9.3: How the Sun Warms the Earth

As we learned earlier, sunlight is a form of electromagnetic radiation. We most commonly observe it as visible light with wavelengths from 400 to 700 nanometers (10^–9 meters, or 1 billionth of a meter). These wavelengths include the colors we see: violet, blue, green, yellow, orange, and red. To understand how the Sun warms the Earth, we also need to consider nonvisible forms of light—wavelengths shorter than 400 nanometers, namely ultraviolet radiation, and wavelengths longer than 700 nanometers, namely infrared light.

Because the gases in Earth’s atmosphere absorb different wavelengths of electromagnetic radiation, some wavelengths of sunlight travel freely through Earth’s atmosphere and others don’t. The selectivity of the atmosphere to different wavelengths of sunlight is referred to as the atmospheric window (NWS 2023).Think of it as a kind of selective filter, blocking some wavelengths of light but not others. If you look closely at an illustration of Earth’s atmospheric window, you will see that not all sunlight reaches Earth’s surface and not all wavelengths of sunlight reach Earth’s surface equally. Gamma rays and X-rays are mostly blocked by Earth’s atmosphere. Ultraviolet, visible, and near-infrared wavelengths are transmitted through Earth’s atmosphere. Mid- to far-infrared wavelengths are blocked. Short- to mid-wavelength radio waves are transmitted, while long-wavelength radio waves are blocked. The blocking (or transmitting) is due to different gases in Earth’s atmosphere. The net effect is that about 20 percent of the Sun’s energy is absorbed in the atmosphere and only about 50 percent reaches Earth’s surface (e.g., Trenberth et al. 2009).

Other than radio waves, the atmosphere is most transparent (least opaque) to visible light (as to be expected given that our eyes evolved to detect this spectrum of light). In contrast, the part of the spectrum known as near infrared (700 to 5,000 nanometers) varies in its transmission through the atmosphere. And mid- to far-infrared wavelengths (5,000 to 350,000 nanometers) are nearly 100 percent blocked by the atmosphere.

Visible and mid- to far-infrared light are the operational parts of the electromagnetic spectrum where heating of our planet is concerned. To simplify the discussion, scientists divide these parts of the spectrum into two parts: shortwave radiation, which includes ultraviolet, visible, and near-infrared wavelengths; and longwave radiation, which includes the mid- to far-infrared wavelengths. Shortwave radiation is transmitted by Earth’s atmosphere. Most longwave radiation is absorbed by Earth’s atmosphere (e.g., NASA 2016a).

The Sun, like all stars, emits all forms of electromagnetic radiation. However, most of the radiation emitted by the Sun occurs in wavelengths between 300 and 2,500 nanometers. In fact, about half the radiation that reaches Earth’s surface from the Sun consists of ultraviolet and visible wavelengths. The other half is near-infrared wavelengths. Thus, the sunlight that reaches Earth’s surface can be classified as shortwave radiation. However, the Earth, like all objects that contain energy, also emits radiation. According to Wien’s Law, the wavelengths of the emitted radiation depend on the temperature of the object: Hot objects emit short-wavelength radiation, while cool objects emit long-wavelength radiation. The superhot Sun emits mostly shortwave radiation, while the much cooler Earth emits longwave radiation. It’s Earth’s emission of longwave radiation and reabsorption of that radiation by gases in Earth’s atmosphere that cause the atmosphere to warm.

The Greenhouse Effect

The filtering of solar radiation discussed above results from gases in our atmosphere. While nitrogen (70 percent) and oxygen (21 percent) make up the bulk of the gases, they absorb very little solar radiation. However, small concentrations of other atmospheric gases absorb significant quantities, especially longwave radiation. Four gases in particular are responsible for absorbing most of the longwave radiation emitted by Earth’s surface: carbon dioxide (CO₂), water vapor (H₂O), methane (CH₄), and nitrous oxide (N₂O). The similarity of these gases to the function of a greenhouse (trapping heat) has earned them the nickname greenhouse gases (NASA 2023c). Scientists refer to the warming of Earth’s surface by the greenhouse gases in the atmosphere as the greenhouse effect. (Watch 17-second animation by NASA.)

Operationally, Earth’s atmosphere acts similarly to a greenhouse except that it doesn’t prevent convection and confine heat like a greenhouse. Shortwave radiation—free to travel through Earth’s atmosphere—reaches Earth’s surface, where it is absorbed. Absorption of that radiation raises the temperature of Earth’s surface. In turn, some of that energy is emitted by Earth’s surface to the atmosphere as longwave radiation. In the atmosphere, that surface-emitted longwave radiation is absorbed by—you guessed it—greenhouse gases. As a result, they heat up. As they heat up, the greenhouse gases re-radiate some of their absorbed radiation. In doing so, they warm the atmosphere and Earth’s surface. In a sense, greenhouse gases recycle longwave radiation, acting as a reservoir of heat that keeps Earth’s surface warm.

