A common way to think about a building's impact is to look at it almost like a living thing. The "life" of the structure begins at construction, continues as people use it as living or work space, and its life ends when builders dispose of materials after demolition. You have to consider not just what materials you're using but where they come from, how they impact the environment as part of the building, and where they'll go when their "life" comes to an end.
Energy efficiency is the most common green building focus, because the benefit is clear and immediate: lower utility bills. Green construction's overall environmental impact is tougher to measure, but there are some good case studies on energy and water usage in certified buildings that can give you an idea of the difference that green building can make.
A 2011 survey of 21 commercial LEED (Leadership in Energy and Environmental Design) buildings showed an average energy savings of 27 percent over more conventional structures. Interestingly, the number of LEED points (given for green construction techniques by the Green Building Certification Institute which determines the certification level) didn't directly correlate to the amount of energy savings [source: Diamond]. That means that a LEED Silver building could be more energy-efficient than a LEED Gold structure, depending on what green building elements the designers focused on.
The other major environmental impact is reducing the carbon dioxide (CO2) emissions in the construction process from processes like producing and shipping materials and running equipment on-site. A February 2009 EPA report looked at green building's potential to reduce these emissions. While the researchers admit that it's hard to measure the construction industry's overall CO2 emissions, the study estimated that through green construction techniques like on-site recycling and using alternative fuels for trucks, the construction industry could reduce their yearly CO2 emissions by almost 8 million metric tons [source: EPA]. To put that in perspective, the U.S. emitted 6,865.5 million metric tons of CO2 in 2010 [source: EPA].
It's pretty clear that green building techniques can reduce energy and C02 emissions, which benefit the environment, but what is green construction's true impact?