Lunar and Planetary Institute

Explore! Ice Worlds - Ice on Earth Background

Ice on Earth Background Information

Ice Is Important on Earth

In our everyday experiences, we encounter water typically in its liquid state. Most of our fresh water, however, exists in its frozen form. About three-quarters of it is found in snow, sea ice, icebergs, ice shelves, glaciers, ice sheets, and soils that remain frozen for two or more years (permafrost). Snow and ice may appear only seasonally at mid-latitudes, but at high altitudes and in the polar regions, frozen water persists year-round as glaciers and ice sheets. Glaciers form in regions where more snow accumulates than melts, such as in high mountain valleys. In the extremely cold polar regions, the glaciers grow to form continent-sized ice sheets. The largest ice sheets cover Antarctica, and smaller ice sheets cover Greenland and part of Iceland. Some of the ice in the Antarctic ice sheet represents the build-up of nearly a million years of snow.

Do You Have Ice in Your Own Backyard?

Do you live at a high altitude under the vaulted ceiling of glacier-topped mountains? Does winter blow snow your way?

Whether you have snow and ice in your backyard is determined by your regional climate. Latitude, elevation, and ocean currents shape your region's temperature, precipitation, and wind patterns — or in short, your local climate. Climate is distinct from weather; while weather can change in a matter of hours or with the seasons, climate is the long-term average weather of a region. The average of thirty years or more of weather determines the climate of a region. Your closet is probably full of clothes, shoes, coats, and hats that are appropriate for your local climate. What you choose to wear on any given day is determined by the weather. If you were to travel to one of the polar regions, you might expect to pack an entirely different set of clothing than what's in your closet. Be sure to check the weather report before embarking on your polar excursion, however; it is the nature of weather to not always fit in with what's expected for a region.

The Poles are Miles Apart

We tend to lump the far-away, frigid polar regions together in our minds as "the poles." While the Arctic and Antarctic share many features, their differences are epitomized by their most charismatic inhabitants: polar bears in the north and penguins in the south.

Floating sea ice dominates the land of the polar bears. The Arctic is an ocean surrounded by land. The land masses of Greenland and Iceland have thick ice sheets, but the ocean is covered by sea ice up to six to nine feet thick. Polar bears live on this floating ice and have easy access to the sea to hunt for seals, fish, and beluga whales. Native peoples likewise live on the sea ice and northern lands. They have hunted in the ocean since prehistoric times. Musk ox, reindeer, caribou, foxes, and wolves live on the land in the lower latitudes of the Arctic.

The tilt of the Earth masks the Sun's warming rays for six winter months and plunges the Arctic into continual darkness. In summer, the tilt of the Earth bares the northern regions to the rays of Sun so that to a polar bear standing on the North Pole, the Sun appears to draw a daily circle around her low in the sky from late March to early September. Temperatures reach an average of 37–54°F (3–12°C). This perpetual morning offers enough warmth to melt some of the sea ice, although some remains throughout the year. The sea ice can expand as more ocean water is frozen, forming an ice sheet reaching to the encircling landmasses of Canada, Greenland, Russia, Alaska, Iceland, Norway, Sweden, and Finland. The coldest temperature recorded in the Arctic was about –90°F (–68°C), and the average wintertime temperature is -30°F (–34°C).

Earth's seasons

The Earth's tilt creates seasons. In summer, the North Pole points toward the Sun to create a 24-hour-long day. In winter, the night is equally long because the pole points away from the Sun.The seasons are reversed for the South Pole. Image courtesy of Athropolis.

On the other side of the world, penguins inhabit a frozen desert completely — and happily for them — free of polar bears (and any indigenous land mammals). Should a penguin stand directly over the South Pole, he too would find a perpetual state of darkness in winter and light in summer; although, those seasons would be in reverse for him. The Antarctic winter descends at the end of March while summer begins in late September.

Directly below the penguin would be an entirely different icy setting than his counterpart, the polar bear, hunts on. Instead of the relatively thin sheet of sea ice over ocean water, thick sheets of ice rest on land in the Antarctic. That ice and land is much higher in altitude than the sea ice at the Arctic; the South Pole is about 15,000 feet above sea level! Not only is the continent high, it is cold! The interior of the continent gets chilly in winter — the lowest recorded temperature for the planet was recorded there at –128.6°F (–89.2°C) — and even the more temperate coastal areas only get up 59°F (15°C) in summer. The Southern Ocean encircles Antarctica with strong air and ocean currents that keep the warmth of higher latitudes at bay.

