Page 361 - Geosystems An Introduction to Physical Geography 4th Canadian Edition

Chapter 11 Climate Change 325
TABLE 11.1 Highlights of the 2013–2014 IPCC Fifth Assessment Report
Key Points of the Fifth Assessment Summary*
• Each of the last three decades has been successively warmer at the earth’s surface than any preceding decade since 1850.
• Over the last two decades, the Greenland and Antarctic ice sheets have been losing mass, glaciers have continued to shrink almost worldwide, and arctic sea ice and northern Hemisphere spring snow cover have continued to decrease in extent.
• The atmospheric concentrations of carbon dioxide (CO2), methane, and nitrous oxide have increased to levels unprecedented in at least the last 800 000 years, primarily from fossil-fuel emissions and secondarily from net land-use-change emissions. The ocean has absorbed about 30% of the emitted anthropogenic carbon dioxide, causing ocean acidification.
• Warming of the climate system is unequivocal. Many of
the temperature changes observed since the 1950s are unprecedented over decades to millennia. it is extremely likely (95%–99%) that human influence has been the dominant cause of the observed warming since the mid-20th century.
• Continued emissions of greenhouse gases will cause further warming and changes in all components of the climate system. limiting climate change will require substantial and sustained reductions of greenhouse gas emissions.
record temperatures in Australia in 2013). In Canada, the warmest year on record is 2010, when the national mean temperature was 3.0 C° above the 1961–1990 baseline based on Environment Canada records (www.ec.gc.ca/adsc- cmda/default.asp?lang=En&n=8C7AB86B-1). Since 1948, an- nual temperatures have fluctuated from year to year, but there has been a steady trend, rising 1.6 C° over the 66-year period.
Ocean temperatures are also rising. As discussed in Chapter 5, sea surface temperatures increased at an average rate of 0.07 C° per year from 1901 to 2012 as oceans absorbed atmospheric heat. This rise is re- flected in measurements of upper-ocean heat con- tent, which includes the upper 700 m of ocean (www .ncdc.noaa.gov/indicators/, click on “warming climate”). This increasing heat content is consistent with sea-level rise resulting from the thermal expansion of seawater (discussed ahead).
Ice Melt
Heating of Earth’s atmosphere and oceans is causing land ice and sea ice to melt. Chapter 17 discusses the character and distribution of snow and ice in Earth’s cryosphere.
Glacial Ice and Permafrost Land ice occurs in the form of glaciers, ice sheets, ice caps, ice fields, and frozen ground. These freshwater ice masses are found at high latitudes and worldwide at high elevations. As tempera- tures rise in Earth’s atmosphere, glaciers are losing mass, shrinking in size in a process known as “glacial retreat” (Figure 11.19; also see photos in Chapter 5, Figure HD 5a, and discussion in Chapter 17).
Earth’s two largest ice sheets, in Greenland and Ant- arctica, are also losing mass. Summer melt on the Green- land Ice Sheet increased 30% from 1979 to 2006, with about half the surface area of the ice sheet experiencing some melting on average during the summer months. In July 2012, satellite data showed that 97% of the ice sheet’s surface was melting, the greatest extent in the 30-year re- cord of satellite measurements.
As discussed in this chapter’s Geosystems Now, perma- frost (perennially frozen ground) is thawing in the Arctic at accelerating rates. Scientists now estimate that between one- and two-thirds of Arctic permafrost will thaw over the next 200 years, if not sooner; these permafrost reserves took tens of thousands of years to form. Warming land and ocean temperatures may also cause the thaw of methane hydrates stored in permafrost and in deep-ocean sediments on the seafloor (discussed in Focus Study 11.1).
Sea Ice In earlier chapters, we discussed the effects of Arctic sea ice on global temperatures. Sea ice is com- posed of frozen seawater, which forms over the ocean (sea ice does not include ice shelves and icebergs, which are made up of freshwater originating on land). Arctic sea ice, also called pack ice, is especially important for global climate owing to its effects on global albedo; re- member that the Arctic region is an ocean surrounded
• Ocean warming dominates the increase in energy stored in the climate system, accounting for more than 90% of the energy accumulated between 1971 and 2010. Further uptake of carbon by the ocean will increase ocean acidification.
• The rate of sea-level rise since the mid-19th century has been larger than the mean rate during the previous two millennia. Over the period 1901–2010, global mean sea level rose by 0.19 m.
• Total radiative forcing is positive and has led to an uptake of energy by the climate system. The largest contribution to total radiative forcing is caused by the increase in the atmospheric concentration of CO2 since 1750.
• Climate models have improved since the Fourth Assessment Report. Models reproduce observed continental-scale surface- temperature patterns and trends over many decades, including the more rapid warming since the mid-20th century and the cooling immediately following large volcanic eruptions.
• Changes in the global water cycle will not be uniform. The contrast in precipitation between wet and dry regions and between wet and dry seasons will increase.
• Global mean sea level will continue to rise. The rate of sea-level rise will very likely exceed that observed during 1971–2010, due to increased ocean warming and increased loss of mass from glaciers and ice sheets.
*Source: Climate Change 2013, The Physical Science Basis, Summary for Policy Makers (SPM), Working group i, Contribution to the Fifth assessment report of the iPCC.
Record-setting summer daytime temperatures are being recorded in many countries (Figure 11.18). For exam- ple, in August 2013 temperatures in western Japan topped 40°C for 4 days, and on August 12th, the temperature in Kochi Prefecture reached 41°C, the highest ever recorded in that country (Focus Study 5.1 on page 136 discusses