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What does “Global Warming” mean? [Greg Laden's Blog]
The Problem with terminology
There is some confusion about the way we talk about global warming. Most of this confusion arises during the communication of science to the public or to policy makers. Part of this confusion rests within the science itself; There is no meaningful confusion about the nature of global warming or how it is observed, but there are some terminological glitches of the kind that arise in science all the time, and that rarely matter to the science itself.
The most commonly used indicator of global warming is a graph that is meant to show the effects of global warming over time. This may be by decade, yearly, or monthly, or using a moving average using a period such as 12-months or some other time period. This is an example:
NASA GISS TEMP ANOMALY 1880-PRESENT
The vertical axis is temperature anomaly, the standard way scientists measure changes in heat over time. Each data point is calculated from thousands of roughly head-height thermometers across the Earth’s land surface, combined with a measure of global sea surface temperatures. As we go back in time there are fewer data sources, and the sea surface temperature is measured differently. During any given year, there are parts of the globe that are underrepresented. The way these deficiencies in the data are addressed varies across the major data sets (from the US, Great Britain, or elsewhere) though all the different data sets use most of the same original raw data. This graph is based on the data provided by NASA’s Goddard Institute for Space Studies (GISS). All of the different data sets show the same thing, a general increase in global surface temperatures, and the variation over time, the up and down squiggles, is similar for all the data sets. They differ only in details.
It is an important fact that as the line indicating global surface temperature going up across the indicated time span, the amount it goes up varies. There are short periods of time when the temperature value goes up rather quickly then drops a bit, and there are periods of time during which the value goes up and down without much increase over several years running.
When we see a brief period (a few years, or a decade, etc.) where the trend varies from the long term trend, we need to ask why this is happening. Most of the dramatic upward spikes turn out to be El Niño years, when the ocean is adding a lot of stored heat into the atmosphere. Most of the periods where the rise in temperature value is somewhat lackadaisical are periods both lacking an El Niño event and having a number of La Niña events, during which the Pacific Ocean is soaking up more heat than average (that heat comes back out during El Niño periods).
There are also periods when the rise in global temperature is attenuated by additional aerosols in the atmosphere. This may be caused by a high rate of volcanic activity or the explosion of a particularly large volcano. Also, at one point, we see general increase in the upward trend that is probably a combination of a) cleaning up some of the human caused aerosols with the Clean Air Act and similar regulatory changes, and b) an increase in the human output of greenhouse gas pollution.
A significant cause in the variation of this signal over time, related to El Niño and La Niña events, is probably the result of one or more long term oscillations in the relationship between the ocean and the atmosphere. I’ve written about recent research addressing this phenomenon here. These multi-decadal oscillations probably explain most of the waviness in the line.
We have recently moved past a period of a relative slowdown in the increase in global surface temperature and are now experiencing a rapid rise in average global temperature. The way this slowdown is discussed is part of the confusion about global warming. To the average person, the term “slowdown” might sound like “decrease,” but it is not a decrease in surface temperatures, but a reduction in the rate at which surface temperatures are going up for a few years.
Nonetheless, the recent slowdown has been exploited by those who argue that global warming is not real, or is not, somehow, caused the way scientists say it is. It has been termed a “hiatus” or a “pause.” This terminology is problematic. A hiatus is a gap. When we “pause” something (like using a pause button) we stop it. The stop button on your music device stops the music. The pause button also stops it. The difference between stop and pause is what happens after you restart it, or a difference in the internal working mechanism. In the old days, for example, if you stop an audio or video tape, the magnetic head that reads the data is lifted off the tape, but if you hit “pause” the magnetic head stays on the tape so restarting is smoother. (If you pause for too long the magnetic head can damage the data on the tape!) Thus, “pause” and “stop” are functionally the same thing when it comes to whether or not the music or video is playing. A “pause” in global warming would be a stop in global warming. That, however, did not happen.