While the Southern Ocean helps make Antarctica the coldest continent, the Arctic Ocean actually helps make the north relatively warmer. Most of the salty water of the Arctic remains liquid since ocean water freezes at a lower temperature than fresh water. And thanks to the high specific heat capacity of water, the ocean water also doesn't cool as quickly as land (just as pizza crust cools to eating temperature more quickly than the liquid sauce). At 30°F, the Arctic Ocean would hardly seem warm to us! Yet its relative warmth, seeping up through the sea ice, is balmy compared to the thousands of feet of ice and frigid land below the penguin's feet.

Antarcticas harsh conditions prevented humans from colonizing the area; people have been visiting for only a few hundred years. Even now, no humans permanently inhabit Antarctica. A treaty signed in 1959 preserves the continent's ecozone and prohibits military activities so that today's 46 participating countries only send peaceful, scientific explorers to visit. Aside from the occasional human and the indigenous penguins, other birds, seals, whales, mosses, lichens, and two types of flowering plants live there.

The Polar Regions Are Changing

The frozen nature of both of these polar realms makes them seem immobile and unchanging. In fact, the dynamic nature of seemingly immovable ice is a shaping factor in these environments, and indeed, for the whole Earth.

On a seasonal scale, sea ice forms and melts at both poles. Sea ice formation is so significant around the coast of Antarctica that winter sea ice doubles the area of that continent.


Sea ice (shown in light blue) and snow (white) melt and extend seasonally at both poles. (Land is represented by grey and ocean, by dark blue.) The extremes in the Arctic (top) and Antarctic (bottom) are shown for the months of March (left) and September (right). The extent of sea ice is at its lowest at the end of the warmer months, which falls in September in the Arctic and March in Antarctica. Scientists are concerned by the decreases in summer sea ice recorded in recent years. Six months of cold and darkness nearly double the area of Antarctica, so that by September the sea ice has grown to the size shown in the bottom right. Sea ice extends over the Arctic Ocean to reach the neighboring landmasses in March, as shown in the upper left. Images courtesy of the National Snow and Ice Data Center.

Over a period of years a hidden property of ice is revealed: ice flows. Glaciers do not merely accumulate snow to expand or melt to retreat; their immense weight causes them to flow like a stream. Furthermore, the pressure of all that ice may cause the very bottom to melt and form a slippery layer. Glaciers both flow and slip down valleys.

As glaciers and ice sheets flow and slip down the land, they change the landscape. They gouge out "u"-shaped valleys; push boulders, gravel, and sand into hills called moraines; and eventually thrust their outer rims out onto the ocean as thick, floating ice shelves.

Ice shelves are anchored to the land by the glacier or ice sheet and float in the ocean at the rims. During the summer, pieces break off the end to form icebergs in a process called calving. The ice shelf continues to flow, as its glacier flows, out into the ocean, but it also serves to shore up the ice behind it and keep the heart of the glacier or ice sheet from flowing quickly to the ocean.

Global Change

In recent years, scientists have observed an alarming trend: ice is melting across the world. Each winter, less sea ice forms on the Arctic Ocean and it melts earlier in the spring. Glaciers in the mountains of New Zealand, Canada, and Alaska are melting back. Ice shelves in Antarctica are collapsing.

When ice melts, it makes water. If that ice is on land, above sea level, the water is added to the ocean, causing the ocean level to rise. Most of the world's — including over half of America's — populations live near coastlines that would be impacted by flooding. Sea levels are already rising, primarily because water expands as it warms in response to global climate change, but also because land-based ice is melting.

Regions vulnerable to Sea Level Rise

Image created by Robert A. Rohde/Global Warming Art.

In Antarctica, the Larsen B ice shelf collapsed into the ocean in February 2002 and the nearby Larsen A ice shelf collapsed a few years previously. Scientists' primary concern is not the melting of this floating ice; sea ice, ice shelves, and icebergs are already in the ocean and have already displaced as much sea water as they ever will. They will not raise sea levels as they melt. Behind melting icebergs and ice shelves, however, remain glaciers and ice sheets. With its ice shelf melted and calved away, a glacier or ice sheet may flow unchecked into the ocean and add its volume of water to the sea. Scientists have found that the ice sheet supported by the Larsen ice shelves is moving eight times faster now than before their collapse. It is the melting and movement of such land ice into the ocean that poses the threat of higher sea levels.

In the Arctic, the extent of summer sea ice set a record low in 2007. While the sea ice extent was slightly higher in the summer of 2008, the minimum levels of Arctic sea ice have been overall decreasing over the last 30 years. Polar bears became the first species to be Federally listed as threatened under the Endangered Species Act in 2008 directly due to global warming.

