Presenting our budget
Kamal Lodaya, Bengaluru
This is the second of a series of articles on the close correlation between the atmosphere, the ocean, and the weather ... and not just on Earth alone.
On February 1st of every year, the Finance Minister presents the Union government's budget in the Indian parliament. The budget gives an account of what the government has done for our economy, how many rupees it has raised in taxes from the people, how many rupees it has spent, and what it plans to do in the coming year. One thing the Finance Minister does not tell us (because the budget is only concerned with India) is how the Indian rupee is faring compared to the basket of other nations' currencies in the world.
To take account of what the Sun does to Earth every day, climate scientists present the Earth's budget. The unit of the Sun's intensity is watts per square meter, how much power is received or sent area-wise. Above the atmosphere, Earth receives solar power with intensity 1.38 kilowatts per square meter. Some solar radiation is reflected back into space by clouds and some is absorbed in the atmosphere. At the ground on a clear summer day we receive 1.36 kilowatts per square meter.
On the ground solar radiation is reflected back into space by snow, ice or other surfaces. Otherwise it is absorbed by Earth's surface. That makes the surface hot. This heat is released from Earth as {infrared} radiation. So outgoing power to space is by sunlight reflection as well as by emitted radiation.
A tiny proportion of the atmosphere is ozone. Ozone high in the atmosphere reflects {ultraviolet} radiation coming in from the Sun's light back into space. This is good for life, because ultraviolet radiation damages crops and marine life, and causes cancer in animals. If we set up colonies of people who will live on the Moon, apart from having a supply of oxygen to breathe, they will have to figure out what to do with the threat of ultraviolet radiation, because there is no ozone on the Moon.
Greenhouse gases in the lower atmosphere (water, carbon dioxide and others) absorb this infrared radiation. It warms up the lower atmosphere. The warmed atmosphere releases more longwave radiation. Some of it goes into space but some of it is in the direction of Earth. It warms up Earth's surface. This is called the greenhouse effect. So at night time, temperature does not drop to -180 degrees C like it does on the Moon's surface when it is not receiving sunlight. Our nights are colder, but even in winter in Antarctica, not colder than -50 C.
The picture shows averages of solar radiation that are reflected, and heat radiation that is emitted by Earth. You can see that more is reflected from the continents (upto 0.7 kilowatts per square meter) and less from the oceans, because the oceans absorb more heat. You can also see that more heat is emitted by the Tropics (upto 0.35 kilowatts per square meter) and less from other regions where the Sun is not overhead.
Greenhouse gases like water (upto 1 per cent), carbon dioxide (0.04 per cent) and methane (even smaller) form a very tiny proportion of the atmosphere. Water molecules disassociate or fall to Earth within a few days, but carbon dioxide and methane stay in the atmosphere for a long time. If their concentration increases, more of these gas molecules prevent the emitted heat from going out into space. This is called global warming. 90 per cent of the atmosphere on Venus is made up of carbon dioxide. It does not let heat escape that easily, and the temperature there is over 400 C, day or night.
About 78 per cent of our atmosphere is nitrogen and 21 per cent is oxygen. They are not greenhouse gases, they do not trap heat emitted by Earth. This has allowed life to flourish on Earth. Compounds of nitrogen and oxygen (such as nitrous oxide) are greenhouse gases. It is important that their proportion remains tiny in the atmosphere.
If you know something about budgets and you have been keeping track of what has been happening, you would have noticed that our solar power budget isn't balanced. Here is a better picture to show this. You can see that almost half of incoming solar power is absorbed by the continents and oceans on Earth. A large part of what happens to this solar radiation goes into what we call climate.
Sources: https://rwu.pressbooks.pub/webboceanography/chapter/8-1-earths-heat-budget/;
https://science.nasa.gov/ems/13_radiationbudget/