Polar Science and Global Warming by Richard Moritz, Polar Science Center
The Polar Regions
Earth has two polar regions – the Arctic and the Antarctic – and each is considerably larger than the lower 48 United States. The most distinctive features of both polar regions are cold climate and abundant snow and ice, caused by the extreme annual variation of sunlight. At the North and South Poles, the sun is below the horizon for six consecutive months, then above the horizon for the next six months. Poleward of the Arctic and Antarctic Circles (currently approximately 66º 34’ North and South) there is at least one 24-hour period each year when the sun is continuously above the horizon, and one when it is continuously below it. These circles are sometimes used to define the boundaries of the polar regions. Other definitions are poleward of treeline and poleward of the line where the average surface air temperature exceeds 10º C in the warmest month of the year.
The central Arctic is an ocean with depths exceeding 4000 meters, topped by sea-ice (frozen seawater) of average thickness 3 meters. The sea-ice moves continually in response to winds and ocean currents, with typical speeds of 5-10 km per day. Tundra, a treeless land of low growing vegetation, covers the northern fringes of the surrounding continents.
The central Antarctic is a continent, covered by a massive sheet of glacial ice (formed by accumulation of snowfall) of average thickness 2000 meters. The glacial ice moves slowly downhill in response to gravity, with horizontal speed on the order of 10m per year. The vast Southern Ocean surrounds the continent, and supports a canopy of sea-ice thinner on average than its Arctic counterpart.
People settled in the Arctic thousands of years ago, and when explorers from lower latitude reached the Arctic, they found established cultures based on subsistence hunting. In the Antarctic the human presence is limited primarily to tourists and scientists who stay for a season or a year.
The North Pole has a six-month-long day followed by a six-month-long night. Seattle has some daylight and darkness in every 24-hour period. What is the total length of time that the sun is above the horizon at each place during a full year?
If there were no atmosphere, which place would receive the most ENERGY in the form of incident sunlight during a full year?
The North Pole becomes much colder than Seattle during the six months of winter darkness. Why doesn’t the North Pole get warmer than Seattle during summer when the sun is above the horizon for six months?
The polar regions are home to a surprising variety of animals and plants. Each species has adapted to the prolonged periods of sunlight and darkness, the low temperature, and the snow and ice. In the central Arctic, polar bears and Arctic foxes roam the surface of the pack ice. In Antarctica penguins inhabit the perimeter of the continent, feeding in the coastal waters and rearing their young on the ice surface. Marine mammals such as whales and seals abound in the Arctic and the Antarctic, though there are differences of species, for example the walrus lives only in the Arctic, while the leopard seal is an Antarctic resident. Caribou, musk ox, grizzly bears and lemmings range over the Arctic tundra.
Polar science is a broad term encompassing the scientific study of any aspect of the polar regions. Science treats phenomena as consequences of general laws, which may be refuted or not refuted by following the scientific method of observation, hypothesis, experiment and measurement. Polar science has its disciplines, sub-disciplines and inter-disciplines, e.g. physics, chemistry, biology, anthropology, sociology, oceanography, meteorology, biogeochemistry, botany, zoology, and ecology. A large fraction of polar science fits well under the heading “environmental science”, which sometimes is taken to mean the study of everything non-human that interacts with humans. Thus defined, polar science encompasses much. It applies equally to the zoologist fastening a tracking device to a polar bear on the Arctic pack ice, as to the theoretical physicist working on mathematical expressions of thermodynamic principles to predict how a gas migrates through the glacial ice sheet on Antarctica. The last 30 years or so have seen a notable increase in research concerned with long term, progressive changes in the polar regions. This increase has been spurred by theories of climate change as a response to increased concentrations of greenhouse gases in the atmosphere, and by observations of large scale environmental change, especially in the Arctic.
How Have the Polar Regions Changed?
Measurements on land and ocean show that the average temperature of the Earth’s surface increased during the 20th century. From 1975 to 2005 the change is estimated to be more than +0.5ºC. The popular name for this change is “Global Warming”. During this same time period the Arctic changed as well – the extent and thickness of Arctic sea-ice declined, the mass of Arctic glaciers decreased, average surface temperature rose over vast expanses of Alaska, Canada and Siberia, shrubs began to replace tundra plants, permafrost soils warmed, stellar sea lion population declined steeply in the Bering Sea, and the size of Caribou herds in Alaska and Canada fluctuated by more than 50%. People living in the Arctic have been affected by these and other long-term changes. For example, reduced sea-ice cover hastens erosion along coastlines and increases the danger and difficulty of traveling and hunting on the ice. Buildings, roads and other infrastructure constructed on permafrost soil are subject to major disruption if the soil temperature rises above the melting point.
