There has been a lot of important climate news in recent weeks and months. In addition to record warmth, an unusually active Atlantic hurricane season, and a devastating string of extreme weather events in the U.S. and around the world, Arctic sea ice has reached a new low in its total volume.
The ice covering the Arctic Ocean goes through a seasonal cycle in which it expands during the winter, reaching its maximum extent in March, and shrinks during the summer, reaching its minimum extent in September. Satellites have been observing the daily coverage of sea ice since 1979, during which time the summer minimum has declined rapidly over the decades. In 2007, the summer minimum dropped by a startling amount compared to previous summers, generating an iconic graph that was splashed across blogs and newspapers around the world (Figure 1). This record still holds, although every year since 2007 has seen below-average summer minima.
According to the National Snow and Ice Data Center (NSIDC), Arctic sea ice reached its minimum extent for 2010 on September 19 at 1.78 million square miles. Although this was the third-lowest extent behind 2007 and 2008, the sea ice set a new and probably more important record by reaching the lowest estimated volume – or total amount of sea ice – since satellite observations began in 1979.
Picturing an ice cube floating in a glass of water is a good comparison. The ice cube has three dimensions. But looking directly down at the glass, you see only the two dimensions that cover part of the surface of the water. When you look at the glass from the side, you can also see that the ice cube has depth, and that most of the ice is below the surface. The same phenomenon holds for sea ice, so if the ice melts from below, it becomes thinner and its total volume decreases.
This year, even though the area of the ocean’s surface covered by ice was a little larger than in 2007, the ice was much thinner, making its total volume much less than in 2007 or any previous year since estimates began in 1979 (Figure 2).
The rapid decline in total ice volume is significant since it takes less heat to melt a small volume of ice than to melt a larger volume. The area of ice cover can recover in one season, as it did in 2009, but the thickness builds up over several years. Consequently, the low volume of ice currently in the Arctic is more susceptible to melting next summer and the summer after that than was the 2007 ice. Consequently, scientists are wondering whether the Arctic could become ice free during the summer much sooner than previously projected.
The opening of the Arctic has enormous implications, ranging from global climate disruption to national security issues to dramatic ecological changes. The Arctic may seem far removed from our daily lives, but changes there are likely to have serious global implications.
- An ice-free Arctic Ocean will absorb more sunlight and convert it to heat, thus amplifying warming.
- The Arctic currently removes CO2 from the atmosphere, but physical and biological changes in the Arctic could cause it to switch to releasing CO2 and CH4 (a very potent greenhouse gas) to the atmosphere, thus amplifying global warming.
- Atmospheric circulation and therefore precipitation and storm patterns may be altered by a warming Arctic and changes in how the ocean interacts with the atmosphere in the region.
- A warmer, ice-free Arctic Ocean with more freshwater from snow and ice melt could change global ocean circulation patterns, thus altering marine ecosystems (i.e. fisheries) around the world and changing patterns of precipitation and storms on a very broad scale.
- More rapid melting of ice on land will accelerate sea level rise and could destabilizing the Greenland Ice Sheet, leading to abrupt and massive sea level rise.
- Countries have begun to compete for access to untapped natural resources in the Arctic. Unlike other international arenas, such as Antarctica, coastal waterways, and space, there are no agreed international rules to govern how different countries will access and utilize the Arctic.
Jay Gulledge is Senior Scientist and Director of the Science and Impacts Program
Several of my previous posts have examined the remarkable weather of the past year, including the unusual U.S. East Coast snowstorms this winter, the wide array of floods and heat waves this summer, and how these can help us understand our vulnerabilities to climate change. The average land surface temperature this summer (June-August) was the warmest on record globally and the fourth warmest on record in the United States.
Now that northern summer has come to a close, we can take stock of just how warm it was. Christopher C. Burt—weather historian, extreme-weather guru, and author—takes a look at temperature records set in the U.S. and around the world this summer in his blog at Weather Underground. Some of his findings include:
- Fifteen (15) U.S. cities recorded their warmest summer (June-August) ever.
- Only one U.S. city (Santa Barbara, CA) recorded its coldest summer.
- Seventeen (17) countries set new records for high temperatures, breaking the previous record of fifteen (15) countries set in 2008.
- No countries recorded a record low temperature.
- The Arctic country of Finland recorded a high temperature of 99°F at the Joensuu airport.
- A town in Pakistan recorded a record high temperature of 128.3°F.
- Los Angeles recorded its highest ever temperature of 113°F this Monday, in spite of an otherwise cool summer.
