Current warming and likely future impacts - Climate Change: Biological and Human Aspects - page 301

largest rivers. New Orleans lies in a basin, much of which is below sea level, and
which at its lowest is 6 m below sea level. Past water management is in no small part
to blame. Since 1930 much of the lower Mississippi has had extensive embankments
to control flooding. These prevent water and sediment from reaching the delta, which
is steadily eroding. Not surprisingly the 2009 USGCRP report emphasised these
concerns.
Events subsequent to 2001 make it worthwhile to look at this last USGCRP con-
clusion more closely. It was thought that New Orleans could survive surges caused by
a category-3 hurricane, but not by any higher categories. Yet on 16 September 2004
Hurricane Ivan (a category-5 hurricane with winds in excess of 240 km/h) hit the
coast some 160 km to the east of New Orleans. Despite this narrow escape a serious
flood incident appeared to be just a question of time. Then, on the 29 August 2005,
the category-5 Hurricane Katrina struck. (Bearing in mind the possible relationship
between hurricanes and sea temperatures discussed above it is interesting - but not
scientific - to note that Gulf of Mexico sea temperatures at the time were, for August,
the highest ever recorded.) Aside from the immediate wind damage caused by Katrina,
sea surges flooded much nearby coastal property. The levy defences around New
Orleans itself were breached so that most of the city was flooded: the flood area
exceeded 400 km 2 . The city effectively shut down and nearly all its inhabitants either
left or subsequently were evacuated. President George W. Bush declared a state of
emergency for Louisiana and surrounding states. The economic losses associated
with Katrina were in excess of $125 billion. Many would have wanted the author-
ities to heed the warnings given years in advance in reports such as those from the
USGCRP (see also 8.5.1).
The USGCRP's 2001 and 2009 reports also looked at commercial and infrastructure
sectors, and, although there is not the space here to review all of them, the section
on agriculture deserves special attention, as the USA is a major supplier of food
globally. In 2000 US products accounted for more than 25% of the global trade in
wheat, maize, soya beans and cotton. Furthermore, cropland occupied some 17% of
the total US land area and in addition pastures and grazing lands accounted for a
further 26% of land area. Economically, agricultural commodities annually exceeded
$165 billion in 1999 at the farm gate and over $500 billion (10% of US GDP) after
processing and marketing. By the time of the 2009 report the gross value of the US
agricultural sector was in excess of $200 billion.
Historically, US agricultural productivity has improved by roughly 1% per annum
since 1950. This success could cause an economic problem for the sector, as profit
margins are tight with little scope for further efficiency gains. The 2001 USGCRP
report focused primarily on plant crops and not animal husbandry, even though (as
the report itself acknowledged) climate change will also effect this dimension of food
production.
The USGCRP reports note that region-by-region climate change will have both pos-
itive and negative effects on different types of agricultural activity. For instance, yields
in the northern midwest are currently limited by temperature, so agricultural produc-
tion, it is thought, would increase with warming. Indeed, the anticipated longer thermal
growing season (TGS) would allow double cropping. The carbon dioxide fertilisation