Stratosphere
The stratosphere is a crucial layer of Earth's atmosphere, situated between the troposphere below and the mesosphere above, extending from approximately 15 to 50 kilometers above the Earth's surface. It is home to about 90 percent of the planet's ozone, which plays a vital role in absorbing harmful ultraviolet radiation from the Sun, leading to a temperature increase with altitude. This temperature gradient contributes to the stable stratification of the stratosphere, significantly influencing atmospheric circulation patterns.
Scientific exploration of the stratosphere has revealed important circulation features, such as the midlatitude westerlies and easterlies, the Brewer-Dobson circulation, and the equatorial quasi-biennial oscillation (QBO). These elements are essential for understanding how energy and ozone are transported within the atmosphere and how they interact with climate systems. Various factors, including volcanic eruptions, solar cycles, and human activities, can perturb these interactions, leading to observable changes in stratospheric temperature and composition.
Recent studies indicate that while stratospheric ozone is expected to recover in the coming decades, this recovery may not fully counteract cooling trends driven by increases in greenhouse gases. The evolving dynamics of the stratosphere are likely to have significant repercussions on tropospheric weather and climate, underscoring the interconnectedness of atmospheric layers and the importance of ongoing research in this field.
Subject Terms
Stratosphere
Definition
The stratosphere is a highly stratified region of the atmosphere that is bounded below by the troposphere and above by the mesosphere. The stratosphere ranges from about 15 kilometers above the Earth’s surface at its lower boundary to about 50 kilometers at its upper boundary. The stratosphere contains about 90 percent of the ozone in the atmosphere. Because of the ozone layer in the stratosphere, which absorbs much of the ultraviolet radiation reaching Earth from the Sun, the temperature increases with increasing height throughout the stratosphere. This ozone-induced increase in temperature with height corresponds to stable stratification, which has a strong impact on the stratospheric circulation.
Significance for Climate Change
Since scientists Richard Assmann (1845–1918) and Léon Philippe Teisserenc de Bort (1855–1913) independently launched balloons and discovered the stratosphere more than a century ago, the stratosphere continues to be probed and measured by a wide variety of scientific instruments, including radiosondes, lidars, rocketsondes, and satellite sensors. These measurements have revealed a region containing a number of prominent circulation features. These features include the midlatitude westerlies in the wintertime hemisphere, the corresponding easterlies in the summertime hemisphere, the Brewer-Dobson circulation, and the equatorial quasi-biennial oscillation (QBO).
The midlatitude westerlies and easterlies affect the planetary wave energy that propagates vertically from the troposphere into the stratosphere, while the Brewer-Dobson circulation, which is characterized by ascent in the equatorial region of the stratosphere and descent in the polar regions, transports ozone from its production region in the tropics to the polar regions. The QBO, which is characterized by an oscillation in zonal winds in the equatorial lower stratosphere, can extend its influence far beyond its seat of origin to affect, for example, Northern Hemisphere surface temperatures. These features, as well as the space-time evolution of the stratosphere, depend vitally on the interactions between radiation, chemistry, and dynamics. Perturbations to these interactions are caused by both anthropogenic and natural forcing and occur over a wide range of timescales.
Changes in stratospheric temperature, which are intimately connected to the radiative, chemical, and dynamical interactions in the stratosphere, have undergone episodic, quasi-periodic, and secular changes over the past several decades. These changes have been attributed to volcanic events, the eleven-year solar cycle, and anthropogenic forcing. Studies have shown that volcanic events can eject aerosols into the lower stratosphere that alter the radiation balance there, resulting in elevated temperatures that can last for a couple of years.
The eleven-year solar cycle has been shown to modulate the temperature and ozone in the stratosphere, which modulate the stratospheric westerlies. The modulation of the westerlies in turn affects the energy propagation of the planetary waves, which through refraction and reflection can impart a downward influence on the troposphere. Anthropogenic forcing has caused ozone depletion and increases in well-mixed greenhouse gases (WMGGs) such as carbon dioxide (CO2), which together have produced a cooling trend in the stratosphere that began most noticeably in the early to mid-1980s. Stratospheric ozone is expected to recover in the coming decades, but not enough to offset the cooling due to the projected increases in WMGGs.
The significance of the stratosphere to climate change hinges on the fact that as the composition of the stratosphere changes, so too will the dynamical circulation. Climate models show, for example, that anthropogenic forcing agents such as WMGGs cause the Brewer-Dobson circulation to strengthen. A strengthening of the Brewer-Dobson circulation will cause the westerly winds to weaken and the temperatures to increase in the extratropical stratosphere. The changes in the winds and temperature in the stratosphere are expected to couple downward to affect the weather and climate of the troposphere.
Bibliography
Baldwin, M. P., M. Dameris, and T. G. Shepherd.“How Will the Stratosphere Affect Climate Change?” Science 316 (2007): 1576–1577. Print.
Cordero, E. C., and T. R. Nathan. “The Influence of Wave- and Zonal Mean-Ozone Feedbacks on the Quasi-Biennial Oscillation.” Journal of the Atmospheric Sciences 57 (2000): 3426–3442. Print.
Cordero, E. C., and T. R. Nathan. “A New Pathway for Communicating the Eleven-Year Solar Cycle Signal to the QBO.” Geophysical Research Letters 32 (2005). Print.
Intergovernmental Panel on Climate Change. Climate Change, 2007—The Physical Science Basis: Contribution of Working Group I to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change. Ed. by Susan Solomon et al. New York: Cambridge U P, 2007. Print.
Labitzke, Karin G., and Harry Van Loon. The Stratosphere: Phenomena, History, and Relevance. Berlin: Springer, 1999. Print.
Nathan, T. R., and E. C. Cordero. “An Ozone-Modified Refractive Index for Vertically Propagating Planetary Waves.” Journal of Geophysical Research 112 (2007). Print.
Schwarzkoph, M. Daniel, and V. Ramaswamy. “Evolution of Stratospheric Temperature in the Twentieth Century.” Geophysical Research Letters 35 (2008). Print.