283
D. Kar, Wetlands and Lakes of the World,
DOI 10.1007/978-81-322-1023-8_19, © Springer India 2013
19.1 The Global Extent
of Wetlands
Wetlands are found on every continent except
Antarctica and in every climate, from the tropics
to the tundra. It is quite diffi cult to estimate the
exact extent of the wetlands in the world mainly
because it depends on the defi nitions used. Also,
it is practically quite diffi cult to quantify the wetlands in the aerial and satellite imageries which
are now the most common sources of data. The
world is blessed with a rich diversity of enormous
number of wetlands. Some of these wetland areas
of the world are unique for their extent, variety
and historical importance. A treatise of these
wetlands has been briefl y discussed in this
chapter.
The extent of the world’s wetlands is generally assumed to be from 7 to 9 million km
2 or
about 4–6 % of the land surface of the earth based
on several estimates. Based on the works of
Russian geographers, Maltby and Turner ( 1983 )
had estimated that >6.4 % of the land surface of
the earth (or 8.6 million km
2
) is wetland. Out of
this total estimated wetland area, about 56 % is
said to exist in the tropical (2.6 million km
2 ) and
subtropical (2.1 million km
2
) regions. Further,
the wetlands have been estimated to occupy 1.0
million km
2 in the sub-boreal region (called temperate region), 2.6 million km
2 in the boreal
region and 0.2 million km
2 in the polar region
(Table 19.1 ).
Notwithstanding the above, other estimates of
the global extent of the wetlands have been developed from various studies on the role which the
wetlands play in regulating the global biogeochemical cycles, especially with regard to the greenhouse
gases. Matthews and Fung ( 1987 ) estimated, on the
basis of global digital databases (1° resolution),
that there is about 5.3 million km
2 of wetlands in
the globe. The study revealed a higher percentage
of wetlands being boreal and a far lower percentage
of wetlands being subtropical and tropical. These
estimates were different from those estimated by
Maltby and Turner ( 1983 ). Further, Aselmann and
Crutzen ( 1989 , 1990 ) estimated that there is 5.6
million km
2 of natural wetlands in the world, with
a higher amount and percentage of wetlands in the
temperate regions than given in either of the earlier
estimates. They used the data from the regional
wetland surveys and various monographs rather
than maps, which had been used by Matthews and
Fung ( 1987 ). It is noted here that these two research
groups had estimated the coverage of rice paddies,
but did not include them in their estimation of total
wetland area. Incidentally, Matthews et al . ( 1991 )
had estimated 1.5 million km
2 of rice paddies,
while Aselmann and Crutzen ( 1989 ) had estimated
1.3 million km
2 of rice paddies. Their estimates of
the extent of the world’s wetlands are 6.8 and 6.9
million km
2
, respectively, by including ricefi elds.
Signifi cantly, the Matthew–Fung estimate ( 1987 )
classifi ed wetlands into fi ve classes, namely ,
(a) forested bogs, (b) non-forested bogs, (c) forested
19
Wetlands of the World
D. Kar, Wetlands and Lakes of the World,
DOI 10.1007/978-81-322-1023-8_19, © Springer India 2013
19.1 The Global Extent
of Wetlands
Wetlands are found on every continent except
Antarctica and in every climate, from the tropics
to the tundra. It is quite diffi cult to estimate the
exact extent of the wetlands in the world mainly
because it depends on the defi nitions used. Also,
it is practically quite diffi cult to quantify the wetlands in the aerial and satellite imageries which
are now the most common sources of data. The
world is blessed with a rich diversity of enormous
number of wetlands. Some of these wetland areas
of the world are unique for their extent, variety
and historical importance. A treatise of these
wetlands has been briefl y discussed in this
chapter.
The extent of the world’s wetlands is generally assumed to be from 7 to 9 million km
2 or
about 4–6 % of the land surface of the earth based
on several estimates. Based on the works of
Russian geographers, Maltby and Turner ( 1983 )
had estimated that >6.4 % of the land surface of
the earth (or 8.6 million km
2
) is wetland. Out of
this total estimated wetland area, about 56 % is
said to exist in the tropical (2.6 million km
2 ) and
subtropical (2.1 million km
2
) regions. Further,
the wetlands have been estimated to occupy 1.0
million km
2 in the sub-boreal region (called temperate region), 2.6 million km
2 in the boreal
region and 0.2 million km
2 in the polar region
(Table 19.1 ).
Notwithstanding the above, other estimates of
the global extent of the wetlands have been developed from various studies on the role which the
wetlands play in regulating the global biogeochemical cycles, especially with regard to the greenhouse
gases. Matthews and Fung ( 1987 ) estimated, on the
basis of global digital databases (1° resolution),
that there is about 5.3 million km
2 of wetlands in
the globe. The study revealed a higher percentage
of wetlands being boreal and a far lower percentage
of wetlands being subtropical and tropical. These
estimates were different from those estimated by
Maltby and Turner ( 1983 ). Further, Aselmann and
Crutzen ( 1989 , 1990 ) estimated that there is 5.6
million km
2 of natural wetlands in the world, with
a higher amount and percentage of wetlands in the
temperate regions than given in either of the earlier
estimates. They used the data from the regional
wetland surveys and various monographs rather
than maps, which had been used by Matthews and
Fung ( 1987 ). It is noted here that these two research
groups had estimated the coverage of rice paddies,
but did not include them in their estimation of total
wetland area. Incidentally, Matthews et al . ( 1991 )
had estimated 1.5 million km
2 of rice paddies,
while Aselmann and Crutzen ( 1989 ) had estimated
1.3 million km
2 of rice paddies. Their estimates of
the extent of the world’s wetlands are 6.8 and 6.9
million km
2
, respectively, by including ricefi elds.
Signifi cantly, the Matthew–Fung estimate ( 1987 )
classifi ed wetlands into fi ve classes, namely ,
(a) forested bogs, (b) non-forested bogs, (c) forested
19
Wetlands of the World