from average for the winter of 2007-2008. Image credit: National Climatic Data Center.

A normal winter for the U.S.
December 2007 through February 2008 was about average in the contiguous U.S.--the 54th coolest winter on record in the 113 year period of record. The average temperature was 33.2°F (0.6°C), which was 0.2°F (0.1°C) above the 20th Century mean. It was the 18th wettest December-February in the 1895-2008 record. New York experienced its wettest winter on record, and the states of Colorado, Connecticut, Pennsylvania, and Vermont experienced their second-wettest winter on record. Only the South received below normal levels of precipitation, mostly due to a dry winter in Texas.

All time winter snowfall records have already been set in some portions of the Northeast U.S. and Wisconsin. As of March 12, Madison, WI had accumulated 92 inches of snow, smashing the previous seasonal snowfall record of 76.1 inches (193.3 cm) of snow in the winter of 1978-1979. Two locations in the Northeast have set new winter snowfall records, and more records will fall if an average amount of snow falls in March. By the end of February, new snowfall records for the season-to-date were also set in both Telluride and Aspen, Colorado.

An exceptionally warm winter in Northern Europe and Asia, cold in Central Asia
Northern Asia and northern Europe experienced an exceptionally warm winter, with Sweden and Finland recording their warmest winters ever, and Norway, its second warmest. Conversely, Tajikistan recorded its coldest winter in 30 years, and heavy snows in Kazakhstan caused severe flooding when they melted. Snow storms and cold weather in China this winter killed 129 people and did over $21 billion in damage.

Why did we see a cool winter, if global warming is occurring?
It is important to understand the difference between weather and climate. Climate is what you expect; weather is what you get. What we experience in one particular season or year is "weather". Weather has a large variation from year to year, with cool seasons and years mixed in with warms ones. "Climate" is the weather measured on scales of tens of years or longer. One cool winter or year is not an indication that the climate is cooling back to normal. The climate is warming, and unless we see a series of several years of cool conditions, this year's cool winter merely represents a normal fluctuation of the weather. Relatively cool weather is to be expected globally during a strong La Niña event in the Eastern Pacific Ocean, and relatively warm weather is expected during an El Niño event. We shouldn't expect to see record warmth for the globe unless an El Niño event is occurring.

Why did we see record snows this winter, if global warming is occurring?
Beware of global warming skeptics trumpeting record snowfalls this winter as an excuse to doubt that global warming is occurring. One should primarily look at global temperatures on a scale of decades to judge the validity of global warming. Dr. Ricky Rood, who writes our Climate Change blog, put it this way in his current blog, Creeping Onset of Spring and in an earlier blog, Water, water, water:

This year has been very snowy in the northern hemisphere. That it is snowy does not suggest that it is colder. If it gets warmer, it does not mean that we no longer see freezing temperatures in places like Michigan. If it gets warmer there is more water in the atmosphere, and when there is precipitation there will be more precipitation, and if it is below freezing, then that precipitation will be ice and snow. The high mountains near the coast, like the Cascades and the Sierra Nevada would expect more snow. This is also true for the high altitudes parts of Greenland and Antarctica. From a climate point of view it is more important to look at snow cover in the late winter and early spring. Is the snow melting earlier?


Figure 2. Average February arctic sea ice coverage as observed by satellites between 1979 and 2008. Image credit: National Snow and Ice Data Center

Arctic sea ice recovers a bit
It will be interesting to see if this year's heavy Northern Hemisphere snow cover melts earlier than usual, as this will have a big impact on the annual Arctic sea ice melt. We're starting off with more ice surface area in the Arctic than in the past four years--February 2008 Northern Hemisphere sea ice extent was greater than each of the previous four years, thanks to cooler than usual temperatures over much of the Canadian Arctic. However, this was still the fifth lowest ice extent on record for the month of February, and 8% below its extent in 1979 when satellite measurements began, according to the National Snow and Ice Data Center. February was the third straight month that a new monthly minimum Arctic sea ice record was not set, following a string of five months in a row where monthly records were set. The extra sea ice extent will help to reduce the amount of melting this summer, but this effect will probably be overshadowed by the fact that natural wind patterns have forced a large amount of thick, multi-year ice out of the Arctic this winter. This has left much of the sea ice very thin, making it very vulnerable to melting. For the first time on record, the edge of thin first-year ice has pushed beyond the North Pole. IF we get another relatively warm and sunny summer in the Arctic in 2008, we will likely see Arctic sea ice loss surpassing last year's astounding collapse.

Annual WeatherDance contest ready for registration!
Armchair forecasters, now's your chance to shine! WeatherDance, based on teams in the men's and women's NCAA basketball tournaments, allows players to predict which team's city will be hotter or colder on game day in each round of the Big Dance. Beginning March 17, players can make their forecasts at the Weather Dance Web site at: www.weatherdance.org. The site will be updated with cities promptly after NCAA seeding announcements. Team selection occurs March 16 for men and March 17 for women. First round Weather Dance selections must be entered by 11:59 p.m. EST March 19. Players can register now and receive periodic reminders as the game progresses.

"Officially, Weather Dance began as a class project to get students involved in weather forecasting, but we kept it around because it got popular. People think they can do better forecasting than the meteorologists. Well, here's their shot!" said Perry Samson, WeatherDance creator, co-founder of the The Weather Underground, Inc., and Professor in the Department of Atmospheric, Oceanic and Space Sciences at the University of Michigan.

This is the third year for the game. Last year more than 2,000 people played. Most play merely for the thrill, but many science teachers involve their classes as part of meteorology units. The winning teacher will receive an invitation and $500 to join the Texas Tech/University of Michigan Storm Chasing team this spring for a day of tornado chasing. Other winners will receive a Weather Underground umbrella or a copy of the book "Extreme Weather," by Christopher C. Burt.

Jeff Masters