Earth's ice is melting because global temperatures, on average, are rising.

Earth's Climate Is a Balancing Act

Earth's temperatures — and by extension, its moderate climate — are shaped by many factors, many of which influence each other. All of Earth's systems, including the energy balance; water, rock, and carbon cycles; and the motions of oceans and atmosphere interplay to create our climate. With so many influences to consider, it is no wonder that climate scientists must use powerful computer models to understand Earth's climate of the past, present, and future.

Luckily, our planet is a warm place. The Sun provides over 99% of Earth's energy; alone, that energy is enough for our planet to reach a rather chilly -2°F (-19°C) or so. (Geothermal energy from the Earth's interioir contributes less than 1% of our energy. Radioactive decay of elements and gravitational energy inside Earth add such a small amount of warming compared to the Sun that it will be ignored here.) It is thanks to a small percentage of all the tiny gas molecules in our atmosphere that Earth is warm enough to inhabit. Naturally-occurring greenhouse gases trap the Sun's energy and keep it from reflecting back into space. Volcanoes and bacteria in natural wetlands are examples of greenhouse-gas producers. Earth is warmed to an average temperature of about 57°F (14°C) by a natural process called the greenhouse effect.

Earth's Climate

Images courtesy of NOAA.

Much of the Sun's radiation is in the visible range of the electromagnetic spectrum, and for the most part, this type of light passes right through our atmosphere. It warms the surface of the Earth, causing Earth to give off a radiation of its own: infrared radiation. The infrared radiation of the Earth is invisible to us. Unlike the higher-energy radiation from the Sun, the low-energy, long-wavelength infrared radiation can't pass back through the atmosphere with ease. Some of it does manage to escape back into space, but most of it is captured by greenhouse gas molecules.

All molecules are able to absorb and emit light energy, but at the molecular scale, that light has to have just the right amount of energy for a particular type of molecule to absorb. Greenhouse gas molecules are all made up of at least three atoms bonded together. They are able to absorb infrared radiation because its energy is just right for causing the atoms of the molecules to move slightly in relation to each other, or vibrate. (The visible light from the Sun was too energetic for the molecules to "catch.") The molecules then emit the energy as infrared radiation, which is often caught by another greenhouse gas molecule or the surface of the Earth.

Carbon dioxide, methane, nitrous oxide, and even water vapor are greenhouse gases. While the atmosphere is 78% nitrogen and 21% oxygen, greenhouse gases make up only a tiny fraction of the air we breathe. For instance, carbon dioxide makes up almost 0.04% of the atmosphere; methane is more efficient at absorbing infrared radiation from the Earth but makes up only about 0.0002%. Water vapor comes and goes in the form of clouds, fog, and humidity. It is highly variable, and may represent between 1-4% of the atmosphere at the surface.

In addition to the greenhouse effect, other factors help moderate the Earth's temperatures by absorbing more of the Sun's energy or reflecting it. These factors have influenced each other and changed to create Earth's evolving climate over time.

Earth's Temperature

 Earth's temperatures are a result of the balance between various warming and cooling influences. Earth image courtesy of NASA Goddard Space Flight Center Image/Reto Stöckli.

Earth's global surface temperatures are rising at an unprecedented rate. The past century has seen an increase of a little more than 1°F (0.74°C). A degree may not seem large to us, but we are accustomed to thinking locally. Local temperatures change with the weather, season, and time of day, often much more than a degree in a single day. Global changes in temperature are averages that take into account the large local variations and represent a change in the balance of factors that shape Earth's climate. Today's global temperatures are the highest of the past 500 years, perhaps even for the past millennium.

Temperature change is nothing new; the Earth has undergone many changes in global temperature in its past. Changes in Earth's orbit, in addition to less influential changes in the Sun's intensity, outgassing from volcanoes and other sources, and changes in ocean currents, have resulted in cycles of cooling and warming. Certain eras in the Age of the Dinosaurs were warmer than today, and the ice ages were colder. However, none of these periods saw such a drastic change in global average temperature over a short period of time as today. Large changes in temperature occurred in the last million years during the glacial cycles, but the global warming at the end of an ice age is thought to have taken 5,000 years. In addition, these changes were all due to natural factors.