Scientists want answers to big questions: Are the changes progressive, as appears to be the case for sea-ice extent, or up-and-down as with the Porcupine Caribou population? Do the changes differ from one part of the Arctic to another? How are the changes related to one another, and to global warming? What caused the changes? To get good, scientifically-tested answers usually requires in-depth research. For example, it seems plausible that the Arctic warming and reductions of ice are related to global warming, but how does this work, and should the Arctic warm faster or slower than the rest of the planet? Also, if the warming is truly global, how can science explain what has happened in the Antarctic?
Changes in the Antarctic differ. Antarctic sea-ice extent shows no sign of progressive change during the past 30 years. Although surface temperature increased over the Antarctic Peninsula and the nearby Larsen-B ice shelf broke up, the average surface temperature of Antarctica did not change significantly. The concentration of stratospheric ozone above Antarctica in austral springtime declined significantly during the 1980’s and 1990’s, driven by chemical reactions involving Freon and other man-made compounds that release chlorine and bromine into the stratosphere, leading to the famous “Antarctic Ozone Hole”, but this explanation would work with or without global warming.
The leading explanation for recent global warming is greenhouse warming associated with changes in the chemical composition of the atmosphere. Observations show that the chemical composition of the atmosphere has changed, with progressive increases in the amounts of carbon dioxide, methane, chlorofluorocarbons (CFC’s) and nitrous oxide. All of these gases absorb and emit thermal infrared radiation, so that in order to balance the incoming sunlight, the earth’s surface has to be warmer than it would be if these gases were absent. In the next section we’ll describe how these gases can affect temperature over the entire surface of the earth.
How do we Know What Caused the Changes?
The surface temperature of the earth, averaged over the globe and over many years, is determined by an energy balance. Sunlight (visible, ultraviolet and photo-infrared radiation) is by far the largest energy source that heats the earth. Only a little sunlight is absorbed by the atmosphere, most of it either reaches the earth’s surface, where some of it is absorbed, or is reflected back to space by clouds and gas molecules, so the source of heating is concentrated near the earth’s surface. The earth emits radiation which escapes into space, cooling the planet. This is by far the largest energy sink. The amount of radiation emitted varies with the temperature: the hotter earth is, the more radiation it emits. In fact the amount of radiation emitted is proportional to the fourth power of the absolute temperature, according to the well-tested Planck theory of radiation. If the amount of sunlight hitting the earth stays constant (which it nearly does), the earth could eventually reach an equilibrium temperature at which the energy source equals the energy sink. If the earth grew any warmer, the energy sink would increase and the earth would cool. If the earth cooled below the equilibrium temperature, the energy sink would decrease and the earth would warm. We can describe this as a stable equilibrium, because small deviations from equilibrium produce a change that acts to restore equilibrium.
Temperature is a measure of the energy of molecular motion, and may be quantified using several different scales, such as the Fahrenheit Scale, the Celsius Scale and the Kelvin Scale. Each scale is measured off in units of degrees, but the ranges of the scales and the sizes of one degree differ. The units are denoted by F, C and K respectively. The size of one degree and the range of the scale are set in such a way that the freezing point temperature (FPT) and boiling point temperature (BPT) of fresh water at standard pressure are:
The Kelvin scale is also called the absolute scale, because theoretically at a temperature of 0ºK, all molecular motion ceases so that it is not possible to achieve a lower temperature than 0ºK or absolute zero. Planck’s theory relates the emission and absorption of radiation to molecular motions, and so it uses the Kelvin temperature scale to express results in simplest form. Mathematically, the total rate at which a body emits energy by Planck radiation is proportional to T4 where T is the temperature in K. Also the wavelength at which most of the energy is emitted is proportional to 1/T.
