Earth Gauge » Seasonal Patterns

Climate Trivia: Marmots and Warming

kraus — Fri, 06 Aug 2010 20:27:12 +0000

Plants and animals that live high up in the mountains have to be able to tolerate strong winds and long durations of snow cover. Most animals, such as the yellow-bellied marmot, deal with long and cold winters by hibernating. Over the last century, the Rocky Mountain region in Colorado has warmed by between 1.5 and two degrees Fahrenheit, with most of this warming happening over the last 30 years. This warming has stimulated a decrease in the amount of time marmots spend hibernating – these animals waking up around 38 days earlier than they did in the early 1980s.

Trivia Question: How have marmots responded to this temperature rise and decrease in hibernation period?

a. They have become larger.
b. They have become smaller.
c. Their population has declined.
d. Their population has grown.
e. a and d.

The correct answer is e. Marmots now have more time to be active, eat and reproduce. As a result, today there are more marmots in the Colorado Rockies and they are bigger than they were several decades ago. Most of the population and size trends have occurred since 2000. There are now three times more marmots living in Colorado’s Upper East River Valley and juvenile marmots are now growing at a rate of 0.7 pounds per year faster than in 2000.

Please visit http://www.earthgauge.net/climate-facts-image-library#7 to download an image of a yellow-bellied marmot in Rocky Mountain National Park. The image is in the public domain.

Seasons: Spring, Summer

Sources: Martens, Chad. “Are Alpine Species DisappearingThe Effects of Climate Change on Alpine Vertebrates in the Rocky Mountains.” Mountain Research Station, University of Colorado, Boulder. Spring 2005 and Ozgul, A et al. “Coupled dynamics of body mass and population growth in response to environmental change.” Nature 466 (2010): 482-483.

Climate Fact: River Temperature Rise

kraus — Mon, 26 Jul 2010 13:22:38 +0000

Earth Gauge Video is available for this tip.

In Brief: Temperatures in local waterways are rising from a combination of factors.

Moving from Chicago to Baltimore, where the average temperature is about ten degrees Fahrenheit warmer, might take a little bit of adjustment but almost certainly wouldn’t be fatal for humans. Most fish and other aquatic species, however, would not survive an equivalent temperature change. Warmer water temperatures have contributed to the elimination of the Brook Trout from many streams in the eastern U.S. In the mid-Atlantic region, temperatures are becoming intolerable for some sensitive species like the Longnose dace and Cutlips minnow. Temperatures are beginning to enter the danger zone even for the relatively tough and tolerant Blacknose dace, the most common species in the region’s urban streams.

Annual mean water temperatures in the nation’s streams and rivers are increasing at an average rate of 0.016 and 0.139 degrees Fahrenheit per year, respectively. The Potomac River around our Nation’s capital is seven degrees warmer than it was in the 1920s and the Delaware River around Philadelphia is 4.5 degrees warmer that it was in 1965. Some likely factors behind the rising water temperatures include:

A Warming Climate: Warming water temperatures are linked to the rise in surface temperatures that have occurred over the same period.
Land Use Changes: Surfaces like concrete, asphalt and rooftops hold more heat more than vegetated ground, making runoff from urban areas warmer than runoff from rural or forested areas. These hard surfaces also prevent water from soaking into the ground, leading to sudden discharges of warm waters into streams and rivers. Urban runoff during a summertime thunderstorm can raise a stream’s temperature by 12 degrees in less than 30 minutes.
Loss of Trees: Fewer trees on stream banks mean that streams receive more direct sunlight, raising their temperatures.
Thermal Power Plants: Increased demand for electricity has led to the construction of more thermal power plants over the last century, and these power plants discharge hot water.
Dams: Dams create large bodies of standing water, which absorb more energy than running water.

Seasons: Spring, Summer, Fall

(Source: Kaushal, SS et al. “Rising stream and river temperatures in the United States.” Frontiers in Ecology and the Environment 2010; 100323112848094 DOI: 10.1890/090037.)

Climate Number: 179 Cubic Miles

kraus — Mon, 26 Jul 2010 13:19:52 +0000

Many of Earth’s great ice masses, which collectively form the cryosphere, are floating on ocean surfaces. There are three main collections of floating ice: the Arctic sea ice, the Antarctic ice shelves and the Antarctic sea ice. All three components have seasonal fluctuations, with the Antarctic sea ice showing the most dramatic differences between winter and summer extents. The Antarctic ice shelves are the edges of the Antarctic continent’s ice sheets that extend out onto the oceans. Every year, in each hemisphere’s respective summer, large portions of the floating ice either melts or breaks off into chunks known as icebergs that float off into the open ocean before melting. In a world with a static climate, about the same amount of ice that melts every summer would refreeze the following winter. Between 1994 and 2004, however, there was on average about 179 cubic miles less floating ice each year, indicating an overall loss of ice and a warming of the oceans and atmosphere. While this loss of floating ice contributes only minimally to sea level rise, such losses may impact ocean salinity, heat distribution and mixing. These changes may in turn lead to changes in the ocean current system, which may have other ramifications for the climate system.