It’s important to put this single year into a broader perspective; if this warmth is just an aberration, then we might be wasting time talking about it. But it is clearly part of a much longer warming trend that has been going on for decades. A recent report from the National Oceanographic and Atmospheric Administration announced that 2009 was one of the ten warmest years on record (since 1880) and that the 2000s was the warmest decade followed by the 1990s and then the 1980s. If the first 9 months of this year are an indication, the 2010s appear poised to continue this upward march in temperatures.
(Figure Source: NOAA’s State of the Climate in 2009, Chapter 2)
Jay Gulledge is Senior Scientist and Director of the Science and Impacts Program
The rough weather of 2010 teaches us that climate change is risky business.
Recently, I posted a blog discussing the possible link between global climate change and two related extreme weather events: the heat wave in Russia and historic flooding in Pakistan. Although there is no method to definitively attribute any single event to climate change, based on documented trends in extreme weather events and research showing that specific types of meteorological phenomena are more common in a greenhouse-warmed world, I said:
“It is reasonable to conclude that, in aggregate, the documented increase in extreme events is partially a climate response to global warming, and that global warming has increased the risk of extreme events like those in Russia and Pakistan. On the other hand, there is no scientific basis for arguing that these events have nothing to do with global warming.”
That’s as far as the science permits me to go with this question. We simply cannot know whether any particular weather event was “caused” by climate change. In recent weeks, however, the media have done their all-too-common “he said-she said” routine of finding one source who says the extreme weather of 2010 is because of climate change and another who says it’s not. This is a meaningless argument that distracts us from what we should be thinking about, which is what these events can teach us about our vulnerabilities to climate change.
Update: Dr. Jay Gulledge is featured on National Journal's Energy & Environment Expert Blogs. Click here to read Dr. Gulledge's take on Climate Risks Here and Now
Last fall I posted a blog about the unusual number and severity of extreme weather events that have been striking around the globe for the past several years. That entry focused on the alternating severe drought and heavy flooding in Atlanta in 2007-2009 as an example of the roller coaster ride that climate change is likely to be. As every dutiful scientist does, I stopped short of blaming those individual weather events on global warming, but I am also careful to point out that it is scientifically unsound to claim that the confluence of extreme weather events in recent years is not associated with global warming; I’ll return to this question later.
The weather of 2010 continues the chaos of recent years. In the past six months, the American Red Cross says it “has responded to nearly 30 larger disasters in 21 [U.S.] states and territories. Floods, tornadoes and severe weather have destroyed homes and uprooted lives …” Severe flooding struck New England in March, Nashville in May, and Arkansas and Oklahoma in June.
There seems to be some confusion out there about weather vs. climate. For example, a Virginia Republican Party video urged citizens to call their Congressmen and tell them how much global warming they got during the big snowstorm a couple of weeks ago. But that doesn’t really make any sense. In simple terms, weather determines whether you need to take an umbrella with you today; climate determines whether you need to own an umbrella. Weather determines whether you need your down coat today; climate determines whether you need to own a down coat. Weather determines whether you turn on your air conditioning unit today; climate determines whether you own an air conditioner. Weather determines whether the plants in your garden have a good day; climate determines what plants will likely thrive in your local environment.
Climate is the long-term average of weather. Weather changes all the time; climates are generally fairly stable, allowing us to make long-term decisions based on the notion that the future climate will be like the past. One unusual weather event does not mean the climate is changing. But many unusual weather events could mean the climate is changing. And climate change will mean that on average, the weather we will have in the future will be different from what we had in the past. That could even mean that record-breaking snowfall events happen more and more often in Virginia and Washington, D.C.
In the past few weeks I’ve posted twice (here and here) on reasons why global warming could be increasing the frequency of heavy snow events in certain parts of the United States (and likely in other similarly situated places around the world).
In a recent post on his WunderBlog (Weather Underground Blog), Dr. Jeff Masters gives his take on this issue. Dr. Masters is co-founder and Director of Meteorology of Weather Underground, a weather service that provides real-time weather information via the Internet. Unlike me, he’s a real weather expert and I highly recommend his blog.
The cold weather continues across much of the Unites States, Europe, and central Asia as the Arctic Oscillation remains in a strong “negative” state, forcing cold Arctic air down to the mid-latitudes. A couple of weeks ago I explained why more frequent heavy snowfall events could be a consequence of global warming for mid-latitude areas near large bodies of water, like Washington, D.C., and Syracuse, New York (see figure).
|The average amount of annual snowfall has been increasing in Syracuse, New York, for most of the past century. (SOURCE: Increasing Great Lake–Effect Snowfall during the Twentieth Century: A Regional Response to Global Warming? Journal of Climate vol. 16, pp. 3535-3342, Figure 1)|
On January 31, I noticed a forecast for lake-effect snowfall around the Great Lakes on Weather.com: “Lake-effect snows are also possible near the central and western Great Lakes today and tonight.”
