Greenhouse effect

The greenhouse effect is a critical process that regulates Earth's temperature. It occurs when specific gases in the atmosphere prevent a planet from losing heat to space. This process raises the surface temperature of the planet. On Earth, this effect is essential for life. Without it, the average surface temperature would be roughly -18 degrees Celsius. Instead, the greenhouse effect maintains an average temperature of about 15 degrees Celsius. This represents a temperature difference of approximately 33 degrees.

To understand the mechanism, we must look at how energy moves. The Sun emits shortwave radiation, which includes visible light and near-infrared wavelengths. Because the Sun is very hot, its radiation has short wavelengths. This sunlight passes through the atmosphere to heat the Earth's surface. The atmosphere and clouds reflect or absorb about 23% of this incoming radiation. The Earth's surface absorbs most of it and then emits longwave radiation, also called thermal radiation.

Unlike the Sun, the Earth is much cooler. Therefore, it emits energy at longer, mid- and far-infrared wavelengths.

Greenhouse gases are the specific components that drive this warming. A gas is classified as a greenhouse gas if it absorbs longwave radiation. While the atmosphere only absorbs 23% of incoming shortwave radiation, it absorbs 90% of the longwave radiation emitted by the surface. This absorption causes energy to accumulate in the atmosphere. Each layer of the atmosphere containing these gases absorbs radiation from below. These gases then emit radiation in all directions, including back toward the surface. This reduces the rate at which the Earth can cool off.

Scientists have worked for centuries to understand this phenomenon. Joseph Fourier proposed the existence of this effect as early as 1824. Claude Pouillet later strengthened this argument with more evidence in 1827 and 1838. In 1856, Eunice Newton Foote conducted experiments showing that air with water vapour or carbon dioxide holds more heat than dry air. She concluded that an atmosphere of such gas would create a high temperature on Earth.

Later, John Tyndall measured the infrared absorption of various gases. He proved that a small proportion of the atmosphere, like water vapour and carbon dioxide, causes the effect. Finally, Nils Gustaf Ekholm applied the term "greenhouse" to the process in 1901.

We can measure the greenhouse effect using different scientific metrics. One way is by measuring the change in temperature. Another way is to measure the energy flux, which is the rate of energy flow per unit area. Currently, longwave radiation leaves the Earth's surface at an average rate of 398 W/m². However, only 239 W/m² actually reaches space. This means the greenhouse effect accounts for an energy flow change of 159 W/m². This represents about 40% of the longwave radiation that leaves the surface but is trapped.

Human activities are now creating an "enhanced greenhouse effect." This happens when the concentration of greenhouse gases increases due to human action. Burning fossil fuels increases the amounts of carbon dioxide and methane in the atmosphere. Since the Industrial Revolution, global warming of about 1.1 degrees Celsius has occurred. Since 1981, the global average surface temperature has increased at a rate of about 0.18 degrees Celsius per decade.

Data from the Mauna Loa Observatory shows that carbon dioxide levels have risen significantly. In 1960, levels were about 313 parts per million (ppm). By 2013, they passed the 400 ppm milestone. This exceeds the natural historical maximum of approximately 300 ppm found in ice core data.

The term "greenhouse" is an analogy, but the physics are different. A real glass greenhouse retains heat by blocking convection, which is the movement of air. In contrast, the greenhouse effect retains heat by restricting radiative transfer. It limits the rate at which thermal radiation is emitted into space. By increasing these gases, humans are changing the Earth's energy balance. This shift affects how much energy stays in our climate system.