For Comparison: 179 cubic miles is about the same size as three million Great Pyramids of Giza.

Seasons: Winter, Spring, Summer, Fall

Source: Shepherd, A et al. “Recent loss of floating ice and the consequent sea level contribution.” Geophysical Research Letters 37 (2010): L13503.

Climate Fact: Mammals on the Move

kraus — Mon, 28 Jun 2010 13:34:51 +0000

In Brief: Warming across the state of Michigan has been followed by an influx of species formerly inhabiting lands to the south.

The Great Lakes region is an ecological “transition zone.” To the north lie boreal forests dominated by conifer trees. To the southwest lie oak savannas and prairies, and to the southeast lie deciduous oak-hickory woodlands. When the climate changes – average temperatures rise or rainfall in a given location changes – animals that are adapted to those conditions will often move to find the conditions for which they are best suited. Temperatures have warmed in the Great Lakes region over the past 30 to 40 years. Since 1985, surface temperatures on Lake Superior have been warming by an average of 1.2 degrees Fahrenheit per decade. Spring conditions are on average arriving earlier in the year and winters are milder, with average minimum temperatures up to 7.4 degrees Fahrenheit warmer than they were in the late 1960s. As a result of these warming conditions, species better adapted to the conditions that were prevalent to the south of Michigan several decades ago – and are now normal in Michigan today – have moved into the state or have expanded their once marginal populations there. Woodland deer mice, for example, used to be dominant throughout the northern part of the Lower Peninsula, but have declined in number since white-footed mice moved in from the south. Flying squirrels, eastern chipmunks and opossums are now common or even abundant in parts of Michigan where they were largely unknown in the 1950s and 1960s.

Seasons: Winter, Spring, Summer, Fall

Source: Myers, P et al. “Climate-induced changes in the small mammal communities of the Northern Great Lakes Region.” Global Change Biology 15 (2009): 1434-1454.

Climate Number: 5.8 million square miles

kraus — Mon, 10 May 2010 14:34:53 +0000

One of Earth’s most dramatic seasonal cycles is the waxing and waning of the sea ice that surrounds Antarctica, the driest, darkest and coldest continent. At its maximum extent at the end of the Southern Hemisphere winter in September, a 6.9 million square mile expanse of ice extends from Antarctica’s shores out into the Southern Ocean. This 6.9 million square mile collection of ice is larger than the solid continent itself, which covers an area of about 5.4 million square miles. By the end of the Southern Hemisphere summer, however, the sea ice has shrunk to about 1.1 million square miles, a difference of about 5.8 million square miles or a 630 percent decrease in area.

For Comparison: 5.8 million square miles of seasonal sea ice is almost enough to cover the lower 48 United States twice over.

Below: Seasonal differences in Antarctic sea ice extent. Image courtesy of the National Snow and Ice Data Center, University of Colorado, Boulder, Colorado.

Seasons: Winter, Spring, Summer, Fall

Sources: The National Snow and Ice Data Center. “All About Sea Ice” Accessed Online 30 April 2010

Climate Number: 1810 Years

kraus — Mon, 05 Apr 2010 14:09:51 +0000

In northern China, how much rain falls each year is controlled by the strength of the Asian Monsoon – the system of seasonal winds that bring moist marine air onto land during the summer and cold dry air out to sea during the winter. When the monsoon system is strong, there is ample rainfall and the region’s rice crops do well. When the monsoon is weak, summertime rainfall decreases and rice production suffers. Researchers recently discovered an over 11-foot-long stalagmite in Wanxiang Cave, located in a semi-arid area of northern China. Stalagmites form on cave floors as water seeps through the soil and picks up minerals along the way. Channels in the ground flow and drip water into subterranean caves, depositing the minerals into icicle-like formations. Because the monsoon is an annual cycle, there are distinct layers like tree rings that mark each year of deposits. The bigger the layer, the more minerals were deposited that year and the more rainfall there was. These deposits also accumulate large amounts of uranium, which decays into thorium at a uniform pace. Analysis of the ratio of uranium to thorium in each of the different layers can give an absolute (within 2.5 years) date when the deposit was laid down. This method tells us that the 11-foot stalagmite in Wanxiang Cave has been growing for 1810 years. Not surprisingly, periods of thin deposits (indicating a weak monsoon and low rainfall) corresponded to historical periods of hunger and social unrest while periods of thick deposits (strong monsoon, ample rainfall) corresponded to historic periods of good crop production and abundant food, population growth, and social stability.

For comparison: When the stalagmite began to grow, the Roman Empire was beginning its long process of decline. In modern day California, the Grizzly Giant Sequoia tree in Yosemite National Park was just sprouting. Modern Hinduism was established in India around this time.

