The last week has seen a flood of stories on clean energy's prospects—stories that make your head spin with their conflicting tales of renewable energy's prospects of ending our dangerous addiction to fossil fuel power from coal and gas.


A renewables transition will "happen without Trump" because of market forces—or can't do the job and is shaping up as "likely very costly." (Both from the same day's New York Times). Cities, states and businesses are filling in the leadership vacuum created by the Trump administration—or they are falling far short. Wind is making it impossible for fossil fuels to compete in Texas power markets—or Texas will continue to be the biggest carbon emitting state of all because wind is too unreliable. (Both from Bloomberg).

And all of these perspectives are coming from scientists and analysts who are pro-clean energy and favor strong action to protect the climate—this is not a fight ginned up by ExxonMobil, Peabody Coal or climate denialists from the Heartland Institute.

So what's the argument? Where do we stand on the ability of clean, renewable energy sources to eliminate the risk to the climate posed by continuing reliance on coal, oil and natural gas?

That depends on the question you ask. If you look at where we are today, our current emission rates are far too high. If continued unchecked, they will rapidly destabilize the weather and increase climate risks to catastrophic levels. (Mathematicians call this the function). If you look at the progress we are making, the future looks brighter, but still quite scary. The commitments governments made at the Paris climate agreement, and the trends for deployment of clean energy vs. fossil fuels, all show future emissions declining, but not declining enough to stabilize the atmospheric concentrations of carbon dioxide. (This question, "How fast are we progressing?" is what mathematicians call the first derivative). But if you compare the pace of progress this year with that pace five years ago, you can see that decarbonization is accelerating. Not only are we cutting emissions, we are cutting them faster with each passing year. If we continue to accelerate that progress long enough, then we can look forward to eliminating fossil fuel carbon dioxide emissions and stabilizing atmospheric concentrations of carbon dioxide. (Mathematicians call this measure of acceleration "the second derivative").

Let's apply these three measurements to the most heated of this week's controversies, the attack by a group of prominent climate and energy scientists on journal articles by Stanford scientist Mark Jacobson which argued that wind, solar and hydro could enable the U.S. to eliminate all fossil fuels and nuclear energy from its electricity mix, without any significant increase in costs.

I'm not going to get into the debate about whether Jacobson's article met scientific standards or was too speculative—I'm not qualified. But the media coverage of the debate has missed the point. Jacobson described a scenario in which we get 100 percent of our power from renewables by 2055 with technologies he thinks will be available by that date. Jacobson's critics disagree—but the lead contributor to their article, Christopher Clack previously published his own trail-breaking journal article saying that we can cut carbon emissions by 80 percent with renewables by 2030. Clack's article argued that this would require connecting different regions of the U.S. with transmission lines—a technology clearly available today. We would need 60 percent wind and solar to do this—hydro, nuclear and natural gas would make up the rest.

So both sides of this media-hyped debate agree that, using today's renewable technology plus transmission, we can cut utility sector carbon emissions by 80 percent by 2030. (President Obama's much criticized Clean Power Plan, now suspended by the Trump Administration, by comparison, envisaged cutting utility emissions by only 32 percent by 2030).

So what Clack and Jacobson disagree about is what happens between 2030 and 2055. How likely is it that new storage technologies will enable us, at no cost, to get rid of the last 20 percent of those emissions—25 or 40 years from now! Even with all of my respect for the scientists on both sides, they can't possibly know the answer to that question. Only 15 years ago, solar power cost about $0.37 kwh. No one anticipated the precipitous drop in costs that followed. Today that same solar power costs a tenth as much so predicting how much electricity storage will cost in 2055 (and storage, plus long distance transmission, is the key to enabling renewable power to meet 100 percent of our needs) is simply not possible.

If, in the next 15 years, battery or other storage costs drop as fast as solar did for the last 15, Jacobson's vision is clearly viable. The second derivative can get us there—but today we are only at 10 percent wind and solar. We have a long way to go.

And we know is that racing ahead to install as much solar and wind as the grid can handle will drive the costs of renewables down even further—and lower utility bills. Both Clack and Jacobson agree that getting 60 percent renewable reliance is feasible and cheaper. So it's premature to ask "will we need some remaining natural gas or nuclear or can we go 100 percent renewable?" And we are making money—as well as cutting carbon—every step of the way.

How much of a difference would such an acceleration of renewable energy in the utility sector make to the climate? Well, cutting utility sector emissions by 80 percent would fulfill the entire Obama Administration Paris pledge (the first derivative). But it would also require doubling the speed at which we have cut utility emissions in the past decade (the second derivative).

How do we make sure that happens? That's the important question—not what do we do about the last 20 percent of those emissions when we get to 2030. We can be reasonably certain every guess we make about that today will prove wrong—however carefully peer reviewed.

So this is the wrong argument to be having. Fortunately, the stakeholders who are the target of the fight—policy makers—are asking the right question and coming up with the right answer. At its Miami convention this week, the U.S. Conference of Mayors unanimously agreed to a resolution, initiated by the Sierra Club, calling for 100 percent renewable power not by 2055—but by 2035. All of America's largest cities just got on board the renewables express—its second derivative just got a boost.

That's leadership.