Climate Change 2001: Synthesis Report

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4.17 Many natural and managed ecosystems may change abruptly or non-linearly during the 21st century. The greater the magnitude and rate of the change, the greater the risk of adverse impacts.

4.18 Changes in climate could increase the risk of abrupt and non-linear changes in many ecosystems, which would affect their biodiversity, productivity, and function. For example, sustained increases in water temperatures of as little as 1°C, alone or in combination with any of several stresses (e.g., excessive pollution and siltation), can lead to corals ejecting their algae (coral bleaching; see Figure 4-3 and Question 2), the eventual death of the corals, and a possible loss of biodiversity. Climate change will also shift suitable habitats for many terrestrial and marine organisms polewards or terrestrial ones to higher altitudes in mountainous areas. Increased disturbances along with the shift in habitats and the more restrictive conditions needed for establishment of species could lead to abrupt and rapid breakdown of terrestrial and marine ecosystems, which could result in new plant and animal assemblages that are less diverse, that include more "weedy" species, and that increase risk of extinctions (see Question 3).

WGII TAR Sections 5.2, 6.4.5, & 17.2.4

Ecological systems have many interacting non-linear processes and are thus subject to abrupt changes and threshold effects arising from relatively small changes in driving variables, such as climate. For example:

  • Temperature increase beyond a threshold, which varies by crop and variety, can affect key development stages of some crops and result in severe losses in crop yields. Examples of key development stages and their critical thresholds include spikelet sterility in rice (e.g., temperatures greater than 35°C for more than 1 hour during the flowering and pollination process greatly reduce flower formation and eventually grain production), loss of pollen viability in maize (>35°C), reversal of cold-hardening in wheat (>30°C for more than 8 hours), and reduced formation of tubers and tuber bulking in potatoes (>20°C). Yield losses in these crops can be severe if temperatures exceed critical limits for even short periods.
  • Mangroves occupy a transition zone between sea and land that is set by a balance between the erosional processes from the sea and siltation processes from land. The erosional processes from the sea might be expected to increase with sea-level rise, and the siltation processes through climate change and other human activities (e.g., coastal development). Thus, the impact on the mangrove forests will be determined by the balance between these two processes, which will determine whether mangrove systems migrate landward or seaward.

WGII SAR Sections 13.2.2 & 13.6.2

WGII TAR Sections 5.3, 10.2.2, 15.2, & 17.2

4.20 Large-scale changes in vegetation cover could affect regional climate. Changes in land surface characteristics, such as those created by land cover, can modify energy, water, and gas fluxes and affect atmospheric composition creating changes in local/regional climate and thus changing the disturbance regime (e.g., in the Arctic). In areas without surface water (typically semi-arid or arid), evapotranspiration and albedo affect the local hydrologic cycle, thus a reduction in vegetative cover could lead to reduced precipitation at the local/regional scale and change the frequency and persistence of droughts.

WGII TAR Sections 1.3.1, 5.2, 5.9,, 13.2.2, 13.6.2, & 14.2.1
Figure 4-3: The diversity of corals could be affected with the branching corals (e.g., staghorn coral) decreasing or becoming locally extinct as they tend to be more severely affected by increases in sea surface temperatures, and the massive corals (e.g., brain corals) increasing.
WGII TAR Section 17.2.4

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