, 2:01 PM GMT on October 04, 2011
An unprecedented ozone hole opened in the Arctic during 2011, researchers reported this week in the journal Nature. Holes in the Antarctic ozone layer have opened up each spring since the early 1980s, but the Arctic had only shown modest springtime ozone losses in the 5% - 30% range over the past twenty years. But this year, massive ozone destruction of 80% occurred at altitudes of 18 - 20 kilometers in the Arctic during spring, resulting in Earth's first known case of twin ozone holes, one over each pole. During late March and portions of April, the Arctic ozone hole was positioned over heavily populated areas of Western Europe, allowing large levels of damaging ultraviolet rays to reach the surface. UV-B radiation causes skin damage that can lead to cancer, and has been observed to reduce crop yields in two-thirds of 300 important plant varieties studied (WMO, 2002.) The total loss of ozone in a column from the surface to the top of the atmosphere reached 40% during the peak of this year's Arctic ozone hole. Since each 1% drop in ozone levels results in about 1% more UV-B reaching Earth's surface (WMO, 2002), UV-B levels reaching the surface likely increased by 40% at the height of this year's hole. We know that an 11% increase in UV-B light can cause a 24% decrease in winter wheat yield (Zheng et al., 2003), so this year's Arctic ozone hole may have caused noticeable reductions in Europe's winter wheat crop.
Figure 1. Left: Ozone in Earth's stratosphere at an altitude of approximately 12 miles (20 kilometers) in mid-March 2011, near the peak of the 2011 Arctic ozone loss. Right: chlorine monoxide--the primary agent of chemical ozone destruction in the cold polar lower stratosphere--the same day and altitude. Image credit: NASA/JPL-Caltech.
What caused this year's unprecedented Arctic ozone hole?
Earth's ozone holes are due to the presence of human-emitted CFC gases in the stratosphere. The ozone destruction process is greatly accelerated when the atmosphere is cold enough to make clouds in the stratosphere. These polar stratospheric clouds (PSCs) act like ozone destruction factories, by providing convenient surfaces for the reactions that destroy ozone to occur. PSCs only form in the 24-hour darkness of unusually cold winters near the poles; the atmosphere is too warm elsewhere to support PSCs. Stratospheric temperatures are warmer in the Arctic than the Antarctic, so PSCs and ozone destruction in the Arctic has, in the past, been much less than in the Antarctic. In order to get temperatures cold enough to allow formation of PSCs, a strong vortex of swirling winds around the pole needs to develop. Such a "polar vortex" isolates the cold air near the pole, keeping it from mixing with warmer air from the mid-latitudes. A strong polar vortex in winter and spring is common in the Antarctic, but less common in the Arctic, since there are more land masses that tend to cause large-scale disruptions to the winds of the polar vortex, allowing warm air from the south to mix northwards. However, as the authors of the Nature study wrote, "The persistence of a strong, cold vortex from December through to the end of March was unprecedented. In February - March 2011, the barrier to transport at the Arctic vortex edge was the strongest in either hemisphere in the last ~30 years. This unusual polar vortex, combined with very cold Arctic stratospheric temperatures typical of what we've seen in recent decades, led to the most favorable conditions ever observed for formation of Arctic PSCs. The reasons for this unusual vortex are unknown.
Figure 2. Global lower stratospheric departure of temperature from average since 1979, as measured by satellites. The large spikes in 1982 and 1991 are due to the eruptions of El Chicon and Mt. Pinatubo, respectively. These volcanoes ejected huge quantities of sulphuric acid dust into the stratosphere. This dust absorbed large quantities of solar radiation, heating the stratosphere. Stratospheric temperature has been generally decreasing in recent decades, due to the twin effects of ozone depletion and the accumulation of greenhouse gases in the lower atmosphere. During Jan - Aug 2011, Earth's stratosphere had its 3rd coldest such period on record. Image credit: National Climatic Data Center.
