Greenhouse Effect – the hidden players

 kw: analytical projects, greenhouse effect, global warming, absorption spectra, saturation

Reading a book about agriculture led me to thinking about the "hidden" greenhouse gases. I am sure almost everyone has read or heard that methane is 80 times as potent as carbon dioxide as a greenhouse gas. I recently learned that nitrous oxide (laughing gas, also a dental anesthetic) is between 250 and 300 times as potent as carbon dioxide. Both of these gases are produced by agricultural activity, so they have increased in the past 200 years as agriculture has been increasingly mechanized, and as chemical fertilizers have been used in ever-increasing amounts. (I generated this image using Leonardo AI; it is free of copyright restrictions)

I researched in several sources to find answers to these questions:

• What were the concentrations of nitrous oxide and methane prior to the Industrial Revolution?
• What are their concentrations now?
• How to they affect global warming?
• Are there other greenhouse gases we should be concerned about?

To simplify the text, I will dispense with formatting the numbers in chemical formulas as subscripts. Thus, CO2 = Carbon Dioxide, CH4 = Methane, and N2O = Nitrous Oxide (Nitrogen has several oxides; only this one is important here).

Here is the connection with agriculture: The middle-American farm belt was created by plowing the prairie and planting grain crops. Today, by far the most important crops are corn and soybeans. The thick, rich prairie soils contained a 10,000-year store of CO2, deposited by the roots of grasses and held there as they decomposed. Plowing the prairie released the CO2 at a pretty steady rate over the past century. It is still going on. Plowing also releases stored CH4.

When I lived in South Dakota in the 1970's and early 1980's, most of the agriculture in the state was cattle ranching, with some grain crops being grown in the eastern third. Since that time seed companies have developed strains of corn and soybeans that can better resist drought, begin growing at lower temperature and ripen faster. South Dakota cattle ranches are being plowed and sown with grains at a steady rate.

Secondly, overuse of nitrogen fertilizer causes much of the "extra" to be converted to N2O. Large amounts also go downstream and contribute to the Dead Zone offshore of the Mississippi Delta.

Thirdly, cattle produce a lot of methane, and the reduction in cattle numbers in the Dakotas is more than offset by continued increases elsewhere; also, plowing the prairie releases CH4, and all this is added to the amount released by fossil fuel production. I have yet to see a credible analysis of all the sources of CH4.

Yet all we ever hear about is the rise in concentration of CO2 alone. This is indeed significant, from about 280 ppm in the 1700's to about 440 ppm today. This "baseline increase" is (440-280)/280 = 0.57, a 57% increase in the past century or so. 

What of CH4 and N2O? Let us first convert them to equivalent CO2. I'll leave out a lot of words and summarize the figures:

1. CH4 as a GHG is 80x as effective as CO2. Current CH4 concentration is 1.9 ppm; times 80 that is equivalent to 152 ppm CO2. In the 1700's, CH4 was 0.72 ppm, or CO2 equivalent (CO2eq)  of 57.6 ppm.
2. N2O as a GHG is ~280x as effective as CO2. Current N2O concentration is 0.34 ppm; times 280 that is equivalent to 95.2 ppm CO2. In the 1700's, N2O was 0.27 ppm, or CO2eq of 75.6 ppm.

Added together, these two gases presently have CO2eq of 247. The preindustrial level was 133. Let's add these to CO2 to see the real picture of the greenhouse effect at these two times:

• Preindustrial: 280+133 = 413 ppm CO2eq
• Today: 440+247 = 687 ppm CO2eq
• (687-413)/413 = 0.66, a 66% increase in CO2eq

The actual increase in CO2eq is greater than the effect of CO2 alone. Suppose we could reduce CH4 and N2O to preindustrial levels. This would subtract 114 ppm CO2eq, for 573. Then (573-413)/413 = 0.39, or 39% increase in CO2eq, compared to preindustrial. To put this in context according to the mental model held by "climate crisis" folks, for CO2 only, a 39% increase over 280 ppm would be 389 ppm. That is about where we stood in 2011; it winds back the clock sixteen years!

Let us focus a moment on N2O. By itself, increase in the concentration of this gas is responsible for about 20 ppm CO2eq, the last nine years of increase. This is nearly all due to overfertilization. Guess which industry complex is bigger and has a stronger lobby in DC than oil and gas? Agriculture plus agrichemicals (particularly fertilizer). I have read in more than one place that without artificial nitrogen-based fertilizer, the world's farmland could support no more than four billion people. It is very complex to analyze just how much fertilizer could be reduced to still support the current world population, but reduce nitrate runoff and outgassing of N2O into the atmosphere. For the moment, I just have to leave these thoughts unfinished. If we could come up with a plan, powerful interests would oppose it.