Seasons: Winter, Spring, Summer, Fall

Source: Zhang, P et al. “A Test of Climate, Sun and Culture Relationships from an 1810-Year Chinese Cave Record.” Science 322 (2008): 940-942 and Kerr, RA. “Chinese Cave Speaks of a Fickle Sun Bringing Down Ancient Dynasties.” Science 322 (2008): 837-838.

Climate Fact: Wind Speed Changes

kraus — Mon, 22 Mar 2010 14:56:52 +0000

In Brief: Higher elevation areas are experiencing the overall global trend of wind speed “stilling” more acutely than surrounding lowlands.

Wind speeds in the mid-latitudes have shown a downward trend over the past 30-50 years, a phenomenon known as “stilling.” Any trends in wind speed have implications for the water cycle, ecosystems and wind energy generation. Wind speeds pick up as the land-surface elevation increases. High elevation areas give birth to the river water that supports about 25 percent of the world’s gross domestic product, and climate changes at high elevations have potential implications for this water. In two high elevation areas, Switzerland and the Loess region of China, the overall trend in wind speed reduction has been more pronounced at higher elevations and during the winter months. For both of these study areas (which have similar climatologies) during the winter months, the wind speed increases by about 11 feet per second for every mile the land surface gains in elevation. Between 1960 and 2006, this wintertime rate of wind speed increase with elevation declined by about one percent. Wind speed is thus another climate variable that is changing faster at higher elevations.

Seasons: Winter, Spring

Source: McVicar, TR. “Observational evidence from two mountainous regions that near-surface wind speeds are declining more rapidly at higher elevations than lower elevations: 1960-2006.” Geophysical Research Letters 37 (2010): L06402.

Climate Fact: Phenology, Adaptation and Climate

kraus — Mon, 22 Mar 2010 14:50:00 +0000

In Brief: Different plants use different signals to tell them when to come out of winter dormancy, which affects their response to climate change.

Phenology is the study of naturally recurring events, such as plants blooming in spring. In moist temperate regions like the Eastern United States, some plants come out of their winter dormancy once temperatures become sufficiently warm – which is usually once there is a certain number of cumulative hours with ambient (air) temperatures above a certain threshold (such as 50 hours over 50 degrees Fahrenheit during a ten day period). Other plants will only respond to a temperature signal once there has been a certain number of winter “chill hours,” or a set amount of hours during the cold season when temperature are near or below freezing. Other plants unleash their flowers and leaves when the days become a certain length – such plants are said to be “photoperiod controlled.” Depending only on temperature as an “alarm clock” is risky for a plant. This is particularly true in regions with variable weather regimes, where the transition from winter to spring can mean several days of temperatures in the 60s that are soon followed by a cold front and a few days of freezing temperatures, which can potentially damage a plant’s young leaves or flowers. Photoperiod and chill hour “alarm clocks” keep plants from sprouting their sensitive leaves and flowers at the wrong time. Because different plants use different “alarm clocks,” not all species respond to warming temperatures in the same way. Most plants and animals do appear to be responding to warming temperatures, with most species at temperate latitudes advancing their spring activities by 2.5 days per decade since 1970.

Season: Spring

Sources: Korner, C and Basler, D. “Phenology under global warming.” Science 19 (2010): 1461-1462 and Menzel, A et al. “European phenological response to climate change matches the warming pattern.” Global Change Biology 12 (2006): 1969-1976.

Climate Trivia: Earth’s Green Season

kraus — Mon, 08 Mar 2010 15:01:01 +0000

In the Northern Hemisphere, deciduous trees are beginning to come out of their dormant season and unfurl their leaves. Soon, the greys and browns that characterize America’s broadleaf forests during winter will be replaced the by the greens of spring and summer. Over the last four decades, there has been a global trend in the length of the “green” season, or the period between when leaves emerge in the spring and when they turn color and drop in the fall.

Trivia Question: Since 1970, Earth’s “green” seasons have become…

a) longer
b) shorter

The correct answer is a. Earth’s “green” season – the combined average length of both the Northern and Southern Hemisphere green seasons – is now on average 15 days longer than it was in 1970. This trend has been linked to warmer temperatures, milder winters and higher concentrations atmospheric carbon dioxide.

Seasons: Late Winter, Early Spring

Source: Peñuelas, J et al. “Phenology Feedbacks on Climate Change.” Science 324 (2009): 887-888.

Climate Trivia: El Niño Events and Frost Days – Great Basin

kraus — Mon, 08 Mar 2010 14:54:12 +0000

Winter is ending and the growing or “frost free” season is almost here! The frost free season is defined as the continuous period of the year when the temperature does not drop below freezing. When this season starts and how long it lasts have important implications for the plants and animals that live around us, especially plants we grow for food. In the United States over the second half of the 20th century, the average length of the frost free season increased at a rate of two days per decade. How many frost days there are each year is influenced – as is much of our weather – by the El Niño-Southern Oscillation (ENSO), or the periodic warming and cooling of the eastern tropical Pacific. This warming and cooling changes how air in the upper-atmosphere moves, which in turn affects weather across the United States.

