The pathways of heat
The new face of the polar regionsIn the course of climate change, the polar regions are undergoing a remarkable transformation – more rapidly and more conspicuously than in most other regions of the world. The consequences of the warming so far have been most pronounced in the Arctic, where large areas of the sea ice and snow cover are disappearing, the sea water in many areas is becoming warmer, the permafrost soil is thawing more often and for longer periods, and the glaciers in Alaska, Canada, Greenland, Iceland and Norway are all losing large volumes of ice. In the Antarctic, on the other hand, the trends are distinctively different from one area to another. For example, although researchers have been observing a retreat of the ice shelves and glaciers on the Antarctic Peninsula for decades, as well as diminishing sea ice and rising air temperatures (processes that are in part also influenced by the presence of the ozone hole), the visible signs of change in East Antarctica have only recently begun to take shape in a significant way. In the central region of the continent, however, there has been no evidence of warming thus far. Here, temperatures have remained constant, or have even fallen slightly due to ozone depletion.
- 3.1 > When icebergs melt at their surface, distinctive features revealing that process are formed. Icicles consisting of re-frozen meltwater are one of these; puddles or pools in which the meltwater collects are another.
- The fact that snow, ice, sea, land and atmosphere interact in so many ways with one another complicates the situation for both polar regions, so that it is often impossible to say exactly what is a cause and what is an effect. In the Arctic, for example, it may be reasonable to ask: Is sea ice melting because the ocean has become warmer, or is the water becoming warmer because the insulation provided by the sea ice is no longer present? Presumably both factors play a role, as changes in the polar regions are mutually reinforcing, particularly in the Arctic. Without a doubt, however, the underlying trigger for all of this is a general warming of the Earth that is being caused by massive emissions of greenhouse gases.
Thawing at the North PoleThe year 2015 drew to a close with a sensational meteorological event in the Arctic. On 29 December, in the middle of the Arctic winter, the surface temperature at the North Pole rose within a single day from minus 26.8 degrees Celsius to minus 0.8 degrees Celsius. It presumably rained at the northernmost point on the Earth on the day before New Year’s Eve, based on meteorological measurements in Ny-Ålesund, Spitsbergen, that indicated that a storm had transported warm moist air from the North Atlantic towards the North Pole. Sea-ice buoys drifting at 85 degrees latitude in the Arctic Ocean at the time confirmed these observations. They registered a positive average temperature of 0.7 degrees Celsius. Consequently, on 30 December 2015 it was warmer at the North Pole than it was at the same time in some parts of Central Europe.
Two decades ago, such a remarkable heat incursion into the Arctic would have been an extreme anomaly. Today, however, reports of such exceptional weather events in the high north are becoming more common, especially during the winter. For example, in February 2017, at a temperature of plus two degrees Celsius it rained in Ny-Ålesund, Spitsbergen’s northernmost settlement. Instead of icy polar cold, the inhabitants of the research village experienced the dreary weather more typical of northern Germany. One year later, in February 2018, strong offshore winds combined with warmer-than-average air temperatures off the north coast of Greenland led to a first-ever event. The old sea ice frozen to the coast broke off to form a large polynya. On 24 February 2018, when the polynya reached its greatest width, Greenland’s northernmost weather station at Cape Morris Jesup recorded a daily high temperature of plus 6.1 degrees Celsius. At Berlin’s Tegel Airport the high temperature for that day was only slightly above freezing.
- 3.2 > Greater-than-average warming in the Arctic continues in the year 2018. From February 2018 to January 2019 the average surface temperature in large parts of the northern polar region was as much as five degrees Celsius higher than the average values from 1981 to 2010.
- This capricious weather matches a pattern that meteorologists at the polar research station called AWIPEV (French-German Arctic Research Base operated by the Alfred Wegener Institute for Polar and Marine Research [AWI] and the Polar Institute Paul-Émile Victor [IPEV]) at Ny-Ålesund, Spitsbergen, have identified through long-term observations. Over the past 35 years the air above Spitsbergen has warmed significantly, not only near the ground but also at higher altitudes. The warming of the Atlantic sector of the Arctic has been especially prominent in the winter months. During recent cold seasons, the temperatures on Spitsbergen have averaged 3.1 degrees Celsius warmer than those of ten years ago. Summers, on the other hand, have warmed less markedly, with an increase in air temperature in Ny-Ålesund of 1.4 degrees Celsius per decade, calculated throughout the year.
