Climate change, one of the biggest environmental problems of our age, is caused by the excessive increase in the amount of greenhouse gases in the atmosphere. Greenhouse gases are gases that can absorb and emit infrared light (heat). Greenhouse gases in the atmosphere first absorb heat radiated from the earth and then reradiate the heat. In this way, it causes some of the heat radiated from the earth to be reflected back. If there were no greenhouse gases in the atmosphere, the Earth would be too cold for liquid water to exist on it. Therefore, greenhouse gases are vital for living things on Earth. However, excessive increases in the amount of greenhouse gases in the atmosphere cause the average temperature of the earth to rise and cause climate change. Today, the Earth's average surface temperature is about 1.2℃ higher than it was before the Industrial Revolution. Global climate change has many negative consequences for living things: extreme weather events, melting glaciers, rising sea levels... At the Climate Change Conference held in Paris in 2015, it was decided to take measures to limit global warming to 1.5℃ in order to minimize all these negative consequences. However, recent scientific studies show that 1.5℃ of warming could also have very negative consequences for the Earth.
Researchers propose two measures of climate change. The first is related to the carbon dioxide concentration in the atmosphere. Human activities such as fossil fuel burning cause carbon dioxide, a greenhouse gas, to accumulate in the atmosphere. In the early Holocene, the carbon dioxide concentration in the atmosphere was around 280 mg/L. Today, this value has increased to 417 mg/L. According to researchers, a safe limit for atmospheric carbon dioxide concentration could be 350 mg/L. This limit corresponds to a global warming lower than the 1.5℃ target set at the Paris Climate Conference.
The second criterion is related to radiative forcing. The Earth receives energy from the Sun and itself radiates energy into space. The change in the Earth's energy balance caused by natural and anthropogenic factors that cause climate change is called radiative forcing. Greenhouse gases and aerosols released into the atmosphere and changes in the earth's ability to reflect light are the most important factors that cause radiative forcing. Today, each square meter of the earth receives 2.91 joules (J) more energy per second compared to before the industrial revolution. It is stated that this value should not exceed 1 J.
As a result, both carbon dioxide concentration and radiative forcing values are currently above the limit values.
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Fresh Waters:
Human activities are also causing changes in the freshwater cycle. The amount of fresh water in certain regions is increasing or decreasing. For example, the Colorado River, one of the largest rivers in the American West, no longer reaches the sea due to overexploitation of its waters and drought caused by climate change. The Aral Sea in Central Asia, once one of the world's largest lakes, has also largely turned into a desert.
The latest study proposes two separate criteria for changes in the freshwater cycle. One of these criteria relates to "blue" water (surface and underground water bodies), the other to "green" water (soil moisture that can be used by plants). Before the Industrial Revolution, variations in blue and green waters were around 3%. Only 10.2% of the terrestrial areas had deviations above these values in the amount of blue water and only 11.2% in the amount of green water. Today, these rates have increased to 18.2% and 15.8%, respectively. It is stated that a safe limit for changes in freshwater could be 95% of the values before the Industrial Revolution. Therefore, the safe zone has been exceeded in both blue and green waters.
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Ocean Acidity:
One of the consequences of the increase in carbon dioxide in the atmosphere due to human activities is the increase in the acidity of ocean waters. When there is more carbon dioxide in the atmosphere, the amount of carbon dioxide dissolved in water naturally increases. This causes the amount of carbonic acid dissolved in ocean waters to increase, making the ocean waters more acidic.
Corals, phytoplankton, and many other creatures living in the ocean use aragonite to build their shells and skeletons. Increasing acidity of ocean waters means more dissolution of aragonite, a type of calcium carbonate crystal. Therefore, increasing the acidity of ocean waters poses a threat to the existence of both living things that use aragonite in their shells and skeletons and other living things that feed on these living things. The aragonite saturation state of surface waters is used as a criterion to define a planetary boundary for ocean acidity. Aragonite saturation state less than 1 means that aragonite will dissolve easily. Aragonite-dependent organisms live in waters where the value is above 1.
Before the Industrial Revolution, the n value of the surface waters of the oceans was estimated to be around 3.44. The limit value that should not be exceeded is accepted as 80% of this value. Today, the value of the oceans' surface water is about 81% of what it was before the Industrial Revolution.
Therefore, although its value is within the safe zone, it is very close to the border. If the amount of carbon dioxide in the atmosphere continues to increase in the future, it is inevitable that this value will decrease and the limit will be exceeded.
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Deforestation
Forests play an important role in regulating the climate. However, the total forest area worldwide is gradually decreasing due to reasons such as land clearing and fires. In this planetary boundary, the focus is on the forests in the tropical and temperate regions, as well as the boreal forests in the regions close to the North Pole, which play an important role in the world system. The limit values that should not be exceeded are determined by comparing them with the potential areas covered by forests in the early Holocene period. For tropical, temperate and boreal forests, these values are 85%, 50% and 85%, respectively.
When it comes to tropical forests, forest areas in North and South America are 83.9 times what they were before the Industrial Revolution; Forest areas in Africa have reached 54.3% of what they were before the Industrial Revolution; Forest areas in Asia have fallen to 37.5% of what they were before the Industrial Revolution. The values of forests in temperate climates are as follows: 51.2% in North and South America, 34.2% in Europe, 37.9% in Asia. The values of the northern forests are: 56.6% in America, 70.3% in Eurasia. These values reveal the extent of deforestation in the three main forested regions of the globe. The same situation applies when looking at the total forest areas worldwide. Today, the total forest area on Earth has fallen to 60% of what it was before the Industrial Revolution. When it comes to total forest areas, the limit that should not be exceeded is stated as 75%.