Trivia Question: During El Niño years such as this year, when the eastern tropical Pacific is relatively warm, does the Great Basin region have on average…

a. fewer frost days?
b. more frost days?

The correct answer is a. All other things being equal, the Great Basin region experiences fewer frost days during El Niño years compared to La Niña years.

Seasons: Late Winter, Early Spring, Fall

(Sources: Meehl, GA et al. “Current and Future U.S. Weather Extremes and El Niño.” Geophysical Research Letter 34 (2007) L20704 and Easterling, David. “Observed Climate Variability and Change.” NOAA/National Climatic Data Center. Ashville, NC: 31 January 2007 http://www.ametsoc.org/atmospolicy/documents/Easterling-Observed-Change-Jan-07.pdf)

Climate Trivia: El Niño and Frost Events – Pacific Northwest

kraus — Mon, 08 Mar 2010 14:50:05 +0000

Trivia Question: During El Niño years such as this year, when the eastern tropical Pacific is relatively warm, does the Pacific Northwest have on average….

a. fewer frost days?
b. more frost days?

The correct answer is a. All other things being equal, the Pacific Northwest experiences fewer frost days during El Niño years compared to La Niña years.

Seasons: Late Winter, Early Spring, Fall

Sources: Meehl, GA et al. “Current and Future U.S. Weather Extremes and El Niño.” Geophysical Research Letter 34 (2007) L20704 and Easterling, David. “Observed Climate Variability and Change.” NOAA/National Climatic Data Center. Ashville, NC: 31 January 2007 http://www.ametsoc.org/atmospolicy/documents/Easterling-Observed-Change-Jan-07.pdf.

Climate Trivia: El Niño and Frost Events – Eastern U.S.

kraus — Mon, 08 Mar 2010 14:44:28 +0000

Trivia Question: During El Niño years such as this year, when the eastern tropical Pacific is relatively warm, does the eastern U.S. experience on average…

a. fewer frost days?
b. more frost days?

The correct answer is b. All other things being equal, the eastern U.S. experiences more frost days during El Niño years compared to La Niña years.

Seasons: Late Winter, Early Spring, Fall

Climate Trivia: El Niño and Frost Events – Southern U.S.

kraus — Mon, 08 Mar 2010 14:34:45 +0000

Trivia Question: During El Niño years such as this year, when the eastern tropical Pacific is relatively warm, does the southern U.S. have on average…

a. fewer frost days?
b. more frost days?

The correct answer is b. All other things being equal, the southern U.S. experiences more frost days during El Niño years compared to La Niña years.

Seasons: Late Winter, Early Spring, Fall

Climate Number: Two Tons

kraus — Mon, 01 Mar 2010 14:57:07 +0000

Over the past 250 years, the amount of carbon dioxide (CO2) in the atmosphere has risen from 280 parts per million to almost 400. Plants use sunlight to convert this atmospheric carbon into the sugars and starches that make up their tissues. As the amount of carbon in the atmosphere changes, plant growth patterns change as well. Longer growing seasons, the period of the year when freezing temperatures do not restrict growth, as well as warmer temperatures in general, also affect plant growth. All three of these climate trends (more CO2, longer growing seasons and higher temperatures) have been occurring in the forests of America’s Mid-Atlantic region over the past century. During this period, plant growth in these forests accelerated. Each acre accumulates a certain amount of “biomass” each year, which reflects how much carbon these forests take out of the atmosphere and store in their bodies. Each acre of Mid-Atlantic forest is now accumulating about two more tons of biomass each year than they did in 1900.

For Comparison: Two tons is about the same weight as two mature Hereford bulls.

Seasons: Spring, Summer, Fall

Sources: McMahon, SM et al. “Evidence for a recent increase in forest growth.” Proceedings of the National Academy of Sciences 107 (2010): 3611-3615 and “Forests are Growing Faster, Ecologist Discover; Climate Change Appears to be Driving Accelerated Growth.” Science Daily 2 February 2010. Accessed Online 28 February 2010 )

Climate Fact: Midwinter Storm Track Suppression

kraus — Mon, 22 Feb 2010 15:15:29 +0000

The temperature/pressure difference between the equatorial regions and the poles is at its maximum during the winter months. The energy this difference generates is thought to power the “storm tracks,” or the bands in the mid-latitudes where east to west traveling storms (cyclonic high and low pressure systems) are most common. The storm track over the Pacific brings the western U.S. ample rainfall for much of the fall, winter and spring seasons. One aspect of Northern Hemisphere winter storm behavior that has been somewhat of a mystery is the midwinter suppression of the Pacific storm track. The maximum latitudinal temperature/pressure difference during winter, which means more power for the storms, is reflected in the Atlantic storm track being at its strongest during the winter months. Over the Pacific, however, the midwinter corresponds to an average decrease in the number and strength of these storms by 20 and 14 percent respectively compared to the fall and spring months. One possible explanation for this suppression is a wintertime drop in the number of atmospheric disturbances that make their way from the mountains of central Eurasia to the Pacific. These disturbances can become the storms that move across the Pacific to North America. The sheer size of the Eurasian landmass means that the wintertime high pressure centers that sit in the middle of the continent are strong enough to push the warmer and competing air masses far to the south and away from the mountains where these warmer air masses can generate the disturbances that can ultimately become the Pacific storms.