Scientists generally attribute the current climate change to increases in greenhouse gas concentrations in the atmosphere. Scientists also largely agree that carbon dioxide released to the atmosphere by human activities is the main culprit of global warming. It is released from burning coal, oil, natural gas in power plants, cars, factories, and to some extent, from the clear cutting of forests. Human activities release other greenhouse gases. Methane is released by farm animals, rice paddies, rotting garbage in landfills, mining, and extraction of natural gas. Chlorofluorocarbons (CFCs) are well-known for creating the ozone hole, but are also implicated in their additional role as greenhouse gases in the separate problem of climate change. The fertilizers used to grow our food add nitrous oxide.

Scientists have records of the amount of carbon dioxide in Earth's atmosphere stretching thousands of years into the past. A dramatic increase in the percentage of carbon dioxide in the atmosphere corresponds with the Industrial Revolution and has proceeded to climb sharply in the ensuing years. Today's levels far exceed even the highest levels of the past 750,000 years.

Ice is a handy record-keeper. Air bubbles trapped in ancient ice have allowed scientists to measure the components of Earth's past atmospheres. Scientists drill cores in glaciers and ice sheets and analyze the preserved bubbles of prehistoric atmospheres. Also contained in the core is wind-blown volcanic ash, which is used to date its layers. Slight differences in the kinds of elemental oxygen within the ice tell the scientists how cold the air was when the snow fell

Ice Cores

Ice cores and other data provide evidence that the amount of carbon dioxide in Earth's atmosphere temperature have fluctuated in a cyclical pattern through time. These cycles of cooling and warming are natural, and caused, over the last 750,000 years, primarily by cyclic changes in Earth's orbit. During that time frame we have experienced alternating periods of warmth and periods of glaciations. However, at present, the levels of carbon dioxide in the atmosphere far exceed even the highest levels of the past half-million years. Our global temperature is increasing in response to this added greenhouse gas. Image courtesy of the UN Intergovernmental Panel on Climate Change (IPCC), Third Assessment Report, Climate Change 2001.

The carbon dioxide concentration in the atmosphere has been measured directly since 1957. The instruments at Mauna Loa, Hawaii reflect the seasonal uptake and release of carbon dioxide by plants as "wiggles," but show an overall sharp increase.

Atmospheric Carbon Dioxide

The amount of carbon dioxide in Earth's atmosphere has been directly measured at Mauna Loa, Hawaii for half a century. The data reflects a sharp increase in carbon dioxide to levels higher than our Earth has experienced in the past half-million years — and certainly higher than humans have ever experienced. The seasonal uptake and release of carbon dioxide by plants in the northern hemisphere were captured here as "wiggles." Image courtesy of Robert A. Rohde/Global Warming Art.

Computer Models Help Decode the Complicated Mystery of Climate Change

Climate models are used to weigh the influences of the myriad of factors which shape our planet's climate, including greenhouse gases from natural and human sources, changes in the Sun's intensity, and the complicated interactions of Earth's systems. While the contributions of natural warming are still not fully understood, it is generally agreed that their effects are comparably minor. Changes in solar intensity and volcanoes produced most of the warming from pre-industrial times to 1950, but are not implicated in the current global change. For instance, when Mt. Pinatubo erupted in 1991, the global average temperature dropped by 0.9 °F (0.5 °C) as volcanic particles in the air reflected some of the Sun's energy. (The volcano also released carbon dioxide, a warming agent, but this addition is thought to be small compared to human contributions.) Studies by the National Center for Atmospheric Research (NCAR) attribute less than a third of the current warming to changes in the Sun's intensity.

Data from all realms of science are pulled together to create and validate the computer models. For example, scientists note what species of flowers bloom earlier in the year and in what regions coral reefs die off because of warmer sea surface temperatures. They measure how increasing carbon dioxide is acidifying our oceans as it dissolves to form carbonic acid. Satellites take data on ice cover, precipitation, temperature, and other characteristics of our planet from above.

Tomorrow's World Will Be a Different Place

The clothes in your closet, when you can plant what flowers in your garden, the varieties of local produce you buy at the grocery store, the types of plants and trees growing in your parks, and what wild animals live in or migrate through your area are all determined by your local climate. Imagine how your world might change as your local climate is reshaped by global climate change. While it might be tempting to blame a hot summer's day on global warming, short duration warmer -or cooler-than-average temperatures are a part of Earth's natural charm (just to keep us on our toes!). Scientists are trying to understand how changes in temperature, precipitation, and sea level will impact Earth's diverse regions.

