Economic calamities have a way of focusing our attention on the problems at hand. The first real slowdown of the 21st century is no exception.

One of the encouraging stories emerging from the "Great Recession," though, is the focus on creating "green" jobs to revitalize our economy while limiting the growth of our carbon footprint. Nowhere is this new economic standard emphasized more than in Pittsburgh, which describes itself as having moved beyond steel and rust and toward a future of green. Indeed, this is one of the main reasons the G-20 group of world leaders will meet in Pittsburgh in September.

But another consistent theme sounded by Pittsburgh leaders is the desire to attract more people to the region to fire up the economy. This reflects the rote belief of economic-development types almost everywhere that economic growth requires population growth.

The problem is, it's not true. And it's not green.

Let's start with the simple fact that the U.S. economy is far larger than countries with far more people, such as China and India. So we can rather easily dispose of the idea that a larger population automatically means a larger economy.

OK, but in the U.S. context there is no doubt that a growing population creates more demand for goods and services, more potential employees to make or provide goods and services and, therefore, more potential economic output. Time and time again we have seen the positive consequences of population growth in North America -- local economies ramp up, jobs and wealth are created, roads are expanded, schools are built.

But there also are negative consequences -- suburban wildlands are destroyed, pollution and traffic increase, allergy and disease rates rise, housing prices escalate beyond the means of average wage-earners. All these things lower the quality of life for most of us.

The fact that negative consequences go hand in hand with population growth should give pause to blind acceptance of the notion that more people is always good, fewer people is always bad.

Consider the stories of the two cities represented in last year's Super Bowl -- Pittsburgh and Phoenix. The demographic tale of these two cities is one of similar initial trajectories, but radically different recent trends.

The story of Pittsburgh is one for the ages -- exponential population growth from the early 1800s through the 1920s, then a plateau, and then, since about 1950, an ongoing population decline. Phoenix's story is no less interesting, with rapid population growth beginning around 1900 and continuing to this day.

So have economic trends followed these population patterns?

Yes, but ...

Certainly Pittsburgh enjoyed massive economic growth as its population grew during the boom years for coal, oil and steel. But it also suffered massive environmental destruction. The decline of the steel industry then produced wrenching economic and population declines, but also vast improvements in environmental quality.

Likewise, Maricopa County (where Phoenix is located) has seen massive job growth and environmental degradation, and most economic indicators showed (until recently) continued upward trends that parallel those in population.

But there is more to the story.

As Pittsburgh's population began to slip in the 1950s, at first the city paid a heavy price. Unemployment shot up, per-capita incomes declined and the region's riverfronts were littered with the skeletons of steel mills and partner industries.

More recently, the city has experienced a renaissance that bodes well for southwestern Pennsylvania and for the nation as a whole. Refocusing on education, health care and high-tech industries, Pittsburgh has seen economic growth and environmental revitalization in spite of, and perhaps because of, its ongoing population declines.

An economic development professional might say, "Yes, but imagine how much more economic growth we would have seen if we could have kept or attracted more people." And it is true that continued population growth from the 1930s through the present day may have eased Pittsburgh's transition to a new economy. But at what cost?

Let's look to Phoenix for one possible answer.

There, population growth has continued unabated and the region consistently ranks as one of the fastest growing in the United States. But the costs of this exponential growth have been appalling.

Maricopa County was once pristine desert, with abundant wildlife, clean air and a stunningly beautiful mountain backdrop. So unspoiled was the environment that doctors used to send sick patients to the "Valley of the Sun" to convalesce and recover.

Today, Phoenix abuts Tempe, which runs into Scottsdale and Glendale, which are surrounded by suburbs as far as the eye can see. Temperatures have risen with the paving of the desert and water is in short supply as homes and putting greens demand grassy lawns. The air is polluted and drive times are lengthy as the number of vehicles spirals upwards. Wildlife has been chased away and the desert beauty has been lost to golf courses and cookie-cutter "Southwest-style" housing developments. Today, sick patients in "The Valley" are told to leave, to find a cleaner place to live.

Pittsburgh, on the other hand, has enjoyed a relatively stable and diversifying economy in recent years, its riverfronts have been reclaimed and the smoke has cleared, unveiling lush green hills. No wonder Pittsburgh now ranks as one of the most livable cities in the United States and Phoenix often lands near the bottom of livability measures.

And today, Phoenix is getting slammed by the Great Recession, its vulnerabilities exacerbated by unsustainable growth. Meanwhile, Pittsburgh is relatively more stable, having grown neither too slowly nor too quickly, and, although substantially affected by the global slowdown, it is not suffering as much as previously faster-growing urban centers.

Several weeks ago I attended a "green jobs" event sponsored by Allegheny County Executive Dan Onorato. At the end, one of the panelists closed by saying that Allegheny County had to grow its population to keep its economy healthy. I was left wondering if the speaker was stuck in a past century. For the truth is there are few things more anathema to a "green" economy than unabated population growth.

Pittsburgh has shown that economic growth and revitalization can occur even when population declines. As we move beyond the Great Recession into an unknown future, only a few things are certain. One is that a growing population, whether in Pittsburgh or Phoenix or elsewhere, might mean faster economic growth but almost certainly would mean environmental degradation and a lower quality of life.

We can have a healthy economy as our population remains stable or shrinks. So let's move forward into a real green economy -- one where economic growth and a rising standard of living don't require population growth. And let's offer this message to the world when the G-20 leaders arrive.