Earth Gauge » Ice

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 Number: 120 Meters (394 feet)

kraus — Mon, 19 Jul 2010 14:13:18 +0000

For about the past two million years, Earth’s climate system has been characterized by glacial cycles that last between 80,000 to 120,000 years. These cycles have long periods when the Earth cools and ice sheets build up to their maximums, followed by relatively short warming periods when the ice retreats and then “interglacial periods” like the climate we inhabit today. Glacial maximums are characterized by Northern Hemisphere ice sheets extending from the Arctic all the way down to the Ohio River and central Europe, low carbon dioxide (CO2) concentrations (about 180 parts per million) and sea levels about 120 meters lower than today. Sea levels were lower because the water that is in the oceans today was in ice back then. From 20,000 to 7,000 years ago, Earth “deglaciated” and by about 5,000 BCE all that was left of the Northern Hemisphere ice sheet was a small remnant on Canada’s Baffin Island. Carbon dioxide levels by that point had risen to about 280 parts per million – where they remained until the early 19th century – and sea-levels and coastlines were about where they are today.

For Comparison: Twenty-thousand (20,000) years ago, when sea levels were 120 meters lower than today, the Bering Strait was dry land that served as a bridge for the ancestors of today’s Native Americans, who crossed the strait on foot around 15,000 years ago and settled North America. Indochina (Vietnam, Thailand, Cambodia, Laos) was connected by land to the Indonesian Islands of Sumatra, Java and Borneo. Much of the Florida coastline today was hundreds of miles inland back then. The black line in the image below (Image Courtesy of Exploring the Submerged New World 2009 Expedition NOAA-OER) marks the location of the Florida coastline 20,000 years ago.

Seasons: Winter, Spring, Summer, Fall

Source: Denton, GH et al. “The Last Glacial Termination.” Science 328 (2010): 1652-1655.

Climate Fact: Mammal Diversity During Deglaciation

kraus — Mon, 28 Jun 2010 13:39:20 +0000

In Brief: Climate changes during the period from 15,000 to 12,000 years ago coincide with changes in small mammal communities in western North America.

Earth’s ecosystems changed rapidly 15,000 to 12,000 years ago and ecosystems in the western U.S. were no exception. The retreat of the region’s alpine glaciers during this period, which during glacial periods extended much farther into the valleys of the region’s mountain ranges than they do today, exposed new areas lands to plant colonization. In the Great Basin, a steady warming and drying trend evaporated the glacial age lakes that had dotted the landscape, leaving behind salt flats and the Great Salt Lake. The forests that had grown beside these lakes were replaced by deserts. Other transitions from one type of plant community to another were experienced across the rest of the region as species shifted in response to relatively gradual multi-millennial warming, as well as more rapid but short-lived transitions in response to sudden changes in ocean circulation. Even catastrophic floods periodically swept across the Columbia River Basin, destroying everything in their path. This was also the time when the first humans arrived in North America. During this period, the Pleistocene Megafauna, a group of species that included wooly mammoths, giant ground sloths, American lions and mastodons, went extinct. There has been much debate about whether these extinctions were caused by the arrival of humans with efficient hunting tactics or whether it was the climatic changes of the period that were chiefly responsible. An analysis of changes in small mammal populations, such as mice, gophers and shrews – species that would not have been actively hunted by humans – shows that more specialized species of small mammals were being out-competed and out-populated by more generalized species of mammals which were better adapted at adapting to rapidly changing landscapes. These small mammal population changes occurred before major declines in megafauna populations occurred. This suggests that while human hunting may have played a significant role in the extinctions of the Pleistocene Megafauna, shifts in populations that were already underway due to climate change was also an important factor.

Seasons: Winter, Spring, Summer, Fall

Source: Blois, JL et al. “Small mammal diversity loss in response to late-Pleistocene climatic change.” Nature 465 (2010): 771-774.

