Kapitel 7: NITROGEN AND PHOSPHORUS POLLUTION
Nitrogen and Phosphorus Pollution
Nitrogen and phosphorus are the building blocks of life. Today, nitrogen and phosphorus are also included in the fertilizers produced for use in agriculture. Although they are useful in terms of increasing agricultural production, artificial fertilizers also harm the environment. Fertilizers spread around due to human activities affect both the nitrogen and phosphorus cycle and cause pollution in land and water. It is stated that in order to prevent excessive oxygen depletion in the oceans, the amount of phosphorus carried by fresh waters to the oceans should not exceed 11 million tons per year. Today, the amount of phosphorus carried by freshwater to the oceans is around 22.6 million tons. The upper limit that should not be exceeded for the amount of phosphorus mixed into erodible soils from fertilizers was determined as 11.2 million tons of phosphorus per year. The current value is approximately 17.5 million tons. Therefore, the limit has been exceeded in both criteria.
The limit that should not be exceeded in terms of nitrogenous fertilizers is 62 million tons per year. The uncertainty in the data regarding nitrogenous fertilizers used today is very high. However, according to the Food and Agriculture Organization, the amount of nitrogen in man-made fertilizers used in agriculture in a year is around 190 million tons. This value is well above the limit.
In conclusion, especially after the development of industry and the rapid increase in the world's population, human activities have caused significant damage to nature. The climate is changing rapidly, forest areas are decreasing, the rate of species extinction has increased extremely, the acidity of the oceans is increasing, access to fresh water is becoming more difficult, synthetic chemicals are being released into the environment unconsciously. All of this means that the conditions in which people have been living for a very long time are changing. Scientific studies on planetary boundaries aim to determine the level these problems have reached today and to raise people's awareness of the steps that need to be taken so that the Earth can continue to feed human societies as it did in the past.
The first study on planetary boundaries was published in 2009. Although the titles have remained the same over time, the criteria and threshold values used in the evaluations have been updated in the light of new findings. In a recent paper, quantitative criteria for all planetary boundaries were determined for the first time. The results show that the limits have already been exceeded in terms of biospheric integrity, climate change, synthetic chemicals, nitrogen and phosphorus pollution, deforestation, and freshwater. Although the limits have not yet been exceeded in terms of ocean acidity and atmospheric aerosols, the trend is worsening.
Today, various efforts are being made to overcome the problems caused by human activities. But these efforts often focus on solving a single problem. However, many of the problems are interconnected. For example, climate change also affects the integrity of the biosphere by increasing the rate of species extinction. Similarly, nitrogen and phosphorus pollution is another factor that increases the rate of species extinction. Therefore, it is necessary to approach the problems caused by human activities as a whole. The main factor underlying most of these problems is the increasing growth of human settlements. In the future, there is a need to reduce carbon emissions, prevent deforestation and develop new agricultural practices.
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Aerosols:
Aerosols are solid particles or liquid droplets suspended in the atmosphere. Desert dust, fire smoke, and dust mixed into the atmosphere during volcanic eruptions are some of the aerosol sources. It is difficult to make precise estimates of how much the amount of aerosols in the atmosphere has changed in the post-Industrial Revolution period. However, based on various observational data, it is estimated that the amount of aerosols in the atmosphere has roughly doubled compared to the 1750s. The biggest source of dust in the atmosphere today is the Sahara Desert. However, during the period 5,000-15,000 years ago, this region was filled with lakes, wetlands and vegetation. A measure called aerosol optical depth (AOD) is used to assess aerosol loading in the atmosphere. AOD corresponds to the total reduction in the amount of sunlight reaching the earth's surface due to aerosols. In the past, a regional limit value was determined for South Asia based on observations of rainfall reductions resulting from high AOD values in South Asia, and it was suggested that an AOD value exceeding 0.25 in monsoon regions would cause a significant decrease in rainfall, disrupting the integrity of the biosphere. Today, the AOD value in South Asia is around 0.3 and therefore above the limit value. Due to the lack of data, it is not known whether the same limit value is applicable worldwide. The average AOD value across the Earth today is about 0.14. Not only high AOD values, but also differences in AOD between the Northern and Southern hemispheres can affect monsoon systems, as seen in the West African and Indian monsoons. Various factors, including natural and anthropogenic aerosols, cause significant temperature differences between the Northern and Southern hemispheres. Differences in radiative forcing caused by aerosols also lead to a relative cooling of the Northern Hemisphere and a shifting of tropical rains to the south. Studies on aerosol-climate interactions following volcanic eruptions; shows that during the last millennium, when the AOD in the Northern Hemisphere was high and the AOD difference between the hemispheres was large, the Northern Hemisphere received less precipitation, while during periods when more aerosols were released in the Southern Hemisphere, the Northern Hemisphere received more precipitation. These results are consistent with observations and global climate models that show that average precipitation in the tropics decreases after large volcanic eruptions. In the 6th Assessment Report of the Panel on Climate Change (IPCC), published in 2021, the decrease in monsoon rains between 1950 and 1980 was partially associated with anthropogenic aerosol emissions in the Northern Hemisphere. Based on these findings, the latest published study suggests using the difference between the annual mean AOD values of the hemispheres to measure aerosol loading.
Today, the annual mean AOD difference between the hemispheres is about 0.076. Due to the seasonal increase in dust storms in the spring and summer periods in the Northern Hemisphere, this value increases to 0.1. Before the Industrial Revolution, the difference between the annual averages is estimated to be 0.03. The limit value that should not be exceeded for aerosol loading was determined as 0.1. However, it is stated that scientific studies are needed to fully understand the effects of AOD differences and that the limit value may need to be updated. The current value currently remains below the set limit.
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