Current warming and likely future impacts - Climate Change: Biological and Human Aspects - page 281

estimated that some 16% of reef-building corals died worldwide. When the thermal
stress is not so extreme only the thin-tissued, branching acroporid and pocilloporid
corals have been bleached, leaving larger coral species (such as Porites spp.). Thermal
stress factors need to be added to the other pressures (such as pollution) on corals. The
conservation of 845 reef-building coral species has been assessed using International
Union for the Conservation of Nature (IUCN) Red List Criteria. Of the 704 species
that could be assigned conservation status, a third (32.8%) are in extinction risk
categories (Carpenter et al., 2008).
At the other end of the gradient, Antarctic terrestrial ecosystems have seen
visually marked changes in response to warming. These include the colonisation
of previously bare or newly exposed ground by mainly mosses, as well as by other
plants. Associated with this colonisation, soil invertebrate animals have also spread.
At the moment there are few examples of exotic species colonising Antarctica,
although those that have are mainly found close to geothermal sources of warmth
and the continent's margins. In addition to this climate impact there are a number of
human-imported species. Climate change favours increasing the chances of imported
species becoming established. As for native species, because many Antarctic species
are already at their lower thermal threshold, future warming is likely to see their
numbers and range increase, spreading away from the coast. Having said this,
imported species will offset part of this range expansion as coastal colonisation takes
place and the imported and native species compete.
Meanwhile, in the peri-Arctic environment of Siberia, satellite analysis of some
515 000 km 2 has been carried out since the early 1970s and compared with images
taken between 1997 and 2004, in an area containing Arctic lakes (about a seventh of
this Siberian biome). The results showed a decline from 10 882 large lakes (those 40

ha) to just 1170; that is, approximately 11% of the original number (Smith et al., 2005).
However, most did not disappear altogether but shrank to sizes below 40 ha. Total
regional lake surface area decreased by 93 000 ha (approximately 6%). One hundred
and twenty-five lakes vanished completely. None of these have refilled since 1997-8.
However, where the permafrost has remained continuous (more to the north) lake
area has increased by 13 300 ha (

>

12%). Nonetheless, the more southerly declines
in lake area have outpaced the more northerly gains. What appears to be happening
is that the more southerly permafrost soils have ceased being permanently frozen, so
allowing the lakes to drain. Then again, further to the north the increased precipitation
expected with global warming (see section 6.6.6 on ocean and atmosphere circulation)
is enabling lakes to become larger. Both the decline in the more southern and the
increase in the more northern Siberian Arctic lake areas appear to be caused by
climate change and warming. Such changes have significant implications for soil
carbon storage.
By the end of the 20th century, thaw lakes (as opposed to permanently frozen
lakes) comprised 90% of the lakes in the Russian permafrost zone. North Siberian
lakes differ from many of those in Europe and North America because they are on top
of organic-rich strata. Russian and North American researchers (Walter et al., 2006)
reported that thawing permafrost along thaw-lake margins accounted for most of the
methane released from the lakes and estimated that the 1974-2000 expansion of thaw
lakes, which took place along with regional warming, increased by 58% in the two

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