But levees are not the only things that protect coasts. Nature offers protection, too. Coastal marshes absorb the wind energy and waves of storms, weakening their impact inland. And while it is expensive to maintain constructed defences, wetlands rebuild themselves.
Protection from storms is just one of many services that ecosystems provide us services we would otherwise have to pay for. In 1997, a team of scientists decided to estimate how much they are actually worth. Worldwide, the team concluded, the price tag was $33 trillion equivalent to $48.7 trillion in todays dollars. Put another way, the services ecosystems provide us whether shielding us from storms, preventing soil erosion or soaking up the greenhouse gases that lead to global warming were twice as valuable as the sum of the gross national product of every country on Earth in 1997.
We basically said, Its an imprecise estimate, but its almost definitely a pretty big number, and weve got to start paying attention, said Robert Costanza, a professor at Australian National University who led the study.
That study proved hugely influential. Many governments large and small started to take the value of ecosystem services into account when they planned environmental policies. But the study also set off a lot of debate. Some economists argued that it was based on bad economics, and some conservation biologists said that calculating price tags was the wrong approach to saving ecosystems.
Seventeen years later, the debate is being re-energised, just as the US becomes immersed in an intense fight over the Obama administrations effort to tackle the emissions that scientists say could threaten many of these ecosystems. Costanza and his colleagues have updated the 1997 estimate in a new study, published in the May issue of the journal Global Environmental Change, and concluded that the original estimate was far too low. The true value of the services of the worlds ecosystems is at least three times as high, they said.
Coral reefs, for instance, have proved to be much more important for storm protection than previously recognised. They also protect against soil erosion by weakening waves. Costanza and his colleagues now consider the services provided by coral reefs to be 42 times more valuable than they did in 1997. They estimate that each acre of reef provides $995,000 in services each year for a total of $11 trillion worldwide.
Most of the 17 services that Costanza and his colleagues analysed in 16 kinds of ecosystems including tropical forests, mangroves and grasslands also turned out to be more valuable. When they added up their new figures, they came up with a global figure of $142.7 trillion a year (in 2014 dollars).
But they also had to take into account the fact that many ecosystems have suffered since 1997. Costanza and his colleagues estimate that the worlds reefs shrank from 240,000 square miles in 1997 to 108,000 in 2011.
If coral reefs and other ecosystems were still as healthy as they were in 1997, the value of their services today would have been higher: $165.8 trillion.
In other words, the damage weve inflicted on the natural world have wiped out $23 trillion a year in ecosystem services. To put that loss into perspective, consider that the gross domestic product of the US is $16.2 trillion.
I think this is a very important piece of science, said Douglas J McCauley of the University of California, Santa Barbara.
That is particularly high praise coming from McCauley, a scathing critic of Costanzas putting price tags on ecosystem services.
This paper reads to me like an annual financial report for Planet Earth, he said. We learn whether the dollar value of earths major assets have gone up or down.