There are similar reports from almost all other parts of the Arctic, and their central message is clear: The northern polar region has been warming more than twice as fast as the rest of the world over the past 50 years, and the trend is continuing. Researchers have observed the largest temperature increases during the winter. For example, in January and February of 2016 the temperature north of 66 degrees latitude was five degrees Celsius above the average monthly value for the years 1981 to 2010. From October 2017 to September 2018 it was 1.7 degrees Celsius warmer all across the Arctic than in the reference period from 1981 to 2010.
- 3.3 > Warming of the Earth due to climate change is not uniform geographically. During the period from 2006 to 2015, for example, temperatures in the Arctic rose twice as fast as those in the rest of the world, whereby the warmer temperatures in the northern polar region were primarily recorded during the winter months. The average temperature was more than three degrees Celsius higher than the average for the period from 1850 to 1900.
Zettajoule is a unit of measure used to refer to especially large amounts of energy that cannot be reasonably expressed in the basic energy unit of joules. A zettajoule is equal to 1021 joules.
Greenhouse gases are warming planet EarthThe warming of the Earth is human-induced and is a result of the unchecked emission of greenhouse gases such as carbon dioxide, methane and nitrous oxide. Since the onset of industrialization, humankind has discharged an estimated 2220 billion tonnes of carbon dioxide into the atmosphere (from 1876 to the end of 2017). This very persistent greenhouse gas is produced primarily by the burning of fossil fuels such as coal, petroleum and natural gas. But it is also released in cement production, the draining of wetlands, and in the deforestation of wooded areas for agricultural and livestock use. As a result of these activities, the concentration of this gas in the Earth’s atmosphere has risen by a factor of 1.5 in recent centuries. In 1750 the value was 277 parts per million (ppm), while present concentrations are around 410 ppm.
The planet’s self-cooling mechanisms are disrupted by the enrichment of carbon dioxide, methane and laughing gas in the atmosphere. This means that the Earth’s surface can no longer simply radiate large portions of the incoming solar energy back into space as long-wave heat radiation, and a kind of heat congestion occurs close to the ground. This has been disturbing the Earth’s climate system at least since 1970, because since that time the planet has been absorbing more radiation than it can release. The average radiation balance value since then has been calculated at around plus 0.4 watts of solar energy per square metre.
In recent centuries around 93 per cent of this additional radiative energy has been absorbed by the oceans and distributed through their depths. The remaining energy has contributed to warming of the air and the continents, so that the global average surface temperature has risen by about one degree Celsius over the past 120 to 170 years. The greenhouse gas carbon dioxide alone is responsible for around 50 per cent of this warming. Methane contributes 29 per cent and laughing gas around five per cent. The remaining 16 per cent is attributed to other substances such as carbon monoxide, halogenated and fluorochlorinated hydrocarbons, and soot particles.
- However, the whole Earth has not warmed uniformly. This is due to the distribution patterns of land and sea areas. The sun heats land surfaces and the overlying air layers more rapidly than it does the large seas. At the same time, however, the ground stores less energy than sea water, and so it also cools down again faster. The oceans are therefore significantly slower in reacting to climatic changes than the atmosphere. The cooling effect of the Antarctic ice masses also plays an important role. Their far-reaching influence on the climate of the southern hemisphere may be one reason why the effects of climate change became apparent earlier and more prominently in the more land-dominated northern hemisphere than in the sea-dominated southern hemisphere. While the first signs of warming appeared in the Arctic as early as the 1830s, for example, the temperatures in Australia and South America remained steady through the turn of the century. In the Antarctic region, it was not until the 1950s that meteorologists began to report rising temperatures on the Antarctic Peninsula and in the West Antarctic.
However, slightly higher local temperatures are not necessarily indicative of general climate change. Scientists can only speak in these terms when a clear and sustained temperature curve – over a period of at least 30 years – exceeds the boundaries that were previously defined by naturally occurring climatic fluctuations. In the Arctic this became clear as early as the 1930s, earlier than in any other region of the world. This was followed by the tropics and the mid-latitudes of the northern hemisphere, where the distinct warming signal was first seen in the 1950s, and then by Australia and Southeast Asia, where mounting evidence for climate change was observed around 60 years ago.
Over the remainder of the world, with the exception of central Antarctica, global warming has been developing at full force since the beginning of the 21st century. Since then, reports of record temperatures have been increasing, and major climate research institutions have begun to rank the warmest years. The list so far is led by the years 2015, 2016, 2017 and 2018. The Arctic region itself experienced its five warmest years from 2014 to 2018.
