Thinking Point
Dr J. Srinivasan
The writer is a former Chairman at the Divecha Centre for Climate Change, Indian Institute of Science, Bangalore, and an eminent climate researcher. Known popularly as J.S., Srinivasan is an icon and an inspiration to climate researchers worldwide.
Climate change is an important issue because it is going to be the biggest environmental challenge we are going to face in the 21st century. This problem is more complex than the issue of air pollution. Air pollution is a local problem and hence can be tackled by a local solution, but climate change is a global problem. Hence, it demands cooperation among all the countries on earth. This is because global warming is caused by carbon dioxide, and this gas rapidly mixes in the atmosphere. Hence, carbon dioxide released anywhere on the earth has an impact on the climate everywhere. For a long time, the cooperation necessary to tackle global warming was not forthcoming because there was a lot of misunderstanding about what causes climate change.
The reason why there is so much misunderstanding is that climate change can occur due to both natural and man-made causes. To differentiate between them is not easy. Both natural and human-induced causes had an impact on climate change in the 20th century, and one could not really distinguish between them. In the first two decades of the 21st century, it is becoming clear that human-induced causes are beginning to dominate climate change. There is confusion between factors that cause global climate change versus factors that influence local climate change. Climate change in urban areas is mainly on account of a change in land cover, and this is as important for urban climate as an increase in carbon dioxide.
Weather has a time scale of a few hours to a few days and is influenced only by regional factors, but climate is not same as weather. Climate is the statistics of weather averaged over time. The averaging period is usually 30 years, but we can ask whether the climate next year will be different from the current year. By this we mean whether the distribution of temperature, humidity and rainfall will be different in the two years.
The climate of the earth, it is influenced by many factors. There are a few external factors that influence climate. One is how much energy is emitted by the Sun. Next is the Sun-Earth geometry. We all know that the Earth goes around the Sun in an elliptic orbit, and that this orbit changes in time. That influences the climate. Many internal factors influence the earth’s climate. Volcanic eruption can cool the earth. The heat transfer between the ocean and atmosphere is very important as that influences how the atmosphere gains water vapour from the ocean. The reflectivity of the Earth and the ocean is another factor because the earth is covered with vegetation, ocean, and ice, and how much solar radiation it reflects has a large influence on climate. This is called earth’s albedo (i.e., the percentage of total solar radiation reflected by the earth). Finally, atmospheric chemistry influences climate. There are many minor constituents like Carbon Dioxide, Methane, and Ozone which influence earth’s climate by trapping the radiation emitted by the earth.
During the past 8,000 years, the earth’s climate has been remarkably stable. The fluctuations of the global mean temperature were around half a degree Celsius. Between 15,000 years and 9,000 years ago, there was a sudden transition from a very cold climate to the present climate. During the last ice age, Earth was about 50 degrees C colder than the present now. The transition to the present occurred slowly and took about 8,000 years. The temperature increase was at the rate of 1 degree C per thousand years. This is natural climate change, and no human beings were involved. In the last hundred years, the global mean temperature has increased by 1oC in 100 years.
The ice cores in Antarctica contain air bubbles whose composition has been measured. The most remarkable part about this is the fact that the changes in temperature, carbon dioxide, and methane are very similar to each other. This was the first evidence that minor gases like carbon dioxide and methane, which are in parts per million, can have some impact on the earth’s climate. This convinced everybody that gases like carbon dioxide, though at very low concentrations in the earth's atmosphere, play a very important role in the earth’s climate. Over the last 400,000 years, the earth managed to maintain the concentration of carbon dioxide between 280 to 300 parts per million(ppm) for a long time. Today we have allowed the carbon dioxide concentration to go above 400 ppm, which is 120 ppm above the highest value seen in the last 400,000 years Today, the concentration of carbon dioxide is changing very rapidly and changes are taking place 10 to 100 times faster than the natural changes that occurred in the past.
A common misconception is that since carbon dioxide is a minor constituent in the earth’s atmosphere and hence cannot affect earth’s climate. This is not true because the two major gases in the earth’s atmosphere, namely, nitrogen and oxygen, happen to be completely transparent to the radiation emitted by the sun and the earth and hence do not play a dominant role in the earth’s climate. The earth’s climate is controlled by four minor gases: water vapour, carbon dioxide, methane and ozone. Carbon dioxide does not absorb the sun’s radiation, but absorbs the infrared radiation (which we cannot see) emitted by the earth. Hence, it is called a greenhouse gas because it behaves like the glass in a greenhouse. Among the greenhouse gases in the atmosphere, the most important gas is water vapour. About 50% of the greenhouse effect is due to water vapour. Carbon dioxide is next, and other gases play a minor role.
Water vapour is very important because it is the dominant greenhouse gas in the earth’s atmosphere. The laws of thermodynamics dictate that the amount of water vapour in the atmosphere is controlled by temperature. On the other hand, gases like carbon dioxide, methane, and ozone do not condense and hence their amount in the atmosphere can be controlled. The amount of water vapour in the atmosphere is not determined by how much we release by human activities, but by the global mean temperature.
The global mean temperature in 2024 was more than 1.5 o C warmer than in 1890. The increase in global mean temperature has led to an increase in heat waves on the earth. They are occurring more frequently and becoming longer and more intense. If we look at the consequences of temperature rise, we see that natural disasters such as storms, floods, and droughts in Asia have gone up from 100 to 350 in the last 30 years. Most climate models used to predict the future climate indicate that by the end of the 21st century, we will get at least 3 to 4 o C warmer than in 1890. Climate models show that due to global warming, the developed countries like Europe, America, and Russia will benefit as their agricultural crop yield will go up, while in the developing countries it will go down.
Climate change is not the only issue we are dealing with in this world. There are also issues of water quality, soil degradation, and air quality. They are all connected. If we stop building coal power plants, we will improve air quality. If we reduce the use of chemical fertilisers and groundwater in agriculture, we will stop soil degradation. We must remember that the type of development path taken during the last 100 years was short-sighted. What the threat of global warming has shown us is that there is an alternative way to develop society. This understanding will lead us to a better future without global warming and environmental degradation.
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