What Tom Murphy doesn't show you

Tom Murphy (the physicist at Do the Math Blog) frequently makes the point that U.S. energy use has increased by about 2.9 percent per year since 1650. Here's the graph he shows. He then makes a big deal about how straight it is.

Then he goes from that to saying that, if it continued for another 400 years, the seas would boil from all the heat.

But what he doesn't show you--what he fails to point out--is that the graph is NOT linear (does NOT show exponential growth) at the end.

Here's that same graph, from 1850 to the present.

Do you see the nonlinearity? Well, let me focus in even further. Here is a graph from 1950 to the present.

Now the non-linearity is easily viewable. But let's make it even clearer that Tom Murphy's graph has absolutely zero predictive power. This is the graph from 1950 onward, but not on a logarithmic scale.

You can see that, if you used Tom Murphy's extrapolation, you'd ALREADY be off by a factor of two in predicting U.S. energy use in 2010. So Tom Murphy's extrapolation of the energy trend that he claims to exist is completely specious. It appears true, but it's completely false. There is no trend that can be extended even 4 years into the future, let alone 400.

Comments

[Mark Bahner]

I wrote, "There is no trend that can be extended even 4 years into the future, let alone 400."

"Jammyman" responds, "He's not extrapolating the future. He's showing why extrapolations are useless."

Bullshit. He's saying that energy growth has tracked GDP growth for the past 400 years--i.e., energy growth has grown exponentially for the past 400 years, along with economic growth--so energy growth should continue growing exponentially if economic growth continues exponentially. That's simply wrong, because energy growth in the United States has NOT been increasing exponentially for the past 400 years, as Tom Murphy claims. It stopped increasing exponentially more than 30 years ago. Tom Murphy doesn't tell the reader that.

"And you have missed his point completely."

No, I know his point completely, and it's nonsense. He says the world economy can't continue to grow exponentially forever. But that's wrong, and he even *knows* it's wrong, because he doesn't ever say what he thinks the limit to economic growth is?

The current world per-capita GDP is about $10,000. Does Tom Murphy think the limit is $20,000? $40,000? $100,000? $1,000,000? More than $1,000,000?

"Sheesh."

Indeed. I guess Tom Murphy gets away with much of what he writes, because people like you don't read very carefully.

P.S. If you really think what you write has merit, why do you comment with a fake name?

[Jammyman]

"There is no trend that can be extended even 4 years into the future, let alone 400."

That is *exactly* Tom Murphy's point.

He's not extrapolating the future. He's showing why extrapolations are useless.

And you have missed his point completely.

Sheesh.