The impact of ozone depletion
The ozone is a thin layer of triatomic oxygen molecules located within the atmosphere which is capable of absorbing lethal ultraviolet (UV) radiation from the sun. Ozone occurs naturally within the stratosphere, and it accounts for about 90% of the total ozone molecules in the atmosphere, compared to the tropospheric ozone which forms a major air pollutant and accounts for only10%. Ozone layer in the atmosphere extends vertically up to about 50Km, and there are approximately 12,000 ozone molecules per 1 billion molecules of air, while less quantity exist in the troposphere of about 20-100 molecules per billion molecules of air.
Stratospheric ozone is formed through a continuous complex process of photochemical reaction involving the splitting of oxygen molecules into two oxygen atoms by solar energy and each atom further combines with oxygen molecules to produce ozone. Because the process is sunlight dependent, more ozone is produced at lower latitudes due the concentration of high solar radiation around the equator; as such ozone is continually produced and destroyed in these complex chemical reactions. The global distribution of ozone depends on conditions such as the availability of bromine and chlorine in the atmosphere, high solar intensity and latitudinal location that favour the production of the molecules.
Without this layer, UV-B radiation when reaching the earth is capable of damaging plant and animal tissues, increasing the risk of health problems such as skin cancer in humans as well as destroying both terrestrial and aquatic ecosystems.
Depletion of the ozone has been observed over the years due to the release of chemical substances into the atmosphere by humans. In 2005, scientists have observed the increase in ozone depleting substances (ODS) which results to the thinning of this protective layer over Arctic and Antarctic poles by about 30 – 50%, and a global average reduction of about 3 – 6% compared to the pre – 1980 levels. The process of depletion begins with the release of the ozone depleting substances (ODS) such as chlorine and bromine and chlorofluorocarbons (CFCs) mostly from human sources. These gases further accumulate into the atmosphere for some time depending of their resident times and then transported to the stratosphere through vertical mixing. These non-reactive gases are further converted into reactive compounds by UV radiation, then chemical reactions takes place to destroy the ozone layer. Finally, these gases are transported back to the troposphere where they are removed through precipitation.
Climate change and ozone layer depletion are interlinked because ozone itself is a greenhouse gas and together with other ozone depleting substances such as bromine (Br) and chlorine (Cl) contribute to global warming. Therefore any changes in the atmospheric concentration and distribution of ozone will have significant impact on the global climate system.
Release of these (ODS) substances including carbon dioxide and chlorofluorocarbons has a cooling effect on the stratosphere. This cooling effect favours the chemical reactions in chlorine and bromine thereby contributing to the formation of Polar Stratospheric Clouds (PSC), a condition that results in the depletion of ozone.
Studies have proved that the decrease in stratospheric ozone observed over Antarctica led to changes in the interactions between the stratosphere and the earth. These changes alter the atmospheric circulation particularly the North Atlantic oscillation (NAO), which in turn has an effect on variation of climate around the Atlantic.
Depletion of the ozone has another significant effect on the global biogeochemical cycles which has profound effect on the climate system. Increase in the amount UV-B modifies the carbon cycle by affecting the uptake of CO2 by plants during photosynthesis, as well as carbon storage in plants tissues as biomass.
Because the terrestrial ecosystem serves as a net sink for carbon, changes in the amount of UV radiation is capable of disturbing the photosynthetic and respiration processes which link the atmospheric carbon and terrestrial carbon uptake and release. Within the terrestrial ecosystems, certain plant species become more susceptible to increased UV radiation, hence reducing their ability to capture and store atmospheric carbon dioxide.
Furthermore, a change in the UV radiation increases the rate of productivity of soil micro organisms such as fungi thereby increasing the rate of carbon release from biomass decomposition. This accelerated turn over time of carbon through this process of photo degradation or photo transformation decreases the storage capacity of the soil as a major carbon sink, as such contributing to global warming. Scientific projections from models suggest a major shift in global ecosystems from cooler and wetter to warmer and drier conditions in response to climate change-UV interaction.
Another important linkage between ozone depletion and climate change is the alteration of the marine biological pump of atmospheric carbon dioxide into the ocean bottom under the influence of UV radiation. Coloured dissolved organic matter (CDOM) present in aquatic primary producers which is useful in absorbing UV in the ocean undergoes photo bleaching under higher dose. Thereby resulting in the loss of the pigment and consequently allow more UV penetration into the ocean and reduce the ability of aquatic plants to fix carbon during photosynthesis.
Also, thermal stratification of ocean waters occur as a result of increased CO2 from human- induced emissions decreases mid-water oxygen around the depth of 200-800m, which affect carbon uptake by the oceans. This stratification affect vertical mixing of substances such as bromocarbons found in tropical waters. Under the influence of UV, certain ozone depleting reactive radicals such as bromine oxide (BrO) are produced.