Season: Winter

Source: Penny, S et al. “The Source of the Midwinter Suppression in Storminess over the North Pacific.” Journal of Climate 23 (2010): 634-648.

Snow in a Warming World

kraus — Fri, 19 Feb 2010 21:09:51 +0000

Snowfall and snow cover have direct effects on transportation, soil freeze/thaw cycles, water availability, flood frequency, water quality, wildlife, forest fires and more.

Far from being just a passive product of prevailing climatic conditions, snow cover also influences climate by changing the surface albedo, the amount of solar radiation a surface reflects. The presence of snow, because snow cover reflects more sunlight than bare ground, changes the surface conditions that influence how the atmosphere around us moves – which affects the weather responsible for the snow itself.

Because of snow’s influence on our daily activities and weather, it is important to understand the mechanics behind this phenomenon, as well as why snowfall and snow cover vary from region to region and from year to year.

Where Does it Snow and Why?

How do Changes in Climate Affect Snowfall and Snow Cover?

Where is More Snow Falling?

Where is Less Snow Falling?

What Causes Snowfall to Vary from Year to Year?

Conclusion

Where Does it Snow and Why?

Two “ingredients” are necessary for snowfall to occur:
• Temperatures between the cloud base and the ground must be around or below freezing;
• A sufficient amount of moisture must be present in the air.

When one of these ingredients is lacking, it becomes the limiting factor for snowfall. How much of each ingredient is present will vary from region to region and from month to month.

Left: Temperatures below or near freezing throughout the lower part of the atmosphere are a necessary condition for snowfall.

Image Courtesy of NOAA.

For example, in the Pacific Northwest, with a prevailing jet stream and surface winds blowing in from the relatively warm Pacific Ocean, winters generally have lots of moisture but not the cold Arctic air that pushes temperatures below freezing. Only the higher elevations (around 2,500 feet and higher) consistently receive snowfall. As result, the region’s mountains hold some of the world’s largest snow accumulations, while in the valleys, which may be only a few dozen miles away, snow is rare. Every ten years or so, a north wind from Canada will pump cold air into the region, giving sea-level cities like Seattle significant snow.

The opposite situation is prevalent in the upper Midwest, which during the winter generally receives a flow of cold air from Canada that keeps temperatures well below freezing. Because this flow originates from land and not the ocean, there is often not enough moisture in the air for snow to occur. Intrusions of warm air from the Gulf of Mexico as well as moisture picked up from the Great Lakes occasionally provide the humidity necessary for snow to form there.

How Do Changes in Climate Affect Snowfall and Snow Cover?

Changes in the frequency and intensity of snowfall occurred over the last century as the world warmed. Because snow’s limiting factors differ from region to region, discussions on the effects a warming trend will have on snowfall are most appropriately conducted with a regional focus. Warming trends do not necessarily mean the whole U.S., a geographic unit with many distinct climate zones, should expect either more or less snowfall.

Where is More Snow Falling?

The Great Lakes Region: As the cold late fall and early winter winds from Canada blow through the Great Lakes region, they pick up moisture from the Great Lakes and deposit it downwind as snow in places like Marquette, Mich. (which gets an average or 180 inches per year) and Syracuse, N.Y. (which gets an average of 120 inches per year) – two of the snowiest places in the U.S. Until the latter part of the 20th century, this lake effect snow would stop around mid-winter in most years when the ice cover had grown across the lake surfaces, thus cutting off the snow’s source of moisture. Today, surface waters in Lake Michigan-Huron are 3.6 degrees Fahrenheit warmer than they were in the late 1970’s and Lake Superior’s surface is 5.3 degrees warmer. The average annual extent and duration of ice cover on these lakes has declined. Lake Superior’s average winter ice cover has fallen from 25 to 15 percent over the last 30 years. Snowfall trends for areas downwind from the lakes climbed during this period when open water during the winter became more prevalent. Syracuse, N.Y., for example, gets an average of 50 percent more snow each winter than it did in the early 20th century.

Left: Trend in the average annual extent of Lake Superior’s ice cover.

Image Courtesy of Austin, JA and Colman, SM, 2008.

The East (Lee) Side of the Rocky Mountains: A combination of more upslope events (i.e. an increase in the number of frontal systems moving across and dumping precipitation over the Rocky Mountain region) and the general increase the region’s cold season humidity have both been correlated with an observed increase in snowfall. This increase is most pronounced in the eastern parts of Colorado and New Mexico.