Climate change

Climate change will alter the temperature, precipitation, and sea levels, which will, in turn, impact human health, agriculture, forests, water resources, coastal areas, and species and natural areas. Image courtesy of United States environmental protection agency (EPA)/Philippe Rekacewicz, UNEP/GRID-Arendal

Scientists use mathematical computer models to predict how the various warming and cooling factors will shape tomorrow's climate. Changes in the Sun's intensity, ice and cloud cover, volcanoes, greenhouse gases, natural biological influences, and human activities all interact in complicated ways.

The thawing of Earth's freezers — the polar regions — will have far-reaching effects. The nature of the polar regions makes them more sensitive to the consequences of climate change than warmer latitudes. Reflective white ice and snow will melt into dark rivers and oceans that better absorb the Sun's energy. Like freezer overdue for defrosting, increasing temperatures will expose organic matter long locked away in the frozen ground of the arctic tundra. This permafrost will thaw and plant matter decomposing in the resulting marshes will release the greenhouse gas methane.

Antarctic sea ice is as necessary to penguins as forests are to songbirds. If the 3.6 °F (2 °C) rise in global temperatures predicted over the next 40 years comes to fruition, essential nesting and feeding grounds will have melted away. The warming would translate to a 50% decline of emperor penguins. The Pt. Géologie colony that increased this species' fame through the movie March of the Penguins is declining as northern Antarctic temperatures increase. With less sea ice, Adélie penguins have a shorter journey from their nests of rock to fetch food from the ocean for their chicks. However, Adélie penguins are adapted to the cold and overall are harmed by increasing temperatures. They face a loss of 75% with the predicted temperature rise. Climate change adds to the problems of pollution and over-fishing of the Southern Ocean.

Arctic sea ice is predicted to continue disappearing.Commerce by sea will have entirely new opportunities for transport through the opened Arctic Ocean, but the changes for humans and animals dependent of the ice are grim. According to a study by the United States Geological Survey, the predicted loss of Arctic sea ice in future years may result in the loss of 2/3 of the polar bear population by the middle of this century.

Arctic Conditions

Present and projected minimum sea-ice extent and vegetation types for the Arctic and neighboring regions. Image courtesy of the UN Intergovernmental Panel on Climate Change (IPCC), Fourth Assessment Report, Climate Change 2007.

Changes will vary across the globe. More warming is expected in the interiors of continents and in the northern latitudes of the Northern Hemisphere than at the coastal regions and tropics. Heat waves are expected to become more intense. Higher temperatures will lead to faster evaporation, and rain, when it occurs, may fall in the form of heavy downpours. More precipitation may provide additional water to some regions, but floods and droughts are also expected to become more frequent. Storms may increase in intensity; in addition, rising sea levels will impact coastal areas. Crops may experience longer growing seasons and fewer frosts. The warmer temperatures and increased carbon dioxide in the air may help some crop varieties, but they, too, have a point at which it is too warm to survive. The ranges of plants and animals, biodiversity, and migratory patterns are expected to continue to change in response to climate change. Pests, parasites, and diseases are likely to thrive in the warmer temperatures, much to the irritation of the plants, animals, and humans they prey upon. While heat waves may contribute to heat-related illness and death, milder winters may be a benefit for health issues. Beach erosion, reduced snowfall, and changes in flora and fauna may limit opportunities for outdoor excursions, but warmer temperatures will provide more opportunities to venture outdoors.

Humans Have the Power to Stabilize Global Change

Humans clearly have an impact on the global environment and the ecosystems it supports. Our use of fossil fuels, such as coal and oil, has added carbon dioxide to the atmosphere and warmed our planet. Now, our influence can be used to stabilize or reduce global warming!

Use of fossil fuels pervades our everyday life and it is challenging to know where to begin reducing it. Not only do fossil fuels power our cars and school buses, coal often produces the electricity that runs our air conditioners and charges our cell phone batteries. In addition, fossil fuels are often used in the production and transportation of our goods before we even take them home from the store. For instance, fossil fuels are used for the energy and materials to create plastic bottles and transport heavy drinking water across the country to the local grocery store.Thus, not only does driving less and conserving electricity help combat global warming, so does being a savvy consumer of local produce and recycled goods.

In addition to carbon dioxide, the greenhouse gases methane and nitrous oxide are by-products of everyday practices. Methane, a natural waste product of certain microbes living in the intestines of cattle, is released by these animals in large amounts. Stocking up on protein from fish, and especially beans and other vegetables, instead of beef is one way grocery shoppers can help slow global warming. Human-produced fertilizers break down in the soil and release nitrous oxide, so composting the vegetable clippings from that high-protein bean salad to use as natural fertilizer has a further positive impact on climate change.


Last updated
October 19, 2009


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