Lest you get the impression that the greenhouse effect is bad, know that the natural greenhouse effect is a good thing—even an essential thing. Because of the greenhouse effect, Earth experiences an average surface temperature of roughly 59°F (15°C). Without the greenhouse effect, Earth’s average surface temperature would be 0°F (−18°C).

Atmospheric Scattering

While the greenhouse effect is the principal means by which Earth warms, other processes can diminish or enhance the amount of sunlight that reaches Earth’s surface. When sunlight encounters the outermost part of Earth’s atmosphere, it begins to interact with suspended particles and gases. Clouds (which are suspended water and ice), particles, and atmospheric gases may absorb, reflect, and scatter sunlight. Preferential scattering of blue wavelengths by atmospheric gases makes the sky appear blue. Similarly, scattering produces those gorgeous orange-red sunsets we often observe (e.g., NASA 2022).

But scattering can also limit the amount of sunlight that reaches Earth’s surface. The most effective light-scattering agents in the atmosphere are aerosols, a group of solid or liquid particles less than a tenth the width of a human hair (less than 1 micrometer). Aerosols prove important in the formation of clouds—acting as cloud condensation nuclei—sites of condensation for water vapor. Aerosols—a form of air pollution—originate from a number of manmade and natural sources, including transportation (e.g., trains, planes, and automobiles), industrial activities (e.g., coal burning), agricultural practices (e.g., fertilizers), volcanoes (e.g., gas emissions), mineral dust (suspended by winds), ocean waves (e.g., sea foam), and even phytoplankton, which produce chemicals that act as cloud condensation nuclei (e.g., Boucher et al. 2013).

One type of aerosol—black carbon, formed from the incomplete combustion of fossil fuels—has received a lot of attention for its role in reducing the reflectivity of Arctic ice, causing it to melt faster (e.g., Hansen and Nazarenko 2004; Winiger et al. 2019). In the lower atmosphere, black carbon and aerosol pollution have severe health effects (e.g., Liu et al. 2018; Kusumaningtyas et al. 2018; Groma et al. 2022). On the other hand, aerosols in the upper atmosphere may actually be cooling Earth. Scientists estimate that cooling by as much as 0.9 to 1.98°F (0.5 to 1.1°C) has occurred because of atmospheric aerosols (e.g., Samset et al. 2018). Ironically, as efforts to reduce air pollution take hold, we can expect additional (and rapid) increases in Earth’s surface temperature. Decreases in aerosol emissions during the pandemic were likely responsible for increases in surface temperatures in eastern China (e.g., Yang et al. 2020). Nevertheless, uncertainty remains over the role of aerosols in Earth’s climate. Ongoing research and new approaches promise to improve our understanding in this important area of research (e.g., Li et al. 2022).

Albedo: The Reflectivity of Earth’s Surface

Shortwave radiation traveling through Earth’s atmosphere may also be reflected. Reflection changes the direction of a beam of light—similar to scattering (although it does not involve light–particle interactions). Reflection of sunlight by clouds, aerosols, particles, and Earth’s surface reduces the energy heating Earth’s surface. Reflection and backscattering (scattering in the opposite direction from which light rays arrive) reduce the sunlight reaching Earth’s surface by about 30 percent. Add 20 percent absorption by clouds, particles, and atmospheric gases, and you can see why only about half the sunlight at the top of Earth’s atmosphere makes its way to Earth’s surface (e.g., Trenberth et al. 2009; Liang et al. 2019).

Of course, the amount of sunlight reflected from Earth’s surface depends on the properties of its surface. Scientists define the reflectivity of Earth’s surface by a property known as albedo—the ratio of reflected light to incoming light. Light-colored surfaces reflect a greater quantity of light than dark-colored surfaces. Thus, light-colored surfaces have a higher albedo than dark surfaces. Places with snow and ice have a high albedo and reflect more sunlight than the ocean or land, which have a low albedo. Urban areas have variable albedos, depending on the materials used to build them and the presence (or absence) of vegetation. For example, Downtown Los Angeles has a higher albedo than surrounding areas because it has less vegetation, which tends to absorb sunlight (e.g., Taha 1997; Vahmani and Ban-Weiss 2016).

