Top of the world: The human influence on Earth’s middle and upper atmosphere
Climate change is not only sweeping across Earth’s continents and oceans; it’s also percolating upward. Trends in the atmosphere far above the surface don’t influence humans as directly as do a heat wave or a hurricane. But high-altitude shifts in circulation and temperature—plus the solar-driven electromagnetic storms known as space weather—do affect radio signals, satellites, and other aspects of our technologically driven culture. Changes in the upper atmosphere also serve as important clues in the greenhouse detective story.
From its earliest days, NCAR’s roster of scientists included experts in upper-atmosphere and solar behavior as well as ground-level weather and climate. These realms stayed largely separate until the last decade. Today, the challenge of understanding the full depth and height of Earth’s atmosphere is driving some of NCAR’s boldest interdisciplinary research.
The realm of global cooling
As people watch and worry about global warming where we live, the opposite trend unfolds far above us. Beyond the troposphere—the lowest 10–16 km (6–10 mi)—much of Earth’s atmosphere is actually getting chillier. NOAA satellites show that temperatures in the lower stratosphere, from about 10 to 30 km (6–18 mi), have cooled by as much as 1.7° Celsius (3.0° Fahrenheit) since 1979. Most of this is a result of ozone depletion, or what’s commonly called the ozone hole.
Higher still, in the upper stratosphere and lower thermosphere, carbon dioxide (CO2) is the surprising culprit behind cooling. As humans add massive amounts of CO2 to the air by burning fossil fuels, some of it gradually sifts upward. Most of Earth’s radiation is absorbed by greenhouse gases in the troposphere and reradiated as heat, so there’s little of it left to be absorbed by CO2 located in or above the stratosphere. However, the molecule still radiates energy to space, so the net result of a CO2 buildup at higher altitudes is a chilling effect.
For satellites, there’s a silver lining in this cooldown. Back in 1989, NCAR space physicist Raymond Roble and his colleague Robert Dickinson (now at Georgia Institute of Technology) predicted that the thermosphere would cool and contract as greenhouse gases increased. Their forecast was borne out by the tracks of satellites that zip through the thermosphere in low-Earth orbit, around 500 km (300 mi) up. Over time, the atmosphere literally drags these satellites downward, but the amount of drag has lessened in recent years—a sign that the thermosphere is indeed thinning and cooling.
Will the future bring even smoother sailing for satellites? Liying Qian, Roble, and Stan Solomon (NCAR’s High Altitude Observatory) and Timothy Kane (Pennsylvania State University) recently became the first to answer this question numerically. The team projected that greenhouse emissions would produce a 3% thinning of the thermosphere by 2017. Besides carbon dioxide, the NCAR-PSU team also accounted for a full 11-year cycle of solar activity, which has its own warming and cooling effects on the thermosphere.
“Satellite operators noticed the solar-cycle changes in density at the very beginning of the space age,” says Solomon. “We are now able to reproduce the changes using NCAR models and extend them into the next solar cycle.” The intensity of the upcoming cycle was itself a source of intense debate in 2006–07. Traditional statistics-based forecasts suggested a fairly mild cycle ahead, while a new dynamics-based model created by NCAR’s Mausumi Dikpati and colleagues pointed to a stronger-than-usual cycle. Solomon’s team based their work on the Dikpati outlook, so if a weaker cycle comes to pass, satellites could glide through the thermosphere with even more ease.
Up and down: tracking waves, tides, and electrons
Even if our climate were stable, the middle and upper atmosphere would be a challenging region to study. Dozens of satellites have kept an eye on the lower atmosphere for decades, but only a handful have monitored the stratosphere, mesosphere, and thermosphere.
Starting in the late 1990s, scientists from across the specialties now housed in NCAR’s Earth and Sun Systems Laboratory began collaborating on a model that would simulate the chemistry and physics of the atmosphere from top to bottom. After years of development, the Whole Atmosphere Community Climate Model (WACCM) is now coming into its own as one of the world’s leading tools for studying links between the lower, middle, and upper atmosphere.
Among the many processes being brought into sharper focus with WACCM is the role of gravity waves. Just as a rock dropped into a pond sends out ripples, air disturbed by mountains, thunderstorms, collapsing fronts, or jet streams can trigger oscillations that propagate outward and upward through the stratosphere and beyond (see graphic). These waves have scales between a few miles and thousands of miles and periods anywhere from minutes to days. While the larger scales can be simulated explicitly in today’s climate models, the smaller ones—those less than a few hundred miles across—cannot.
“Small-scale gravity waves are very important to making atmospheric models work right, but they’re very poorly understood,” says NCAR’s Richard Carbone, head of The Institute for Integrative and Multidisciplinary Earth Studies. At a 2006 TIIMES retreat, a group of 30 experts stressed the need to accurately portray the sources of these gravity waves. Mountain-induced waves already appear in most global models, but those generated by thunderstorms and fronts are trickier. Since climate models struggle with these short-fuse events, modelers often introduce an approximated but plausible representation of storm-related waves as a substitute for depicting them directly.
Over the last several years, NCAR’s Jadwiga Richter has developed a better strategy for modeling thunderstorm-generated gravity waves. She’s now added the parameterization to WACCM with encouraging results. WACCM also benefits from a new scheme for depicting frontal-based waves, one devised by Martin Charron and Elisa Mancini at Germany’s Max Planck Institute for Meteorology. “WACCM is the first global model to include physically based representations of all these wave sources,” says Richter.
There’s much more work to be done in making WACCM a true whole-atmosphere model. With support from NSF and the Office of Naval Research, Hanli Liu is leading NCAR’s push to extend the top of WACCM from 140 km (87 mi) to 500 km (310 mi). The model’s new domain will incorporate the upper thermosphere as well as the collocated ionosphere, the soup of plasma (electromagnetic gas) vulnerable to solar storms. By mid-2007, Liu and colleagues had forged a “reasonably good thermosphere” for WACCM, with the ionosphere next in the queue.
NCAR has modeled Earth’s uppermost atmosphere for years, but connecting it to lower levels is a different challenge. “We believe a lot of the day-to-day variability in the upper atmosphere is related to the lower atmosphere,” says Liu. For instance, gravity waves that propagate from below appear to cause sudden depletions of plasma in the upper ionosphere’s F-region, which radio broadcasters employ to bounce signals to distant locations. A team led by Thomas Immel (University of California, Berkeley) combined NASA satellite data with NCAR’s Global Scale Wave Model to connect thunderstorm-triggered gravity waves and ionospheric turmoil (see graphic). The findings could eventually help scientists issue alerts for potential radio and GPS disruptions.
There’s plenty of interest in WACCM’s lower layers as well. For instance, the model needs to depict atmospheric tides, large-scale gravity waves that pulse upward through the stratosphere and mesosphere with each day’s heating. Tides and other gravity waves often interact in ways that are difficult to observe, let alone model. Scientists also hope WACCM can clarify the critical exchanges between the upper troposphere and lower stratosphere, a region where the seeds for some important ground-level weather shifts appear to sprout. “These challenges demand an integrated model that treats the whole atmosphere consistently,” says Rolando Garcia, leader of the WACCM team.