Monday :: Apr 23, 2007

Getting Dry In A Dry Land

by Christina Hulbe

A new paper in the journal Science projects a transition to a more arid North American southwest within the 21st Century (abstract of Seager et al, 2007, at ScienceExpress and an overview by Seager). The broader headline might be "the wet get wetter and the dry get drier" as the global hydrologic cycle intensifies (pdf).

Drought is a normal feature of climate in most places on Earth. At a simple level, drought is the result of lower than expected precipitation in a region but more rigorous definitions consider both the water deficit with respect to long-term average conditions and the broader impacts of the drought event. A distinction is often made between meteorological drought, defined according to the degree of dryness compared to the regional average, and hydrological drought, in which the effects of precipitation shortfalls on the surface and subsurface water supply are also considered. Following from there, agricultural drought emphasizes effects on farm production.

Here's an example to put the distinctions in perspective: Most of us are familiar with the U.S. high plains droughts of the 1930's and the associated Dust Bowl. We tend to think of it as a decade-long meteorological event but not all of the Dust Bowl years were dry. Across the southern high plains, the exceptionally dry years were 1930–31, 1934, 1936, and 1939–40 but those events were so severe and so closely timed that the region could not recover between them and the hydrological and agricultural drought deepened throughout the decade (eventually, even water well-supported household gardens failed).

The effects of the 1930's drought was exacerbated by overcapitalization in the 1920's and poor farming practices applied to land that was marginal for rain-fed agriculture in the best of years. The high plains drought of the 1950's was more intense than the drought of the 1930's but its impact, while substantial, was less severe. By the 1950's farmers had tapped deep aquifers, soil conservation districts had been created, federal programs were in place to keep farmers on the land in lean years, and the national economy was in fine shape.

A consortium of research groups maintains a real-time drought monitor at the University of Nebraska, Lincoln. UN-L's drought website offers a wealth of drought information.

global precipitation
Precipitation varies dramatically across the planet--from the extreme aridity of the nearly lifeless Atacama Desert to the downpours and immense biodiversity of New Guinea--due to the large-scale "general circulation" of the atmosphere. That circulation is driven by the equator-to-pole gradient in net energy fluxes: the tropics receive more incoming (shortwave) energy than they radiate (longwave) back to space and the opposite is the case at high latitudes.

In and of itself, the equator-to-pole energy gradient would produce a very simple equator-to-pole overturning of the atmosphere but the circulation is complicated by Earth's rotation and by the distribution of continents across the planet. The Coriolis force deflects the moving fluid, yielding jet streams between zones of high and low pressure, and high mountain ranges help direct the zonal flow.

Writ large, the result is three vertical convection cells, the Hadley, Ferrel, and polar cells (nice graphics & explanation). The accompanying accompanying precipitation pattern is: wet tropics, dry subtropics, wet mid-latitues, and dry high latitudes (climate region map). There are important seasonal land-sea contrasts as well, which yield on-shore and off-shore summer and winter circulations.

the Hadley Cell
Year-round solar heating in the tropics keeps the ocean and land surface relatively warm, which in turn warms air near the surface. The warm air carries water vapor and those two attributes result in relatively buoyant air masses that rise to great altitudes (water vapor is lighter than O2 and N2, the main components of dry air). As the rising air cools, the vapor condenses and falls back to the surface as rain.

The rising tropical air dries out and moves north and south, toward higher latitudes. As that air begins to sink back toward Earth's surface over the subtropics, it warms. The warm, dry air drives evaporation, yielding the subtropical deserts.

Air masses returning toward the equator gain moisture along the way, recharging to fuel the deep tropical convection that produces dramatic clouds (Space Shuttle photograph) and tropical rain forests. The low-latitude circulation is called the Hadley cell. Some of the sinking air flows toward higher latitudes, where increasing moisture content again drives vertical convection and precipitation.

changing climate in the southwestern U.S.
The North American southwest is arid because it is under the dry, descending limb of the Hadley cell (and far from the moisture brought onshore to the Gulf states). Global warming has two effects that both work to increase subtropical aridity. Overall, as the atmosphere warms its moisture content increases, intensifying the existing circulation pattern and expanding the Hadley cell poleward. The expansion of the Hadley cell in turn expands the subtropical dry zones. Expansion of the Hadley cell also causes mid-latitude storm tracks to shift toward higher latitudes, further depriving the subtropical regions of moisture.

In their new paper (abstract), Seager and his colleagues use an ensemble of nineteen climate models involved in the IPCC Fourth Assessment to investigate changes in subtropical moisture driven by global warming. The models followed a mid-range future-CO2 scenario in which emissions rise until about 2050 and decrease slightly thereafter, with stabilization at 720 ppm CO2 by 2100. What they find is a shift, early in this century, toward persistently drier conditions ("severe drying" is how they state it) in the southwestern U.S and northern Mexico (and elsewhere, including southern Europe, the Mediterranean, and the Middle East).

The projected change is different from the historical droughts with which we are familiar. Typical North American drought events are episodic. They are the result of variations in tropical sea surface temperature, primarily in the Pacific, that produce transient changes in atmospheric circulation. What Seager and colleagues discuss is a climate shift toward drier conditions unlike anything in our experience.

cultural impacts of drought
Across the sweep of human history, drought stands out for its ability to drive political, economic, and cultural change. Cities are relocated, governments fall, ways of life change , and communities vanish when the water required to support agriculture disappears. I haven't read the popular books on this subject but I do try to keep up with the relevant scholarly literature. The archaeological stories are, to my mind, complex. It doesn't seem to be just one hardship that gets you, but challenges in combination.

Consider, perhaps, the collapse of the Classic Maya civilization between 750 and 950 AD. No single theory quite explains the abrupt end of that advanced, urban society. The possibilities include overpopulation, drought, and warfare (pdf). Not all of the Mayan cities collapsed at the same time and not all were locked in protracted conflict. The southern Mayan cities--the first to fall--might have persisted longer in the face of a shift to drier conditions on the Yucatan Peninsula (pdf of research paper) if they hadn't been busy fighting each other and perhaps the northern cities lasted longer because they were not.

Both the persistence and intensity of drought are expected to increase in many subtropical regions due to global warming. The Intergovernmental Panel on Climate Change Fourth Assessment's Working Group 2 reported recently (pdf), that drought-related impacts are expected to be hardest in the developing world, where capacity for adaptation is limited. But if history is any guide, whatever it is that's distracting you when you should be preparing for and adapting to climate change probably matters too.

Christina Hulbe :: 7:50 AM :: Comments (17) :: Digg It!