Greenhouse effect

The process begins when visible light from the sun passes through the atmosphere and warms the Earth’s surface. This energy is then re-radiated back toward space, but at a different wavelength—specifically, long-wave infrared radiation. Unlike incoming sunlight, this thermal "glow" cannot easily pass back through certain atmospheric gases.

Greenhouse gases absorb this outgoing heat and re-emit it in all directions. Roughly half of that energy is sent back down to the surface, effectively recycling heat. This creates a thermal equilibrium where the planet stays significantly warmer than it would based on its distance from the sun alone.

It is a common misconception that the greenhouse effect is inherently "bad." In its natural state, it is the primary reason Earth is habitable. By raising the planet’s surface temperature by about 33°C (59°F), the greenhouse effect allows for liquid water and the complex chemistry required for life.

The system is a delicate balancing act. While the moon is roughly the same distance from the sun as Earth, it lacks an atmosphere to trap heat, resulting in a desolate world of extreme temperature swings. The greenhouse effect provides the thermal stability that allows ecosystems to thrive across the globe.

By volume, water vapor is the most abundant greenhouse gas, responsible for about half of the total effect. however, its concentration depends on temperature—warmer air holds more water, creating a feedback loop. It doesn't "drive" the climate; it amplifies changes caused by other factors.

Carbon dioxide ($CO_2$), though present in smaller quantities, is the most important "forcing" gas because it persists in the atmosphere for centuries. While methane and nitrous oxide are more potent at trapping heat on a molecule-for-molecule basis, the sheer volume and longevity of human-emitted $CO_2$ make it the primary lever shifting the planet’s energy balance.

The concept was first proposed by Joseph Fourier in 1824, who realized the atmosphere must act as an insulator. By 1859, John Tyndall identified the specific gases—water vapor and carbon dioxide—that were responsible for trapping heat. He was the first to prove that even trace amounts of these gases could change the climate of the entire planet.

In 1896, Swedish scientist Svante Arrhenius performed the first manual calculations to predict how doubling atmospheric $CO_2$ would affect global temperatures. His estimates were remarkably close to modern computer models. This historical lineage shows that our understanding of the greenhouse effect is rooted in fundamental Victorian-era physics, not just modern computer simulations.

Since the Industrial Revolution, the concentration of $CO_2$ in the atmosphere has increased by nearly 50%. By burning fossil fuels, humans are taking carbon that was buried for millions of years and injecting it back into the active cycle. This increases the "optical thickness" of the atmosphere for infrared radiation.

The result is a planetary energy imbalance: Earth is now absorbing more energy from the sun than it radiates back into space. This surplus energy doesn't just warm the air; over 90% of it is absorbed by the oceans, leading to rising sea levels, altered weather patterns, and the rapid melting of polar ice.