At this point in my analysis I wondered what other greenhouse gases exist, and how they might modify the picture. As it happens, nothing much. Here is a table I worked from for the figures above, which adds six greenhouse gases that, together, are sometimes written about in very scary terms, but have no practical effect at present:

First, ground level Ozone (O3) has a modest Global Warming Potential (GWP: 1.5 x CO2), and exists in the 1-10 parts per billion range, so it is not effectively a greenhouse gas. Then, the industrial chemicals Sulfur Hexafluoride (SF6) and Nitrogen Trifluoride (NF3) have very high GWP, but exist at levels of a few parts per trillion. To totally eliminate them would reduce CO2eq by much less than one percent (see the black text at the bottom of the table)

Various fluorinated refrigerants, those highlighted in brown, have very high GWP, but also exist at levels of a few parts per trillion, so together, they also amount to less than one percent (the brown text). Thus, they present no useful "targets" for ameliorating the greenhouse effect.

My aim here has been to back off a few steps to see a bigger picture. As it happens, this points a finger where none has been pointed before, at farmers. A significant proportion of the increase in CO2eq results from farm practices. In particular, far too many farmers use more fertilizer than their crops really need. There is too much of, "a little more might help." No, it doesn't, it harms. It even harms the farmer, who spends more than needed on fertilizer that isn't helping.

I have a philosophical point to end with. I think that the greenhouse effect will prove to be more beneficial than otherwise. The "father of greenhouse warming", Svante Arrhenius, thought so. Another degree or two of warming is likely to make more of Canada and Siberia amenable to crop production, and let's not forget South Africa and Argentina. On another note, I saw an article recently with a headline, "550,000 will die of extreme heat." The subhead said, "The greatest cause of early death." The article never mentioned that 4.6 million will die from cold. Nine times as many! The subhead is, quite simply, a lie, and the article is utterly one-sided deception. I suspect many of those 4.6 million would love for their home country to be a little warmer.

Unwarranted Extrapolation?

 kw: analyses, greenhouse effect, climate crisis, carbon dioxide, logistic curve

Introduction

We are daily exhorted to worry about the climate, about "carbon pollution", about the "existential crisis" of "human caused global warming". As I have written elsewhere, before I was a teen I had learned how to analyze the greenhouse effect in the atmosphere of Earth caused by water vapor and carbon dioxide. Decades later I read that if the "windows" in the spectrum between the edges of the absorption bands of carbon dioxide were "closed", the total warming would be less than 4°C (7°F). Still later a series of "official" reports were issued, based primarily on computer modeling, making projections of the warming caused by increased carbon dioxide, with low, moderate, or high estimates of the temperatures that are expected. The reports are dire, the warnings are shrill. They are overdone.

This essay is not a quantitative analysis with lots of math or equations. It is conceptual, but, I hope, not simply "arm waving".

Greenhouse Gases

Let's look first at the two primary gases that affect Earth's temperature by modifying how light from the Sun is reflected and absorbed.

Water Vapor

In the past, one would see in the literature of atmospheric warming by the Sun a statement such as, "Without water in the atmosphere, the average temperature of Earth would be colder by 33°C or 59°F." This makes water vapor by far the most important greenhouse gas. We'll see more in a moment.

Carbon Dioxide

Once the infrared absorption spectrum of carbon dioxide was determined, a few scientists calculated how its concentration in the atmosphere might be adding to the greenhouse effect caused by water vapor. One of these was Svante Arrhenius. Late in the 19th Century he published his calculation that if the amount of carbon dioxide in the atmosphere were doubled (it was at that time about 300 ppm), average temperature of Earth's atmosphere would rise by between 1.5°C and 2°C (that's 2.7°F to 3.6°F).

