A black-necked stork (Ephippiorhynchus asiaticus) hunts for fish in a marsh in Rajasthan, India. Photo: Jack P. Blair/National Geographic

The question of whether to drain a wetland to make way for a shopping mall or a cornfield, or to instead leave the wetland intact, often seems like a no-brainer: the “development” options have a clear dollar value, but the wetlands themselves do not.

But therein lies a big problem.

Wetlands do vital work.  They recharge groundwater, keep rivers and lakes clean by filtering out pollutants, and provide habitat for birds and wildlife.  About half of North America’s bird species depend in some way on wetlands.

Wetlands also mitigate the impacts of droughts and floods, a function increasingly important as the world experiences more weather extremes under a changing climate. They act like a sponge, absorbing and holding heavy rains, and then release that water more gradually to surrounding streams and aquifers.  In this way they can lessen damage from both floods and droughts.

In the Mississippi River Basin, for example, the loss over many decades of 35 million acres of wetlands (an area the size of Illinois) in the upper watershed likely contributed to the impacts of the epic flooding in 2011, as well as of the extreme low flows during the drought the following year, when ship navigation between St. Louis and Cairo was at risk of shutting down.

The problem is that most of the time no one pays for the services wetlands provide.  Lacking tangible, market or “dollar” value, they are drained, filled and lost to other uses.

According to a new study in the journal Global Environmental Change, the global area of freshwater wetlands and floodplains shrank by nearly two-thirds between 1997 and 2011, from an estimated 165 million hectares (408 million acres) to 60 million hectares (148 million acres).

The study team, led by Robert Costanza, a professor at the Crawford School of Public Policy at Australian National University in Canberra, examined changes in the area covered by different ecosystem types, or biomes, over this period.  Then, using the best estimates they could assemble from the literature of the value of the services performed by each of these ecosystem types, they calculated the change in value.

For freshwater wetlands and floodplains, that amounted to an estimated loss in value between 1997 and 2011 of some $2.7 trillion per year (in 2007 dollars).

Taking into account all marine and terrestrial land types – from coral reefs to wetlands to tropical forests—the value of ecosystem services has dropped by $20.2 trillion per year over this 14-year period.

Still, the contribution of ecosystems to human well-being is very large.  The research team estimates the total value of global ecosystem services in 2011 at $120 trillion/year, compared with the 2011 global GDP of  $75.2 trillion/year (both figures in 2007 US dollars).

This current assessment updates a 1997 study published in Nature, and also led by Costanza, that estimated the global value of ecosystem services at $33 trillion/year in 1995 US dollars ($46 trillion/year in 2007 US dollars).

Constanza and his team are careful to point out that their estimates are approximate (though likely conservative) and that applying the same unit value to an entire area of a certain ecosystem type (whether wetlands, forests or coral reefs) lacks the spatial specificity needed to make local decisions.

These estimates are very useful, however, for raising awareness about the value of ecosystems to human welfare and the global economy – and the foolishness of allowing them to be whittled away at such a rapid rate.

Originally published at National Geographic Newswatch.