The oceans are warmingThe fact that global warming so far has been comparatively moderate at around one degree Celsius can mainly be attributed to the world’s oceans. For one thing, the oceans in the past have absorbed 30 per cent of the carbon dioxide emitted by humans and thus noticeably buffered the progress of the greenhouse effect. For another, the oceans possess an enormous capacity to store heat. This is a result of the physical properties of salt water as well as the sheer magnitude of water in the oceans. An example calculation: 1000 times more heat energy would be required to warm all the world’s oceans by one degree Celsius than would be needed to heat up the atmosphere by the same amount.
Furthermore, the oceans react very sluggishly to changes in the environment because their water masses circulate and are repeatedly cooled down as they pass through the polar regions. It therefore usually takes around ten years for the surface water of the oceans to adjust to globally rising air temperatures. Centuries to millennia, on the other hand, are required before the additional heat reaches the deep sea.
- 3.4 > The world’s oceans continuously absorb immense amounts of heat energy. While this heat was initially stored almost exclusively in the upper water layers, it has now been shown to reach deeper levels.
- Based on recent research, the oceans have absorbed about 436 zettajoules of thermal energy since 1871. That is equal to a thousand times the amount of energy that humans presently consume each year. In the past 25 years alone, the oceans have absorbed so much heat that, if they were only ten metres deep, it would theoretically have been sufficient to warm the seas by 16.25 degrees Celsius. It is because the average depth of the oceans is almost 3700 metres that the warming is limited to what has been observed to date.
Nevertheless, the trend is clear. For decades the water in all the world’s oceans has been continuously getting warmer. Most of the heat energy is retained in the upper 700 metres of the water column, but it must be noted that the temperature sensors used for these measurements could not be deployed any deeper prior to the year 2005. Since then, however, autonomous drifting buoys, called “ARGO Floats” (Array for Realtime Geostrophic Oceanography) have been widely deployed. The data from these reveal that the water masses at depths between 700 and 2000 metres are also warming up significantly everywhere, with potentially serious consequences for the global ocean-current conveyor belt. Thermohaline circulation can be weakened by ocean warming in two ways. For one, added heat lowers the density of the water due to thermal expansion. The water becomes lighter. Secondly, the same effect of lowered density results when seawater is diluted with freshwater from increased rainfall or melting of the glaciers in Greenland and Antarctica. Both of these factors, freshwater influx and increasing water temperature, inhibit the sinking of water masses in the North Atlantic and in the Southern Ocean, and this can suppress the driving forces of thermohaline circulation.
- 3.5 > The world’s oceans do not all absorb the same amounts of heat energy. The differences can be easily recognized when the oceans are divided into a number of different measurement regions. The regions far to the south absorb especially large amounts of heat.
Tracking heat in the polar seasFor the polar regions, warming of the world’s oceans is of crucial importance: More heat is being transported into the Arctic and Antarctic regions today than in the past through the ocean currents flowing towards the poles. The Atlantic water flowing into the Arctic Ocean, for example, has become verifiably warmer since the early 1990s. In order to track the pathways of heat into the Arctic Ocean, German and Norwegian Scientists set up a transect of oceanographic survey sites across the Fram Strait at 79 degrees north latitude in 1997, from the west coast of Spitsbergen to the northeast coast of Greenland. At each of the 16 sites in this array of moorings, the temperature, current speed and salinity of the inflowing and outflowing water masses are measured throughout the water column. These data show that the water of the West Spitsbergen Current coming from the North Atlantic is on average one degree Celsius warmer when it passes through the Fram Strait into the Arctic Ocean today than it was when the long-term measurements began 20 years ago. Evidence of this warmer water is already present throughout the entire Eurasian Basin.
Sea-surface temperatures have also risen in most of the ice-free areas of the Arctic Ocean. This is why, today, the sea here not only freezes over later in the year, but the sea-ice also melts earlier, leaving large areas of the Arctic Ocean free of ice for longer periods in the summer. This enables them to absorb more solar energy, which in turn promotes a further increase in temperature.
- 3.6 > The crane on a research ship heaves a mooring chain out of the sea with devices attached for water sampling (top) and for collecting phytoplankton (bottom).
- The Southern Ocean holds a key position in the Earth’s climate system because without the cooling and overturning of water masses in the Antarctic region the oceans would not be able to store as much heat and greenhouse gases as they currently do. The sinking of heavy water represents the only possibility of transporting heat and carbon dioxide from the upper water layers to greater depths for long periods of time, and in the Antarctic this occurs on a much larger scale than in the North Atlantic. Researchers have been documenting a ubiquitous rise in water temperatures in the Southern Ocean since the 1950s. Its magnitude indicates that the sea south of the 40th parallel has absorbed significantly more heat from the atmosphere than all other marine regions combined.
