Troposphere

Definition

The troposphere is the lowest layer of the Earth’s atmosphere. It typically extends from the Earth’s surface to an altitude of about 10-16 kilometers high, varying by latitude. In the tropics, the troposphere extends relatively high, whereas, in the polar region, its upper boundary is significantly lower.

Because gravity weakens as the distance from a body increases, the atmosphere’s density and pressure decrease exponentially with altitude. Most of the air molecules in the atmosphere are located in the atmospheric layers closest to Earth’s surface. As altitude increases, the number of atmospheric molecules per unit of volume decreases, so the air becomes thinner.

By contrast, air temperature changes as a function of altitude are much more complex. The temperature of Earth’s atmosphere decreases with altitude to the tropospheric boundary, 10-16 kilometers high. From that point to about 20 kilometers high, there is a “pause” in the temperature change, as temperature remains relatively constant. Above 20 kilometers, the temperature begins to increase. From about 45 to 50 kilometers up and from about 80 to 90 kilometers up, there exist two other zones of relatively constant temperature. Between them, from 50 to 80 kilometers up, the temperature decreases with height. Above 90 kilometers, the temperature increases with height throughout the rest of Earth’s atmosphere. These zones of different types of temperature change define the structural layers of Earth’s atmosphere: the troposphere, , stratosphere, stratopause, mesosphere, mesopause, and thermosphere.

Because more than 90 percent of atmospheric molecules are located in the troposphere, most weather and convection happen within this layer of the atmosphere. During the daytime, the Earth’s surface warms relatively quickly. This radiative heating is necessary for convection to be generated. Warm, moist air rises, condensing when the air temperature drops at mid-to-high levels of the troposphere. This pattern of convection and condensation is the core process generating weather and storms.

During the nighttime, the air near the Earth’s surface cools faster than that above it. Very low in the troposphere, radiative cooling can cause a temperature inversion, in which a warm layer of air is situated above the cool air near the surface and below the cold air of the upper troposphere. This temperature structure acts as a lid in the mid-troposphere, limiting convection. This phenomenon is the reason storms do not typically occur late at night or early in the morning.

Significance for Climate Change

The importance of the troposphere to global warming and climate change is twofold. First, most greenhouse gas (GHG) emissions, both natural and , are released into the troposphere, close to Earth’s surface. Since gravity keeps more than 90 percent of air molecules within the troposphere, most of the emitted GHGs will stay there. Second, because the troposphere is in direct contact with Earth’s surface, tropospheric reflections of global warming will be the most sensible and evident atmospheric reflections.

In the early 2000s, scientific opinion was split concerning the state and future of the troposphere and climate change. While some scientists predicted global warming would cause a significant increase in tropospheric temperatures, other studies, based on the analyses of satellite measurements and radiosonde (weather balloon) data, showed no evidence of tropospheric warming. These studies were criticized by those who argued that results indicating no warming, or even cooling, in the troposphere were based on incomplete analyses because they failed to take into account the cooling effect exerted by the stratosphere on the troposphere. However, some scientists believed that the overall temperature of the troposphere could remain unchanged or even decrease while the Earth’s surface temperature rises. They argued that an increase in atmospheric GHG concentrations may result in increased cloud cover. These clouds would increase Earth’s albedo, reflecting solar radiation back into space and acting as a cooling influence. In 2021, the scientific opinion continued to vary on the topic, but new data emerged suggesting significant warming of the troposphere. Because of human GHG emissions and the continued depletion of the ozone, the troposphere rose approximately fifty to sixty meters per decade beginning in the last half of the twentieth century. This change may have been influenced by volcanic activity and typhoons, but the largest factor impacting this increase was rising temperatures in the troposphere, consistent with the rate of increase on the Earth’s surface due to human factors.

Most weather occurs inside the troposphere. Thus, any changes affecting the troposphere will inevitably affect Earth’s weather patterns, which is evident in the increased cloud cover and a 5-10 percent increase in the Northern Hemisphere’s extratropical precipitation reported in 2023. As the example of tropospheric temperature studies illustrates, however, it is difficult to predict the precise nature of these changes and their effects. If the troposphere is warm, it will extend vertically, and scientists predict that this will cause much stronger convective storms because an extended troposphere will allow deeper convection and thicker clouds. This is perhaps one reason that the Intergovernmental Panel on Climate Change (IPCC) predicted more severe storms would accompany future climate warming. Another possible effect of tropospheric warmth is an extension of tropical regions poleward. Such an extension would fundamentally alter the environment of many regions on Earth, with significant socioeconomic consequences.

Bibliography

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