Bill Chameides

Could the heat coming from your refrigerator be contributing to global warming?

Anyone who has given a public lecture on climate change has almost certainly gotten this question:

Whenever you operate a motor or turn on a light, you generate heat. How do you know that that heat is not a significant source of global warming?

It's a good question. To answer it, let's start with a review of one of my favorite topics.

Energy, Energy, Where's the Energy

It takes energy to do just about everything we do -- walk, run, bike, eat, turn on the microwave, light the stove, heat our homes. You get the picture. Question: What happens to the energy after we're done with it -- after it did the work we needed done? Does it just disappear? Is it gone?

It can't be. We know that from one of the most basic laws of physics -- the first law of thermodynamics -- which states that energy is conserved. You can't create energy and you can't destroy it; you can only convert it from one form to another. And so it may sometimes seem that the energy we used has disappeared, but it has not it; it's just been converted to another form.

And we know from another law -- the second law of thermodynamics -- that eventually the energy will find its way to its most disorganized and random form: heat (basically just molecules moving around randomly and bumping into each other).

Consider: You’re riding your bike. Chemical energy from your food is turned into mechanical energy moving your bike. But if you stop pedaling, your bicycle slows and eventually stops. Why? Friction dissipates the mechanical energy. Where did that mechanical energy go? Heat energy (feel your tires).

Consider: We burn some natural gas to create electricity to light a light bulb. The photons (or light energy) from the bulb radiate away from the bulb and hit your face and "disappear." Does that mean that the energy in the light disappeared or was destroyed? Of course not, it shows up as heat on your face.

Now try this: Rub your hands together. You have to work to do that. Why? Friction resists the motion. You probably noticed your hands got warm. The mechanical energy used to move your hands has been converted to heat energy.

In every case of energy usage, if you follow the energy trail, keeping in mind that energy can never be destroyed, you will find heat energy at the end of the trail.

And so, the question: if we've got all that heat from our energy usage -- all that thermal pollution -- how do we know that it's not a cause of global warming. As I said, it's a good question, but one easily answered. It's a matter of scale.

How Much Energy Are We Talking About?

Consider the following:

• The amount of energy that the Earth gets from the sun is about 174,000 terawatts (a terawatt is a trillion watts).
• The amount of sunlight that is absorbed at the Earth's surface is about 90,000 terawatts.
• The extra heating to the atmosphere by greenhouse gases (what we call radiative forcing) is a tiny fraction of that: only 1,000 terawatts. Still, that is large enough to cause the climate to warm appreciably.
• The amount of energy we use annually is about 15 terawatts.

That's right: the upper bound of heat that can be generated from energy use is 1.5 percent of the radiative forcing from greenhouse gases and only 0.009 percent of the energy from the sun. It pretty easily follows that any contribution to global warming from energy usage must be dwarfed by that from the greenhouse effect. End of story, right? Not quite.

New Study Says There Is a Climate Effect

Taking what might at first seem to be a contrarian stance, Gaung Zhang of the Scripps Institute of Oceanography and coauthors reported last week in the journal Nature Climate Change the results of a series of model simulations assessing the effect on global temperatures of all the heat from our energy generation.

Their results show higher surface temperatures in the fall and winter over North America and Eurasia in simulations in which heat from our energy usage was included compared to their control run without.

In some places and times, the temperature increase was found to be as large as 1 degree Celsius (1.8 degrees Fahrenheit).

That's a lot, the average global warming since the Industrial Revolution is only about 0.8 degrees Celsius (about 1.5 degrees Fahrenheit).

Holy Heat, Batman

Wow, is this a major turnaround in climate science? Not really. Sure, in the model runs with the heat included, there were warmer regions, but there were also cooler regions. To a close approximation the warmer and cooler regions canceled each other out. The authors write: "global mean air surface temperatures are insignificant for both the annual mean … and seasonal mean."

So what's going on? The authors reason that the addition of heat in and around major population centers in the Northern Hemisphere leads to a "change in atmospheric circulation … consistent with … warming in the mid- and high-latitudes and cooling south of 40 ^oN."

In other words there's not significantly more heat in the atmosphere with the extra heat, it's just being moved around differently. The authors are seeing regional-scale changes in temperature but not a significant net warming on a global scale.

How could such a small amount of energy have such a large effect on global circulation? It comes down to location.

Much of our energy consumption is concentrated near the Northern Hemisphere's jet stream. If you pay attention to the weather maps and you live on the mid- and high-latitudes, you know that small perturbations to the path of the jet stream can have a huge impact on the local weather and the local temperature.

So Are These Findings a Waste of Time Then?

Remember those hypothetical questions posed at the top of the post? Well, the answer, as you’ve likely guessed by now, is "no": the heat generated from operating a motor or turning on a light or keeping the food in your fridge cold is not a significant source of global warming.

But that doesn't mean that the findings by Zhang et al are irrelevant. Quite the contrary. Climate scientists are continually evaluating climate models by comparing observations and model calculations. The authors find that including the impact of thermal pollution in modeling simulations reduces the differences between observed and simulated surface warming trends.

This could turn out to another small piece of the jigsaw puzzle that is the climate system.

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