Current observations of change related to climate

Research from the Commonwealth Scientific and Industrial Research Organisation (CSIRO) in Australia shows the relationship between sea surface temperatures and bleaching of several coral species in the Great Barrier Reef over the past 140 years, and projections into the future (see Figure 12.2). The data are clear that we are now in a transition period, when the frequency of conditions conducive to bleaching are increasing rapidly, and will soon be in a range where bleaching conditions occur annually. The link between high sea surface temperatures and bleaching of corals has been established unequivocally, and the phenomenon has been shown to be circumtropical in nature.

Figure 12.2 Projected outlook for coral reefs as ocean temperature rises as a result of climate change

Source: Hoegh-Guldberg, 2005.

The widespread occurrence of land-based glacial retreat is also well documented. Glaciers in both northern latitudes in North America, Europe and Asia are known to be receding. Moreover, glaciers at high elevations in mid-latitudes and in the subtropics and tropics are also receding. Widespread change in the climate system is the only forcing factor that can explain the simultaneity of these changes over such broad ranges of latitude and elevation. Some of these changes (for example, the retreat of Chacaltaya Glacier in Bolivia) have direct impacts on human well-being, in this case, reduction of one of the major water supplies for La Paz.

The most recent assessment by IPCC's Working Group II (IPCC, 2007b) has extensively documented a wide range of physical and ecological impacts that are clearly associated with changes in the physical climate system. These range from reductions in mountain snowpack in the US Pacific Northwest to the loss of Arctic sea ice and unexpectedly rapid break-up of several ice shelves in the Antarctic. In addition to these, there are more purely biological changes, such as the earlier migration of many species of birds and butterflies, and longer growing seasons in temperate latitudes. Over 1500 studies are documented by IPCC, and two extensive meta-analyses show conclusively that the ecological and biological changes that have been documented are consistent with changes in the climate system itself.

One marine ecosystem change that has recently been documented is also due to the accumulation of excess CO2 in the atmosphere, although it is not directly related to changes in the climate system per se. Because the chemistry of ocean surface waters quickly comes into equilibrium with the changing atmospheric concentration of CO2, a reduction in the average pH of surface ocean waters of about 0.1 pH units has been observed over the last 150 years. If concentrations of atmospheric CO2 continue to rise at about the same rate as is occurring at present, ocean pH could drop another 0.1 units over the next several decades (see Figure 12.3).

It is already known that reductions in average ocean surface pH of this magnitude do not reflect the full range of pH losses in different ocean basins. In addition, the reactions by which marine organisms, including corals and free swimming plankton, extract calcium carbonate from solution in seawater to construct their skeletons are themselves pH-dependent. The reduction in oceanic pH results in those organisms being less capable of building their skeletons. While of obvious importance for corals and plankton, there are almost certainly going to be impacts from this phenomenon all through marine food chains. However, the consequences for fisheries and other marine organisms are not known at this time with any certainty.

700-

600-

cl

500

cl

o

400-

300-

200

"Business as usual" IS92a scenario IPSS, 1995)

"Business as usual" IS92a scenario IPSS, 1995)

pCO2

1850 1900 1950 2000 2050 2100 Year

1850 1900 1950 2000 2050 2100 Year

1850 1900 1950 2000 2050 2100 Year

Figure 12.3 Past and projected changes in ocean chemistry as a result of the rising atmospheric CO2 concentration

Source: Wolf-Gladrow et al, 1999.

There have also been significant increases in the number of major wild fires over the past 50 years (see Figure 12.4). As with many other phenomena, this pattern has multiple causes, including increases in the likelihood of persistent drought and the consequences of decades of fire suppression policy, which has acted to increase the fuel available for ignition. However, variations in fire frequency in forest ecosystems in western North America, at least, have been linked to variations and change in the climate system. This phenomenon parallels one of the most consistent, and apparently robust, model projections that has emerged over recent decades, namely, that there will be an increase in the risk of forest fire as a result of climate change.

Wild fires

Number at events

Clara plotted by decade 50

Number at events

Clara plotted by decade 50

Figure 12.4 Number of major wild fires by continent and decade since 1950

Source: MEA, 2005.

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