Infrared Spectra

My first job, by which I worked my way through a couple of years of college, was performing infrared spectroscopy for a project funded by the Department of Defense. I became quite familiar with the way certain gases absorb and, when heated, emit infrared light. This image (one of many similar) shows the infrared absorption spectra of water vapor and carbon dioxide:

These spectra do not include visible light, which would be a little to the left, between 0.4µm and 0.7µm. The abbreviation "µm" means "micrometers", also called "microns", or millionths of a meter. It is immediately evident why water vapor is a strong greenhouse gas. While it doesn't absorb visible light, it absorbs many wide bands of infrared. The next graphic shows why these gases are important:

This chart, from an article in 2010 by Clive Best, shows the balance between solar radiation and terrestrial radiation. This time the wavelength scale starts at zero, so it includes visible and ultraviolet radiation. About 1/3 of the Sun's light is in the visible range; the cream-colored band in the image shows how it is divided up. The surface of the Earth also emits thermal radiation, at longer wavelengths, which we see in the right half. Simply put, if there were no absorption in the atmosphere, the blue "mountain" would mostly fill in under the blue curve of the three. The "blocked" radiation shows the greenhouse effect.

For clarity, three thermal infrared radiation curves are shown for different surface temperatures:

• Black: 210K = -63°C = -145°F, deep winter cold in central Siberia or Antarctica
• Blue: 260K = -13°C = +9°F, a cold winter day in Denver or London
• Lavender: 310K = 37°C = 99°F, just above body temperature, or a hot summer day in NYC

These three curves, and the red curve for the much hotter Sun, show theoretical emission from a "black body", which is a theoretically perfect absorber and perfect emitter. Just for fun, check out the lavender curve: If you get the flu and your temperature is just a couple degrees above normal, a spectrometer pointed at you would record peak radiation from your skin at about 9µm, with significant amounts in the range 5µm-22µm. To a spectroscopist, the range of wavelengths longer than 5µm, out to about 50µm, is longwave infrared, also known as thermal infrared.

Just below the red-blue section we find a combined spectrum for a generalized atmosphere ("Total Absorption and Scattering"), and then spectra for several gases, starting with water vapor and carbon dioxide. These two together produce most of the combined spectrum shown just above water vapor.

Look carefully at the rightmost absorption band for carbon dioxide. It mostly overlaps the major band for water vapor. This shows that the two gases do not act independently. When humidity is high, the differential effect of carbon dioxide is small, but in dry air, carbon dioxide has a stronger effect.

The other major absorption band for carbon dioxide, between 4µm and 5µm, is near the edges of the thermal radiation bands, so it has even less effect. It is little affected by the nearby band for water vapor.

Let's look briefly at the rest of the gases. I am disappointed that the creator of this chart chose to combine oxygen and ozone. Perhaps it was out of trust that most who read the article would know that the broad absorption at the far left, in the ultraviolet range, is for ozone, and the others are for oxygen. Anyway, it shows that oxygen, which makes up 21% of the atmosphere, is a mild greenhouse gas, while ozone, which is nearly all in the stratosphere, is a strong greenhouse gas, but only at high altitudes. Ozone's absorption of solar UV is the major reason that the upper stratosphere is warmer than the lower stratosphere. 

Methane is next. It looks like a poor absorber, but that is because it occurs in very low concentration, less than two parts per billion (or less than 200,000 times the concentration of carbon dioxide). The fact that it shows up at all warns us that it will be a strong absorber if its concentration rises much. If an atmospheric sample had equal amounts of carbon dioxide and methane, the greenhouse warming by methane would be more than 80 times as much!

Nitrous oxide is also a strong greenhouse gas, but is present at very low concentration, about 300 parts per billion, or less than 1/1,000th the concentration of carbon dioxide.

Rayleigh scattering is a physical effect; the "gas" for this part is the combination of nitrogen and oxygen. A small proportion of light is scattered off the molecules of these gases, and the scattering is stronger for shorter wavelengths. It is the shorter wavelength visible light (blue), scattered from sunlight, that makes the sky blue. As a matter of fact, for any planet with an atmosphere, anywhere in the universe, unless the air is very dusty, the sky will be more blue than the star that is that planet's "sun".

What a Greenhouse Gas Does

Let us consider, for the moment, only water vapor in an otherwise "pure" atmosphere of nitrogen and oxygen. Sunlight heats the surface, which radiates thermal infrared. Much of this is absorbed by the water vapor, which heats the atmosphere. This heat also produces thermal infrared radiation, and to a first approximation, half is aimed downward and half is aimed upward, to eventually escape into space. What goes down heats the ground, so more up-going radiation ensues. This feedback process raises the temperature of everything until the amount of radiation escaping is equal to the amount that descended from the Sun. In short: Sunlight in, IR out, with a "hang up" as some, or most, of the IR cycles between the surface and the atmosphere. The "greenhouse effect" is the "hang up".

