We have all seen the numbers.

Buildings account for 40 percent of energy usage, nearly 40 percent of total CO2 output and 12 percent of potable water consumption. When we add to the equation the amount of waste and vehicle transportation that are tied to buildings, we can sense the magnitude of the impact our nation's buildings have on the environment.

We need to change, but it won't happen overnight.

When we survey the green buildings landscape, we still see the majority of LEED certifications are design and construction projects. This makes perfect sense.

From the moment a new green building is conceived, all involved -- from architect and engineers to sub-contractors -- are all working towards meeting specification requirements and materials as well as resource guidelines before the building opens its doors. But that is well before it has to face the greatest challenge to optimal building performance -- the building's occupants.

We are beginning to see change within this context, albeit not as rapidly as needed.

If we're serious about reducing the environmental impact of buildings, addressing energy independence, using renewable energy resources, conserving water and slowing waste streams, we must make greening existing buildings a greater priority.

In fact, using LEED certification and registration data as an indication of green building market trends, greening existing buildings is finally achieving serious numbers. While existing buildings only represent 4 percent of LEED certified square footage, they now account for 20 percent of LEED registered square feet. This is a positive trend -- spurred by a combination of energy prices, environmental awareness and generational shifts -- that if continued, presents the greatest opportunity to positively impact the environment since green building became prominent early this decade.

There are significant challenges to greening existing buildings. The reality is that green -- especially in an existing building -- is multi-disciplinary. Green, in most cases, requires coordination of competing priorities, from occupant comfort and productivity to energy efficiency waste reduction. All too often, multiple stakeholders have responsibility over different green aspects of a building, while budgets are tight and daily workloads are increasing.

Green initiatives are typically embraced by building occupants, but can be fleeting when behavior change is involved. While environmental responsibility is more visible and desired than ever, building occupants might balk when forced to return silverware and plates to the cafeteria instead of tossing disposables, manually turning on lights if working later due to lighting schedule changes, or giving up personal desktop printer for a networked one.

Ultimately, the greatest challenge to greening existing buildings is financial. While rising energy prices and shifting attitudes are making green efforts a greater priority, most organizations are faced with budgetary constraints and stringent return on investment requirements. In this context, few organizations bundle green retrofit activities, but pursue individual efforts piece meal and make financial justifications based on strict ROI criteria, or through a more qualitative lens such as employee morale.

Look to LEED for Existing Buildings: Operations & Maintenance rating system as a guideline to put it in context. But remember that LEED does not reward pre-retrofit versus post-retrofit improvement; it rewards a building that operates according to specific performance metrics and goals, including energy efficiency performance, indoor environmental quality, and O&M policies and procedures that reflect environmentally responsible, high-performance building standards. In many cases, a significant energy retrofit won't be enough to meet LEED EBOM requirements.

However, whether pursuing LEED certification or not, the rating system is an ideal guideline. If you don't know where to start, here are some ideas:

• Benchmarking the building's energy performance using Energy Star Portfolio Manager.
• Meter as much of the building as possible. If you can't measure how much energy the building is using it's to hard to pinpoint areas of optimization and savings.
• Invest in an energy reporting and monitoring software package.
• Evaluate current policies and procedures regarding areas such as purchasing and recycling.
• Engage the building occupants, and make sure that everyone is aware of what you are doing, why you are doing it, and where you are going.

When it comes to the cost of sustainability measures, some are easier to measure than others.

Energy conservation is truly the dollars and cents of sustainability, and will continue to play that role in the future. In any building there is typically "low hanging fruit" that can yield significant savings, such as lighting retrofits, Building Automation System optimization or mechanical system improvements. These are measurable savings, and can easily be translated into CO2 reduction equivalents that organizations are using as metrics today.

If together we can tie the savings from a retrofit with the often intangible benefits of green -- market differentiation, employee retention, customer loyalty, worker productivity -- we have gone past the tipping point for the green movement. Are we there yet? Not quite, but we're close; we just have a little more work to do.

Ari Kobb heads a range of sustainability, energy and green building initiatives at Siemens Building Technologies.

Images courtesy of Siemens Press Pictures, © Siemens AG, Munich/Berlin