Current warming and likely future impacts - Climate Change: Biological and Human Aspects - page 302

effect would also be an additional positive influence on yields. However, these would
be balanced by yield decline in the southern part of the region by as much as 10-20%
due to a lack of sufficient water that would be needed in the warmer climate.
Much of the nation's meat, wheat and fibre is produced on the Great Plains,
including 60% of the country's wheat, 87% of sorghum and 36% of its cotton. In
the 20th century temperatures across the northern and central Great Plains increased
by more than 1 ◦ C and by up to 2 ◦ C in parts of Montana and North and South
Dakota. The eastern portion of the Great Plains has seen precipitation increased
by more than 10% and Nebraska and Texas both had higher levels of rainfall at
the century's end compared to its beginning. Climate models predict that in the
21st century temperatures would rise throughout the region, with the largest rise in
the western parts of the Great Plains. Precipitation generally is likely to continue
to increase, except in the lee of the Rocky Mountains. Furthermore, the effects of
temperature increases on potential evapotranspiration (the theoretical maximum level
of evaporation and plant transpiration with unlimited water and optimal nutrients) are
likely to outstrip the extra supply of water received through increased precipitation.
The result is likely to be a net decline in soil moisture from levels in 2000, especially in
Louisiana and Mississippi. Maintaining water supplies through supply management
throughout the year and ensuring water quality are likely to become key issues for
the region, especially in the south.
The USGCRP 2001 report (relying, as it did, on the sometimes contradictory
Hadley and Canadian models of the late 1990s) did not raise concerns as to lack of
water in some parts of the west. Instead, it concluded that more atmospheric carbon
dioxide, higher temperatures and increased precipitation are likely to increase crop
productivity and extend the TGS. However, it did warn that the patterns of potential
crop productivity are likely to shift. It notes that fruit and nuts make up 32% of
the west's crop value (with a third of that from grapes alone) and that changing the
principal locations of such crop production may well be problematic, bearing in mind
that such crop plants can take decades to become established. Here, if anywhere, there
is an urgent need for accurate local forecasting of likely climate change. This changed
with the 2009 report concluding that drought frequency and severity are projected
to increase in the future over much of the USA, particularly under higher-emissions
scenarios. Increased drought will be occurring at a time when crop water requirements
also are increasing due to rising temperatures. Water deficits are detrimental for all
crops.
The USGCRP's report in 2001 concludes that without adaptation many crops will
grow better by the end of the 21st century, especially cotton and grapefruit (but not
oranges), and pastures will improve. However, the 2009 report mentions that water
shortage, especially in the summer, is a cause of great concern. Barley and oats are
unlikely to improve or will fare badly under future conditions. Potato and orange
production could also see some experience decline. If adaptation is made then crops
could see a 10-100% increase in productivity. Such adaptations importantly will
necessitate changing which crop grows where and changes in water management.
This last will often involve interstate cooperation and considerable cost, whereas a
minority of farmers will have to accept abandoning their land.