The storage of large amounts of heat over a number of decades has other consequences as well. Based on long-term measurements along the prime meridian, German polar researchers have been able to determine that the entire water column in the Weddell Sea, and particularly the deepest water layer, the Antarctic Bottom Water, has been warming since the 1990s. Similar observations have been made in other Antarctic marine regions and scientists have now ascertained that, at depths below 1000 metres, the Southern Ocean has warmed faster in the past three decades than the global oceanic average.
The reason for this warming is still unclear. Is it primarily caused by warming of the atmosphere above the Southern Ocean? When the air temperature rises the sea is not able to release as much of its own heat to the atmosphere. Furthermore, the wind conditions change over the sea, which can increase or decrease the speed of certain ocean currents and in turn influence deep-water formation. Or is the increase in temperature at depth more likely caused by the influx of warmer waters into the Southern Ocean? Presumably all of these factors contribute to some extent.
It is remarkable that researchers can now track the Antarctic-wide warming of deep water northward to beyond the equator. The heavy water masses flow there after they have filled up the deepest level of the Southern Ocean.
More fog, more cloudsThe influx of warmer waters along with rising air temperatures in the polar regions is resulting in intense warming of the seas there. The warmer an ocean becomes the more water will evaporate from its surface. The water-vapour content of the air increases, amplifying the greenhouse effect and increasing the probability of fog and cloud formation. Both of these phenomena, particularly in the Arctic, prevent the loss of heat energy into space and therefore promote the warming process.
In spring, for example, the snow cover on the Arctic sea ice is melting earlier as a result of higher atmospheric humidity and cloud formation, and the sea ice is thus also melting earlier. In summer, low-hanging clouds and fog promote warming on the surface of the remaining sea ice. Modelling suggests that a diminished sea-ice cover in autumn tends to increase the formation of clouds over the Arctic Ocean, with the consequence that the newly formed ice is thinner at the beginning of winter than it would be with less cloud cover.
Meteorologists at the AWIPEV polar research station in Ny-Ålesund cannot yet say whether the thickness, altitude or consistency of the cloud cover over Spitsbergen has changed because the necessary measurements have only been carried out for a few years. But from the daily weather-balloon launches that have taken place since 1993 to altitudes of 30 kilometres, they know that the air has become warmer and contains more moisture. The scientists report that the island’s climate today, even in winter, is actually more maritime than truly extreme Arctic.
Recent studies support this local perception: Trends in cloud cover vary from region to region, but the Arctic climate has become wetter in many areas. Both the humidity and the amount of precipitation have increased. Researchers see this as a sign that more atmospheric moisture from the middle latitudes is reaching the high north today. They predict a continued increase for the future. Because warmer air masses are able to store more moisture, higher rates of evaporation can be expected over the ice-free areas of the Arctic Ocean, along with more precipitation. The latter will result in a rise in water level in the Arctic rivers. The researchers also expect that summer rain will reduce the albedo of the sea ice and further enhance melting of the ice.
- 3.7 > Atlantification: As a result of the decreasing sea ice in the Barents Sea, warmer Atlantic Water is advancing further north into this marginal sea of the Arctic Ocean, causing a retreat of the characteristic Arctic sea zone.
The Atlantic sends out its tentaclesThe heat-driven changes in the Arctic Ocean are particularly noticeable in the Barents Sea, the northern European gateway to the Arctic Ocean. The 1.4 million-square-kilometre marine area between Svalbard, Norway and the Russian archipelago of Novaya Zemlya has traditionally been separated into two regions with contrasting sea-ice conditions and water-column configurations.
The water masses in the northern part of this sea are vertically layered in typical Arctic fashion. This pattern is characterized by sea ice floating upon a surface layer of cold, rather low salinity water, below which lies another cold but more saline layer called the halocline. These two layers protect the ice floes from the warmer, deeper currents. By reflecting a large portion of the incident solar radiation, the white sea-ice cover prevents large-scale warming of the uppermost water layer during the summer months.
In the southern part of the Barents Sea, however, the sea ice and the cold surface layer are both absent. Here, warm saline water from the Atlantic Ocean flows northward at the sea surface. It loses its heat to the atmosphere, which inhibits the formation of new sea ice in the winter. Furthermore, the ice-free water surface absorbs large amounts of solar energy during the summer months. In August 2018, for example, the surface-water temperature in the southern Barents Sea was eleven degrees Celsius. This was between one and three degrees Celsius higher than the average summer temperature for the years 1982 through 2010. This warming has major consequences.
