text: Studies compiled by ecologist Robert Costanza in the 1990s argued strongly that even just considering the most basic seventeen of these services , the combined value of the ecosystems of the earth was worth more ( US$ 33T ) each year than the whole human exchange economy ( US$ 25T ) at that time ( 1995 ) .
simplified: Studies compiled by Robert Costanza in the 1990s argued strongly that even just considering the most basic seventeen of these services , the combined value of the ecosystems of the earth was worth more ( US$ 33T ) each year than the whole human exchange economy ( US$ 25T ) at that time ( 1995 ) .