Part 1: What is climate and why is it changing? The greenhouse effect:
Climate and Earth’s energy balance
Weather is what we experience when we go outside at any given moment, including temperature, precipitation, humidity, etc. Climate refers to these patterns over three decades or more. Global climate depends strongly on temperature, which in turn depends on our planet’s energy balance, that is, the amount of energy absorbed by the sun minus the amount of energy radiated back into space. How much energy is radiated back into space depends on many factors, such as chemistry of the atmosphere, clouds and Earth’s albedo. Scientists use the term albedo to describe how much light a planet or surface reflects back. A planet completely covered with snow would have an albedo of nearly 100%, while a completely dark planet would have an albedo of nearly 0%. The current albedo for Earth is ~31%.
Since we know the incoming energy from the sun and the albedo of the Earth, we can easily calculate what the temperature of the Earth would be if we disregard the effects of the atmosphere. For those interested, I show this calculation at the end of this article. Without the atmosphere, the global average temperature would be -19.5 °C. Our actual average temperature is ~15 °C, which is 34.5 °C warmer than the calculated value. This discrepancy is due to the greenhouse effect of the atmosphere. So in principle, the greenhouse effect is a good thing that has made our life on this planet possible in the first place. But how does it work?
The greenhouse effect
Due to the short wavelength of sunlight, it can easily penetrate the atmosphere. When the sunlight hits the Earth’s surface, part of it is reflected back into space, the rest is absorbed by the ground. Solar energy absorbed at Earth’s surface is radiated back into the atmosphere as heat, i.e. as infrared radiation. Infrared radiation has a longer wavelength, which means that greenhouse gases can absorb it, warming the atmosphere. The reason why greenhouse gases can absorb infrared radiation is because of their atomic structure. They are all made of 3 or more atoms held loosely enough together to vibrate and thus absorb infrared radiation. Eventually, the vibrating molecules release the radiation back to the Earth’s surface, to other greenhouse gas molecules, or out to space. The following figure shows: The more greenhouse gases there are in the atmosphere, the greater the likelihood that infrared radiation will be absorbed by them and heat up the atmosphere. So the basic equation is: more greenhouse gases = a warmer Earth.
Not all greenhouse gases have the same heat absorption capacity, so they are calculated in CO2e (CO2 equivalents):
- Methane (CH4) is ~30 times stronger than CO2
- Nitrous oxide (N2O) is ~300 times stronger than CO2
- Fluorinated gases are ~23000 times stronger than CO2
The natural greenhouse effect is very important and has kept the Earth’s climate in a balance that has allowed humans to live on Earth for thousands of years. We produce a lot of additional greenhouse gases from burning fossil fuels, raising livestock, deforestation, and other areas of our industry like cement production. This additional human-caused greenhouse effect could, in the long run, put the Earth’s climate in a new state that is uninhabitable for us if we are not careful.
Calculation of Earth’s energy balance without atmosphere:
The incoming solar radiation is called “solar constant”. It can be measured by satellites and is 1361 W/m². In order to calculate the total amount of energy arriving at Earth, we need to know how much area is being lit. The following figure shows that the amount of light intercepted by our spherical planet is exactly equal to the amount that would be intercepted by a flat disk of the same diameter.
This means that the total energy intercepted by the Earth can be calculated using the following formula:
with K_S = solar constant and R_E = Earth radius.
Some of this energy is directly reflected by the Earth. Since albedo represents the energy reflected from the Earth, 1-albedo is equal to the energy absorbed by the Earth:
The Stefan-Boltzmann law tells us how much infrared energy the Earth emits per unit area. In this case, the area is the surface of the earth, because the earth rotates, and therefore all sides of the earth heat up and radiate energy:
with σ = Stefan-Boltzmann constant and T = temperature.
The law of conservation of energy tells us that the energy emitted must be equal to the energy absorbed. If we then solve this equation for the temperature we get:
Now we only have to insert the values and we get the temperature of the Earth:
If we convert this value we get ~-19.5°C.