Studies have shown that, in addition to increasing temperatures and greater amounts of inflowing Atlantic water over the past two decades, the changes are also encroaching further to the north. This advance is facilitated by the drastic decline in sea ice in the northern Barents Sea throughout the year. Because significantly less sea ice is now being formed here in winter than it was at the beginning of the 21st century, there is a diminished input of freshwater into the sea during the normal melting periods in spring and summer. As a result, the temperature and density differences between the surface layer and the deeper layers are disappearing. The once clearly distinguishable water masses are now mixing more often with one another, and the warm Atlantic waters from below more frequently reach the sea surface. There, the higher surface temperatures delay or prevent the formation of new sea ice. When there is less ice available for melting in the spring, the weakened layering of the water masses allows warm Atlantic water to well upward, which in turn inhibits the formation of new ice in autumn. This thus becomes a self-reinforcing process, and scientists refer to it as one of the many “positive feedbacks” acting in the Arctic climate system.
Extra Info Does more freshwater in the Arctic Ocean weaken the Gulf Stream?
- But there is a second important effect of the stronger and deeper mixing of the water masses in the Barents Sea. The Arctic Ocean as a whole loses more heat to the atmosphere because it can be cooled to greater depths through the constant mixing. Until now this process has been primarily typical only for the North Atlantic. In the long term, this change could even result in a northward shift of the elements of North Atlantic overturning circulation into the Arctic Ocean, resulting in even more warming of the Arctic than is already occurring.
Disappearing sea ice and the emergence of a water column without distinctly layered water masses – the Arctic Ocean, as a result of climate change in the Barents Sea, is losing two of its most notable characteristic features. Researchers are now referring to this as “Atlantification” of the Barents Sea, which will bring with it a fundamental change in the living conditions in this marine region. Some climate simulations suggest that the northern Barents Sea may be completely shifted to Atlantic mode by the end of this century. Based on their own observations, however, Norwegian scientists predict that this systemic change could occur much sooner. If the sea ice continues to shrink at the rate it has over the past two decades, such a large amount of freshwater will be lacking that the northern Barents Sea will no longer have clearly stratified water layers by the year 2040, and Atlantification of this water body will be complete.
The Barents Sea, however, is not the only marginal sea of the Arctic Ocean into which warm water is advancing. The Labrador Sea off the east coast of Canada as well as the Bering and Chukchi Seas off the coast of Alaska are warming at comparable rates. In all four of these marine regions the summer surface temperatures are now rising by one degree Celsius per decade. Furthermore, sea ice is receding in all four regions, the ice-free water surfaces are absorbing more solar energy, and warm water masses from below are more frequently reaching the surface. It is therefore extremely difficult to distinguish the individual processes from their effects. What is certain is that climate warming has set into motion processes in the Earth’s climate system that are mutually reinforcing in their effects, and that are becoming increasingly evident, especially in the Arctic region.
Extra Info Dust and soot particles on the skyway to the Arctic
Arctic amplification – a fatal chain reactionWhich effects contribute to amplification, and to what extent, are matters of substantial debate in the scientific community. Some researchers argue that the drastic warming is primarily due to the decreasing snow and sea-ice covers in the Arctic. The fewer lighter areas there are, they say, the lower the reflectivity in the Arctic, and the more solar energy remains in the polar region to drive changes in the oceans and atmosphere. Others point out that the warmer air above the Arctic absorbs more water vapour, therefore enhancing cloud formation, which in turn impedes the radiation of heat energy back into space. Depending on the season and kind of clouds, however, the effect of this could also be reversed such that the cloud cover has a cooling effect.
Both arguments are valid and each can be verified by measurements. The actual explanation for the amplification presumably lies in the interaction of all of these factors, the magnitudes and effects of which vary not only with the seasons but also from region to region. Moreover, the climate system is not only complex but its individual components also interact with each other in an extremely chaotic way, which greatly complicates the identification of causes and effects. Scientists refer to this as climate noise, climate fluctuations, or the natural variability of the climate system.
It is certain that air and ocean currents today transport more heat and moisture into the northern polar region than they did in the past. According to a widely held hypothesis, this reduces the general temperature contrast between the high and middle latitudes. This contrast, in turn, is the energy source for the polar jet stream. This slightly undulating band of strong winds normally circulates around the Arctic region parallel to the equator between 40 and 60 degrees latitude, and like a protective wall it prevents warm southern air masses from encroaching into the Arctic.
