The first rays of light ending the long Arctic winter began the processes that rapidly brought Arctic stratospheric ozone levels to record low levels the spring. The stratosphere, deprived of heat from the lower atmosphere by greenhouse gases and La Nina weather patterns, cooled to unusually low temperatures this winter. Polar stratospheric clouds formed at the exceedingly low temperatures of below negative 80 Celsius. Ultraviolet light hit chemically complex, chlorine-containing ice molecules that began a chain reaction of ozone destruction. Within a few weeks ozone levels in the Arctic stratosphere had plunged to the lowest levels ever measured.
Beautiful but ominous, ozone destroying, rainbow colored, Polar Stratospheric Cloud
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Photo Credit: National Geographic
| This year’s Arctic ozone depletion is caused by industrial CFCs and related compounds. Production of these chemicals was banned by the Montreal Protocol. Ozone loss was particularly large this winter due to unusually low temperature, which results in the presence of clouds in the polar stratosphere. Reactions on the surface of these clouds transform chlorine containing breakdown products of CFCs into compounds that aggressively remove ozone. Even though the Montreal Protocol has successfully banned the production of CFCs and related compounds, chlorine levels in the Arctic stratosphere are only about 5% below the prior peak level, due to the long atmospheric lifetime of CFCs (removal takes 50 to 100 years). The Arctic ozone layer will remain vulnerable to depletion for the next several decades, particularly following unusually cold winters. In contrast, temperatures within the Antarctic stratosphere are cold enough, each winter, to lead to widespread occurrence of stratospheric clouds that are part of the chain of events that causes the Antarctic ozone hole that occurs each spring.
The stratosphere has been observed to cool, following the rise of greenhouse gases (GHGs), because heat that would otherwise reach the stratosphere is trapped below, warming the surface. The situation for the Polar Stratosphere is more complicated because of dynamical heating by waves generated in frontal systems. For several years, however, scientists have noted that the coldest winters in the Arctic stratosphere are getting colder, a development that enhances the ozone-destroying efficiency of the remaining CFCs and could be linked to rising levels of GHGs. “The current winter is a striking continuation of this tendency. Hence, we are not surprised by the fact that such massive ozone depletion has now occurred above the Arctic”, says Rex. |
El Nino (warm water along the equatorial Pacific Ocean) warmed the stratosphere in 2010. In 2011 La Nina (cooler than normal upwelling waters in the equatorial Pacific) brought cooling to the stratosphere. A huge ozone hole opened up in the Arctic in 2011.
The Montreal Protocol to greatly reduce production of ozone depleting CFC gases has successfully reduced the size of the spring Antarctic ozone hole, but the Arctic ozone hole is still growing. Eventually the Arctic ozone hole is expected to diminish but the cooling Arctic stratosphere due to increasing greenhouse gases has counteracted the benefits of reduced CFC levels.
| GENEVA 5 APRIL 2011 (WMO) — Depletion of the ozone layer- the shield that protects life on Earth from harmful levels of ultraviolet rays - has reached an unprecedented level over the Arctic this spring because of the continuing presence of ozone-depleting substances in the atmosphere and a very cold winter in the stratosphere. The stratosphere is the second major layer of the Earth’s atmosphere, just above the troposphere.
The record loss is despite an international agreement which has been very successful in cutting production and consumption of ozone destroying chemicals. Because of the long atmospheric lifetimes of these compounds it will take several decades before their concentrations are back down to pre-1980 levels, the target agreed in the Montreal Protocol on Substances that Deplete the Ozone Layer. Observations from the ground and from balloons over the Arctic region as well as from satellites show that the Arctic region has suffered an ozone column loss of about 40% from the beginning of the winter to late March. The highest ozone loss previously recorded was about 30% over the entire winter. |
