Last month, 360 miles above the Earth, a little-noticed light went dark. It was the third and final laser on NASA's Ice, Cloud and Land Elevation Satellite (ICESat), developed and managed by NASA's Goddard Space Flight Center in Greenbelt. For the last 6 1/2 years, ICESat has been using precise laser measurements to determine how much the Greenland and Antarctica ice sheets are contributing to the rise of the global seas and how much the sea ice that blankets the Arctic Ocean is thinning in ways that can affect climate all over the world.

While attempts are under way to restart ICESat's lasers, it is clear that the mission is at or near the end of its life. The situation with ICESat serves as a stark reminder that many of the remarkable capabilities that NASA has developed to help us understand our planet are living on borrowed time.

NASA is best known for exotic projects that explore distant places, but many of NASA's greatest contributions to society have come from its Earth-observing satellites. Their views from hundreds of miles above the Earth provide a global perspective that enables scientists to understand the large-scale processes that govern the Earth's climate, pulling back the curtains on some of our planet's greatest mysteries. Among these are:

* what is happening to the rapidly disappearing ice that blankets the Arctic Ocean;

* how fast the world's seas are rising and why;

* how hurricanes strengthen and weaken;

* the distribution and movement of the world's fresh water;

* what controls the storage and release of world's carbon - the fundamental element of life and a key component of greenhouse warming;

* how well the Antarctic ozone hole is recovering;

* how variable the amount of energy is that the Earth receives from the sun.

I wish the loss of ICESat were just an aberration in an otherwise healthy global observing system, so we could continue to understand how and why our planet is changing. Unfortunately, this is only a hint of things to come.

As of today, 14 of NASA's current system of 15 Earth-observing satellites are past their design lives. This Earth-observing system is dying precisely when we need it most, and the schedule for ushering in the next generation is woefully inadequate. NASA is planning to launch only eight scientific satellites in the next seven years. These will likely provide less than half the observational capability that it has in place today, and some of these missions will carry instruments that are less capable than those now in orbit.

We are at a point in human history in which our actions as a society can determine the planet's climate trajectory for centuries. As a result, the Earth is changing in ways that modern society has never experienced, presenting civilization with its greatest global scientific and technological challenges. Now, more than ever, we need to understand how and why our planet is changing and what those changes mean for life on Earth. We cannot afford to fail in this endeavor, yet without the perspective provided by a healthy system of Earth-observing satellites, our chances of success are severely hampered.

Sadly, the impending demise of the Earth-observing fleet could have been avoided had NASA simply maintained its funding for Earth science at its 2000 level, which reflected the importance given to Earth science during the 1990s. Unfortunately, over the last eight years, other priorities for NASA prevailed, and Earth science suffered more than a 25 percent reduction. Recent actions by Congress and the Obama administration are encouraging, but restoring the NASA Earth science budget to its 2000 level would require an incremental investment on the order of $500 million annually, and even more is needed to make up for lost ground.

Yes, $500 million is a large sum, but it is only about 3 percent of NASA's total budget and considerably less than what Americans reportedly spend on gasoline each day. These resources would enable significant progress toward a robust Earth-observing system and support the research necessary to realize its potential.

Investing this modest amount to understand the Earth's future environment - whatever it may be - is clearly the prudent course. To do otherwise when the risks are as great as they are, and when we have the means, would be reckless, irresponsible and impossible to explain to our grandchildren.

Waleed Abdalati, a former NASA scientist studying the Earth's polar ice cover, is director of the Earth Science and Observation Center in the Cooperative Institute for Research in Environmental Sciences at the University of Colorado, where he is also associate professor of geography. His e-mail is waleed.abdalati@gmail.com.

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