But as the Arctic becomes warmer, the temperature difference between the polar area and the southern regions decreases. As a result, the westerly winds that make up the polar jet stream also weaken. The air flow is thus more easily diverted from its zonal alignment by high- and low-pressure areas, and meanders in large waves across the northern hemisphere (see Chapter 2). This opens the way for two opposing shifts in air masses. Over the North Atlantic and western North America, warm, humid air from the south moves into the Arctic. Over Siberia and the rest of North America, on the other hand, cold polar air from the Arctic penetrates southward into the middle latitudes, bringing with it spells of freezing cold, especially in winter.
At times when the jet stream is weak, it is also more common for shifting high- or low-pressure areas to become stalled and remain in one area for a long time. Such a situation routinely leads to extreme weather events, such as prolonged rainfall with subsequent flooding, or prolonged warm weather and drought such as that which occurred in Central Europe in the summer of 2018.
- 3.11 > In the winter of 2018/2019 the Arctic polar vortex broke down into three smaller vortices within twelve days. In North America, polar air then penetrated far to the south, triggering an extreme cold spell in Canada and the northeast United States.
- Scientists do not yet fully understand the details of this high-impact chain reaction. But there has been great progress. New studies indicate, for example, that the drastic decline in sea ice in the Barents Sea and the Kara Sea has played a decisive role in weakening the jet stream over Europe and Asia. Simply stated, the two marginal seas of the Arctic Ocean absorb so much solar energy in the summer that they do not begin to freeze over until October or November, which is relatively late. By then, however, the exposed waters have released so much heat and moisture into the troposphere that more snow falls over Siberia. The increased snow cover, in turn, enhances the reflectivity of the land surfaces, thus facilitating cooling and the formation of a high-pressure area over Siberia.
To the west, meanwhile, a pocket of warmer temperatures forms due to the heat released by the sea. The jet stream, sweeping through the overlying air layers, is thus deflected to the south, but in part also to the north. The warm-air pocket also presents an obstacle for the planetary waves. Air packages coming from the west shoot upwards here like a skateboard in a halfpipe, and maintain enough momentum to rise into the stratosphere and disturb the polar vortex rotating above the Arctic. Under certain conditions they can even split the vortex.
A breakdown of the polar vortex then weakens the jet stream in the troposphere, causing the obstructing high- and low-pressure areas to linger over Europe and Asia. These then divert cold air to Asia and Europe, and warm air towards the Greenland Sea. The latter effect then logically leads to a rise in the air temperature over the Arctic Ocean, a decline in the number of freezing days, and a less strongly frozen or even melting sea-ice cover.
Arctic scientists are predicting an increase in autumn and winter temperatures of up to four degrees Celsius over the next three decades. A warming of this magnitude would result in large areas of the Arctic Ocean to be ice-free for greater parts of the year. Large areas of permafrost ground would also thaw out. Both of these fundamental changes would have direct consequences for the local ecosystems, as well as for shipping, resource extraction and any other human activities in the Arctic.
Different trends seen in the AntarcticIn the Antarctic, climate change is not generating the kind of uniform warming pattern that is observed in the Arctic. This is probably due to the cooling effect of the continental ice masses, in part caused by their high reflectivity, as well as to the insulating effect of the Antarctic Circumpolar Current. In addition, there are great regional differences between marine-dominated coastal areas and the continental conditions over central Antarctica.
In the Pacific sector of West Antarctica as well as in the region of the Antarctic Peninsula, researchers have been observing an acceleration in the motion of glaciers in recent decades along with diminishing sea ice, rising surface temperatures and, in some places, heavier snowfall. These developments are due both to changes in atmospheric circulation, whereby more heat and moisture are transported towards the pole, and to ocean currents that transport warmer water into coastal areas. Westerly winds over the Southern Ocean are responsible for the increase in atmospheric heat transport. These have been strengthening since the 1970s and have shifted their path poleward, triggered by the rising greenhouse gas concentrations and by increasing and sustained ozone depletion over Antarctica in the spring. Both of these processes have led to a greater temperature difference between the tropics and the southern polar region, which has resulted in stronger winds.
The shift of the westerly winds, however, is not the only climatic change in the southern polar region that is driven by the periodic existence of the Antarctic ozone hole. It is now a well-known fact that the regular depletion of ozone over Antarctica has a fundamental impact on the climate of the region.
