Climate change model scenarios are far from certain. Earth’s climate system is
the product of an interplay of factors, many of them unrelated to the presence of
humans. As numerical modeling becomes more sophisticated, the models are bet-
ter able to incorporate the impacts of the processes by which greenhouse gases are
removed from the atmosphere, the potential for increased humidity (from
increased) evaporation exacerbating the warming, the impacts of the extent and
depths of clouds, and the possible mitigating influence of sulfate aerosols as
released by human activities.Grasslands
Grasslands are a major floralistic association of theEarth. They are characterized
by grasses (Poaceae) and forbs (non-woody plants) of various heights and den-
sities. On some grasslands, trees are interspersed, especially along watercourses.
The climatic cause for widespread grasslands is a greater potential evapotranspira-
tion than precipitation for the year. In such cases, the landscape cannot be domi-
nated by trees because of the moisture stress. Grass species become shorter and
sparser as potential evapotranspiration becomes progressively larger than precipi-
tation. Ultimately, the grasslands grade into desert where the potential evapotran-
spiration is more than twice precipitation. There are also non-climatic reasons by
which grasses can dominate the landscape, such as places in which the soils
contain high concentrations of minerals such as nickel that are toxic to many tree
species.
Geographers usually differentiate grasslands into twobiomesdominated by
grasses. The first is the tropical savanna and the second is the midlatitude grass-
lands. Their characteristics were given in the biomes article earlier in thisHand-
book. There are major grasslands on all the continents save Antarctica. The
regional names are familiar to geographers: Examples include the Great Plains
(North America), pampas (South America), steppe (Russia), veldt (Africa), and
rangelands (Australia). Each continent has its own “flavor” of grassland floral
and faunal species, yet the grassland formations are distinct from those in forests
and other floralistic associations.
Within the last few thousand years, probably 40 percent of continental surfaces
were grasslands but the percentage is declining as a result of the warming of the
planet as it has emerged from the Pleistocene (ice age) and the unintentional
destruction of the ecosystem by overgrazing (seeDesertification). A billion peo-
ple live in the grasslands of the world, so these areas are quite susceptible to
human modifications. Overall, grasslands have declined in their ability to sustain160 Grasslands