Robert Costanza and his colleagues evaluated Earth’s ecosystem services and quantified their values to be more than $33 trillion (1994 dollars). The estimate in 2009 dollars is $44 trillion. Though the figures are considered to be gross underestimates, especially for entities hard to quantify like habitat and genetic resources, they point out that ecosystems provide us with valuable, sometimes irreplaceable, services but that they can continue to do this only to the degree that human impact will allow. When we degrade ecosystems, we reduce their ability to provide these services.