We often hear – and increasingly – talk about global warming, greenhouse effect and greenhouse gases, but what is greenhouse effect exactly?
While other planets in the solar system are either hot or very cold, the surface of the Earth has relatively mild and stable temperatures. We owe this to our atmosphere, the thin layer of gas that covers and protects the planet. However, 97% of climate scientists agree that humans have dramatically changed the Earth’s atmosphere over the last two centuries, resulting in global warming. But to understand global warming, one must first become familiar with the concept of the greenhouse effect.
There is a delicate balance to maintain each day on our planet, involving the radiation that it receives from space and the radiation reflected back into space. The Earth is on one side constantly bombarded with huge amounts of radiation, mainly from the Sun. This solar radiation hits the Earth’s atmosphere in the form of visible light, ultraviolet (UV), infrared (IR) and other types of waves that are invisible to the human eye. UV radiation has a shorter wavelength and a higher energy level than visible light, while infrared radiation has a longer wavelength and a lower energy level.
About 30% of the radiation entering the earth’s atmosphere is immediately returned to space by clouds, ice, snow, sand and other reflective surfaces. The remaining 70% is absorbed by the oceans, the earth and the atmosphere. As they heat up, these three elements release heat in the form of infrared heat radiation, which passes into the atmosphere and into space.
It is this balance of incoming and outgoing radiation that makes Earth habitable, with an average temperature of about 15 ° C. Without this atmospheric equilibrium, the Earth would be as cold and lifeless as the moon, or as hot as Venus. On the moon, which has almost no atmosphere, temperatures vary from a low of – 183 degrees Fahrenheit at night and a high of 224 degrees Fahrenheit during the day. On Venus, which has a very dense atmosphere that traps solar radiation, the average temperature rises to 864 °F.
It is this exchange of incoming and outgoing radiation that is called the “greenhouse effect”, because a greenhouse works in the same way. Incoming UV rays easily pass through the glass walls of a greenhouse, and are absorbed by plants and hard surfaces inside. Lower IR radiation is difficult to penetrate the glass walls and is trapped inside, warming the greenhouse. This effect allows tropical plants to thrive in a greenhouse, even during a cold winter.
But there are on Earth gas molecules that absorb thermal infrared radiation. And when they are present in too large quantities, they can disrupt this thermal equilibrium. These molecules are called “greenhouse gases”. Carbon dioxide (CO2) and other gases such as – among others – methane or nitrous oxide (N2O) form a kind of blanket that absorbs infrared radiation, preventing it from escaping into the space. The balance is broken. This results in a gradual warming of the atmosphere and the surface of the Earth: global warming.
Since the dawn of the industrial revolution in the early 1800s, the burning of fossil fuels such as coal, oil and gasoline have significantly increased the concentration of greenhouse gases in the atmosphere, including CO2. For example, atmospheric CO2 levels have increased by more than 40% since the beginning of the industrial revolution, from about 280 parts per million (ppm) in the 1800s to 400 ppm today. The last time the atmospheric CO2 levels of the Earth reached 400 ppm, it was during the Pliocene age, between 5 million and 3 million years ago.
Combined with increasing levels of greenhouse gases and the resulting global warming, the greenhouse effect then has caused significant climate change: sea-level rise, increasing ocean acidification, extreme weather events, and other serious natural and societal impacts. Here we are.