Climate Trivia: Sea Level and Ice Melt

kraus — Mon, 24 May 2010 13:49:46 +0000

By most estimates, Earth’s sea level rose by 3.5 mm per year between 1993 and 2006. About one-seventh of this sea level rise can be attributed to ice melt on one island – two to three days worth of the summertime melt water from the island could supply the New York Metropolitan area’s water needs for a year!

Trivia Question: Which island is this?

a. Baffin Island
b. Hokkaido (Japan’s northernmost Island)
c. Hawaii’s Big Island
d. Greenland

The correct answer is d. Greenland’s ice sheet, the world’s second largest ice sheet behind the Antarctic ice sheet, has been losing more ice during the summer melt season than it gains during the cold season. For the past few decades, Greenland has been losing about 57 cubic miles of ice each year. For further comparison, this ice melt is about 14 times the annual flow of the Colorado River.

Seasons: Spring, Summer, Fall

Sources: Steffen, K. “Cryospheric Contributions to Sea-Level Rise and Variability.” United States Senate, Washington, DC. 3 May 2007. Accessed Online 21 May 2010

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: 24.2 Teragrams

kraus — Mon, 10 May 2010 14:13:19 +0000

Since the 1750’s, the amount of methane (CH4) in the atmosphere has increased by 250 percent. Much of this methane is emitted from lakes in northern regions. Glacial movement across the far North (north of 45 degrees) during the last ice age leveled the landscape, carved depressions in the bedrock and deposited ice that formed lakes as it melted. About 40 percent of Earth’s lakes are located north of 45 degrees North. Some of these northern regions have almost half of their surface area covered by lakes. On these lake bottoms, the decomposition of organic matter, much of which comes into lakes as the “active layer” of the surrounding permafrost thaws during the warm months, results in significant releases of methane into the atmosphere. In total, 24.2 teragrams (about 53,000,000,000 pounds) of methane is released from northern lakes each year. As temperatures have warmed over the past few decades, the permafrost “active layer” has become deeper in many areas and the total area covered by permafrost has shrunk. Thawing of Siberia’s Yedoma ice complex, where many of these high latitude lakes occur, could result in a total release of almost 50,000 teragrams of methane.

For Comparison: 24.2 teragrams weighs more than two million Boeing 757-200s.

Seasons: Winter, Spring, Summer, Fall

Source: Walter, KM et al. “Methane bubbling from northern lakes: present and future contributions to the global methane budget.” Philosophical Transactions of the Royal Society: A 365 (2007): 1657-1676.

Climate Number: 18 Degrees Fahrenheit

kraus — Mon, 05 Apr 2010 14:12:00 +0000

Earth 13,000 years ago was in the process of thawing from the coldest part of the last ice age. Then, something sudden and catastrophic happened: within a few decades, northern Europe’s average temperature dropped by 18 degrees Fahrenheit. The sudden cold period that followed is called the Younger Dryas, named after the Arctic tundra wildflower that expanded south across Scandinavia at that time. While this cold period was a global event, its most pronounced effects were experienced in northern Europe. Today, the thermohaline circulation in the North Atlantic is driven by sinking cold and salty waters off the coast of Greenland. As these waters sink, they draw up warm waters from the south. The warm waters bring warm air masses to Europe, keeping the continent warmer than it would otherwise be at its relatively high latitude. As the planet was thawing 13,000 years ago, North America’s Laurentide Ice Sheet was melting. This melt allowed previously ice dammed glacial lakes, such as the now dry Lake Agassiz located in modern day Manitoba, to suddenly drain. Shortly after 13,000 years ago, the ice dam that was keeping Lake Agassiz from draining overland into the Arctic Ocean melted, releasing about 2,300 cubic miles of water (almost 24 years worth of discharge from the Mississppi River!) in less than a year. This drainage upset the balance between salt and fresh water that keeps the sinking in the North Atlantic going. As the sinking suddenly stopped, so did the flow of warm waters and air masses to Europe. The interruption of this flow caused the sudden 18 degree Fahrenheit drop in the northern Europe’s temperature.

For comparison: A sudden 18 degree drop in average temperature is equivalent to the climate of Memphis, Tennessee suddenly becoming like the climate of Chicago, Illinois.