According to Clive Best in his article, most of the possible absorption by carbon dioxide was already occurring before the Industrial Revolution. This is expressed in the diagrams above by the portions of the curves that have already reached 100%. Let's look closer at one of the diagrams:

When a portion of the spectrum is below 100% absorption, and the concentration of the gas being investigated is increased, the wings of the absorption band rise, and the area that absorbs 100% increases. The "Area of Interest" in this image is between about 12µm and 14µm, and the absorption by water vapor is usually less than 50% in this range. This is where almost all the action is; this is the source of "global warming" by carbon dioxide! In order to pull the left side of the curve upward until nearly all of it reaches 100% would require increasing the gas concentration by a factor of ten or more.

While we are here, let us take a side trip to Venus. There, the atmosphere is almost all carbon dioxide, at a pressure of 92 atmospheres, or 1,334 psi. That is 220,000 times as much carbon dioxide as Earth's atmosphere. The tapered bands, and the smaller peaks shown, will all be at 100%, and even the little bits of hash near 10µm and elsewhere, plus others not shown here, will be at 100%. The spectrum will be a bunch of rectangles, filling much more of the total space. No wonder the temperature on Venus is above 460°C (860°F)! Of course, it "helps" that Venus gets sunlight twice as intense as that on Earth.

All these details about absorption, and the interactions between the absorption profiles of carbon dioxide, water vapor, and other gases, result in a very complex system. I hope it is evident that, in the range of a few hundred parts per million, up to one or two thousand parts per million, the extra effect of the growing margins of the absorption bands is less and less as concentration increases. There is a point of diminishing returns. Let us discuss the Logistic Curve.

The Logistic Curve

Here is one familiar situation. A weakling wants to get stronger (usually this is a guy, so I'll use male pronouns). He goes to a gym and engages a trainer. He is tested with several weights; let us assume that, for starters, he can bench press 50 pounds. That means he can't even do one pushup. He trains three times weekly, with much grit and determination. After a few months he can "bench" 80 pounds, and in a few more, he attains 100 pounds, and can now do one or two pushups. In another year he finds his one-rep bench press is approaching 200 pounds! He has quadrupled his strength, and he looks a lot better also, more filled out. Question: Can he quadruple it again? 800 pounds? Men who have trained for decades seldom exceed 500 pounds; the record for someone who never took steroids is less than 1,000. Well, then, can he attain 400 pounds? Possibly, but at that point he is likely to have joint or tendon problems. The bones and tendons he started with can't increase as much as the muscles that attach to them. Somewhere in the 200-400 pound range he has a limit.

Let's consider this graph:

The blue line is a much-studied relationship that is not well known outside mathematics and statistics departments. It is called the Logistic Curve. The red curve is an Exponential Curve, such as compound interest. The units of this graph don't matter. Many natural phenomena can be described by setting this curve in appropriate units. For example, if the red curve shows the natural ability of locusts to multiply (let's say the units along X are weeks), the blue curve shows what will happen when they eat everything nearby and must begin to migrate to find more food. They may carry along for a while and reach the "carrying capacity", or "saturation", but somewhere off the right end of the graph, the curve will fall again when the locusts have eaten everything they can reach. Or, consider our ambitious weightlifter. Let's assume "10" represents the point where he started concentrated training. His strength doubles, doubles again, and then he finds it hard to make such great gains. Eventually, he "tops out". If he's lucky, he has done this without pulling any tendons or breaking any bones. That's why he has a trainer…

How does this relate to the greenhouse effect and carbon dioxide? For the next graph I've recast the axes to make a useful analysis, at least in a general way.

The amount of absorption by carbon dioxide as its concentration increases, and the level of added heat that this causes, is also a logistic curve. If we want to make predictions about what might happen with increasing gas concentration, we need to know where we are on the curve. NOTE: The temperature scale is not intended to be highly accurate, but to show relative relationships.

Depending on which horizontal scale we use, the baseline, "pre-industrial" amount, is either the lower black circle or the lower blue circle. We don't know for sure just how much heating was already in place in pre-industrial times, compared to a situation with zero carbon dioxide. Such a "zero situation" has never occurred on Earth. But we can compare two scenarios, one using the black circles and the black scale, and one using the blue circles and the blue scale; both follow the blue curve. The implication of the dotted black curve will also be considered.

