Energy exchange vs. Energy change.
Energy change represents a change of the total global energy. Energy exchange represents an exchange of energy between two parts of the Earth’s systems, without necessarily increasing the total energy.
A seesaw exchanges the potential energy of Person A on one side of the seesaw to Person B on the other side of the seesaw. As Person A falls, they lose potential energy, causing Person B to rise, gaining the potential energy lost by Person A. In this process there is no change in the total energy of Person A+B: energy is merely exchanged between two people (there is a small amount of energy lost due to friction in the center pivot, requiring each person to push off the ground a small amount).
The oceans and the atmosphere represent two objects that also exchange energy, engaging in their own SeaSaw, where in some years the oceans lose energy to the atmosphere, and in other years the atmosphere loses energy back to the oceans. Just as two people using a seesaw merely exchange potential energy back and forth, without changing their total potential energy, so too, during energy SeaSaw cycles the oceans and atmosphere exchange energy. Although the long-term trend is that global warming causes both the oceans and the atmosphere to gain energy, because the energy exchanged between the oceans and the atmosphere can be up to 10 times as much as the energy gained by the atmosphere during a given year, it is usually difficult to estimate the amount of global warming from the temperature change of the atmosphere from one year to the next. To estimate the magnitude of global warming typically requires looking at atmospheric temperature trends from one decade to the next, or more commonly, by using a 10-year moving average. Try out the SkS Temperature Trend Calculator to see how the averaging period affects the temperature trend line.
Examples of SeaSaws are the El-Nino/La-Nina cycles (also referred to as El-Nino Southern Oscillation, ENSO), and the Pacific Decadal Oscillation (PDO). El-Nino cycles typically occur near Christmas. For this reason, South-American fisherman referred to this weather phenomenon as El-Nino de Navidad, which in Spanish means the Christ child. In Spanish, El-Nino therefore means “the little boy”, whereas La Nina means “the little girl”.
During an El-Nino cycle, the oceans transfer sufficient energy to the atmosphere to cause globally-averaged atmospheric temperature to rise by as much as 0.2°C. During La-Nina cycles, the atmosphere transfers sufficient energy to the oceans to cause as much as 0.2°C cooling. The net effect of El-Nino/La-Nina cycles is not global warming, just energy exchange between the oceans and the atmosphere.
A good way to think of Earth’s energy system is to think of using a seesaw on an elevator. While the elevator is on the ground floor, not moving, a seesaw will simply cause two people to go up and down in rhythm, but there is no net upward motion. An upward motion only occurs when the elevator starts moving upwards, regardless of whether the seesaw is being used.
Temperatures going up and down from year-to-year mean very little: it may mean the elevator is moving up, but it may also represent the action of a SeaSaw. Temperatures going up consistently from decade-to-decade are more concerning, because it likely means the elevator is going up. Decadal temperatures have been increasing for the last five decades. And not just going up, but rising rapidly in geological terms, at a rate of about 0.2°C/decade (0.36°F/decade). SeaSaw-style fluctuations in temperature cause local changes to weather that can seriously impact local communities for a year or two. Elevator-style increases in temperature cause global effects that last for a millennium or two.
For additional information, see Questions 9, 10, and 11 of “Climate Change: Evidence and Causes.”
You may hear reference to natural 1500-year temperature cycles. Such natural temperature cycles are topics of past and continuing research, and represent changes to global SeaSaw cycles. These 1500-year cycles usually represent local warming and cooling: where there is a 1500-year cycle of warming in one part of Earth, there will usually be a 1500-year cycle of cooling elsewhere. These well-known 1500-year cycles do not represent global warming, but rather a very complicated energy redistribution on a global scale. Although some will refer to these 1500-year temperature cycles as global warming/cooling, they actually represent global energy redistribution: if the northern hemisphere warms, the southern hemisphere cools in response. See Richard Alley (2007), Annual Review of Earth Planetary Science, 2007. 35:241-272.
Posted by Evan on Thursday, 22 April, 2021
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