Seasons: Winter, Spring, Summer, Fall

Sources: Murton, JB et al. “Identification of Younger Dryas outburst flood path from Lake Agassiz to the Arctic Ocean.” Nature 464 (2010): 740-743 and “River reveals chilling tracks of ancient flood.” Nature 464 (2010): 657.

Climate Number: 229 Trillion Gallons

kraus — Mon, 05 Apr 2010 14:05:47 +0000

Each year, rivers originating in the surrounding mountains and forests send an average of 229 trillion gallons of freshwater into the Gulf of Alaska. The amount of water flowing into the Gulf and when most of the flow occurs affects how salty the waters in the Gulf are. How salty these waters are affects the currents along the shore, which can impact local weather. Salinity variation has also been linked to primary production in the Gulf, which has implications for salmon populations – an important component of the regional economy. Glaciers cover about 30,000 square miles (18 percent) of the Gulf’s drainage area. Melt water from these glaciers currently accounts for about 47 percent of the freshwater discharge, a percentage that has been growing. Over the last few decades, these glaciers have been increasing their annual contribution to the Gulf waters by an average of one trillion gallons each year.

For comparison: 229 trillion gallons is about double the amount of water in Lake Erie.

Seasons: Spring, Summer, Fall

Source: Neal, EG et al. “Contribution of glacier runoff to freshwater discharge into the Gulf of Alaska.” Geophysical Research Letters 37 (2010): LO6404.

Climate Fact: GRACE and GPS Ice Mass Update

kraus — Mon, 22 Mar 2010 14:54:55 +0000

In Brief: The area of ice mass loss on the Greenland ice sheet has been migrating northward since 2005.

As large ice sheets melt and lose weight, the underlying crust “uplifts” or “rebounds” in response. Researchers have been using two key tools to monitor the rates of ice loss on the Greenland Ice Sheet: a) GPS stations located on bedrock next to the ice and b) twin satellites that are part of the Gravity Recovery and Climate Experiment (GRACE). The GPS data monitor uplift directly, but each GPS station can only tell you about the uplift taking place in a local area. Data from GRACE, on the other hand, which uses Newton’s Laws of Gravity to monitor changes in Earth’s mass distribution, can predict crustal uplift by measuring ice mass losses averaged over hundreds of miles. Taken together, these two data sets suggest that the rates of ice mass loss in southeast Greenland are faster now than they were before 2003, a year when destabilizations on the ice sheet likely occurred. Although the rates of ice sheet loss stabilized there by 2006, these glaciers are still contributing more ice to the oceans that they were in the late 1990s and early 2000s. While most of the ice mass loss had been limited to the southern portion of the contienent before 2005, the area where noticeable melting and discharge occurs has been moving north along Greeland’s west coast since that year. Since 2007, the northwest corner of the ice sheet has been shrinking.

Seasons: Winter, Spring, Summer, Fall

Source: Khan, SA et al. “Spread of ice mass loss into northwest Greenland observed by GRACE and GPS.” Geophysical Research Letters 37 (2010): L06501.

Climate Number: 2200 Cubic Miles

kraus — Mon, 01 Mar 2010 14:54:42 +0000

Glaciers have a mass balance. Glaciers lose mass by melting during the warm season (primarily the summer months) and gain mass by accumulating snow during the cold season (centered around the winter months). If a glacier accumulates more mass during the cold season than it loses during the warm season, it is said to have a positive mass balance. If it loses more mass than it gains, it is said to have a negative mass balance. Since 1960, it has become more common for glaciers to have negative mass balance years than positive mass balance years, leading to an overall global trend of glacial retreat. It is estimated that since 1960, the world’s glaciers (this does not include the ice sheets on Greenland and Antarctica) have lost about 2200 cubic miles of ice. Because melt water from these glaciers feeds the creeks and rivers that ultimately flow into the ocean, more glacier melt means higher sea levels. About one-third of the recent 3.1 mm average annual sea level rise is due to glacial melt.

