Global warming XIII – The Carbon Dioxide Control Knob

Wayne,

EPA Administrator Scott Pruitt appeared on CNBC’s Squawk Box on March 9. Here’s one question and answer:

JOE KERNEN (HOST): Let me ask you this, let me ask you one other thing, just to get to the nitty gritty. Do you believe that it’s been proven that CO2 is the primary control knob for climate? Do you believe that?

SCOTT PRUITT (EPA ADMINISTRATOR): No, I think that measuring with precision human activity on the climate is something very challenging to do, and there’s tremendous disagreement about the degree of impact. So, no, I would not agree that’s a primary contributor to the global warming that we see. But we don’t know that yet, as far as — we need to continue debate — continue the review and analysis.

The question is awkward, since no one asserts the premise of the first question. No one says that CO2 is the primary control knob for climate. What knowledgeable people believe is that of the various “control knobs” for the climate, humans are “turning up” the CO2 knob and warming the planet. Thus, by accident, Pruitt’s first word is correct: “No.” After another accidental correct claim, that measuring the effects of human activity on the climate is challenging, the rest of his answer is wrong.

Pruitt asserts that there is “tremendous disagreement about the degree of impact” (of human activity on the climate). This is incorrect. There is general agreement that increasing levels of CO2 in the atmosphere due to people burning carbon fuels is warming the Earth. In the various models, as in all scientific endeavors, models and theories produce somewhat different predictions based upon different starting points, different methods, and different interpretations. All professional studies of the effect of the increasing CO2 agree that it will increase the Earth’s temperature. Some predict an increase of, say, 2 C, in 50 years given no changes in present fossil fuel use, others 4 C. None predict no effect. None predict that there will be a cooling. Pruitt says that because some scientists say 2 C and some 4 C, they have no idea what the warming will be. This is the claim of many opponents of scientific knowledge, that if scientists don’t know everything about a phenomenon to infinite precision, then they might as well know nothing.

All climate researchers would agree that neither they, nor anyone else, knows all there is to know about the climate. Contrary to Pruitt and to many other global warming deniers, the climate researchers know plenty, and, as it typical in research, they have sought the causes of the big effects first. Those are well-known and have been for decades, some more than a century.

Pruitt says that he does not agree “that’s a primary contributor to the global warming that we see.” Now the contraction in this sentence is “that it is” where the “it” refers to the question: carbon dioxide. Filling this in, we see Pruitt’s meaning: “I would not agree that carbon dioxide is a primary contributor to the global warming that we see.” The question from Kernen: “Is CO2 the primary control knob for the climate?” The answer from Pruitt: “No. CO2 is not a primary contributor to global warming.” At least Pruitt says that the globe is warming.

That CO2 is not a primary contributor to global warming is false. The Earth’s climate arises, of course, from many factors. To understand it involves assessing the Earth’s energy balance in accordance with the 1st Law of Thermodynamics or the Conservation of Energy. This great law, known with increasing depth and precision, arose in scientific thought in the 1800s.

To summarize the matter, consider the Earth including its atmosphere as a thermodynamic system. That system contains a certain amount of energy, known as internal energy. (That is, I’m not considering the kinetic energy of the Earth’s motion around the Sun, although I could.) The Earth’s internal energy has various parts, potential energy and kinetic energy. Potential energy is in all the Earth’s chemical bonds and nuclear bonds. Kinetic energy is in the random shaking, jiggling, and zipping about of the atoms and molecules, and directed kinetic energy is in motions such as winds and water currents. Energy flows in and out across the outer boundary of the Earth system. Energy comes to us from the Sun, and the Earth radiates energy to the cosmos. If energy from the Sun exceeds the energy the Earth radiates, the Earth’s internal energy must increase. If that increase occurs in the random kinetic energy part of the Earth’s internal energy, the Earth’s temperature increases.