The "We are in the early stages" scenario (black scale)

Temperature has risen, as gas concentration went from 280 ppm to 420 ppm over the past 175 years, and we find ourselves a bit beyond halfway along the rising portion of the curve. The amount the curve can rise until saturation is about equal to what has already occurred.

The "We are in a later stage" scenario (blue scale)

The upper blue circle shows that we are near the top of the curve. There is a little ways to go, but even doubling or tripling the gas concentration will not push us much further.

I have not been able to discover whether the climate modeling community has picked a spot on a logistic curve, or if they are even using one.

The "This could destroy everything" scenario (black scale, black dashed line)

If the climate disaster prophets are using predictive curves at all, they are probably using one that doesn't peel over at some saturation level, but goes up forever. The portion of the blue curve between the black circles looks pretty straight, straight enough that many folks, not seeing the whole picture, will project a straight line to forever. I am pretty sure that the "impending disaster" predictions that get all the press these days (at least since the first IPCC Assessment Report in 1990) are following a linear extrapolation, such as the dashed black line.

I favor the blue circles, which imply that most of what could happen has happened already. The global climate system may gain another half degree or one degree, but not more.

That is my conclusion, dear readers. I believe we are being bullied into making drastic changes based on a nonsensical extrapolation.

Climate Change or Climate Forcing...redux

kw: musings, greenhouse effect, greenhouse warming, global warming, climate change, climatology, analysis

When I wrote a book review on the subject of climate change a few days ago, I had something in mind, but the review went in a different direction. The title actually didn't fit any more. Here are charts I made to illustrate my thinking on the terminology about "carbon pollution" and all the hype surrounding it, on both sides. Firstly, an nGram of the usage of three popular terms since 1970:

Although warming caused by the greenhouse effect, in particular that caused by carbon dioxide, was quantified by Svante Arrhenius in 1896, only after about 1970 was it brought to public attention. The great promotion of this issue really took off after 1985. This nGram shows that, in print at least, "Greenhouse Effect" was soon superseded by "Climate Change" and "Global Warning". But let's focus on a different term and its scientific synonym:

You can see from the upper chart that the term "Climate Forcing" has been scarcely a blip on the radar. But on the scale of the lower chart we see that about the time "Greenhouse Effect" peaked, "Climate Forcing" began to surge (relatively speaking), and shortly after that, "Anthropogenic Climate Change", a more scientific synonym, came along, but it is quite a mouthful.

It is a pity that Google stopped scanning books in 2008. I'd like to see how these words have fared in these past ten years. Nonetheless, I did a Google search for all these terms and a few others I've thought of in the past day or two (put in quotes to force literal searching), setting the search time to the past year, here is how they score:

• 7 - Anthropogenic Climate (allows "change" or other following words): 488,000
• 8 - Anthropogenic Climate Change: 478,000
• 6 - Carbon Pollution: 523,000
• 1 - Climate Change: 147,000,000
• 9 - Climate Forcing: 366,000
• 4 - Climate Science: 2,310,000
• 5 - Climate Warming: 975,000
• 2 - Global Warming: 56,600,000
• 3 - Greenhouse Effect: 4,360,000
• 10 - Greenhouse Warming: 295,000

The number preceding each term is its rank in this alphabetized list.

Why do people use the words they use? Impact. "Climate Change" and "Global Warming" get the public's attention. But the actual debate is not really about whether climate is changing or the globe (i.e. its atmosphere) is warming. It is about the extent that human civilization contributes to the change or the warming. However, "Anthropo..whatever" is too much of a mouthful, and "Climate Forcing" doesn't have quite the ring of the more popular terms.

But: Climate Forcing is really the best term about which to have a policy debate. The atmospheric climate will change gradually over time, whether the human race is highly civilized or goes extinct. Prior to 1975, the big worry about "Climate Change" was about "Global Cooling". A cooling trend highlighted by the first 15-18 years of weather satellite measurements triggered fears of a new ice age. And we find that the recent Solar Maximum had lower sunspot activity than most prior cycles. Based on historical records, this could indicate a cooling trend because lower solar activity heats the Earth's atmosphere less than average. Sunspot numbers are an indicator (not a cause) of the number of flares and other phenomena that send extra energy our way.