For Comparison: An equivalent to 2200 cubic miles of volume is about 36 million Great Pyramids of Giza, or about six million Sears Towers.

Seasons: Winter, Spring, Summer, Fall

Sources: Global Climate Change Impacts in the United States, Thomas R. Karl, Jerry M. Melillo, and Thomas C. Peterson,(eds.). Cambridge University Press, 2009 and Meier, MF et al. “Glaciers Dominate Eustatic Sea-Level Rise in the 21st Century.” Science Express 19 July 2007 / Page 1 / 10.1126/science.1143906.

Climate Number: 73 Terawatts

kraus — Mon, 01 Mar 2010 14:45:37 +0000

The energy moving in both weather systems and through the wires that power your home can be measured in watts. The Sun heats the Earth causing the fluids of the atmosphere and the oceans to move, creating the winds and currents of Earth’s climate. The vast majority of the energy in the climate system moves through the oceans, where currents of warm and cold waters that dwarf even the largest of Earth’s land rivers transport heat and salt to and from the different ocean basins. Compared to other waters in the Arctic, the Barents Sea, which lies to the north of Scandinavia, has a relatively low amount of seasonal sea ice cover. While the northern portion of the sea freezes over with several feet of ice, the southern portion of the sea remains ice free. A current of water from the North Atlantic brings the basin a sufficient amount of warm water to ward off the Arctic ice’s southerly advance. This current moves about 86 terawatts worth of heat into the Barents Sea, and about 13 terawatts leave the sea through other currents. The remaining 73 terawatts is lost into the atmosphere, making what is known as the Barents Sea Opening a major exit, or release point, for the ocean’s heat storage.

For Comparison: It would take about 61,000 1200 megawatt nuclear power plants – about 140 times the number that exist around the world today – to generate 73 terawatts worth of power. This amount of power is almost 30 times the world’s current electrical generation capacity.

Seasons: Winter, Spring

Sources: Smedsrud, LH et al. “Heat in the Barents Sea: transport, storage, and surface fluxes.” Ocean Science 6 (2010): 219-234 and World Nuclear Association. “Nuclear Power in the World Today.” Accessed Online 28 February 2010

Climate Fact: North American and Eurasian Snow

kraus — Mon, 22 Feb 2010 15:13:48 +0000

Snow is both a product of the weather and a weather maker. It has long been recognized that snow exhibits a cooling effect on local and regional scales. Snow reflects more sunlight than bare ground, meaning that it absorbs less energy. More snow cover also means soils stay moist for longer following the spring melting than they otherwise would. More soil moisture means that more of the sun’s energy that would have been spent heating the ground is instead spent evaporating water. In North America, lots of snow over the continent affects the storm track, or the latitudinal band where travelling cyclonic high and low pressure systems are most common. More specifically, the cold temperatures the snow cover induces eventually force the storm track over North America to veer south. As it does this, the storm track downstream in Eurasia veers north, allowing warmer air masses to penetrate further into the continent than they otherwise would. The presence of these warmer air masses generally means less snow there. Thus, years of above average snow cover in North America tend to be years of below average snow cover in Eurasia.

Seasons: Winter, Spring

Source: Sobolowski, S et al. “Modeled Climate State and Dynamic Responses to Anomalous North American Snow Cover.” Journal of Climate 23 (2010): 785-799.

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

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Climate Number: $16.3 Billion

kraus — Mon, 01 Feb 2010 15:37:37 +0000

When put in 2000 US dollars, freezing rain (ice storm) events in America caused an estimated 16.3 billion dollars in total losses between 1949 and 2000 due to downed power lines, downed trees, agricultural losses, transportation accidents and medical costs from injuries due to slippery conditions. Freezing rain events are most frequent in the Northeast, but are also common across the Midwest and Piedmont regions from North Carolina northward. When freezing rain events hit the Southeast they tend to be accompanied by high dewpoints. This means that while ice storms are rarer in the Southeast, they tend to be heavy and particularly damaging when they do hit. Records kept since the late 1920’s show that ice storms were the least frequent during the 1930’s and rose to a peak in the early 1950’s, showing little or no trend thereafter.

