Just because we can do something doesn't necessarily mean we should.

Yet this homely admonition seems forgotten in the otherwise admirable efforts of cities and regions around the globe to create more sustainable energy policies.

Since 2007, many governments have adopted a policy goal of reducing greenhouse-gas emissions by 80 percent by the year 2050. It's a goal based on solid science about what needs to happen in the Earth's atmosphere to avoid the worst effects of global warming: floods, famine, disease, war . . . the whole biblical catalog.

The United States and many other nations have stated this goal in national policy and in multinational agreements, and it has been adopted by 16 U.S. states and 37 U.S. cities. Philadelphia seems poised to adopt the goal as well.

But, setting aside the right goal for the Earth, what is the right goal for any particular city or region?

It is highly unlikely that the smartest, fairest, or even fastest way to achieve a national goal is to have every state, county, city, and neighborhood adopt the same numerical target.

Yet many cities and regions are doing exactly that, with the "charge of the Light Brigade" attitude of progressive places that want to lead by example. Many are trying to show that it is feasible for their city or region to achieve the "80 by 50" pledge. And that's certainly more responsible than adopting a goal without asking if it's feasible, of course. But it's still not the same as asking what goal a given region should adopt in the first place.

Assumptions about this question are the crux of virtually every energy policy debate right now in cities and regions around the world, and certainly in the Philadelphia area.

Some argue that change here should be effectively zero: Philadelphia should focus on exploiting Marcellus and Utica shale gas and that energy transition is a bridge to nowhere.

Others argue that change should be effectively instantaneous: Philadelphia should focus on exploiting clean-energy technologies and that we should think short off-ramp instead of bridge.

And still others argue that a portfolio of mutually reinforcing energy resources and investments over time is needed and that we need to carefully design the bridge to usefully inform decision makers.

At the core of these arguments is one over-arching question: How fast should we be changing from fossil fuels to low-carbon energy?

The answer to that question will not be found in studies of whether Philadelphia can achieve 80 by 2050. Mayors and governors must engage diverse and often contentious local stakeholders and to do that they need to make local cases for local benefits that exceed local costs.

Eighty by 2050 will never provide the case-making that local leaders need. Why? Because the brutal logic of climate policy is this:

Virtually all the benefits to mitigating carbon emissions will fall outside the city or region that pays for that mitigation. This so-called "collective action" problem is why so many people are rightly pessimistic about our chances as a species. It's also why a goal about carbon emissions doesn't make sense for local policy making.

Instead local leaders need to champion goals based on local benefits. The right target for reducing emissions over time in Philadelphia is the one that maximizes net benefits here on employment and earnings, public health and environmental justice, resilience and adaptation to climate changes already locked into our region.

Mobilizing Philadelphia around a planetary atmospheric target is not going to get us there.

Mark Alan Hughes is the University of Pennsylvania's faculty director of the Kleinman Center for Energy Policy in the School of Design and of the Fels Policy Research Initiative in the School of Arts and Sciences. markalanhughes@gmail.com