Probably the most important rule of process control is, "The better we understand a process, the better we can control it.” While my process control experience is mostly industrial, during the past decade, I gradually got involved with some non-industrial precesses, and found the same principles also apply to their control. In other words, the dynamic characteristics of non-industrial processes can also be described by dead times, time constants, capacities, gains, interactions, etc., and these values must be determined to control them. In recent columns, I've written about cyber terrorism, artificial intelligence, control of self-driving cars and international trade. Here, I'll describe the dynamic characteristics and possible controls for the potentially runaway process of global warming.
We know that during the past century, the global population quadroupled, energy consumption increased tenfold, total production (which without recycling is the amount of future garbage), increased twentyfold, yearly global energy consumption reached the equivalent of burning 12.5 billion tons of oil (3.5 x 1017 BTUs)—on which we spend 10% of the global GDP (25% in the EU, 20% in the U.S.)—and this consumption is expected to double by the end of this century, when the population will exceed 10 billion.
We should also remember that the average solar energy received on Earth is 5 x 1021 BTU per year. If we only consider the half of the solid surface area of the planet that at any one time is facing the Sun, this insolation amounts to about 5,000 times our total energy consumption. In other words, our total energy consumption corresponds to about 0.02% of the global insolation. If we consider the areas of our deserts or the areas of the roofs of our buildings, even at the present low efficiencies of our collectors, we can easily collect what we need.
Now let us look at the heat balance of our planet, which receives 340 W/m2 average energy from the sun (insolation). Most of this insolation is in the form of short-wavelength radiation, such as ultraviolet (UV). This solar energy in part serves to maintain the temperature of the planet (by making up the heat losses to the cold space surrounding the Earth), and partly to support the life on it. The part of the energy reflected back into space is mostly in the form of long-wavelength radiation, such as infrared (IR). Part of this radiation back into space is reflected back to Earth by the gases in the atmosphere ("greenhouse effect"), and this reflected energy also warms the planet.
The temperature control task of this heat balance process is similar to keeping the temperature constant of a water tank that receives a constant heat input (of 340 W/m2), and has a heat loss corresponding to the heat shielding effect of 280 ppm CO2 greenhouse gas concentration in the atmosphere. It is obvious that, if we increase the insulation thickness on this tank, the water temperature in it will rise, and if we decrease it, the temperature will drop.
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In case of the Earth, the "thickness of the insulation" is a function of the concentration of the greenhouse gases in the atmosphere, which during the past half-million years never exceeded 280 ppm of CO2 despite ice ages, volcanic activity or meteroid impacts, while today it is 400 ppm (Figure 1). In the case of our planet, this increase in the Earth's "insulation" causes global warming, and from a process control perspective, this insulation is the only manipulated variable available to us to keep the temperature constant.
The Earth started to be in thermal imbalance at the start of the Industrial Revolution, when mankind started to burn the fossile fuels that had accumulated on our planet during the past millions of years. After the start of this thermal imbalance, the "insulation thickness of our water tank" started to increase, as less and less energy was reflected back into space. As the accumulation of greenhouse gases such as methane (CH4), nitrous oxide (N2O), water vapor (H2O) and carbon dioxide (CO2) increased, they reflected more and more energy back to Earth, thereby increasing the temperature of the surface of the planet (Figure 2). Some 93% of this extra energy has been absorbed by the oceans, 3% by the continents, 3% by the ice at the poles and 1% by the atmosphere. The surface of the oceans warm slower, but this warming rate rises as we go deeper down (I do not understand why).
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Naturally, switching from the use of fossil fuels to green energy reduces the heating of the planet by reducing the greenhouse effect (Figure 3). The atmosphere thermally insulates the globe from the surrounding low temperature of space. In spite of this thermal insulation, some of the surface radiation escapes to space and cools the Earth, while some of it is reflected back to Earth by the greenhouse gases and heats it. Additional cooling is provided by melting of the ice at the poles, but it is only temporary because this cooling will stop once all the ice is melted.
We don’t precisely know the time constants, gains and reaction rates of this heat transfer process. We do know that these time constants are long and the reaction rates are low in all such large-capacity processes, but we also know that they can accelerate with time. What the models of global warming sometimes disregard is the fact that the dynamics of this process will drastically change when all the ice at the poles has melted (which, according to our scientists, has never occurred during the past million years). At that point, like when the ice melts in our gin and tonic at the pool, the temperature will jump and the consequences of that will be much more severe than the slow rising of the ocean levels until then. Some models project that it can destroy human civilization.
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Therefore, we have an absolute limit on our controlled variable—the surface temperature of the planet—that must not be exceeded. That limit, according to the scientific experts of 195 nations who were meeting in Paris in 2015, is a temperature rise of 2 °C. This, according to some of the models, corresponds to an athmospheric CO2 concentration of about 510 ppm. Calculating the time to get there will give an idea of how much time we have to convert to a clean and sustainable energy economy. This calculatioon is difficult because even when we start sending less CO2 into the athosphere (even if the man-made generation of greenhouse gases were stopped), the greenhouse gases already in the atmosphere will continue warming the planet for many years. According to some estimates, the residence time of some of the CO2 the atmosphere will exceed a century.
Therefore, it seems to me that this is somewhat like a batch process where the content of the reactor is being heated, but when the temperature of the content reaches a certain limit, the batch is done and its limit temperature must not be exceed. While we do not fully understand this process, we know two things:
1. The rate at which we are approaching that control limit can be estimated based on measuring the rate at which the ice is melting at the poles.
2. The manipulated variable (the control valve) of this loop is the combined CO2 emission of mankind, and the sooner we close that control valve (stop burning fossile fuels), the better.
You might recall that earlier columns have described the reversible fuel cell (RFC), which has to be fully developed before we can fully convert to solar energy. This is because the technology for storing and transporting solar energy is an absolute necessity for converting to an all-solar economy.
