"Our greatest responsibility is to be good ancestors."

-Jonas Salk

Friday, July 25, 2008

Limits to Clean Energy

I've been meaning to talk about the comparison between carbon-based energy and clean energy in terms of global warming. One way this comes up is when the delusionists bring up "heat island" effects. Somebody who doesn't understand that the greenhouse warming is already observed will pipe up that "maybe it's just the amount of energy we're using".

Well let's get some numbers. A CO2 doubling is usually treated as a top-of-atmosphere imbalance of about 4 W/m^2 with fast (century delay or shorter timescale) feedbacks included. What would the comparable number be for ordinary, sensible, net-emission-free energy usage?

Well, here's a site claiming "Using 1995 figures provided by the World Bank, in that year, the world's energy consumption totaled 316 quadrillion BTUs." OK that's 316,000,000,000,000,000 = 3.16e17 BTUs = 9e16Watt-hour = 9e13 KwH = 9e13/7e9 KwH/capita-yr = 12800 KwH/capita-yr = 12800/(365*24) Kw/capita = 1.4 Kw. So the average person and all his or her support infrastructure currently burns about 1400 watts, night and day. That seems believable.

Then the world wattage is 1.4e3 * 7e9 = about 1e13 W. The area of the world is 5.1 e8 km^2 = 5.1e14 m*2. So the direct heating of existing energy is on the order of 1e13/5e14 W or about 1/20 watt per square meter. Compare this with 2 watts of anthropogenic greenhouse forcing, on its way to 4.

This is in line with what I got all the other times I worked it through, it's just verging on noticeable but is certainly not comparable to anthropogenic greenhouse forcing. Even if everyone lived at much higher US power consumption levels, this would still be a small forcing, about 1/3 W/m^2, comparable to observed solar variability.

But in the 8 July 2008 issue of EOS, whose website, proudly proclaiming its mission for the advancement "through unselfish cooperation in research, [of] the understanding of Earth and space for the benefit of humanity." doesn't make available to nonmembers, Eric Chaisson of Tufts and Harvard puts a different spin on this story. He suggests that this comfortable margin is not as comfortable as all that under conventional growth scenarios.

He points to theories that 1) economic growth is tightly coupled to energy growth and 2) economists believe healthy economic growth is at least on the order of 1%/annum sustained. Suppose we stipulate these ideas. When does non-greenhouse anthropogenic global warming become a problem? High school level computations suffice for an estimate on the order of 450 years for a global warming of 10 degrees Celsius. Higher growth rates bring that point much closer. And nothing in the assumptions allows the warming to stop there.

Accordingly, even in the total absence of an anthropogenic greenhouse effect, the world cannot sustain indefinite increases in energy use. Either the coupling of growth to energy or the growth itself will necessarily stop. Blithely ignoring the discount rate and thinking like a geophysicist, Chaisson concludes as follows:
Even acceding that the above assumptions can only be approximate, the heating consequences of energy use by any means seem unavoidable within the next millennium - a period not overly long and within a time frame of real relevance to humankind.

More than any other single quantity, energy has fostered the changes that brought forth life, inetlligence, and civilization. Energy also now sustains society amd drives our economy, indeed grants our species untold health, wealth and security. Yet the very same energy processes that have enhanced growth also limit future growth, thereby constraining solutions to global warming. Less energy use, sometime in the relatively near future, seems vital for our continued well-being, lest Earth simply overheat.

Got that? It's a fundamental limit to growth from which there is no escape (short of escape velocity) It won't cut in soon but if none of the other ones do, this one will eventually show up. The future of the planet has fundamental limits.

Update: Tidal notes that this limit does not apply to earth-based renewable energy, i.e., directly or indirectly solar. The question of what fraction of that we can appropriate is not obvious to me, but the total is vast. A quick follow-up on the calculation above indicates that solar forcing (after relection) is about 4000 times human energy consumption. We appropriate a good fraction of it already for food, though.

