Background and context

The state of the Earth system has clearly been changing rapidly over the past several decades. The Fourth Assessment Report of Working Group I of the IPCC (IPCC, 2007a) has summarized the available scientific information on recent changes in the Earth's physical climate system, and concluded that a significant human influence on climate is unequivocal. It is no longer possible, for example, to model changes in climate during the 20th century without accounting for the influences of changes in human forcings from greenhouse gases and aerosols. This is powerful evidence that human actions are already affecting the climate system, and there is a very clear expectation that this influence will continue to increase, especially if emissions and atmospheric concentrations of greenhouse gases also continue to increase. IPCC's analyses of simulations carried out using a wide range of atmosphere—ocean general circulation models demonstrate clearly that, over the next century, the Earth is very likely to experience both rates of change and absolute magnitudes of change in the physical climate system that far surpass any that human societies have experienced, and indeed that surpass natural rates of change seen in hundreds of thousands of years.

At the same time, there have been equally dramatic changes in the Earth's complement of natural resources and ecosystems. The Millennium Ecosystem Assessment (MEA, 2005), which is the most recent and most comprehensive scientific assessment of the planet's natural resources, provides documentation of unprecedented changes in natural resources over the past 40—50 years. For example:

• more land has been converted to cropland since 1945 than during the 18 th and 19th centuries combined;

• 20 per cent of the world's coral reefs have been lost and 20 per cent degraded over recent decades;

• 35 per cent of mangrove area has been lost over recent decades;

• the amount of water held in reservoirs has quadrupled since 1960;

• withdrawals of water from rivers and lakes have doubled since 1960.

From the standpoint of terrestrial ecosystems, one of the most dramatic changes in the status of ecosystems and natural resources is the dramatic land-cover change that has taken place over past decades. In the MEA (2005) and in Lepers et al (2005), the locations of deforestation and forest degradation, and of cropland expansion and contraction were carefully documented for the time period 1980-2000. Figure 12.1 (Plate 16) shows one of the results of this analysis, which required harmonizing regional and global datasets, and then portraying them on the same GIS grid so that the top 10 per cent of pixels in which changes were seen could be identified (more methodological details are available in Lepers et al, 2005).

The results of this analysis are very clear. Major areas of tropical and subtropical forests were lost over this time period in South America, Central America, Central and East Africa, and throughout Southeast Asia. However, in addition, there was substantial loss and degradation of forests across Russia, from its Eastern European borders to the Russian Far East. These regions include some of the most species-rich regions of the world (for example, the Amazon Basin), as well as the largest expanse of boreal forest. It is also important to note that many areas not showing change over this period were significantly transformed in prior periods.

During the same time period, there were also extensive changes in the land devoted to cropland. Significant expansions of croplands occurred in South America, Central and East Africa, and Southeast Asia, whereas cropland in East Asia actually decreased in area. Data for the US indicate a patchwork quilt of

Figure 12.1 Main areas of deforestation and land degradation from 1980-2000 (see Plate 16 for color version)

Source: MEA, 2005.

increases and decreases, partially depending on the specific definitions of cropland used in the different data sets that were harmonized. The very beginnings of the large expansion of soy agriculture in South America were just beginning to be detected in the MEA analysis.

Marine ecosystems experienced similarly large and rapid changes. The MEA concluded that many of the major open ocean fish stocks have already been depleted from overfishing and are no longer being harvested in a sustainable manner.

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