Very cold Arctic stratosphere might lead to
severe ozone loss in the spring of 2005

Colder than usual meteorological conditions in the Arctic ozone layer at about 20 km altitude prompted scientists from all over Europe to alert the public on Friday 28 January 2005. In a common statement they expressed their concern that substantial ozone loss might occur in the Arctic later this spring if the cold conditions persist. The severity of Arctic ozone loss in spring is largely determined by the extent of conditions cold enough for the existence of Polar Stratospheric Clouds (PSC) during the winter months. The overall extent and persistence of such conditions to date in the 2004/05 winter is larger than it has ever been during the past 40 years. Inhabited regions affected by Arctic ozone depletion could extend from Scandinavia down to Central Europe or even further south, since Arctic air masses regularly drift to lower latitudes. Severe ozone depletion will lead to increased levels of harmful UV-B radiation in these regions.

Chemical ozone destruction

The use of halogen-containing substances, such as chlorofluorocarbons (CFCs) and halons has led to an increase in the atmospheric concentration of chlorine and bromine. The substances can cause ozone depletion. The destruction of the ozone layer by man-made chlorine and bromine is most effective under very cold conditions. Rapid ozone loss can occur when temperatures drop below about
-78°C, a value that is sometimes reached in the Arctic ozone layer at about 20 km altitude in winter. Since ozone destruction also requires sunlight, the ozone loss process starts after a cold winter when the sun returns to polar latitudes in spring. So far one has not experienced the same amount of ozone loss in the Arctic as one observes in the Antarctic. Conditions during Antarctic winters are usually considerably colder than during Arctic winters and an ozone hole regularly forms during spring in the Antarctic. So far ozone loss in the Arctic has been much more limited mainly due to the warmer winter conditions there.

Can there be clouds in the stratosphere?

Since early December 2004 ozone scientists have observed unusually low temperatures in the Arctic at 20 km altitude. At low temperatures clouds can form in the stratosphere. The amount of water vapour in the stratosphere is very low, only 5 out of one million air molecules are water molecules. This means that under normal conditions there are no clouds in the stratosphere. However, when the temperature drops below -78°C clouds that consist of a mixture of water and nitric acid start to form. These clouds are called polar stratospheric clouds (PSC) of type I. On the surface of the clouds particles chemical reactions occur that transform passive and innocuous halogen compounds (e.g. HCl and HBr) into so-called active chlorine and active bromine (e.g. ClO and BrO). These active forms of chlorine and bromine cause rapid ozone loss through catalytic cycles where one molecule of ClO can destroy thousands of ozone molecules before it is passivated through the reaction with nitrogen dioxide (NO2).

When temperatures drop to -86°C or colder clouds that consists of pure water ice will form. These ice clouds are called PSC type II. Particles in both cloud types can grow so large that they no longer float in the air and they fall out of the stratosphere. In doing so they bring nitric acid with them. Nitric acid is a reservoir that liberates NO2 under sunlit conditions. If NO2 is physically removed from the stratosphere (a process called denitrification), active chlorine and bromine can destroy many more ozone molecules before they are passivated. The formation of ice clouds will lead to more severe ozone loss than caused by PSC type I alone. On the surfaces of the larger ice particles halogen species are much more effectively activated. The ice particles have large fall velocities and physically remove water from the ozone layer (dehydration).

In the extremely cold Antarctic stratosphere PSCs of both types are formed and this is the reason for the total destruction of ozone in the height range from 14-20 km. During cold Arctic winters PSC type I can form over relatively large areas (up to 15 million sq. km), but PSCs of type II are much more rare and cover much smaller areas.

This Arctic winter the vertical extend of PSCs is very large. Measurements from Sodankylä (Finland) have shown unusual thick PSC layers between 16 and 24 km, and the measurements show the presence of ice particles and indicate the onset of dehydration.  Further measurements of PSCs have been made at Andøya (Norway), Kiruna (Sweden) and Ny-Ålesund (Spitzbergen).

Mother-of-pearl clouds represent a special type of polar stratospheric clouds. In this photo we see such clouds above southern Norway at sunrise on 5 January 2005. Mother-of-pearl clouds show up in the stratosphere, about 20-25 km above the ground, in lee-waves that form when strong westerly winds blow over the Norwegian mountains. The colours are caused by diffraction around the ice particles that these clouds consist of (the so-called corona effect). Despite their beauty they forebode ozone destruction through conversion of passive halogen compounds into active species that destroy ozone. Photo: Geir Braathen, Norwegian Institute for Air Research.