Above: Snowfall trends for 1930-31 to 2006-07, with dotted areas indicating negative trends and white areas positive trends. Numbers are the magnitude of the respective trends, or the per year percentage departures from the 1930-31 to 2006-07 mean.

Image Courtesy of Kunkel, et al., 2009

Where is Less Snow Falling?

The Southern Snowfall Margin: Oklahoma, Arkansas, Tennessee and North Carolina are considered to be at the southern margin of where wintertime temperatures are cold enough for snow to form. Areas near this southern margin experienced declines in snowfall over the 20th century.

The West: Most of the West’s water supply originates from snowpack. How much snow falls during the winter, when the snow melts and how quickly that melt happens are vital factors for the region’s water resources. Warm, northern-moving currents off the Pacific Coast bring ample moisture to the West during winter. Warming temperatures have corresponded to a decrease in the proportion of annual precipitation falling as snow, decreases in mountain snowpack accumulation and an increase in the elevation of the snowline (the elevation where conditions become cold enough for rainfall to turn to snowfall). Snowfall trends have been particularly pronounced in the Pacific Northwest (PNW), with some mountain stations now reporting snowfall totals less than half of what was reported in the 1930’s. The proportion of annual precipitation falling as snow in the PNW has been declining at a rate of almost nine percent per decade since the 1950’s. Warmer temperatures and less late season snowfall are causing snow cover to melt earlier in the year. In the PNW, the date when snow starts to melt now happens an average of 16 days earlier in the year; in California and Nevada it is happening about nine days earlier. As a result, for the West as a whole, the average date when the spring “pulse” of meltwater is first observed in streams is happening about 20 days earlier than it did in the 1950’s. Earlier snowmelt and peak-annual river flows pose challenges for water managers in the West.

Above: Linear 1950-1997 trends for snow water equivalent, or the amount of snow remaining on the ground, on
April 1 when the melt season across most of the West is underway.

Image Courtesy of Mote, et al. , 2005.

What Causes Snowfall to Vary from Year to Year?

Weather variability is controlled by a variety of cycles: the daily cycle, the annual cycle and the millennial orbital cycles that are believed to largely drive the ice ages. Multi-annual cycles, during which concentrations of heat in the oceans and atmosphere “move around,” occur on periods between the annual cycle and the millennial cycles and are possibly influenced by cycles in solar output occurring on periods of around 11 years and 80 years. A few multi-annual cycles have been identified as particularly important for understanding America’s year-to-year snowfall variability.

The El Niño-Southern Oscillation: Probably the strongest and most ubiquitous single source of interannual variability, the El Niño-Southern Oscillation (ENSO) is a change in the heat distribution in the tropical Pacific Ocean. The eastern tropical Pacific cools and warms over a period of three to seven years. When it is cool, it is considered to be in a La Niña phase; when it is in a warm, it is considered to be in an El Niño phase.

This cooling and warming affect the circulation in the upper atmosphere and the strength and position of the Northern Hemisphere storm tracks that bring winter storms to the U.S. La Niña phases tend to push the storm tracks north, bringing more winter storms to the northern U.S. El Niño events mean a more southern storm track and more wintertime moisture for the southern U.S. These variations in the position of the storm tracks mean variations in the regional frequency of the winter storms that often accompany snowfall.

El Niño phases correspond to more frequent East Coast Winter Storms, or Nor’easters, which form around Cape Hatteras and travel north along the East coast, bringing high winds and snowfall.

The Arctic Oscillation: In the Northern Hemisphere, how much atmospheric mass is (or how many air molecules are) concentrated at the mid-latitudes and the poles varies from day to day and from year to year. This is reflected in changes in the difference in atmospheric pressure between the mid- and high latitudes, which influences weather in the U.S. Specifically, a greater difference in atmospheric pressure means a stronger jet stream. A stronger jet stream works to “block” the Arctic air masses that would otherwise descend into mid-latitudes and the U.S. A weaker jet stream weakens the blocking, allowing the frigid air to penetrate further south.

When the pressure difference between mid and high latitudes is large, the Arctic Oscillation (AO, the term for this shifting of atmospheric mass) is considered positive and the high latitudes are cooler than normal and the mid-latitudes warmer than normal. When the pressure difference is small, the AO is negative and the higher latitudes are warmer than normal and the mid-latitudes cooler than normal. Another way to remember this is a positive AO relates to positive temperature anomalies in the U.S. and a negative AO relates to negative temperature anomalies in the U.S.

Cities like Chicago and Boston have many more extremely cold days during negative AO phases. The recent late-December to early-January cold snap occurred when the AO was strongly negative. Because it controls how much cold Arctic air reaches the mid-latitudes, where this air mixes with warm and wet air masses, the state of the AO has implications for snowfall in America. In generally snow-deprived Atlanta, there are five times as many days when trace snowfall occurs during negative phase winters versus positive phase winters.

