WASHINGTON — Tom Murphy, an associate physics professor at the University of California-San Diego, likes to compute big and daunting numbers.

So when he grew concerned about how the nation could sustain its spiraling energy consumption, he decided to chart the use of fuel alongside the estimated U.S. gross domestic product since the year 1650 (http://bit.ly/PEdrgn).

That led him to the alarming realization that exponential economic growth cannot continue into the decades and centuries ahead because of constraints on energy supplies, said Murphy, whose Website is named Do the Math: Using physics and estimation to assess energy, growth, options (http://physics.ucsd.edu/do-the-math/).

One way to understand the crucial role of energy in today’s world is to see what happens if it’s unavailable.

People might think that, because energy costs comprise a little over 10 percent of the economy, a 10 percent cut in energy availability would be relatively harmless, amounting to a 1 percent impact.

However, that energy enables so many things that in a severe energy crunch, “you’re going to see something larger” – a sort of multiplier effect on the economy, Murphy said.

Murphy said he believes that the world is “approaching a trap,” with energy supplies unable to keep up with demand and creating a need to divert some precious energy for use in developing alternatives, such as tens of millions of fuel-efficient cars and new forms of renewable energy sources, he said.

That, in turn, could create a new problem.

“If things happen the way they typically have, where high oil prices trigger recession, who’s going to buy these expensive electric cars?” he asked.

Murphy decided to study his home gas and electric meters to get an idea how much Americans might save.

Just turning off the pilot light on their furnace during the summer saved so much natural gas that they decided to set their furnace on 55 degrees in the southern California winter, too, he said.

Mothballing their electric clothes dryer, they now watch for a sunny day to hang their wet laundry outside. Next, Murphy personally assembled and installed solar panels on their roof and put batteries in the garage to store some of that energy.

That gave the Murphys off-the-grid power for their refrigerator, computers, television and stereo, slicing their total electricity consumption to a quarter of that used by similar San Diego homes.

Murphy said he sees “all kinds of warning signs that we’re entering a new phase of declining oil supplies,” and he lamented “the lack of long-term thinking” among U.S. and world leaders who he fears won’t react until a crisis hits.

“No politician from any side,” he said, “has said the things that placate me, that I want to hear, that make it clear that they understand the perils, and they have a plan that just might involve some sacrifice.”

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