For Comparison: 16.3 billion dollars is around the same amount collectively pledged by all participating parties to fight global poverty following the 2008 U.N. anti-poverty summit. It is also roughly the same amount as NASA’s annual budget.

Seasons: Winter

Source: Houston, TG et al. “Freezing rain events: a major weather hazard in the conterminous United States.” Natural Hazards 40 (2007): 485-494.

Climate Fact: Earth’s Ice and Tipping Points

kraus — Mon, 25 Jan 2010 14:35:59 +0000

Ice masses maintain their own local climate through several mechanisms. One mechanism is known as the ice-albedo feedback mechanism: ice is more reflective than surrounding rock or ocean and the more reflective a surface is, the less sunlight it absorbs and the less it warms. Highly reflective ice surfaces promote the cold conditions that allow the ice to exist in the first place. A second mechanism has to do with a glacier’s height. Higher elevations are cooler and for every 340 feet of elevation gain or loss on a glacier, the temperature at the glacier’s surface will rise or fall by between one and 1.8 degrees Fahrenheit. These two phenomena are types of “positive feedback mechanisms” – an initial loss or gain in an ice mass triggers further losses or further gains. Ice masses are also influenced by what are known as “negative feedback mechanisms” – an initial loss or gain in an ice mass triggers effects that make further losses or further gains less likely. A retreat of the Arctic Sea ice, for example, exposes more ocean water. While this increased exposure allows more sunlight to be absorbed (a positive feedback mechanism) it also means that more heat can leave the ocean, thus cooling the waters (a negative feedback mechanism). Another example of a negative feedback mechanism is glacial retreat causing warming, but that warming leading to increased moisture transport to the glacier, greater snow accumulation and glacial growth. Better understanding of these feedbacks and their interactions with other feedbacks will enable better prediction of future trends in Earth’s ice masses.

Seasons: Winter, Spring, Summer, Fall

Source: Notz, D. “The future of ice sheets and sea ice: Between reversible retreat and unstoppable loss.” Proceedings of the National Academy of Sciences 106 (2009): 20590-20595.

Climate Fact: Antarctica’s Subglacial Lakes

kraus — Fri, 15 Jan 2010 16:02:11 +0000

Beneath the Antarctic ice sheet lie some of Earth’s final frontiers – networks of subglacial lakes, many of which have been isolated from the atmosphere for as long as 15 million years. Outlet channels allow these lakes to periodically drain into the ocean, refill and drain again. The largest of these lakes, Lake Vostok, lies about 2.5 miles below the surface of the East Antarctic ice sheet and is about the size of Lake Ontario. Recently, subglacial lakes have attracted the attention and imagination of much of the scientific community for two primary reasons:

• Ice Stream Stability: Ice streams are areas of continental ice sheets where inland ice flows rapidly into the ocean – they can be characterized as “rivers of ice.” Subglacial lakes are an important component of ice stream dynamics. A series of large lakes sit at the onset of the Recovery ice stream, which comprises eight percent of the East Antarctic ice sheet, providing the initial “lubricant” for ice destabilization and movement (which occurs at a rate of about 320 feet per year). The periodic drainage of these lakes can lead to periodic accelerations in ice flow as well. Better understanding the relationship between subglacial lakes and the ice that covers them is crucial to predicting future rates of continental ice loss and sea level rise.

• Unique Ecosystems: Because subglacial lakes have been essentially untouched by sunlight, oxygen and other ecosystems for millions of years, the life that does exist in these lakes is unique and potentially analogous to early life on Earth, particularly life that survived in extensive glacial periods of Earth’s distant past (500-1,000 million years ago). Samples taken from outlet water flowing from a subglacial lake 500 yards below Taylor Glacier in West Antarctica reveal that the microorganisms living there use a series of reactions with sulfate and ferric iron to “breathe” and metabolize the limited organic matter in this virtually oxygen-free environment. Similar reactions have been performed in laboratories, but no where else on Earth have such ecosystems been found. The scouring of the iron rich rocks by the massive ice sheets is thought to be the source of the nutrients that feed this life.