A hiatus or a pause in global warming is at present physically impossible. Our climate system operates in such a way that increasing the amount of human generated greenhouse gas pollution, all else being equal (or more or less equal), will increase the global heat imbalance and force the surface temperatures upwards. The implication of a “pause” or “hiatus” (stopping, or a gap in, warming) is that global warming is not happening for a period of time, as though the physical process stopped working, and the implication of that is that physics does not work the way climate scientists know it works. This is why “pause” is so beloved a meme in the denier community. If there is a pause, the science must be wrong. But even if there were sufficient aerosols from a huge volcanic eruption to actually lower global surface temperatures significantly for a short time, the greenhouse gasses previously and continuously added by humans would remain and continue to exert an upward effect on temperature. Once the aerosols settle, which does not take long, the added greenhouse gasses will remain for many decades (even centuries) and warming will continue until an equilibrium is reached. That would not be a pause or hiatus, just a bit more wiggling in the line marching ever upward.
Process vs. pattern
Global warming is a process. More greenhouse gasses along with the resulting positive (heat increasing) feedback effects that accompany that increase cause a heat imbalance and the parts of the Earth that can absorb heat from the sun directly or indirectly become warmer. Global warming is also a pattern. It is a pattern we observe in the average global temperature measures such as the surface temperature measurement described above.
If you can show that the pattern as observed is not as expected, that would bring into question the process of greenhouse gas-caused changes in the Earth’s heat, right? Well, no. The answer is no because the way the process works and the way we generally observe the pattern are not the same thing.
Look at these two graphs.
OHC
GMST
The upper graph shows the temperature of the part of the ocean, the upper 700 or 2000 meters, that can be warmed indirectly by increasing greenhouse gas pollution over a period of time. The lower graph shows the same thing for the sea surface and the atmosphere. These graphs are dramatically different in the x-axis. The ocean heat content graph only goes back to the late 1950s, while the surface graph goes back to 1880. This is because of difference in the data that are available.
Now, have a look at this graph:
BUBBLE GRAPH SHOWING DISTRIBUTION OF HEAT IMBALANCE
This graph shows the relative percentage of the overall warming that occurs in the ocean vs. the atmosphere and a few other systems. The ocean heat graph above, which shows no recent period of time during which heat does not go up, represents over 90% of the heat increase due to global warming. The surface represents only a small percentage. In the following graph, I simply cropped each of these two graphs to show only 1960 to the present, then scaled them so the surface graph and the ocean graph are in proper relationship to each other on the vertical axis. This is a bit hokey but it makes the point:
HYBRID GRAPH SHOWING GMST AND OHC SCALED
Wow. When we refer to the process of global warming, we are referring to changes in the Earth’s heat balance, which is a combination of what climate scientists call “forcings” (not my favorite term but it is the one in use) that move heat imbalance either up or down. Human caused greenhouse gas pollution is a positive forcing, which in turn causes a number of other feedbacks, also positive. So the total result of greenhouse gas pollution is an increase in temerature over time until some future point where the forcing stops (because we stop using fossil fuels) and the heat imbalance eventually settles out (far into the future). Aerosols from other human pollution or volcanoes, etc., force in the opposite direction. The net outcome has been, for decades, an increase in temperature. The cobbed together graph above shows that all of the forcing combined has resulted in a steady upward increase in temperature. The graph also demonstrates that even large up or down deviations in the pattern of surface temperature are not especially relevant to the total process of warming.
When we refer to the pattern of global warming, however, we generally do not refer to overall changes in heat balance, but in practice, we refer to changes only in global means surface temperature. Why? Because that is the measure for which we have data over a long period of time. It is like this. Say you want to know if your child has a fever. You may put a thermometer in the child’s mouth, or some other orifice, or put a heat sensitive strip on the child’s forehead, or an ear thermometer in the ear. In so doing, you have measured the child’s temperature, right? Well, no. What you measured is the temperature of the child’s mouth, or distal large intestine, or forehead, or ear drum. You don’t call the nurse hot line and say, “Help, my child’s mouth is 104 F, what do I do?” You use the measurement you took of one tissue or body part to estimate the child’s overall body temperature. Well, actually, you use the measurement you took to see if your child’s temperature-related homeostasis is off. In any event, you used a measurement of a small part of your child’s body to estimate internal temperature.
When climate scientists show you a graph of mean surface temperature and talk about global warming, they are not ignoring the ocean. They are simply measuring the surface as an indicator of an ongoing change in the Earth’s heat imbalance, using data that are available, understood, and cover a long time period. So they are talking about the process of global warming (which involves the oceans, the air, the sea surface, the ground under your feet, ice, etc) but using a readily available and useful tool to track it.