The wavelength of maximum radiation emitted by any object, including earth, varies with temperature, again according to the Planck theory. The hotter the object, the shorter the wavelengths of most of the radiation. The sun, with a surface temperature of nearly 6000º K, emits mostly visible light. The earth is much cooler, around 288º K at the surface, so it emits mostly thermal infrared (IR) radiation which has longer wavelengths than visible light. The most abundant gases in the atmosphere (nitrogen, oxygen and argon) don’t absorb IR radiation, but the trace gases water vapor, carbon dioxide, methane, nitrous oxide and CFC’s are IR absorbers. So a lot of the radiation emitted by the earth’s surface heats the atmosphere instead of passing through to space. These trace gases also emit IR radiation, some of which propagates up into space, and some of which propagates down to be absorbed by the earth’s surface, adding to the surface energy budget. To reach equilibrium, the earth’s surface has to emit more IR radiation than it would if there were no atmosphere, thus its temperature is higher. On average, temperature decreases with height from the surface upwards into the atmosphere. Thus the IR radiation emitted to space by the trace gases (which must balance the incoming sunlight), is emitted at a temperature below that of the earth’s surface. In other words, to achieve energy balance with an atmosphere containing absorbing gases, the earth’s surface has to be warmer than it would be without the atmosphere.
This effect has been called the greenhouse effect (although a more appropriate name is the atmosphere effect), and the trace gases that absorb the IR radiation are called greenhouse gases. Water vapor is the most abundant greenhouse gas, and its concentration in the atmosphere is limited by condensation into cloud droplets. Air temperature exerts strong control on condensation. The controls on abundance of carbon dioxide, methane, CFC’s and nitrous oxide are more complicated, depending on chemical processes much slower than condensation. Chemical analysis of air samples over the past 40 years or so shows progressive increases in all of these gases at stations all over the globe, including in the Arctic and Antarctic. The increase of atmospheric CO2 is due mostly to combustion of coal, oil and gas, production of cement, and the burning and decay of trees cut down to clear land. The increase of CFC’s is due to their use in refrigerators, freezers and as propellants in aerosol sprays. These changes increase the greenhouse effect and in the absence of any other effects would cause earth’s surface temperature to rise. In this way the activities of people and societies can influence global climate. The predominant view among climatologists (people who use science to study climate) is that this man-made enhanced greenhouse effect is an essential part of the explanation for the global warming. Thus to figure out how global warming is related to change in the Arctic and Antarctic, the scientist needs to consider the man-made greenhouse effect as a factor that can cause change.
The enhanced greenhouse effect can warm the polar regions by affecting IR radiation locally, and by warming the air and ocean water that enter the polar regions from lower latitudes. These factors can change the surface energy balance and increase the net melting of snow and ice, thus reducing their coverage. Snow and ice are bright surfaces that reflect a large percentage of sunlight. If the coverage of snow and ice decrease, more sunlight is absorbed, heating the polar surfaces even more. The percentage of sunlight reflected from the surface is called albedo, and the process by which warming lowers albedo to produce more warming is called snow and ice albedo feedback. It is a positive feedback because an initial change (warming or cooling) causes a bigger change (warming or cooling). Acting by itself a positive feedback would cause exponential growth of small initial deviations of temperature from equilibrium. Acting in concert with other feedbacks, a positive feedback can increase the size of the temperature change necessary to restore equilibrium when greenhouse gas abundance is changed. The strongest feedback in the global energy balance is a negative one that has already been mentioned: as the earth and its atmosphere warm (or cool), they emit more (or less) IR radiation to space which acts against the warming (or cooling) to restore equilibrium.
One expects the snow and ice albedo feedback to operate in regions where warming can produce a large change in the surface albedo, that is within and on the margins of the Arctic and Antarctic where snow and ice are abundant. For this reason, climatologists want to know if the polar regions have warmed more than the rest of the earth. When changes in temperature in the polar regions exceed the changes over the rest of the earth, the phenomena is called poleward amplification of temperature change.
The interpretation of observed changes in surface temperature, snow, ice, permafrost, plants and animals is complicated because many other factors besides greenhouse warming can have an effect. Climate-changing factors include variations in the output of the sun, variations in the amount of soot and other particles (called aerosols) in the atmosphere, and changes in the earth’s surface caused by agriculture, industrialization and urban development. These factors are often grouped with the man-made greenhouse gases as external forcing factors, because they depend mainly on processes that are outside the workings of the atmosphere and the ocean. Even if none of these factors changed, the earth’s climate would not be constant, because the atmosphere and the ocean are turbulent fluids that vary all by themselves. We call this turbulent kind of variation natural variability. Over time periods relevant to climate change (let’s say 10 years and longer), it is impossible to predict this natural variability in detail. The best we can do is predict its statistical properties, for instance its average value, standard deviation or probability distribution.
