Dunes and Climate - Dune Worlds: How Windblown Sand Shapes Planetary Landscapes - page 243

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Dunes and Climate

A number of factors are required to form a dune, and thus
the existence of a dune tells us that those factors are, or
were once, present. If those factors are not present today, it
follows that they must have occurred in the past, and so
dunes can serve as windows into the past.
A principal factor in controlling whether fine particles
(which in many places means soil that sustains agriculture)
is rainfall, both directly (in that damp ground is sticky and
sand moves less effectively) and, more importantly, by the
indirect role of soil moisture on vegetation. Plants bind the
soil with their roots, the organic matter from the breakdown
of dead plant matter improves the sand's cohesion, and the
aerodynamic shielding by the plant's branches and leaves
reduces the wind stress applied to the soil. Due to all these
factors, perhaps more than variations in windiness (which
may yet be occurring), climate change via rainfall variation
can alter the mobility of large areas of sand. The planetary
perspective can help set our changing climate in context: in
particular, just as Venus is the solar system's 'poster child'
for greenhouse warming, Titan has a hydrological cycle that
resembles an extreme version of Earth's, in the direction of
which Earth's climate may be evolving. Specifically, a
warmer atmosphere can hold more water vapor, and thus
surface evaporation (driven by more-or-less fixed sunlight)
takes longer to recharge the atmosphere with moisture after
a rainstorm. But with more moisture available, the storm
when it comes is more violent. Thus a warming atmosphere
may see the uncomfortable double blow of heavier rain-
storms with longer droughts between them. Whereas the
*1 m/yr of rainfall cycles through the *2 cm of precipi-
table moisture in Earth's atmosphere, resulting in a few
showers a month, Titan's thick atmosphere can hold several
liquid meters' worth of methane vapor, yet the meager
sunlight drives only a few centimeters per Earth year of
evaporation. Thus Titan's rain is manifested as massive
downpours separated by centuries. In that context, the
apparently high mobility of sand on Titan is not difficult to
understand.

Just as 'global warming' does not mean every single
place sees a monotonic increase in temperature, there may
be 'winners and losers' in the game of mobility. Some
present-day deserts on Earth may become more easily cul-
tivated, and the greening of the prairies a couple of hundred
years ago, turning barchans [whose topographic signature is
still present (Wolfe and Hugenholtz 2009)] into vegetated
parabolic dunes is an example. But likely the majority of
areas will see average soil moisture drops and consequent
mobilization of sand (Fig. 21.1 ), which is generally bad
news for people other than aeolian geomorphologists. One
study in particular (Thomas et al. 2005) forecasts that
pastoral and agricultural areas in the Kalahari basin which
lie on presently inactive dune sands will see significantly
enhanced dune activity by 2039, and that by 2099 dune-
fields from northern South Africa to Angola and Zambia
will become highly dynamic.
The Earth likely saw less precipitation overall during the
Ice Ages, when conditions were colder and the atmosphere
could hold less moisture. Furthermore, because so much
water was locked up in polar and continental ice sheets, the
sea level was lower by around 100 m, and thus many areas
such as the Persian Gulf, which are currently shallow sea-
beds, were in fact dry land and could act as sand sources.
The overall sand dune activity on the planet was likely
much more vigorous than today, forming once-active dunes
which are now vegetated and stabilized (such as the
Nebraska sand hills). The same periods also saw the long-
distance transport of dust, resulting in the deposition of
thick loess in China, the Rhine Valley, Missisipi, Alaska
and elsewhere.
In all this, then, dunes give us a limited but nonetheless
valuable window into the past. Models can help us decode
the dune patterns and layers into prior wind regimes. The
same sort of exercise has been tried at Mars (where global
mapping of dunes has been done systematically, e.g.,
Hayward et al. 2007, 2009), although there is sufficient
widespread disagreement between the models and the dunes