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What if you were told that the United Nations -- and its scientists and economists -- had already identified the biggest opportunity for greenhouse gas emission reductions?

And that the U.N. had further determined that the key strategies for pursuing that opportunity were also the least costly to society?

And finally -- what if these greenhouse gas emission reduction strategies worked in tandem with successful economic development of emerging economies, and to the long-term resiliency of communities?

It would be fair to say that you might expect action on this solution. We certainly do. The opportunity that the U.N., the Intergovernmental Panel on Climate Change (IPCC), the International Energy Agency (IEA), and countless other climate and economic experts are talking about is the world's building stock. Experts have identified the building sector as having the single greatest potential for greenhouse gas emission reductions. They have also determined that energy efficiency -- including efficiency in new and existing buildings -- is a highly profitable strategy with economic development benefits for entire societies.

Even President Felipe Calderon, the President of Mexico and host of the recent U.N. climate talks in the city of Cancún, declared efficiency strategies as those with "positive net present value." In other words, good for business. At the same event hosted by Calderon, World Bank President Robert Zoellick called energy efficiency "a diamond." (And then reminded us that a diamond is really just coal under pressure).

These words from such renowned leaders carried a refreshing optimism at COP-16 -- shorthand for the 16th Conference of the Parties to the U.N. Framework Convention on Climate Change -- especially as countries remained gridlocked on bigger issues like "Kyoto or No Kyoto." They underscored the message that we were there to carry: We can, and must, act now to capture the opportunities presented by the built environment. And we can do so while advancing socioeconomic goals such as energy access, the priority of most of the developing world.

While the Cancún talks produced some serious progress on difficult issues -- that's something no one can deny -- governments must now get to work back at home on climate change. For clear reasons, the planet and the economy is in desperate need of an ambitious binding agreement to hold countries accountable to emission reductions and to support a new, global clean energy economy. But we can't wait for that to act. While Kyoto lingers on the brink, there are actions that can be taken today, both domestically and internationally, to begin unlocking efficiency in the built environment.

Momentum is building. Recently, with the help of the U.S. Green Building Council and the World Green Building Council, a new coalition of businesses and environmental NGOs formed to begin pushing for action on buildings and other infrastructure as a central strategy for tackling climate change. The Global Leadership in Our Built Environment (GLOBE) Alliance is advocating for accelerated investment in the built environment and for policy frameworks that will address buildings, transportation, and cities to reduce emissions and increase resiliency to climate impacts. GLOBE will working on a national and international level to influence decision-makers. Things like the United Nations Foundation's "Building Block Approach" seem to be gaining traction as implementable, common-sense pathways to begin immediate work on climate change. The Mexican Government has already started on a path toward low emission development through a variety of programs designed to boost affordable, efficient, low-carbon housing, which they showcased proudly at COP-16.

We encourage other governments to do the same. In the U.S., for example, there are tools at our government's disposal to begin greening our existing building stock that will create jobs and capture those emission reductions, like green mortgages and a national building energy labeling program, just to name a few. Every country can devise their own specific tools tailored to local circumstances. But no country can afford not to start with buildings -- both new and existing -- if they want to make real progress for the climate and for the economy.

Roger Platt is the Senior Vice President for Global Policy and Law at the U.S. Green Building Council, Jane Henley is President and CEO of the World Green Building Council.