A major challenge for climatologists is to estimate how much each factor contributes to a given change in climate. Perhaps the mostly widely used approach is to simulate climate changes running a global, coupled atmosphere/ocean/ice/land climate model on a computer, with one or more of the causal factors held constant, then compare the simulated climate changes with observations. This approach has shown that the global warming of the past 40 to 50 years does not appear in the simulated climates unless the increase in greenhouse gas concentrations is included as a factor. The simulations also show that natural variability can cause variations in average temperature that are similar in size to the temperature change caused by external forcing. Polar scientists often study how the climate models simulate the Arctic and the Antarctic, for example to see if the model simulations produce poleward amplification, and to compare this amplification with observations.
During 1987-1995 the average surface air pressure over the Arctic Ocean was low compared to preceding and following decades, weakening and shifting the pattern of surface winds, which is normally clockwise (anticylonic) over the region. The anomaly in the Arctic was found to be part of a pattern extending over most of the Northern Hemisphere called the Arctic Oscillation (AO). The winds during 1987-1995 forced the sea-ice motion into a pattern that flushed large amounts of old, thick ice out of the Arctic Ocean into the North Atlantic. This ice was replaced by younger, thinner ice which is more susceptible to melting away during summer. Analysis shows that this fluctuation in the AO had a large influence on the downward trend in sea-ice extent. But this downward trend has continued even though the Arctic air pressure has returned to a pattern closer to its pre-1995 state. The connections between the AO and global warming are not clear, and ongoing research aims to determine whether this is part of natural variability or caused by external forcing. Researchers are also exploring the possibility that the response of the sea-ice to the AO is non-linear, such that once the sea-ice is sufficiently thin, even if the forcing is withdrawn, the sea-ice does not return to its previous equilibrium thickness. This has been described as the ice cover going beyond a tipping point, analogous to the tipping of a glass of water.
To most people, ice is a familiar substance, simply the solid form of water. A closer look reveals that ice is quite strange. For example, water is the only substance on earth that becomes less dense when it solidifies. If it were not this way, ice on lakes and seas would sink instead of float! In fact, water is the only substance on earth that occurs naturally as a gas (water vapor), a liquid and a solid. The latent heat of ice melting is the energy required to change a unit mass of ice into liquid water at the melting point temperature. The latent heat of ice is among the highest of all substances on earth that can change from solid to liquid. Most chemicals dissolve readily in water, which is sometimes called the universal solvent. But when liquid water solidifies, dissolved chemicals are not incorporated into the ice crystal lattice. Instead they are expelled with some of the liquid water as a brine that contains all the dissolved chemicals but only some of the original water. The freezing point temperature of saltwater brine decreases with increasing salinity, i.e. the more concentrated the brine, the lower the temperature required to freeze the water in the brine. This explains why salt is sometimes sprayed on the surface of roads covered by snow and ice – the salt lowers the freezing point below the ambient temperature, so that the snow and ice melt into a briny slush. The slush allows better contact between the tires and the road, and is easier to move with a plow.
When freezing happens in the saltwater of the ocean, globs of pure ice form and some of the brine gets trapped in pockets in between. Thus sea-ice is a mixture of pure ice and brine. If the temperature drops, the freezing point temperature of the brine will have to drop to maintain equilibrium. Since the freezing point temperature is roughly proportional to the brine concentration (salinity), this means the brine has to get saltier. Therefore some of the brine will have to freeze into pure ice, leaving behind all the salt and a reduced amount of liquid water with a higher salinity and a smaller volume. For the same reasons, brine pockets expand when sea-ice warms. This process increases the heat capacity of sea-ice, i.e. the amount of energy needed to change the temperature of one kilogram of ice by one ºC. During summer the ice may warm so much that the brine pockets expand right through to the ocean water underneath, so that the brine can drain into the ocean, freshening the ice. In this way ice which survives a summer melting season becomes less salty. In fact, Arctic sea-ice that has survived two or more summers tends to make excellent drinking water because nearly all the salt has drained back into the ocean.
Polar Research Around Puget Sound
Scientists in the Puget Sound region have conducted polar research for many decades. In the late 1950’s the University of Washington participated in the International Geophysical Year (IGY) with scientists conducting measurement program on the pack ice of the Arctic Ocean and the glaciers of Alaska. In subsequent decades the research expanded within the UW, and other organizations became established in the region. Visit the Links page for a list of the local research organizations.