How the ozone hole alters the Antarctic climateThe Earth has its own sunscreen – a filter composed of ozone. Lying in the stratosphere it almost completely absorbs the shortest and therefore highest-energy rays of the sun, thus preventing this ultraviolet radiation (UV rays), invisible to humans, from reaching the Earth’s surface. Without this natural protective screen, life on the Earth would hardly be possible because when UV rays penetrate the skin or other protective layers of plants, animals and people, they can damage the immune system and genetic material deep within their tissues.
Ozone is a highly reactive gas whose concentration in the Earth’s atmosphere gradually starts to increase above an altitude of ten kilometres. It is most dense at an altitude of 30 to 35 kilometres. Nevertheless, the total proportion of the gas in the atmosphere is extremely low compared to other gases, as illustrated by this calculation: If one were to take an air column that extends from the ground to outer space and subject it to normal atmospheric pressure at a temperature of zero degrees Celsius, all of the ozone it contains would yield a layer just three millimetres thick.
This fact makes the influence of the ozone layer on the Earth’s climate all the more remarkable. In fact, ozone not only absorbs the incoming UV rays; depending on its altitude, as a greenhouse gas it also absorbs heat energy that is radiated from the Earth. The more ozone an air package contains, the more UV rays or heat radiation it can absorb, and the more strongly it heats up parts of the atmosphere. Conversely, this means that if the ozone concentration in the stratosphere decreases, the surrounding air masses cool down.
- 3.12 > Ozone depletion over the Antarctic primarily occurs during a few months of the year. It begins in August and reaches its peak in September and October. Then, in November, the ozone concentration rises again and the hole closes.
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Assault of the free radicalsIt is precisely this phenomenon that scientists have been observing since the ozone layer over the Antarctic began to thin out regularly at the end of (southern) winter and the ozone hole began to appear in September and October. It is due to man-made gases (chlorofluorocarbons and brominated hydrocarbons) that have been used – or are still being used – as propellants, refrigerants or solvents, and contain chlorine or bromine compounds that can destroy ozone. For these gases to unleash their destructive power, however, special conditions are necessary that are only present during the long, dark winters in the polar regions. Therefore, ozone holes can only occur in the Antarctic or, in some exceptional cases, also in the Arctic.
First, the air temperature in the stratosphere must fall below minus 78 degrees Celsius. Such low temperatures only occur in winter, and usually only inside the polar vortex. The polar vortex is a high-altitude depression that forms over a polar region as a result of high thermal radiation into space and the associated accumulation of cold air during the polar night. It extends from the upper troposphere into the stratosphere. The air masses at this altitude contain very little water vapour, but droplets of sulphuric acid are present that mostly entered the stratosphere at some time as a result of volcanic eruptions. At temperatures below minus 78 degrees Celsius, residual water and nitric acid condense on these droplets and freeze. Millions upon millions of acid crystals are formed. From the ground, the crystal accumulations are recognized as polar stratospheric clouds. Colloquially, this celestial phenomenon is called mother-of-pearl clouds.
- 3.15 > Mother-of-pearl clouds shimmer in rainbow colours in the sky over the Antarctic. The clouds composed of acid crystals form when temperatures fall below 78 degrees Celsius. They then trigger ozone depletion.
- These clouds are the chemical factories of the stratosphere. Chemical reactions take place on their crystal surfaces which convert the otherwise harmless propellants and refrigerants to highly reactive gases. These are stable as long as it remains dark. But at the end of the polar night, when the sun rises above the horizon again, they begin to decay and release chlorine or bromine radicals, each of which destroys many thousands of ozone molecules. Bromine is 60 to 65 times more effective in this process than chlorine. The high point of this assault by radicals above Antarctica usually occurs in mid-October, and it does not end until the sun warms up the air masses within the polar vortex, the mother-of-pearl clouds dissolve, and more ozone-rich air flows in from the mid-latitudes. The radicals then lapse into a kind of summer dormancy. They react with nitrogen dioxide, which is also brought in with the inflowing air, to form chlorine nitrate (ClONO2) or bromine nitrate (BrONO2), then remain inactive until the next winter.
Researchers consider an ozone hole to be present when the ozone concentration in the stratosphere falls below a threshold value of 220 Dobson Units. This unit of measurement, named after the British physicist and meteorologist Gordon Dobson (1889–1976), denotes the total sum of ozone molecules in the atmosphere above a given point on the Earth. For comparison: 220 ozone molecules correspond to 220 Dobson Units, and in terms of the example calculation above this would be equal to a pure ozone layer with a thickness of 2.2 millimetres. Before the first occurrence of the ozone hole, the average ozone concentration in the Antarctic was 250 to 350 Dobson Units. Today, during the Antarctic spring, it regularly sinks to a low value of around 100 Dobson Units.