Record cold January  

The cold conditions have worsened during the month of January and the last few days the geographical extent of both types of PSC have reached values which are much larger than ever observed in the Arctic.

"I got concerned when I first saw the temperature forecasts for the Arctic stratosphere about a week ago," says Dr. Markus Rex, an ozone scientist at the the Alfred Wegener Institute for Polar and Marine Research in Potsdam, Germany. "Overall, measured by extent and persistence of conditions for PSC formation, the situation is now colder than anything I have seen in the Arctic before" concludes Dr. Rex. He added that initial analyses of measurements from the international ozonesonde station network confirm that ozone loss has started. Due to the unusual situation these measurements will be intensified.

Dr. Bjørn Knudsen, a meteorologist at the Danish Meteorological Institute in Copenhagen states that these forecasts now have been confirmed by meteorological analyses from the European Centre for Medium Range Weather Forecasts (ECMWF) and radiosonde observations. “Meteorological forecasts can tell us what the situation will be during the next ten days,” says Dr. Knudsen.

Is there a link between ozone and climate?

Recently, climate researchers discovered a tendency towards colder stratospheric conditions in the Arctic winters over the past four decades. Particularly the cold Arctic winters became colder during this period resulting in considerable ozone loss during some Arctic winters of the 1990s. The current Arctic winter further extends the observed cooling trend and could end with unprecedented ozone loss. It is currently not clear whether the change in climate conditions in the Arctic is related to anthropogenic emissions of greenhouse gases. While these gases tend to warm the atmosphere at the surface, the well known greenhouse effect, they radiatively cool the stratosphere. However polar stratospheric temperatures are also influenced by many other climate processes, which are not well understood, and so the net effect of the increasing levels of greenhouse gases cannot be predicted at present. Should further cooling of the Arctic stratosphere occur, increasing levels of Arctic ozone depletion can be expected for the next couple of decades.

What will happen to the ozone layer in the future?

Over the next few decades, the drastic measures to protect the ozone layer that were implemented by a series of international treaties starting with the Montreal protocol in 1987 will pay off: Since the production of CFC, halons and methylbromide, the most harmful ozone depleting substances, is basically banned worldwide now, the concentrations of these gases in the atmosphere will go down over the next decades. But it will probably take about half a century until atmospheric levels of the CFC and halons, that have been released by mankind in the past, will fall below harmful levels. In the meantime the state of the ozone layer will remain vulnerable to any cooling trend of the polar stratosphere. One should also keep in mind that the variability from one year to the next is quite large in the Arctic. The last four winters have been relatively mild and one has observed moderate ozone loss. With the present high levels of chlorine and bromine, the ozone layer will also be highly vulnerable if major volcanic eruptions should take place, as it was seen in the years following the eruption of Mt. Pinatubo in June 1991.

What will happen this winter?

“The meteorological conditions we are now witnessing resemble and even surpass the conditions of the 1999-2000 winter,” says Dr. Neil Harris of the European Ozone Research Coordinating Unit, Cambridge, UK, and one of the coordinators of the SCOUT-O3 project. “However, it is still too early to predict the temperature development in February and March, which are the crucial months for ozone loss in the Arctic. We will watch the development closely from day to day, and the public and our authorities will be informed if the situation becomes worrying,” concludes Dr. Harris.

More information

Contact persons:

Dr Geir Braathen, Norwegian Institute for Air Research

Phone:  (47) 6389 8180 or (47) 9517 7125;

Dr. Claus Brüning, DG Research, European Commission

Phone:  (32) 2 295 4484;

Dr. Martyn Chipperfield, University of Leeds,

Phone: (44) 113 343 6459;

Dr. Neil Harris, University of Cambridge,

Phone:  (44) 1223 311797;

Dr. Esko Kyro, Finnish Meteorological Institute

Phone:  (358) 40 552 7438;

Dr. Niels Larsen, Danish Meteorological Institute

Phone:  (45) 3915 7414;

Prof. J.A. Pyle, University of Cambridge

Phone:  (44) 1223 336473;

Dr. Markus Rex, Alfred Wegener Institute, Potsdam

Phone:  (49) 331 288 2127;


More information on the ozone layer problem can be found at:

Ozone Hole Tour:

UN Environment Programme:

World Meteorological Organisation: 

Antarctic ozone hole:

Real-time chemical calculations:


More information on the SCOUT-O3 project can be found at: 

The ALOMAR Observatory at Andøya, Northern Norway keeps an eye on the development of the Arctic stratosphere and the ozone layer. NILU operates several instrruments at this observatory. On this photo we see some of the lidar instruments in operation.