Greenhouse gases cause stratospheric cooling
When ozone absorbs UV light, it heats the surrounding air. Thus, the loss of ozone in recent decades has helped cool the stratosphere, resulting in a feedback loop where colder temperatures create more PSCs, resulting in even more ozone destruction. However, in 1987, CFCs and other ozone-depleting substances were banned. As a result, CFC levels in the stratosphere peaked in 2000, and had fallen by 3.8% as of 2008, according to NASA. Unfortunately, despite the fact that CFCs are falling in concentration, the stratosphere is not warming up. The recovery of the ozone layer is being delayed by human emissions of greenhouse gases like carbon dioxide and methane. These gases trap heat near the surface, but cause cooling of the stratosphere and increased formation of the PSCs that help destroy ozone. We need only look as far as our sister planet, Venus, to see an example of how the greenhouse effect warms the surface but cools the upper atmosphere. Venus's atmosphere is 96.5% carbon dioxide, which has triggered a hellish run-away greenhouse effect. The average surface temperature on Venus is a sizzling 894 °F, hot enough to melt lead. Venus's upper atmosphere, though, is a startling 4 - 5 times colder than Earth's upper atmosphere. The explanation of this greenhouse gas-caused surface heating and upper air cooling is not simple, but good discussions can be found at Max Planck Institute for Chemistry and realclimate.org, for those unafraid of radiative transfer theory. One way to think about the problem is that the amount of infrared heat energy radiated out to space by a planet is roughly equal to the amount of solar energy it receives from the sun. If the surface atmosphere warms, there must be compensating cooling elsewhere in the atmosphere in order to keep the amount of heat given off by the planet the same and balanced. As emissions of greenhouse gases continue to rise, their cooling effect on the stratosphere will increase. This will make recovery of the stratospheric ozone layer much slower.
Greenhouse gases cause cooling higher up, too
Greenhouse gases have also led to the cooling of the atmosphere at levels higher than the stratosphere. Over the past 30 years, the Earth's surface temperature has increased 0.2 - 0.4 °C, while the temperature in the mesosphere, about 50 - 80 km above ground, has cooled 5 - 10 °C (Beig et al., 2006). There is no appreciable cooling due to ozone destruction at these altitudes, so nearly all of this dramatic cooling is due to the addition of greenhouse gases to the atmosphere. Even greater cooling of 17 °C per decade has been observed high in the ionosphere, at 350 km altitude. This has affected the orbits of orbiting satellites, due to decreased drag, since the upper atmosphere has shrunk and moved closer to the surface (Lastovicka et al., 2006). The density of the air has declined 2 - 3% per decade the past 30 years at 350 km altitude. So, in a sense, the sky IS falling due to the greenhouse effect!
Since any increase in solar energy would heat both the lower and upper atmosphere, the observed drop in upper atmospheric temperatures in the past 30 years argues against an increase in energy coming from the sun being responsible for global warming. The observed cooling of the upper atmosphere is strong evidence that the warming at Earth's surface is due to human-emitted greenhouse gases that trap heat near the surface and cause compensating cooling aloft. It should also give us additional confidence in the climate models, since they predicted that this upper atmospheric cooling would occur. Keep in mind, also, that 2010 was tied for Earth's hottest year on record, and the amount of energy coming from the sun during 2009 - 2010 was the lowest since satellite measurements began in the late 1970s. There has been no long-term increase in energy coming from the sun in recent decades, and the notion that global warming is due to an increase in energy coming from the sun simply doesn't add up.
The development of an ozone hole in the Arctic is a discouraging reminder that humans are capable of causing harmful and unexpected planetary-scale changes to the environment. A 2002 assessment of the ozone layer by the World Meteorological Organization concluded that an Arctic ozone hole would be unlikely to occur, due to the lack of a strong Arctic vortex in winter, and the fact CFCs levels had started to decline. "Day-to-day temperatures in the 2010 - 11 Arctic winter did not reach lower values than in previous cold Arctic winters," said lead author Gloria Manney of NASA's Jet Propulsion Laboratory in Pasadena, Calif., and the New Mexico Institute of Mining and Technology in Socorro. "The difference from previous winters is that temperatures were low enough to produce ozone-destroying forms of chlorine for a much longer time. This implies that if winter Arctic stratospheric temperatures drop just slightly in the future, for example as a result of climate change, then severe Arctic ozone loss may occur more frequently." I might add that its a very good thing CFCs were banned in 1987, or else the Arctic ozone hole would have opened up much sooner and would have been far worse. It is highly probable that we will see future nasty climate change surprises far more serious than the Arctic ozone hole if we continue on our present business-as-usual approach of emitting huge quantities of greenhouse gases. Humans would be wise to act forcefully to cut emissions of greenhouse gases, as the cost of inaction is highly likely to be far greater than the cost of action.
Manney, G.L., et al., 2011, Unprecedented Arctic ozone loss in 2011, Nature (2011), doi:10.1038/nature10556
Weather Underground Ozone Hole FAQ
World Meteorological Organization (WMO), "Scientific Assessment of Ozone Depletion: 2002 Global Ozone Research and Monitoring Project - Report #47", WMO, Nairobi, Kenya, 2002.
Zheng, Y., W. Gao, J.R. Slusser, R.H. Grant, C. Wang, "Yield and yield formation of field winter wheat in response to supplemental solar ultraviolet-B radiation," Agricultural and Forest Meteorology, Volume 120, Issues 1-4, 24 December 2003.