What is the greenhouse effect? What is the criterion for classifying a gas as a greenhouse gas?

Understanding the role of greenhouse gases is fundamental to comprehending how the greenhouse effect contributes to the warming of our planet. These gases, which include carbon dioxide (CO2), methane (CH4), nitrous oxide (N2O), and fluorinated gases among others, act like a blanket around Earth. They allow sunlight to enter the atmosphere but prevent the heat that the ground emits from completely escaping into space.

It's essential to know that not all gases are greenhouse gases. For a gas to qualify as a greenhouse gas, it must fulfill certain criteria: it has to absorb and re-emit infrared radiation, must be in a gaseous state at Earth's average ambient temperature, and should be transparent to ultraviolet sunlight. This selective filtering is what keeps our planet warm enough to sustain life. Without these gases, Earth would be an icy, desolate place.

The Earth primarily receives energy from the Sun in the form of visible light and ultraviolet radiation. But once this solar energy hits the Earth's surface, it doesn't all stay there. Instead, the surface absorbs some of the energy and heats up, then radiates energy back into the atmosphere in the form of infrared radiation, which we often perceive as heat.

Greenhouse gases are especially good at absorbing this infrared radiation. Once they absorb it, they re-radiate it in all directions, including back towards the Earth's surface. This process is what makes the Earth's surface warmer than it would be if the gases weren't present. Understanding how infrared radiation works help us grasp the reason behind things like the warmth of a sunny day or why we experience warmth from the pavement even after the sun has set.

When we discuss the Earth's average temperature in the context of climate studies, we're talking about the combined temperature of the Earth's surface, which includes the land and the oceans. The greenhouse effect plays a pivotal role in regulating this average temperature. Without it, Earth would have an average temperature of about -18 degrees Celsius (0 degrees Fahrenheit) instead of the current roughly 14 degrees Celsius (57 degrees Fahrenheit).

The balance between the amount of solar radiation entering our atmosphere, the proportion absorbed by the Earth, and how much infrared radiation is trapped by greenhouse gases determines the Earth's overall temperature. It's a delicate balance, and even small changes in the concentration of greenhouse gases can lead to significant changes in climate patterns, which is why it's so critical to monitor and manage our emissions.