These days, it seems like skyscrapers are in a race to be the greenest, as well as the tallest. New York City's Hearst Tower is largely made from recycled steel and uses rainwater for 50 percent of its needs. China's 71 story Pearl River Tower (pictured below) will soon use wind, sun and geothermal energy to power itself, and even the Empire State Building, one of the world's oldest skyscrapers, is currently undergoing an energy retrofit facelift to stay in the race.

To be the greenest skyscraper on the block, designers are incorporating cutting edge energy and water saving technologies like helical wind turbine technology, thousands of solar panels, sunlight-sensing LED lights, rainwater catchment systems and even seawater-powered air conditioning. One building awaiting construction is the Burj al Taqa "Energy Tower" (interior pictured below). With a 197-foot roof turbine and 161,459 square feet of solar panels, this 68 story skyscraper, if built, would create all its own power on site.

Skyscrapers located in well-designed urban centers earn sustainability points for saving space and creating density by sending growth up instead of out. As mixed-use buildings located in mixed-use zones, skyscrapers allow residents to be within walking distance of office buildings, shopping centers and mass transit options.

But skyscrapers were born in a time when energy, labor and resources were cheap. Despite new found ways to create efficiency, these projects still use enormous amount of material and energy during construction, as well as after completion to run internal systems like water pumps, interior lighting and elevator operation. Perhaps green tech fixes and density aren't enough to make a skyscraper bright green. But these buildings can make for great laboratories for the biggest and sturdiest systems we've got. If we can make a skyscraper carbon neutral, maybe other green building challenges will start to seem less intimidating.