The Sun radiates energy to us across 90 million miles of space. The Sun is an incandescent sphere with a surface temperature of about 6000 K, where the K is short for Kelvins. Scientists prefer this scale, whose degrees are the same size as the more familiar Celsius scale (used in sensible nations) and are 9/5th of the size of the Fahrenheit scale (used in the United States). But the zero of the Celsius scale, the melting or freezing temperature of water, is 273 K. Zero K is the unreachable absolute zero. As an incandescent object at this temperature the Sun radiates most of its energy in the infrared, (the short wavelength infrared around 1 millionth of a meter). It radiates throughout the visible, with a peak in the green (around ½ a millionth of a meter). And it radiates some energy in the ultraviolet. To our eyes, the Sun is white. At the top of the atmosphere, the imaginary boundary of the thermodynamic system, power from the Sun is about 1 kW/m2.

The atmosphere is, for the most part, transparent to this incoming radiation. Clouds, dust, and other things reflect some of it back through the system’s boundary. Some parts of the surface also reflect the radiation upwards, such as ice and snow. All of the rest must be absorbed by the Earth, land and sea.

To take a convenient value, consider that the surface of the Earth is 300 K, which is not far off. The Earth radiates energy upward because of its temperature, just as the Sun radiates energy upward from its surface. The Earth’s surface is about 20 times cooler than the Sun’s. Thus, the peak of the Earth’s upward radiation has a wavelength 20 times longer than does the Sun. Twenty times ½ millionth of a meter is 10 millionths of a meter. That’s the peak, but there is energy moving upward from 5 to 20 or 30 micrometers.

If the Sun’s power to the Earth, with a (blackbody, as the physicists say) spectrum at 6000 K, exceeds the Earth’s outgoing power with a blackbody spectrum at 300 K, the Earth’s surface temperature must increase: 310 K, 320 K, and this increase in temperature will increase the outgoing power until it equals the incoming power.

Water vapor and CO2 in the atmosphere add important details, without changing the picture that an imbalance of power in and out leads to temperature changes. These gases absorb energy in the long wavelength infrared, and the molecules re-radiate that energy in all directions. Some outgoing power returns to the Earth.

Consider, for the moment, if there were no water vapor or CO2. Scientists can estimate the Earth’s surface temperature in this case that will balance power in and out. With no water vapor or CO2, the Earth’s surface temperature would be about 260 or 265 K, -5 or -10 C, 20 F! That is, the world’s oceans would be frozen.

If water vapor and carbon dioxide absorb some of the out-going power, then the Earth’s surface must radiate more so that what makes it into outer space equals the incoming power from the Sun. As the Earth’s surface temperature increases, the upward moving power increases. The water vapor and carbon dioxide still absorb and re-radiate some, but the surface temperature will increase until that fraction that passes all the way through the atmosphere equals the power coming the other way. These natural processes are the greenhouse effect. They are the reason that the Earth’s temperature is about 18 or 20
C, or 295 K or thereabouts, and the oceans are liquid.

I don’t mean to say that the entire matter is simple. The details matter, and variations in any of the effects I’ve mentioned and in some I haven’t may and do influence the energy and power balance. That’s why the Earth’s climate scientists have been working so hard for decades.

The general picture I’ve explained, however, forms the basis for our understanding of the Earth’s temperature.

Of the many possible control knobs, the Sun’s output power may vary, the Earth may shift in its orbit, the Earth’s axis may shift, volcanoes may add particles and gases to the atmosphere. Any of these and more may vary and have varied in the Earth’s history. It happens to be the case, that humans have been transforming ancient sunlight, stored in the chemical bonds of fossil fuels, into energy to power our civilization, producing water and carbon dioxide and some other stuff. We have grabbed onto the CO2 control knob, and we have begun turning it up. Of course, the various natural forces and effects still operate. But the climate change science deniers do not explain why this twist of the carbon dioxide control doesn’t increase the temperature.


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