To see depictions of Antarctica’s subglacial lake networks and Lake Vostok, visit http://www.earthgauge.net/climate-facts-image-library#4. These images come from the National Science Foundation and are in the public domain.

Seasons: Winter, Spring, Summer, Fall

Sources: Grom, Jack. “Ancient Ecosystem Discovered Beneath Antarctic Glacier.” ScienceNOW Daily News 16 April 2009. Accessed Online 14 January 2010 and Bell, RE et al. “Large subglacial lakes in East Antarctica at the onset of fast-flowing ice streams.” Nature 445 (2007): 904-907 and Mikucki, JA et al. “A Contemporary Microbially Maintained Subglacial Ferrous ‘Ocean.’” Science 324 (2009): 397-400 and Christner, BC et al. “Limnological conditions in Subglacial Lake Vostok, Antarctica.” Limnology and Oceanography 51 (2006): 2485-2501.

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: Antarctica’s Moisture Sources

kraus — Tue, 05 Jan 2010 21:35:41 +0000

Once water is evaporated from the ocean or a moist land surface, it may spend days traveling through the air. Complicated systems of winds at different levels of the atmosphere can transport moisture (as well as other gases and dust) from the point of origin to remote locations thousands of miles away. While about 30 percent of the moisture that rains or snows over Antarctica originates in the Southern Ocean close to the continent, the rest comes from latitudes north of 50 degrees South (about the same latitude as the southern tip of New Zealand). Ten percent comes from north of 30 degrees South (about the same latitude as Durban, South Africa). The higher elevations closer to the center of Antarctica have mean moisture origin sources north of 44 degrees South. During the summer, when there is less sea ice, more of Antarctica’s precipitation originates from the waters around the continent.

Seasons: Winter, Spring, Summer, Fall

Source: Sodemann, H and Stohl, A. “Asymmetries in the moisture origin of Antarctic precipitation.” Geophysical Research Letters 36 (2009): L22803.

Antarctica Climate Number: 300,000 Years

kraus — Tue, 05 Jan 2010 21:33:17 +0000

For the first half of the Cenozoic (the era spanning 65 million years ago to today), Earth was too warm to support ice sheets and sea levels were much higher than today. Then, about 34 million years ago, the Earth crossed a threshold. Over a period of about 300,000 years, the temperature dropped and ice sheets began to form on Antarctica. While most of the ice formed in the highlands of East Antarctica, some ice probably formed in West Antarctica, which is much closer to sea-level. The amount of ice on Antarctica has both grown and shrunk significantly over the past 34 million years, but the ice sheet covering East Antarctica has been relatively stable for about the past three million. The water in both ice sheets came from the ocean; as the ice sheets formed during this 300,000 year period, sea level fell by 220 feet, creating much more land area.

Seasons: Winter, Spring, Summer, Fall

Wilson, DS and Luyendyk, BP. “West Antarctic paleotopography estimated at the Eocene-Oligocene climate transition.” Geophysical Research Letters 36 (2009): L16302 and Katz, ME et al. “Stepwise transition from the Eocene greenhouse to the Oligocene icehouse.” Nature Geoscience 1 (2008): 329-334.

Antarctica Climate Number: 7.2 million cubic miles

kraus — Mon, 04 Jan 2010 15:11:48 +0000

Ninety (90) percent of Earth’s ice sits on top of Antarctica, a 5.4 million square mile continent. Virtually all of this area is covered by an ice sheet that can be three miles high with an average thickness of 1.24 miles, giving it an approximate total volume of 7.2 million cubic miles.

For Comparison: The amount of water in just one cubic mile of ice is equivalent to three days worth of discharge from the Mississippi River Delta. If all 7.2 million cubic miles of ice melted, enough water would flow into the oceans to raise sea levels by 275 feet!

Seasons: Winter, Spring, Summer, Fall

Source: ScienceDaily: Science Reference. “Antarctic Ice Sheet.” Accessed Online 4 January 2010