Robert Constanza and his colleagues have put a price tag of US$33 trillion a year on

Robert Constanza and his colleagues have very recently tried to put price tags on nature's life-support services. Researchers have put an average price tag of US 33 trillion a year on these fundamental ecosystems services, which is largely taken for granted because they are free. This is nearly twice the value of the global gross national product GNP which is (US 18 trillion).