Page 372 - Geosystems An Introduction to Physical Geography 4th Canadian Edition

336 part II The Water, Weather, and Climate Systems
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1900 1950 2000 2050 2100
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(a) Models suggest that the smallest amount of warming corresponds to the lowest CO2 emissions scenario (RCP2.6). Warming is greatest under the RCP8.5 scenario, with the highest CO2 emissions and strongest positive radiative forcing of temperature.
(°C) –2–1.5–1–0.5 0 0.5 1 1.5 2 3 4 5 7 9 11
(b) Possible temperature responses in 2081–2100 to high emission scenario RCP8.5.
(c) Possible temperature responses in 2081–2100 to low emission scenario RCP2.6.
RCP8.5 RCP6.0 RCP4.5 RCP2.6 20th century
Mean temperature change for high emission scenario RCP8.5
Mean temperature change for low emission scenario RCP2.6
▲Figure 11.27 AOGCM scenarios for surface warming during this century. The lowest temperature impacts occur when atmospheric greenhouse gases are held constant at 2000 levels. The lowest–CO2-emission scenarios are B1 and a1T, and correspond to the lowest amounts of global warming. The highest-emission scenarios are a1F1 and a2, corresponding to the greatest temperature increases. [Based on iPCC, Working group i, Fourth Assessment Report: Climate Change 2007: The Physical Science Basis, Figure SPM-5, page 14.]
solar variability and volcanic output alone (shaded blue area), with no anthropogenic forcing.
The model using both natural and anthropogenic forcings (including greenhouse gas concentrations) was the one that produced the closer match to the actual observed temperature averages. Natural forcings alone do not match the increasing temperature trend. These models are consistent with the hypothesis regarding the effect of human activities on climate; the primary an- thropogenic input to Earth’s climate system is increased amounts of greenhouse gases.
Future Temperature Scenarios
GCMs do not predict specific temperatures, but they do offer various future scenarios of global warming. GCM- generated maps correlate well with the observed global warming patterns experienced since 1990, and a variety of AOGCM forecast scenarios were used by the IPCC to pre- dict temperature change during this century. Figure 11.27a depicts four temperature scenarios presented in the IPCC Fifth Assessment Report, each with different conditions of radiative forcing. Each Representative Concentration Pathway, or RCP, is identified by the approximate radia- tive forcing it predicts for the year 2100 as compared to 1750; for example, RCP2.6 denotes 2.6 W/m2 of forcing. Each RCP correlates with certain levels of greenhouse gas emissions, land use, and air pollutants that combine to produce the forcing value. For RCP2.6—the lowest
level—forcing peaks and declines before 2100. This sce- nario could occur with major reductions in CO2 emis- sions and actions to remove CO2 from the atmosphere. RCP4.5 represents stabilization of forcing by the year 2100. For RCP6.0 and RCP8.5, high-emission scenarios of continued heavy fossil-fuel use, radiative forcing does not peak by the year 2100. According to these models, con- tinued CO2 emissions and other human activities that en- hance radiative forcing are the scenario that would cause the greatest amount of warming over the 21st century. Figure 11.27b and c compare these two scenarios.
Sea-Level Projections
In 2012, NOAA scientists developed scenarios for global sea-level rise based on present ice-sheet losses coupled with losses from mountain glaciers and ice caps world- wide. These models projected a mean sea-level rise of 2.0 m by 2100 at the high end of the range, with 1.2 m as the intermediate projection. For perspective, a 0.3-m rise in sea level would produce a shoreline retreat of 30 m in some places; a 1.0-m rise would displace an estimated 130 million people.
In Canada, sea level is not rising at the same rate ev- erywhere on the coast. In response to glacial unloading, some coasts are still rebounding faster than global sea level is rising, and sea level is falling locally. However, there are parts of the coastline that have been identified as “highly sensitive” to sea level rise (Figure 11.28). A
Global surface warming (C°)