Stop Building. Now.

In May, for the first time in human history, average carbon dioxide levels in the earth's atmosphere reached 400 parts per million. As widely reported earlier this month, if temperatures keep rising at current rates by mid-century the coldest years could be warmer than the hottest of the past.

Media coverage of these gloomy revelations overlooked a key contributor -- buildings.

According to estimates, buildings use more energy than any other single industry, accounting for half of all energy and three quarters of electricity consumed in this country. Nearly half of all CO2 emissions come from buildings, compared to a third from transportation and less than a fifth from industry. You might think cars and factories are the problem, but buildings are the bigger culprit.

Much is being done to combat this. For example, over the past decade the US Green Building Council's Leadership in Energy and Environmental Design (LEED) rating system has helped cut annual carbon emissions by nearly ten million tons. But this progress may not be enough. Last year, Preservation Green Lab issued a landmark report showed that it can take up to eighty years for the greater energy efficiency of a LEED-certified building to overcome the harmful climate effects created during the building's construction. Making buildings slightly more efficient isn't enough.

At this watershed moment in the history of the atmosphere, here's a modest proposal: Stop building altogether.

Every year, the US builds some five billion square feet of new construction, even though some 13 million housing units remain unoccupied year-round. According to the US Census Bureau, the 2012 gross vacancy rate was nearly fourteen percent, and the Metropolitan Institute forecasts a surplus of up to 22 million homes over the next couple of decades. Harvard economist Edward Glaeser has estimated that if construction halted entirely it would take three years for the number of households to catch up to the number of houses. We build too much, and much of it goes unused.

So what if we instituted a nationwide moratorium on all new construction. Not forever, just for a while--say, a few years. What would happen?

Because new housing tends to occur on suburban greenfield sites, growth often exacerbates sprawl, increases emissions, and destroys habitats. Barred from this kind of development, the design and construction industry would be forced to turn its attention to improving the nearly 300 billion square feet of existing building stock.

In the US alone, the building industry represents an economy worth hundreds of billions of dollars. Imagine if this were applied exclusively to adaptive reuse, historic preservation, and restoring infrastructure. Virtually overnight, blight and sprawl would disappear, and the state of communities would dramatically improve.

Energy consumption would drop drastically. The aforementioned Preservation Green Lab study concluded that reusing an existing building can be nearly fifty percent more efficient with resources than building a new one.

An overabundance of vacant buildings drags down property values, spurs deterioration and vandalism, and drains public resources to police and maintain the unoccupied properties. With a cap on new development, demand could rise as supply remains unchanged, so housing prices could go up, stimulating economic growth and higher quality.

If no new construction proved to be impossible, we could allow new building under certain conditions, the same way companies trade carbon credits to get around emissions restrictions. Consider two exceptions to the rule:

First, new construction could only occur on brownfield (previously used) or reclaimed infill sites. Preference would be given to sites where soils are contaminated and remediation would be required. This move would preserve virgin land and habitat, restore soil health, and increase density, which enhances communities and the health of residents by encouraging more interaction and casual exercise.

Second, we could require any new construction to be "net zero" in energy and emissions. On-site clean, renewable energy production, through photovoltaics and other sources, would eliminate any additional greenhouse gases. The cost and quality of these technologies could become much more affordable as supply and demand rise. Any architect or builder not interested in renovation or preservation suddenly would become an expert in high-performance design, and the intelligence and innovation of the entire industry would skyrocket.

For the building industry to stop building is radical, to say the least. But in the face of new knowledge about the state of the climate, radical change is exactly what we need.

Lance Hosey is Chief Sustainability Officer with the global design leader RTKL and a member of the AIA Committee on the Environment. His latest book is The Shape of Green: Aesthetics, Ecology, and Design.