western Canada, Eurasia, and the United States. The primary driver of these changes is the warming in most of the regions evaluated, with lesser contributions 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 1ºC (relative to 1950-2003) 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 availability of fuel. Uncertainties include understanding of local soil moisture changes with global warming. Over time, extensive warming and associated wildfires could exhaust the fuel for fire in some regions, as forests are completely burned. {5.4}

Rising atmospheric CO₂ 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 calcium carbon mineral, for atmospheric CO₂ 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 CO₂ 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}