Conversely, climate change also has a significant influence on ozone layer depletion. This influence may either accelerate or decelerate the ozone process of recovery. Climate change induces the formation of Polar stratospheric clouds around the high latitudes which when exported to mid-latitudes generate further depletion of the ozone around such areas. Studies have shown that radiative forcing from global warming may help the ozone to recover because it tends to reduce the formation of such clouds that interact with gases in the atmosphere to destroy the ozone. Evidence was observed in the reduction in the loss of ozone over Antarctica between 2001 and 2004 during the spring period. Since ozone depletion is the principal cause of reduction in temperature of the stratospheric ozone by about (-0.17°C/ decade), increase in the emission of Green House Gases (GHG) into the atmosphere will have a warming effect thereby reversing this loss. Reactions involving compounds of halogen are directly affected by UV-B and climate change. Halomethane emissions attributed to climate change react with UV-B and consequently regulate ozone availability in the atmosphere. Climate change induced increase in temperature stimulates the release of methyl bromide and methyl iodide from certain species of plants under the influence of UV radiation. Also, climate change result in the alteration of the global hydrological cycle by increasing the rate of precipitation and eutrophication of organic carbon into rivers and streams from land. Mineralisation of this organic material takes place under the influence of UV to further release carbon into the atmosphere and contribute to global warming.
In addition, global warming caused by human-induced increase in Nitrogen oxide (NO), Carbon monoxide (CO), and Methane (CH4) from bush fires increases the rate of production of ozone in the troposphere. As such global warming may increase the amount of aerosols present in the atmosphere which subsequently affects the rate of ozone photolysis by about 6-11%.
Other natural factors contributing to climate change such as volcanic eruption and variation in sun-spot activity affect ozone layer depletion. Because ozone depletion in the stratosphere is formed under the influence of solar energy, any increase in the amount of radiation coming from the sun will increase the amount of ozone in the atmosphere. Variation in the 11-year sun spot activity indicate an observed increase and decrease in ozone concentration with corresponding maximum and minimum solar cycles respectively. Furthermore, The Brewer-Dobson circulation is responsible for the transport of sulphur gases from volcanic eruptions into the stratosphere. The ascending branch of this circulation transport gas from the tropics upwards while the descending branch return the gases back to the troposphere in the high latitudes.
Volcanic eruptions also release sulphate gases into the atmosphere. These gases significantly reduce the rate of propagation of incident radiation from the sun and decrease the production of ozone. Other natural factors such as the release of methyl bromide into the atmosphere from rice cultivation deplete the ozone and thus increase the penetration of UV radiation.
There is a strong relationship between UV radiation, carbon and nitrogen cycling which has a significant climate change implications. Increase in UV can affect the nitrogen cycle through changes in the rate of organic matter decomposition of nitrogen containing compounds through nitrogen fixation. Nitrogen compounds such as ammonia and nitrate are continuously cycled within the biosphere in series of complex processes. Dissolved organic Nitrogen (DON) reacts with UV radiation to break it down into more soluble ammonium compound through the process of photoammonification. All these processes determine rates of carbon uptake and decomposition in the global carbon cycle. Report from the World Meteorological Organisation (WMO 2003) indicate feedback mechanisms from increasing water vapour into the atmosphere, which increases the availability of odd-hydrogen radical that leads to ozone depletion by disturbing nitrogen and chlorine cycles.
Within the marine ecosystem, hydrolysis of bromine and iodine takes place by photolysis reaction in the ocean to produce ozone depleting substances. Marine phytoplanktons like algae found in tropical waters emit halogen compounds into the troposphere. Also, the interaction between UV-B radiation and the sulphur cycle contribute to climate change. Pollutants such as dimethyl sulphide (DMS) and carbonyl sulphide (COS) are emitted as aerosols that have cooling effect on the atmosphere.
Climate change can also affect the propagation of planetary waves into the atmosphere (Rhind et al.,2005a;2005b; Scott and Polvani, 2004: Scott et al., 2004). Climate models suggest a significant impact of climate change on troposphere-stratosphere interaction. Studies by Rhind et al. (2001) estimated in about 30% in this interaction resulted from doubling of carbon dioxide amount in the atmosphere. Estimation by Scaife (2001) shows a decadal increase of about 3% as a consequence of climate change. All these interactions have profound effect on the transport of ozone depleting substances into the stratosphere as well as their removal from the stratosphere back to the earth surface.
In order to minimize or eliminate the impacts of ozone layer depletion, the Montreal Protocol on Substances that Deplete the Ozone Layer was signed in 1987, and then came into force in 1989. Under this agreement, various nations that signed up the treaty pledged to reduce the production and consumption of harmful halogen gases .This reduction target begins with the slowing down the production and then their eventual phase out through the use of substitute gases. The use of ozone friendly Hydrochloroflourocarbons (HCFCs) was adopted to substitute the use of CFC-12 in the manufacture of refrigerants and foam making agents.
The Montreal Protocol has successfully achieved a reduction in the concentration of chlorine in the global atmosphere in the late 20th century. Another important achievement is the reduction in the production of methyl chloroform and CFCs to a near zero level at the global scale. Towards the end of this century, substances such as methyl chloride and methyl bromide are expected to be eliminated from the atmosphere due to the projected stabilisation and subsequent reduction in their production.
Complete recovery of the ozone to pre 1980 level is expected under strict compliance to the Montreal Protocol by the middle of this century, with slower recovery rate predicted by computer models around the “Antarctic ozone hole”.
In conclusion, human induced climate change and ozone layer depletion are closely inter-related. With ozone depletion exacerbating the rate of global warming while climate change continues to deplete the ozone. Therefore necessary measures must be taken under the Montreal and Kyoto Protocol provisions to reduce the emission of ODS and other green house gases in order to save the planet from consequences of further warming effects on human health and the environment.