Negative and Positive Feedback Loops

Processes internal to a system that modify how it responds to change are known as feedback loops. Feedback loops can slow down or speed up changes in a system. The Earth system and its seven subsystems (Chapter 1) exhibit numerous feedback loops. Of prime concern here are the ones that affect Earth’s energy budget.

Feedback loops that reduce or reverse the impact of a change are known as negative feedback loops. The thermostat in your house provides a familiar example. When the house gets hot, the A/C kicks in. When the desired temperature is reached, the thermostat turns the A/C off. Negative feedback loops reduce extremes in a system. They help to maintain more constant and stable conditions. Scattering by atmospheric particles represents a negative feedback loop because combusion of fossil fuels produces particles that reflect shortwave radiation and reduce the warming effects of the resultant greenhouse gases. If we decrease our burning of fossil fuels, we’ll produce fewer particles and Earth will warm as more shortwave radiation reaches Earth’s surface. Of course, reduced concentrations of greenhouse gases (and the resultant cooling) will more than compensate for any increases in heating.

Changes that occur in a system that amplify or accelerate an effect are known as positive feedback loops (positive as in the same direction, not the “You’re doing a great job” kind of positive feedback). During ice ages, when a greater proportion of Earth is covered with ice, more sunlight is reflected from Earth’s surface. The increased albedo leads to even more cooling and more ice and even more cooling. Alternatively, losses of ice due to warming of Earth’s surface lower Earth’s albedo and allow more sunlight to be absorbed. Greater warming means even greater loss of ice and even more warming. This type of positive feedback worries climate scientists, as global warming will cause melting of the ice caps and greater warming.

At some point, feedback loops within a system (especially positive feedback loops) may cause it to spin out of control, so to speak. The system becomes unstable in its current state and reorganizes into a different form. Scientists refer to such events as tipping points, a change in the state of a system that is irreversible and unstoppable. A roller coaster makes a good example. As the car ascends to the highest point on the track—click–clack, click–clack, click–clack—you nervously anticipate what’s ahead. You reach the top, your wits still about you, but then, there’s that moment when the car crosses the point of no return. You’ve reached the tipping point. OMG! The entire coaster lets go and you scream at the top of your lungs, praying you’re not about to die.

Climate scientists fear the Earth system has multiple irreversible tipping points (e.g., Heinze et al. 2021; Franzke et al. 2022). The predicted impacts will be severe: permanent loss of ice sheets, disruption of ocean circulation, and mass extinction of corals, among them. In the scientists’ words, “the evidence from tipping points alone suggests that we are in a state of planetary emergency: both the risk and urgency of the situation are acute” (Lenton et al. 2019).

Earth’s Temperature Is Rising

While atmospheric greenhouse gases occur naturally as a result of volcanic activity, decomposition of organic matter, evaporation of water, and oceanic exchanges (among others), these natural processes cannot account for the measurably significant rise in greenhouse gases since 1850. Human activities, especially the burning of fossil fuels (i.e., petroleum, natural gas, and coal) and deforestation, the removal of trees, have added greenhouse gases to the atmosphere.

For the 800,000 years prior to the industrial era (about 1850), atmospheric CO₂ never exceeded 300 parts per million (e.g., Lindsey 2022). Fast-forward 170 years, and the concentration of atmospheric greenhouse gases has risen to more than 400 parts per million, an increase of nearly 50 percent. On April 26, 2022, atmospheric CO₂ reached a record-setting 422.06 parts per million at the Mauna Loa Observatory in Hawaii (CO2.earth 2021). Atmospheric CO₂ has now reached its highest concentration in 23 million years (Cui et al. 2020).

What happens to Earth’s surface temperature when the concentration of greenhouse gases in the atmosphere rises? As you might predict, the addition of greenhouse gases to the atmosphere increases the amount of radiation absorbed by greenhouse gases and re-emitted to Earth’s surface. If more longwave radiation remains in our atmosphere, then it stands to reason that Earth’s temperature will rise. That’s exactly what we see. Climate scientists calculate that Earth has warmed by an average of nearly 2.0°F (1.1°C) over temperatures from 1850–1900 (IPCC 2021). That’s the warmest Earth has been in 125,000 years (Tollefson 2021).

One way to visualize what happens as a result of an increase in atmospheric CO₂ is to consider what it’s like on an ordinary day at the mall versus the mall during holidays. On an ordinary day, you can walk through the mall without bumping into people. On busy shopping days, you have to push and shove your way through crowds of people. Heat rays moving through a crowded greenhouse gas atmosphere take longer to travel into space and Earth’s heat reservoir fills up.