As a result, the term “global warming” to many climate scientists means “overall positive heat imbalance most of which is in the ocean.” To some scientists, “global warming” refers to global mean surface temperature change, and the ocean is viewed as a reservoir where heat is stored, waiting in the pipeline to come out later. That is really the same thing, and both acknowledge the difference between surface temperature measurement and ocean heat content measurement. When you actually look in the published literature, you see no confusion or changes in terminology. You really don’t see the term “global warming” being used very often when referencing measurements. If the measurement being discussed is the heat in the ocean, you see the term “ocean heat content” (OHC). When the measurement being discussed is the surface temperature record, you see a term like “Global mean surface temperature” (GMST). And, you don’t even see these terms used on their own; At some point in the scientific paper, the actual data set used to derive these values is specified.
This does not imply that the difference between ocean heat content and global surface temperature is not important. It is very important. For one thing, we live at the surface. Heat waves are a phenomenon of the atmospheric temperature at the same location it is measured by all those thermometers. Tropical storms form more frequently or grow larger with a higher sea surface temperature. A warmer atmosphere holds more water. Relative changes at different latitudes in surface temperature appear to have changed how weather systems behave, giving us the phenomenon known as “weather whiplash” causing frequent droughts (short or long term) and regional inundation with exceptional rain or snow. The deeper (down to 2000 meters) heat in the ocean does not directly cause those things.
But, the heat in the ocean does contribute when it comes out, like during an El Niño, adding to the surface heat. And, there are other effects of heat in the ocean, causing important ecological changes including ocean acidification. It isn’t really that complex. Both the ocean (down to 2000 meters or so) and the surface (SST and the bottom of the atmosphere over land) are warming. The total amount of heat that the ocean can absorb is huge (because water holds more heat than air). The heat moves back and forth between the sea and the air. Numerous effects occur. The whole shebang together is global warming, but we often represent the phenomenon as changes in surface temperatures because that is an excellent measurement.
Perhaps we should not use the term “global warming” for the pattern of surface temperature changes, because global warming is bigger than that, it includes more stuff. But the surface is where we are at, and just like we say “my child has a fever” because our child’s forehead feels hot and the ear drum is verified as warmer than it should be with an optical thermocouple, we can refer to the GMST and speak of global warming. Because it is.
On several occasions, I have had lengthy discussions with colleagues who are full time well respected climate scientists about this terminology. As far as I can tell, two things are true: 1) left on their own, these scientist would probably sort out into two groups who prefer two different ways of using terms like “global warming” and “surface temperature” and such; and 2) this matters about as much as what color pocket protector they are using to hold their mini-slide rules. It does not matter at all. As discussed above, when doing the science, the systems to which one refers and the data one makes use of are specified unambiguously and differences in the way these terms may bleed out into public (and policy making) space are not important. Outside the science, in that public and policy making space, the terms do become important, but not because they have problems. They become important only because they are being used, exploited, by deniers of science to misdirect and mislead.
And, as I suggested at the beginning of this essay, this is normal. Terminological messiness occurs in all areas of science. I once copied out all of the glossary definitions of major phenomena in population genetics, from a well written textbook produced by a well respected population geneticists, into a handout. Every definition from that glossary contradicted or complexified at least one other definition. It was a mess. But it was also real. Each term had been introduced at a different time by different scientists for different reasons with different problems trying to describe something somewhat different. Had a committee of population geneticists sat down for two years, long after the science had been worked out in some detail, and generated a glossary of terms (like “bottleneck,” “genetic drift” and “founder effect”) there would be no contradictions or ambiguities (though there might be a few black eyes along the way). Even the term “gene” has multiple and often contradictory meaning in use, and that gets worse when dealing with the literature over a couple of decades. This emerged over time as we learned more about what a “gene” actually is. Yet, population geneticists and DNA experts do not have any problem doing the science. These are just quirks that emerge at the interface of the rational pursuit of truth in science and this crazy thing we have called language.
Source: http://scienceblogs.com/gregladen/2015/06/07/what-does-global-warming-mean/
