Synthesis Report - Question 5

Climate Change 2001: Synthesis Report

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Figure 5-2: After CO2 emissions are reduced and atmospheric concentrations stabilize, surface air temperature continues to rise by a few tenths of a degree per century for a century or more. Thermal expansion of the ocean continues long after CO2 emissions have been reduced, and melting of ice sheets continues to contribute to sea-level rise for many centuries. This figure is a generic illustration for stabilization at any level between 450 and 1,000 ppm, and therefore has no units on the response axis. Responses to stabilization trajectories in this range show broadly similar time courses, but the impacts become progressively larger at higher concentrations of CO2 .

WGI TAR Sections 3.7, 9.3, & 11.5, & WGI TAR Figures 3.13, 9.16, 9.19, 11.15, & 11.16

The lower the stabilization target for atmospheric CO2 , the sooner emissions of CO2 would need to decrease to meet it. If emissions were held at present levels, carbon cycle models indicate that the atmospheric concentration of CO2 would continue to rise (see Figure 5-3).

  • Stabilization of CO2 concentrations at any level requires ultimate reduction of global net emissions to a small fraction of the current emission level.
  • Stabilization of atmospheric CO2 concentrations at 450, 650, or 1,000 ppm would require global anthropogenic CO2 emissions to drop below the year 1990 level, within a few decades, about a century, or about 2 centuries, respectively, and continue to decrease steadily thereafter (see Figure 6-1).

These time constraints are partly due to the rate of CO2 uptake by the ocean, which is limited by the slow transport of carbon between the surface and deep waters. There is sufficient uptake capacity in the ocean to incorporate 70 to 80% of foreseeable anthropogenic CO2 emissions to the atmosphere, but this would take several centuries. Chemical reaction involving ocean sediments has the potential to remove up to a further 15% over a period of 5,000 years.

WGI TAR Sections, 3.7.3, &
5.6 A delay between biospheric carbon uptake and carbon release is manifest as a temporary net carbon uptake. The main flows in the global carbon cycle have widely differing characteristic time scales (see Figures 5-1 and 5-4). The net terrestrial carbon uptake that has developed over the past few decades is partly a result of the time lag between photosyntheticcarbonuptakeand carbonreleasewhenplants eventuallydieanddecay. For example, the uptake resulting from regrowth of forests on agricultural lands, abandoned over the last century in the Northern Hemisphere, will decline as the forests reach their mature biomass, growth slows, and death increases. Enhancementofplantcarbon uptakeduetoelevated CO2 or nitrogen deposition will eventually saturate, then decomposition of the increased biomass will catch up. Climate change is likely to increase disturbance and decomposition rates in the future. Some models project that the recent global net terrestrial carbon uptake will peak, then level off or decrease. The peak could be passed within the 21st century according to several model projections. Projections of the global net terrestrial carbon exchange with the atmosphere beyond a few decades remain uncertain (see Figure 5-5). WGI TAR Sections 3.2.2-3 & 3.7.1-2, & WGI TAR Figure 3.10

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