Environmental Engineering Reference
In-Depth Information
western Canada, Eurasia, and the United States. The primary driver of these
changes is the warming in most of the regions evaluated, with lesser con-
tributions from changes in precipitation. {5.4}
Areas of the United States that are particularly vulnerable to increases
in wildfire extent include the Pacific Northwest and forested regions of
the Rockies and the Sierra. Studies are limited in number but suggest that
warming of 1°C (relative to
19
50-2003) is expected to produce increases
in median area burned by about 200-400% (see Figure O.6).
Some dry
grassland and shrub regions (for example, in the southwestern United States)
may experience a decrease in wildfires, because warming without increases
in precipitation would reduce biomass production and hence limit the avail-
ability of fuel. Uncertainties include understanding of local soil moisture
changes with global warming. Over time, extensive warming and associ-
ated wildfires could exhaust the fuel for fire in some regions, as forests are
completely burned. {5.4}
Ocean Acidification
Rising atmospheric CO
2
alters ocean chemistry, leading to more acidic
conditions (lower pH) and lower chemical saturation states for calcium
carbonate minerals used by many plants, animals, and microorganisms to
make shells and skeletons.
Ocean acidification is documented clearly from
ocean time-series measurements for the past two decades. Surface ocean
pH has dropped on average by about 0.1 pH units from pre-industrial levels
(pH is measured on a logarithmic scale, and a 0.1 pH drop is equivalent
to a 26% increase in hydrogen ion concentration). Additional declines of
0.15 and 0.30 pH units will occur if atmospheric carbon dioxide reaches
about 560 ppm and 830 ppm, respectively (see Figure O.7). Polar surface
waters will become under-saturated with respect to aragonite, a key cal-
cium carbon mineral, for atmospheric CO
2
levels of 400-450 ppm for the
Arctic and 550-600 ppm for the Antarctic. In tropical surface waters, large
reductions in calcium carbonate saturation state will occur, but waters
will remain super-saturated for projected atmospheric CO
2
during the 21st
century. Subsurface waters will also be affected, but more slowly, governed
by ocean circulation; the fastest rates will occur in the upper few hundred
meters globally and in polar regions where cold surface waters sink into
the interior ocean. {4.9}