If we assume that we can appropriate 10% of that energy effectively at most (allowing some for a biosphere, some for food production, and some for insurmountable inefficiency in the conversion process) that leaves some 8 or so doublings. At 1 % growth that is about 560 years until we run out of renewables. Still in the same ballpark as Chaisson's numbers. At 3% growth in a pure renewables scenario, we run out of sources of renewables in 180 years.


John Mashey said...

Well, let's be careful with terminology.

If you believe Ayres&Warr, it's not energy, it's work = efficiency*energy that affects GDP growth (i.e., it's a big chunk of Total Factor Productivity, which is about 60% of the growth.)

This is not an argument with the fundamental idea of limits, just that efficiency matters a great deal.

I think I pointed you at some of this before, but let me know if not, and I can post some URLs.

tidal said...

I must be having a brain cramp here. I've read similar critiques in the past, but I don't really understand where the extra heat is coming from? Leaving aside nuclear or possibly geothermal, and calling "clean energy" solar PV, solar thermal, wind, tidal, maybe biofuels, etc. Where does the "extra" heat come from? Whether we intercept and use some fraction of the solar, hydro, rotational, gravitational energy fluxs and dissipate low-grade heat in the process, OR it's not used by us and just dissipates on it's own... how would we be net "heating" the planet? I can get how maybe some of the visible light that might otherwise get reflected would be converted to infrared, but it seems so tiny... Is this just a function of "those tiny things add up with geometric growth"?

I can see where nuclear or fossil fuel burning adds heat, but I have trouble comprehending it with the truly "clean energies". Like I said, probably a brain cramp - I am just trying to grasp this...

By the way, these kind of conclusions are genuinely depressing. I remember reading a similar description in Sayres' online manuscript, now called "Unearthed: The Economic Roots of our Environmental Crisis", that you pointed to on the globalchange group last year. It's a somewhat interesting read, but every turn toward "solution" is eventually met with another "it's an environmental/resource dead end!"... It's hard even reading it.

Michael Tobis said...

Tidal, I think your point is sound. I am not entirely sure how much energy we can extract from the sun; there does seem to be an entropy limit somewhere.

Jordan said...

Your industrial heat by-product limit to growth only applies if you assume all of humanity's energy use remains on Earth. Any scenario that involves continuing exponential growth in energy use quickly gets into Dyson sphere territory (as with any good piece of science fiction). It's certainly still a limit to Earth-based energy growth. In any case, my point is that you can't argue against magical thinking like infinite growth by pointing out flaws in the logic, because magic can always fix any problems - that's why it's magic.

Michael Tobis said...

Jordan, I actually touched on the space question: "no escape short of escape velocity".

I wonder what the earth, at the bottom of a gravity well, will have to sell to the people in space. The number of launches from earth will be severely limited, presuming the space elevator idea will go nowhere...

EliRabett said...

Beat cha to it. However two serious points:

1: Chaisson is using the majik of compound growth. It is not clear that this will happen.

2: Space based power is shorthand in some circles (Marshall Institute) for large lasers in space. Chaisson is very anti star wars

3: I got a better title for my post too.

Michael Tobis said...

Tidal, Chaisson acknowledges your point: "Note that utilizing solar energy that naturally affects Earth (including solar-driven tides, wind and waves), without generating any further energy via nonrenewable supplies, would not cause additional heat."

After that statement he immediately points out that solar collectors in space do not qualify as renewable under this definition. Those or fusion, which I have always thought of as the long-range hopes for energy growth, both tax the viability of the biosphere.

So we should treat 100% renewables as the goal. That's a lot of energy, but it's diffuse, and the biosphere needs some of it.

David B. Benson said...

Realistic numbers:

World human population stale at 9 billion of us.

Total energy requirement (not including food), all sources: 800 exajoules.


Not probable.

ac said...

Michael, what was your estimate of the fraction of solar energy currently appropriated for food?

Michael Tobis said...

ac, humans appropriate the majority of the net primary productivity of the earth, not leaving much slack in biotically active areas, but there is plenty of sunshine in the larger barren areas.

It is a nice question. I don't have a ready answer. Anyone?

tidal said...

On the off chance that comments are still open, an article about Eric Chaisson in the Boston Globe. "In it" had them scooped by 5 months! ;)