So, how big a factor is Climate Forcing? Let's call the Climate Forcing Factor the CFF. The way the media report things, one group called "climate deniers" would say the CFF is close to zero. In the same media, a group called "established science" claims the CFF is "most" or "nearly all" of the difference, in the range 50%-90%. Putting aside my conviction that the media are rather incredibly biased, we can instead identify the poles of the debate as "Large CFF" and "Small CFF" factions. Who is right? Do we have a way to know?

We don't, actually! But we can dig out an indication or two.

This article in ScienceDirect states that variations in sunspot activity account for about 40% of long term temperature rise in Norway over the past century, with a probable range of 25-56%. For a different portion of the North Atlantic, the range is 63-72%. This ought to please the Large CFF folks.

Remember the ozone hole? Starting about 30 years ago colleagues of mine at DuPont determined the great amount of damage being caused to the ozone layer some 15 miles (~25 km) overhead, and this triggered research efforts at DuPont and other chemical companies to find new refrigerants for air conditioners and new propellants for aerosol cans. The ozone "hole" was a dramatic thinning of this layer mainly over Antarctica, but spreading halfway to the equator, and there was a similar, but smaller thinning over the Arctic. But we need to be clear: the "ozone layer" isn't pure ozone; it is where ozone is concentrated to a level of about one part in 100,000 (0.001%); it is still mostly nitrogen. Ozone at sea level is around 1/30th of this, about 1/3,000,000th. Chlorine from refrigerants and propellants in use before 1980 had reduced the level of ozone over the poles by about 2/3, and elsewhere by about 20%. "Ozone hole" is the dramatic term that refers to the reduction of ozone from 1/100,000 to 1/300,000 over Antarctica during the southern summer.

Ozone is funny stuff. It is created from oxygen by ultraviolet light (UV), and then it absorbs UV, which heats it up. So the more ozone, the more the atmosphere is heated from the top. Specifically, at subtropical latitudes, surface temperature averages about 300K (27°C or 81°F), while 15 km (9½ mi) above, air temperature has fallen to about 200K (-73°C or -100°F). Ozone and other stratospheric gases absorb UV and some IR to raise the temperature back to 300K by about 50 km (30 mi) altitude. This warm gas in the mid- to upper stratosphere emits thermal radiation (longwave infrared) both upwards and downwards, which heats the air below a little. The gradual increase in stratospheric ozone levels over the past 30 years have contributed a little heating, but I have not found a rigorous analysis of the matter. "About a degree" is a general statement I have read. This is a factor that tends to please the Small CFF folks.

These things indicate that the CFF is unlikely to be greater than 50%, and is probably closer to 25% or less. I would not say, "close to zero", so I am not in the extreme Small CFF crowd, but neither do I favor Large CFF. As I have stated elsewhere, I learned that if we were to raise carbon dioxide levels to, say, ten times their present level, the amount of greenhouse heating would not exceed 4°C or about 7°F. That is quite significant. Is it enough to end civilization? I don't think so, but it will definitely change it. We are unlikely to find out, though. If we were to burn all the fossil fuels that we currently know about, it would no more than double the amount of carbon dioxide that we have already emitted. That's another way of saying that we have already burned about half the global reserve of fossil fuels. In rough terms, it means we have so far doubled atmospheric carbon dioxide, from around 200 ppm to around 400 ppm. Once we run out of natural gas, oil and coal—should we continue freely burning them—the level could become around 600 ppm. I don't think we have enough fossil fuel available to push that to 1,000 ppm, where some people begin to feel the effects.

For all that, we must continue to find other sources of energy, on all fronts. No source of energy is perfect. Wind farms (currently 4% of global electricity generation) disturb wind patterns, heat the air that passes through the fan blades, and kill migrating birds; solar panels turn about 15-20% of sunlight into solar energy and the rest is turned into heat, and much of this would be in desert areas where the sand usually reflects 75% of the light right back out into space; geothermal energy is "clean" from a heat perspective, because the heat will emerge from the earth anyway, but using geothermal energy causes pollution of surface water and ground water, a whole lot of pollution; and so forth. The more we learn about all these things, the better we can select energy generation methods that cause the least harm. That, and that alone, will reduce the CFF. It will probably never be zero, until human population is zero.

Postscript: Do you know what the global average temperature is? I am a geophysicist. Including the whole planet, the average temperature is about 4,000K (over 7,000°F). We need a different term for "global average atmospheric temperature", and we need to always specify at what elevation; is it surface, or at the average elevation of continental plains (about half or 2/3 a kilometer), or some other "standard" height?