Editors’ Vox is a blog from AGU’s Publications Department.
The stratosphere, one of the Earth’s atmospheric layers, is essential to understand due to its significant influence on the global climate system. Given that the composition of the stratosphere is highly influenced by air transport and circulation, scientists have developed a measure called “age of air” to quantify how long air has been in transport.
A new article in Reviews of Geophysics explores the development and use of this stratospheric age of air metric. Here, we asked the lead author to give an overview of the stratosphere, how scientists use the age of air metric, and what questions remain.
In simple terms, what is the stratosphere and what makes it a particularly interesting layer of the atmosphere to study?
The stratosphere is the second layer of the atmosphere from the ground, at about 20 to 50 kilometers high. Below, in the troposphere, temperatures decrease with height, but in the stratosphere, it gets warmer when moving higher up. This is mostly because of the stratospheric ozone layer, which absorbs high-energy sunlight that warms the stratosphere, and, at the same time, protects us from harmful UV radiation.
The dynamics of the stratosphere are very interesting: the stable stratification (i.e., temperature increase with height) inhibits the formation of “weather”, as we know, it in the troposphere. Nevertheless, the stratosphere is not boring. Certain atmospheric waves formed in the troposphere can propagate upward, and drive a gigantic heat pump in the stratosphere, transporting air upward in the tropics, poleward, and downward over the poles. The existence of such a circulation was first postulated in the 1940s and 1950s and has been named after its proposers the “Brewer-Dobson circulation”.
How does the “Brewer-Dobson circulation” help us to understand the composition of the stratosphere?
Hemisphere-wide transport circulation is important to understand how trace gases are distributed in the stratosphere.
The hemisphere-wide transport circulation is important to understand how trace gases are distributed in the stratosphere. For example, ozone is formed by photochemistry, thus primarily in the tropics, where most sunlight is available. However, ozone concentrations maximize at mid- to high latitudes, and this is thanks to transport by the Brewer-Dobson circulation. Indeed, when Brewer and Dobson took observations of ozone and water vapor in the stratosphere, the only way they could explain them was by a large-scale poleward circulation. However, at this time it was not clear how this circulation could be driven physically, and it took the science community another few decades to understand the dynamical driving of the circulation.
What is the “age of air” and how can it be observed and measured?
One of the problems with the Brewer-Dobson circulation is that it is difficult to observe it directly, since the mean velocities associated with the slow upward motion in the tropics are on the order of millimeters/second. Furthermore, processes other than the slow overturning circulation are important for understanding total transport of air masses, like fast mixing of air between tropics and extra tropics.
One way to quantify this total transport circulation is by average transport times, i.e., measuring how long it takes air to move from its entry into the stratosphere in the tropics to another point in the stratosphere. This transport time is commonly known as “age of stratospheric air”.
One advantage of age of air is that it can be deduced from certain observable trace gases. Specifically, if the concentration of a trace gas rises steadily in the troposphere, one can measure the delay of concentrations in the troposphere versus in the stratosphere. This delay equals the age of air, but strictly only if the trace gas has ideal properties of a linear increase in concentration and no chemical sinks or sources. In the real world, we have some tracers that almost fulfill those conditions, but not exactly, one example being carbon dioxide. Correcting for the non-ideal properties of tracers when deriving age of air from observations is possible, but it is a bit of an art, as we summarize in our new review paper.
How has data on the age of air advanced understanding of processes in the stratosphere?
Many trace gas measurements have been collected via aircraft, balloon, or satellite observations, and we can use them to deduce age of stratospheric air.
Over the last few decades, many trace gas measurements have been collected via aircraft, balloon or satellite observations, and we can use them to deduce age of stratospheric air. This puts us in a situation that we now have a good observational data base on how “old” the air is in the stratosphere on average. As described above, age of air is a measure of the total transport strength in the stratosphere, including many different processes. This is both good and bad: on the one hand, age of air is very well suited to test if global climate models do a good job in simulating transport. On the other hand, solely based on age of air, it is difficult to say which process is how important for the total transport time. Coming up with additional diagnostics of how we can better disentangle the role of different processes for total transport, i.e., for age of air, has been a focus of research in recent years, with good progress.
How is age theory being applied beyond the stratosphere?
The concept of age of air as a measure of transport times from a defined surface to a certain point in a fluid can be used for many geophysical circulations systems. For example, it is commonly used by oceanographers to measure when a water parcel was last in contact with the surface – in this case, the age is considerably longer, on the order of centuries or even millennia, compared to a few years for air in the stratosphere.
Why is understanding stratospheric circulation important for projecting the impacts of climate change?
Changes in the concentration of trace gases in the troposphere under climate change, such as carbon dioxide, methane or nitrous oxides, will be communicated to the stratosphere via the transport circulation. There, those trace gases can have effects on stratospheric temperature via radiation, but also on chemistry, leading to changes in the stratospheric ozone layer or stratospheric water vapor.
Stratospheric circulation and its changes are important in order to understand how the stratospheric ozone layer will be influenced by climate change.
Moreover, there is a long-standing consensus between climate models that the stratospheric Brewer-Dobson circulation will speed up in response to climate change, with consequences for how fast trace gases are transported into and through the stratosphere. Thus, stratospheric circulation and its changes are important in order to understand how the stratospheric ozone layer will be influenced by climate change, directly impacting our UV shield. Moreover, changes in the concentrations of ozone and water vapor, particularly in the lower stratosphere, lead to changes in the overall radiation, directly impacting climate change on the surface.
What are some of the remaining questions where additional modeling, data or research efforts are needed?
A topic of much debate over the last decades is whether the acceleration of the Brewer-Dobson circulation simulated by climate models can be detected in observations. The problem here is that the trends are on the order of a few percent per decade, so we need very high precision data for many decades to detect a trend. Given many uncertainties in deducing age of air from trace gas measurements, at the moment we must conclude that our database is not good enough to derive long-term trends in age of air, at least in the middle stratosphere. Thus, continuous collection of high-quality data of stratospheric trace gases will be necessary to make progress on detecting long-term changes in the stratospheric circulation.
—Hella Garny ([email protected]; 
0000-0003-4960-2304), Institute of Atmospheric Physics, German Aerospace Center (DLR), Germany