Cooling in the centre, warming on the Antarctic PeninsulaThere are immediate consequences related to temperature developments in the stratosphere and the underlying troposphere when the ozone layer over the Antarctic begins to thin out near the end of the polar night. Initially, the air in the lower stratosphere hardly warms up at all. Without the ozone an important greenhouse gas that absorbs the Earth’s long-wave heat radiation is missing. The air layers in the lower stratosphere are therefore now as much as ten degrees Celsius cooler than in the years before the ozone hole developed.
Since the 1990s, the cooling of the lower stratosphere has led to far-reaching climatic changes in the Antarctic region. The influence of ozone depletion is so widespread that since that time scientists have been able to attribute a large portion of the changes in the temperature patterns in the Antarctic to the ozone hole. One example of this is the slight drop of surface temperatures in the centre of the Antarctic continent. This is because the underlying troposphere also tends to cool down more easily as a result of ozone depletion in the stratosphere.
The sustained cold in the lower stratosphere, however, also prevents a timely collapse of the polar vortex. Instead, its lifespan is increased, which also lengthens the time period of ozone depletion. At the same time, the ozone-related cooling of the lower stratosphere amplifies the temperature contrast between Antarctica and the tropics. This causes changes in the atmospheric circulation patterns. Winds in the stratosphere strengthen and the tropopause above Antarctica descends, which causes direct changes in the weather patterns. The tropopause also influences the way high- and low-pressure areas line up and expand. The band of westerly winds over the Southern Ocean has shifted further to the south, while the temperature and precipitation conditions have changed in some coastal areas of Antarctica, especially in the summer.
Since the discovery of the ozone hole in 1985, summer temperatures along the Antarctic Peninsula have risen noticeably, coincident with a retreat of the sea-ice cover. Especially in the Bellingshausen Sea and the waters to the west and northeast of the Antarctic Peninsula, researchers are recording significantly shorter periods of sea-ice cover than 30 years ago. Scientists have also discovered that storm paths and mid-latitude rains have shifted to the south in the wake of the westerly winds. Both of these phenomena influence the water temperatures and currents in the Southern Ocean. Today, for example, considerably more water is being circulated through the Antarctic than in the 1990s. Further north, in the subtropics, the Hadley Cell has increased in size as a result of the changes. It also now rains more there. The climatic impacts of ozone depletion in the Antarctic stratosphere thus extend far beyond the boundaries of the Antarctic region.
Ozone holes are rare in the ArcticReports of ozone loss over the Arctic are quite rare because the stratosphere in the high north is considerably warmer than in the Antarctic, and the northern polar vortex is much less stable. Thus, only in very few exceptional cases do the super-cold conditions occur that are absolutely necessary for the formation of polar stratospheric clouds. For example, scientists observed remarkably low ozone concentrations above the Arctic in the spring of 2011 and in January and February 2016, when the temperature in the stratosphere dropped to minus 90 degrees Celsius. As a result, more than a quarter of the ozone was destroyed.
An agreement is workingOverall, the concentration of ozone in the stratosphere has been increasing steadily for several years now. This positive development is a result of the signing and implementation of the Montreal Protocol of 16 September 1987. The Protocol restricts or bans worldwide the production of a range of substances that deplete the ozone layer. Model simulations have shown that without this ban on the production of ozone-depleting substances a large ozone hole would have formed over the Arctic in 2011. Smaller holes in the Arctic ozone layer would by now have become a recurring problem.
Thanks to the international agreement, the amount of ozone-depleting substances in the atmosphere has been reduced and the ozone layer is slowly recovering. Outside of the polar regions, for example, the ozone values in the upper stratosphere, at an altitude above 40 kilometres, have increased by several per cent since the year 2000. Researchers now believe that by the year 2030 the ozone layer over the northern hemisphere will recover and rise again to the levels observed in 1980. Over the southern hemisphere this process will probably take 20 to 30 years longer. The Antarctic ozone hole has not increased in size in recent years. This positive outcome is attributable to the Montreal Protocol. The hole is still a feature of the climate system and will continue to appear in the coming years. However, it should slowly become smaller and become a thing of the past by the year 2060, provided that all of the stipulations of the Protocol continue to be met.