On December 19, Washington, D.C. was buried by two feet of snow, setting a new record for snowfall during the entire month of December and paralyzing the city for three days. As my neighbor and I shoveled out from the storm, he stopped for a moment, grinned, and asked, “So what happened to global warming?” Boy was he surprised when I said, “Glad you asked,” and launched into a 15-minute oratory on why global warming might mean more, not less, extreme snowfall for some parts of the world.
Before continuing, I need to reiterate that no single weather event can be attributed to global warming. So the question here is not, “What caused the heavy snowfall on December 19?” The question is, “Is heavy snowfall or unusually cold weather inconsistent with global warming?”
You need two things to create heavy snowfall: moist air and cold air. The two generally don’t occur in the same air mass because cold air can’t hold much moisture. So you need two air masses, one that is warm and moist and one that is cold and dry, to collide with each other. That is exactly what happened over the Mid-Atlantic region on December 19.
This condition is not only consistent with global warming, but it can be expected to occur more frequently in certain places as a direct result of global warming. It takes warmth to generate moist air. First, you need warmth to evaporate enough water from lakes or oceans to generate a massive snowstorm. Second, you need warm air to keep the water vapor aloft so that it doesn’t rain out before it finds a cold air mass to collide with. When I asked weatherman Joe Witte where the moisture that ended up in my snow shovel came from, he said, “Some of the moisture came out of the Gulf of Mexico AND some from the warm Atlantic ocean with the VERY warm (70s!) Gulf Stream along the East coast acting as a hot plate for evaporation of moisture into the cold dry air.”
|(SOURCE: NOAA polar-orbiting satellite data compiled by Rutgers University Coastal Ocean Observation Lab)|
Although the past few weeks have been very cold in the eastern United States, Joe pointed me to NOAA satellite measurements that found sea surface temperatures in the Gulf of Mexico and the Atlantic Ocean to be 1 to 3 °F warmer than normal during the week before the big snowstorm hit (see figures above). There is strong scientific evidence showing that, on average, the oceans are warmer today than they were a century ago because of human-induced global warming. So the warm ocean temperatures that fed the heavy snowfall are consistent with global warming. In fact, because of global warming, we should expect such conditions to be more common today than in the past and even more common in the future as warming continues.
So where did the cold, dry air come from? Global warming is about changes in long-term averages and not about single events; it does not mean an end to cold weather. Instead, it means that cold weather will become less frequent and hot weather more frequent when averaged over decades. In fact, both of these trends have been observed over the past 50 years in the United States and globally. So, even with global warming we will have cold winters, just fewer of them. It is also important to remember that a cold winter here doesn’t mean a cold winter everywhere. In fact, many parts of the world, including the Arctic and the tropics, are having an unusually warm winter. The current cold snap is concentrated in the mid-latitudes of the northern hemisphere, and there will always be the potential for cold Arctic air masses to visit the mid-latitudes from time to time.
The current cold snap is related to a known weather pattern called the Arctic Oscillation. When the Arctic Oscillation switches between “positive” and “negative” states, it simply shifts heat between the Arctic and the mid-latitudes. Scientists call this kind of pattern “internal variability,” and it does not change the total amount of heat in the climate system. Internal variability can create strong differences in the weather from year to year and place to place, but these shifts average out to zero net climate change over decades. Only a net change in the total amount of heat in the climate system can change the long-term average climate, and that is the nature of global warming.
When the mid-latitudes get periodic blasts of cold Arctic air, global warming makes it more likely that the cold air from up north will collide with moist, warm air from down south, creating more heavy snowfall events in mid-latitude areas near large bodies of water. A similar phenomenon is affecting the Great Lakes region. Syracuse, New York is one of the snowiest places in the country, but it and other areas around the Great Lakes are getting even snowier! Because the Great Lakes are getting warmer, they are icing over later and melting earlier than they used to. Without the ice, water can evaporate and enter the atmosphere over the lakes later in the fall and earlier in the spring. When winds blow this moist air over the land where temperatures are lower, we get the famous “lake effect” snow. With more open water during the winter, more lake effect snow is falling.
These are the cold facts of global warming.
Jay Gulledge is Senior Scientist and Program Manager for Science & Impacts