The Pacific Decadal Oscillation: The Pacific Decadal Oscillation (PDO) describes changes in the distribution of sea surface temperatures in the North Pacific on a period of 50-60 years. Like ENSO, this sea surface temperature shift affects atmospheric circulation. The effects of this shift are most pronounced on weather in the western U.S.

Warm phases of the PDO generally shift the storm track northward, giving areas like Alaska more wintertime precipitation and areas like the Pacific Northwest less. Cool phases do roughly the opposite. Warm phases also mean warmer temperatures in the Pacific Northwest, so more of the already reduced precipitation falls as rain instead of snow. Reduced snowpack during warm years results.

Above: North Pacific surface temperature anomalies during positive and negative phases of the Pacific Decadal Oscillation.

Image Courtesy of NOAA.

Oscillation Interaction: How snowfall in the U.S. varies from year to year depends largely on how the different phases of these oscillations interact. For example, a winter with an El Niño driven southerly storm track and strong intrusions of Arctic air driven by a negative AO is likely to have above normal snowfall in the mid-Atlantic and southern snowfall margins. On the other hand, these same regions are not likely to receive much snow during a winter with a La Niña driven northerly storm track and a positive AO.

Conclusion

The specific factors that bring together the conditions necessary for snow vary from region to region. Understanding these regional differences is necessary to understand how snowfall can vary as the global climate changes. Some regions have experienced more snowfall during the recent warming trend while other regions are now receiving less. Multi-annual oscillations and the interactions between these oscillations are key variables in regional snowfall occurrence.

Special thanks to Joe Witte and Steve Tracton for their contributions to this paper.

Sources

Austin, JA and Colman, SM. “A century of temperature variability in Lake Superior.” Limnology and Oceanography 53 (2008): 2724-2730.

Austin, JA and Colman, SM. “Lake Superior summer water temperatures are increasing more rapidly than regional air temperatures: A positive ice-albedo feedback.” Geophysical Research Letters 34 (2007): L06604.

Burnett, AW et al. “Increasing Great Lake-Effect Snowfall during the Twentieth Century: A Regional Response to Global Warming.” Journal of Climate 16 (2003): 3535-3542.

DeGaetana, AT et al. “Statistical Prediction of Seasonal East Coast Winter Storm Frequency.” Journal of Climate 15 (2002): 1101-1117.

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Climate Number: Five Trillion Gallons

kraus — Mon, 01 Feb 2010 15:39:54 +0000

While commonly considered two separate lakes, Lakes Michigan and Huron are actually hydrologically one body of water – they are connected at the Straits of Mackinaw and rise and fall in unison. Since 1980, Lake Michigan-Huron has been warming and annually averaged surface temperatures are now 3.6 degrees Fahrenheit warmer than they were in the late 1970’s. This warming has been accompanied by less wintertime ice cover and more evaporation from the Lake’s surface. While about half of this 25 percent increase in evaporation is accounted for by increases in summertime evaporation, more water is evaporating from the lakes during the spring, fall and winter seasons as well. The decline in ice cover means that there is now more liquid water exposed to late fall and winter winds, which pick up the moisture from the Lake and deposit it down-wind in the form of lake effect snow. Increases in Great Lake lake effect snow have been observed during this same period when ice cover has declined. This 25 percent annual increase in evaporation from the Lake means that over five trillion more gallons of water are going into the atmosphere from the lakes each year than during a typical year in the 1970’s. This increase helps to explain why lake levels have been declining over the last few decades despite above average regional precipitation totals. Water levels in Lake Michigan-Huron have been hovering around long-term record lows since 2000.

For Comparison: Five trillion gallons is about the same amount of water stored in the reservoir created by China’s Three Gorges Dam on the Yangtze River. Five trillion gallons is also about how much water is allocated annually to seven western states and Mexico under the 1922 Colorado Water Compact, which is based on estimates of the annual flow of the Colorado River at that time.

Seasons: Winter, Spring, Summer, Fall

Sources: Hanrahan, JL et al. “Connecting past and present climate variability to the water levels of Lakes Michigan and Huron.” Geophysical Research Letters 37 (2010): L01701 and National Geographic: Global Action Atlas. Colorado River Project Summary (2009). Accessed Online 1 February 2010 and Dangerfield, Whitney. “Snapshot: Yangtze River.” Smithsonian.com 1 September 2007. Accessed Online 1 February 2010 and Source: Burnett, AW et al. “Increasing Great Lake-Effect Snowfall during the Twentieth Century: A Regional Response to Global Warming.” Journal of Climate 16 (2003): 3535-3542.

