10/21/2010 04:57 pm ET | Updated May 25, 2011

Climate Change: The Upside of Getting Old

Crossposted with

When it comes to global greenhouse gas emissions, population size really does matter.

Demographers have some good news for us and some bad. The good news is that human populations will not grow without limit and should reach about nine billion by mid-century. The bad news is the same as the good -- human populations will stabilize at about nine billion -- a whole heck of a lot of people for our little blue planet, about two Chinas worth on top of today's population.

Of course, there are uncertainties in this projection. Depending on how things play out, by 2050 human population could reach as much as ten and a half billion or as little as seven and a half billion. The possible range at the end of the century is even larger: the upper limit set at 14 billion and the lower limit set at five and half billion with the central or best estimate being a little less than about nine billion.

global population projection through 2300

Projections of global population out to 2300. (Source: United Nations, World Population in 2300 [pdf])

Growing Populations Generally Mean Growing Carbon Emissions, but There Are Exceptions

A new paper published last week in the Proceedings of the National Academy of Sciences by Brian O'Neill at the National Center for Atmospheric Research, Boulder, Colorado, and colleagues investigates how changes in population and population demographics, such as age and urbanization, would impact global greenhouse gas emissions. O'Neill et al used a dynamic energy- and economic-growth model that is responsive to global household changes across nine broad regions.

I guess you don't have to be a climate scientist or a demographer for that matter to know that the more people we've got on the planet, all things being equal, the more emissions we'll have.

Currently we emit about 8.8 billion tons of carbon per year. O'Neill et al find that, were the population to reach the upper limit of their projections, global emissions could be as much as 12 to 17 billion tons of carbon in 2050. Alternatively, if population followed the lower limit, global emissions could be reduced by as much as 16 percent below current levels in 2050.

So clearly, population matters, but what's more interesting than these numbers in toto is O'Neill and colleagues' finding that the changing character of population also affects carbon emissions.

Aging population suppresses emissions: "Aging populations," the authors explain, "are associated with lower labor productivity or labor force participation rates ... which ... leads to slower economic growth" and by extension less carbon dioxide emissions. It's also true that an aging population is a less fertile population and so a less populous one.

Urbanization does the opposite: As populations become more urbanized, on the other hand, emissions increase because urban labor, which tends to be more productive, leads to more economic growth. This result is somewhat surprising since urban lifestyles tend to be more energy efficient, but, the authors argue, enhanced income and therefore increased consumption in urbanized economies result in increased emissions.

Populations Getting Older, Becoming More Urban

As we look to the future, we can anticipate a human population that is more aged (especially in industrialized countries) and more urbanized (especially in developing economies like those of China and India). The authors' best estimates are that the urbanization trend will dominate over the aging trend and so, in the absence of policies to reduce emissions, they will increase substantially -- to about 10 to 14 billion tons of carbon per year in 2050 for medium population projections (or up to 60 percent over current emissions).

If you're concerned about climate change, as I am, this definitely falls into the "bad news" category -- some scientists estimate that in order to avoid dangerous climate change, global carbon emissions not only cannot increase; they will need to decrease by 50 percent or more by 2050.

So, I am worried about climate change. But there is an upside. They say that worrying accelerates the aging process and, according to O'Neil, aging populations lead to lower carbon emissions. Doesn't that mean that by worrying I am doing my bit to hold carbon emissions down? If only it were that easy.