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The Difference Between Energy, Work and Power – and Why it Matters to Climate Prediction
Guest essay by Eric Worrall
How often have you heard claims that a warmer [climate] will be more energetic – that we shall all experience more violent storms, more rainfall, more storm damage, because the atmosphere is “absorbing more energy”?
Such claims are based on a fundamental misunderstanding of the nature of energy.
Consider an electric flashlight battery. An electric battery contains energy, but that energy doesn’t do anything – the battery can sit on a shelf for months, ready to be plugged into your flashlight. But plug the battery into your flashlight, and leave it on, within hours the battery will be dead – all its available electrical energy has been used up.
Similarly, a battery plugged into a motor also uses up its stored energy.
Motor powered by a battery.
How does this example of a battery powered motor relate to climate change?
In physics terms, the Earth’s climate is also an engine. Instead of a battery, the climate is powered by solar energy – sunlight falls on the daylight side of the Earth, is converted to heat, and is then radiated away into space. The means by which that energy is transported from warmer regions to cooler regions before being radiated into space is the sum of all of the world’s climate phenomena – wind, rain, ocean currents are all part of the global climate engine which is powered by our sun.
Earth’s climate is powered by the sun
The point to remember is energy from sunlight is delivered at a constant rate to the Earth’s surface, because solar output is more or less steady, and the size of the Earth doesn’t change. The power of the climate system, the rate at which work can be performed by the climate system, is constant.
Global warming – accumulation of energy in the form of heat in the Earth’s atmosphere – does not affect the amount of energy which is available to power the Earth’s climate. If the accumulated energy were to affect the climate, by contributing to more violent weather, it would be expended, just like the stored chemical energy in a battery is expended when it is connected up to a flashlight or a motor.
The way that energy is used by the Earth’s climate might change if the world warms. For example, Milankovitch cycles – the slow changes to distribution of sunlight caused by wobbles in the Earth’s orbit – likely have a profound effect on global climate, triggering the growth and recession of ice ages. But the orbital wobbles described by Milankovitch cycles don’t affect how much sunlight in total strikes the world, they only affect the proportion of the total sunlight budget which is received by the Northern and Southern hemisphere during different seasons.
The implications of a total Earth climate energy budget are profound. A warmer world might experience more rainfall. But there has to be a tradeoff. Evaporating water to produce rainclouds requires an enormous amount of energy. If energy is diverted from the global climate budget into producing more rain, less energy is available to power wind and ocean currents.
There is freedom of movement for small changes. A lot of sunlight which strikes the Earth is bounced straight back into space, with minimal impact on the climate. If the albedo, the reflectiveness of the Earth were to drop substantially, more sunlight might be absorbed, which might make more energy available for wind, rain and ocean currents. But there is no guarantee a warmer world would have a significantly lower albedo. Some albedo might be lost if polar ice melts, but polar ice doesn’t receive that much sunlight, compared to the amount of sunlight received by tropical regions. Any drop in albedo due to melting polar ice would likely be more than compensated by increased tropical cloud cover.
Awareness has grown over the last few years in the climate community of this thermodynamic limitation. A lot of climate scientists have started qualifying scary predictions of wilder weather, by saying storms will become more violent, but they will occur less frequently. But as I noted at the start of this post, far too often the qualification is lost. Many people believe weather will simply grow more violent without constraints if the world warms, because they don’t understand the hard limits imposed by the Earth’s total climate energy budget.
Warmer climatic conditions which are more prone to thunderstorm formation might result in more frequent but weaker storms, with less damaging winds. But a boring prediction like that wouldn’t fit the climate science narrative that global warming will make everything worse.
The following scientific paper explores limits to the Earth’s climate energy budget in more detail;
Constrained work output of the moist atmospheric heat engine in a warming climateRead more (requires registration): http://science.sciencemag.org/content/347/6221/540.full
Incoming and outgoing solar radiation couple with heat exchange at Earth’s surface to drive weather patterns that redistribute heat and moisture around the globe, creating an atmospheric heat engine. Here, we investigate the engine’s work output using thermodynamic diagrams computed from reanalyzed observations and from a climate model simulation with anthropogenic forcing. We show that the work output is always less than that of an equivalent Carnot cycle and that it is constrained by the power necessary to maintain the hydrological cycle. In the climate simulation, the hydrological cycle increases more rapidly than the equivalent Carnot cycle. We conclude that the intensification of the hydrological cycle in warmer climates might limit the heat engine’s ability to generate work.
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