Climate Fact: Musk Ox Parasites and Warming

kraus — Mon, 25 Jan 2010 14:31:48 +0000

In the Canadian Arctic, Musk Oxen endure the long winters and short summers that characterize one of Earth’s most extreme environments. The animals have spent millennia adapting to the brutal cold, but now increases in temperature are presenting new problems. A parasitic species of nematode dwells in the musk oxen lungs and too many nematodes can inhibit respiration in the oxen making them more vulnerable to predators. The nematodes have a larval stage in slugs that live on the tundra and require a certain amount of heat to reproduce and move from larvae to adult. Traditionally, the cold has made it difficult for an adult nematode to reproduce each year and instead they would only reproduce once every two years. From the late 1970’s to 1990, temperatures were generally not warm enough to allow the nematodes to reproduce every year. Around the late 1980’s however, a threshold was reached and from 1990 to 2003 the nematodes reproduced most years and their populations expanded both in number and range. A 50 percent decline in the study area’s musk oxen population was observed from 1988 to 1994.

Seasons: Winter, Spring, Summer, Fall

Source: Kutz, SJ et al. “Global warming is changing the dynamics of Arctic host-parasite systems.” Proceedings of the Royal Society B 272 (2005): 2571-2576.

Climate Fact: Antarctic Sea Ice

kraus — Wed, 13 Jan 2010 14:42:55 +0000

Much attention has been given to the decline of sea ice over the North Pole, which fell to a September minimum of 1.6 million square miles in 2007, about 40 percent below normal levels. On the other side of the world, the sea ice that extends from Antarctica’s continental ice sheets out over the ocean fluctuates between an average summertime (March) minimum extent of about 1.1 million square miles to an average of 6.9 million miles at the end of winter (September). In contrast to the Arctic ice, the average annual extent of the southern hemisphere ice has actually grown since the late 1970s at a rate of around one percent per decade. This trend has been linked to:

•   Ozone Depletion: The most pronounced rates of ozone depletion have occurred over Antarctica, where the ozone hole forms during the spring months. While the strong westerly winds that “trap” frigid air around the continent during winter make the ozone hole possible, the hole itself works as a feedback by accentuating the pressure difference between the continent and the mid-latitudes of the Southern Hemisphere. This works to strengthen the winds responsible for the ozone hole in the first place.
•   Wind Shifts: The accentuation of the pole to mid-latitude pressure difference linked to ozone depletion has deepened several of the continent’s low pressure zones, strengthening some of the winds that blow from the continent over the ocean during the autumn months. This has led to increases in sea ice over several of Antarctica’s coastal regions.
•   Freshwater on the Ocean Surface: Increased precipitation around Antarctica and melting of the glaciers that sit on the land have led to freshening of the ocean surface waters. This promotes ice formation.

Shifts in the winds have also caused decreases in sea ice extent in some areas of the continent – specifically parts of the Southern Ocean adjacent to the Indian Ocean and the Amundsen-Bellingshausen Sea sectors. These losses have been more than compensated for by gains in other areas.

Seasons: Winter, Spring, Summer, Fall

Source: Turner, J et al. “Non-annular atmospheric circulation change induced by stratospheric ozone depletion and its role in the recent increase in Antarctic sea ice extent.” Geophysical Research Letters 36 (2009): L08502.

Climate Fact: The Ozone Hole and Climate

kraus — Mon, 11 Jan 2010 15:28:14 +0000

Near the center of Antarctica in the polar vortex, strong westerly winds that blow in a circle around the continent during winter trap an envelope of air near the South Pole, prohibiting this air from mixing with warmer air masses closer to the equator. The extreme cold in the vortex causes clouds to form in the lower part of the stratosphere. Conditions in these stratospheric clouds are just right for a complex series of chemical reactions to take place, resulting in the destruction of ozone molecules and the formation of the ozone hole. This hole is at its maximum during the austral (Southern Hemisphere) spring months of September through December; stratospheric ozone concentrations during these months can fall by 33 percent. Once temperatures warm sufficiently, the strong westerly winds slow and the polar vortex breaks up, allowing ozone rich air to blow in and ozone poor air to blow out. This movement of ozone poor air is noticeable in parts of New Zealand and South America, where ozone concentrations can temporarily drop by 10 percent.

While the vortex is a local phenomenon, the strength and annual duration of the westerly winds that create the polar vortex are influenced by a larger phenomenon called the Southern Annular Mode (SAM), the difference in atmospheric pressure between 40 and 65 degrees South. When this difference is relatively large, the westerly winds around Antarctica are particularly strong, leading to a stronger vortex and more ozone destruction. High concentrations of ozone, however, can affect the movements of air between the stratosphere and troposphere, ultimately affecting the SAM itself. Better understanding this “coupling” between the SAM and the ozone hole will be needed for better weather and climate prediction, as well as for predicting future ozone concentrations.

Seasons: Winter, Spring, Summer, Fall

Sources: Sparling, B. “The Antarctic Ozone Hole.” NAS Educational Resources. 2001. Accessed Online 10 January 2009 and Fogt, RL et al. “Intra-annual relationships between polar ozone and the SAM.” Geophysical Research Letters 36 (2009): L04707 and Son, SW et al. “Ozone hole and Southern Hemisphere climate change.” Geophysical Research Letters 36 (2009): L15705.