Can human activities have any effects on worldwide phenomena such as depletion of the ozone layer and climate change?
There is overwhelming evidence that human activities are influencing global phenomena.
Natural environmental cycles often span thousands of years but most scientific measurements are only available for the past one to three centuries. It is not easy to accurately determine the influence of humans on any natural activity. In the case of the ozone layer, the depletion of the ozone over Antarctica cannot be explained by natural cycles alone but is caused by the increase of man-made chemicals in the stratosphere. The relationship between these chemicals (e.g. chlorofluorocarbons, also known as CFCs) and ozone depletion has been shown by experiments in laboratories, numerical modelling studies and by direct measurements in the atmosphere (see Figure 8a).
By absorbing the infrared radiation emitted by Earth, some substances influence the natural energy flows through the atmosphere. The greenhouse gases, such as carbon dioxide, although only a tiny fraction of the atmosphere, play an important role in this process. Carbon dioxide (CO2) is produced when fossil fuels are used to generate energy and when forests are burned. Measurements show that its concentration has increased by almost 30% over the past 250 years. In the mid-20th century, the concentration of CO2 was 280 ppmv, and it is currently 390 ppmv, and increasing at about 3% per decade. Methane and nitrous oxide emitted from agricultural activities, changes in land use, and other sources are also important (see Figure 8b).
The increase in greenhouse gases contributes to climate change in the form of increased temperatures (about 0.6°C near Earth’s surface over the past 150 years) and a rise in sea level. Models of the climate change induced by emission of greenhouse gases predict that the global temperature will rise between 2 and 6°C in the next century. If this happens, the change would be much larger than any temperature change experienced over at least the last 10,000 years.
There is overwhelming evidence that human activities are influencing global phenomena.
Natural environmental cycles often span thousands of years but most scientific measurements are only available for the past one to three centuries. It is not easy to accurately determine the influence of humans on any natural activity. In the case of the ozone layer, the depletion of the ozone over Antarctica cannot be explained by natural cycles alone but is caused by the increase of man-made chemicals in the stratosphere. The relationship between these chemicals (e.g. chlorofluorocarbons, also known as CFCs) and ozone depletion has been shown by experiments in laboratories, numerical modelling studies and by direct measurements in the atmosphere (see Figure 8a).
By absorbing the infrared radiation emitted by Earth, some substances influence the natural energy flows through the atmosphere. The greenhouse gases, such as carbon dioxide, although only a tiny fraction of the atmosphere, play an important role in this process. Carbon dioxide (CO2) is produced when fossil fuels are used to generate energy and when forests are burned. Measurements show that its concentration has increased by almost 30% over the past 250 years. In the mid-20th century, the concentration of CO2 was 280 ppmv, and it is currently 390 ppmv, and increasing at about 3% per decade. Methane and nitrous oxide emitted from agricultural activities, changes in land use, and other sources are also important (see Figure 8b).
The increase in greenhouse gases contributes to climate change in the form of increased temperatures (about 0.6°C near Earth’s surface over the past 150 years) and a rise in sea level. Models of the climate change induced by emission of greenhouse gases predict that the global temperature will rise between 2 and 6°C in the next century. If this happens, the change would be much larger than any temperature change experienced over at least the last 10,000 years.
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