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This blog is expressly directed to readers who do not have strong training or backgrounds in science, with the intent of helping them grasp the underpinnings of this important issue. I'm going to present an ongoing series of posts that will develop various aspects of the science of global warming, its causes and possible methods for minimizing its advance and overcoming at least partially its detrimental effects.

Each post will begin with a capsule summary. It will then proceed with captioned sections to amplify and justify the statements and conclusions of the summary. I'll present images and tables where helpful to develop a point, since "a picture is worth a thousand words".

Wednesday, October 27, 2010

The CO2 Bathtub

Summary:  Previous postings have detailed the findings that a) even if humanity stops burning fossil fuels that produce atmospheric CO2, simply continuing to use existing facilities, such as power plants and automobiles, will add new CO2 to the atmosphere for several decades and lead to increased warming of the planet; and b) in reality, humanity’s demand for ever increasing amounts of energy will lead to much higher levels of atmospheric CO2 over the coming decades than at present.  In order to visualize this, here we imagine that the atmosphere is a bathtub containing CO2 at the present concentration.  We assess the factors that cause the bathtub to add more CO2 or that help drain it.  As noted, the bathtub will fill with more CO2 in coming decades. This model shows clearly that it is not enough merely to cut back on CO2 production, but that it is critical to cease completely the burning of fossil fuels for energy as soon as practical.

Introduction:  In earlier postings (Note 1)  we have discussed the current status of global warming  due to greenhouse gas (mainly carbon dioxide, CO2) accumulation.  Davis and coworkers (discussed here) have shown that even if installation of all new energy facilities worldwide that rely on fossil fuels were to abruptly stop right now, the continued emissions from existing facilities and machines would continue feeding new greenhouse gas into the atmosphere, leading to further global temperature rise.  In contrast to this limited hypothesis, Hoffert, in his commentary on the Davis article (presented here) , emphasizes the very dire situation that we will actually face in the future because of the accelerating addition of new CO2-emitting facilities.  Recent predictions of future global warming from the United Kingdom and the United Nations Intergovernmental Panel on Climate Change suggest that average global temperatures could rise 4 deg C (7 deg F) or more from today by the end of the century.  Such a drastic change in climate would lead to serious unfavorable effects on human populations around the world.

The CO2 Bathtub.  Suppose that the earth’s atmosphere is a bathtub, containing air with  CO2 at the present concentration of 390 parts per million (ppm; this means that out of 1 million molecules of all the gases in the atmosphere, 390 are CO2).  In this imaginary model, if the level of CO2 were to decrease the bathtub would be less full, and if the level of CO2 were to increase, it would ultimately overflow (see image below).  Thus, in this image, the bathtub represents the world’s CO2 level at the present time, and further represents the baseline for the additional CO2 accumulation anticipated by the climate scientists.  The CO2 bathtub at its present level also correlates with global warming (the planetary long-term average of measured temperatures) which is now about 0.7 deg C (1.3 deg F) above the average temperature that prevailed prior to the beginning of the industrial revolution, when the CO2 concentration was 280 ppm.

The Atmospheric CO2 Bathtub, showing pre-industrial (280) and current (390) CO2 levels.
Bathtubs are installed with faucets, delivering fresh water to the tub; and with drains that, when open, empty the contents of the bathtub.  In our imaginary bathtub, there is a “faucet”, made up of all the planetary processes that deliver new atmospheric CO2 into the bathtub.  There is also a “drain” that is partially “open”, such that some of the CO2 delivered into the atmospheric bathtub by the “faucet” is depleted.  In the previous posting we discussed the “virtuous cycle” of CO2 production and removal that was operating during the millennia before the onset of the industrial revolution.  This cycle operated with a dynamic equilibrium, maintaining the atmosphere, or in our present image, the bathtub, at the constant level of about 280 ppm CO2, or about 72% of its present level.  We will not consider the processes underlying this virtuous cycle any more here.  Rather, let’s mention the newer man-made processes that contribute to filling or emptying the bathtub. 

The Bathtub’s “Faucet”.  By far the largest contributor to the “faucet”, adding new atmospheric CO2, is the worldwide burning of fossil fuels for energy.  This energy provides mechanical motion in transportation vehicles of all kinds, including cars, trains and airplanes.  It is also used significantly in electrical appliances and apparatuses, and in heating and air conditioning of living and working spaces.  An additional contributor to the CO2 “faucet” is the large scale destruction of tropical forests, primarily by burning, to clear land for agricultural use or other purposes.  It’s important to note that this burning is on a scale that vastly exceeds the processes of dynamic CO2 equilibrium taking place in the primordial “virtuous” cycle.

The Bathtub’s “Drain”.  The major process “draining” atmospheric CO2 from our bathtub is the absorption, or dissolution, of atmospheric CO2 into the oceans of the planet.  A certain fraction of atmospheric CO2 dissolves into the oceans, but this process does not remove all the additional CO2 that we add by burning fossil fuels.  The absorption of CO2 into the waters of the oceans itself has detrimental environmental effects which will be discussed in future postings.   Another potential process that might contribute to “draining” atmospheric CO2 from the bathtub could be reforestation undertaken by humans.  This activity occurs in addition to the “virtuous” cycle of primordial times.  Its effect requires decades to become significant, however, since the planted trees have to grow to a size that would draw significant amounts CO2 out of the air.

The Current Level in the Bathtub.  At the present time, the “level” in our imaginary bathtub is not staying steady, but rather is rising in the tub.  The people of the world are burning so much fossil fuel that the level of atmospheric CO2 is rising by about 2 ppm per year, corresponding to about 0.5% per year, even though the “draining” processes remove part of the added gas.

The Future Level in the Bathtub.  As noted in the Introduction, Davis, Caldeira and Matthews, Science Vol. 329, pp. 1330-3, 2010; see Note 2 and see also have shown hypothetically that even if installation of all new energy facilities worldwide that rely on fossil fuels were to abruptly stop right now, the level of atmospheric CO2 in our imagined “bathtub” would continue to rise, in view of the continued emissions from existing facilities and apparatuses.  This would continue to lead to further global temperature rise for several decades.  Only as the presently installed base of fossil fuel facilities and apparatuses ages and is removed from service would these processes level off and the level in the “bathtub” begin to fall.

In reality, however, we know that new facilities that consume fossil fuels are constantly being added to the energy economy. The United Kingdom National Weather Service (Met Office) reported in 2009 that CO2 emissions are proceeding at a higher rate than was thought only a few years earlier.  In other words, not only is the current level in our “bathtub” increasing at about 0.5% per year, but the rate of rising of the level is most likely going to increase for the indefinite future.  Very briefly, this can be ascribed to increased demand for energy in developed countries of the world, and to the addition of vast new portions of the global population to the demand for energy-requiring apparatuses and living conditions, populations that earlier had minimal demands for energy.  Indeed, the Met Office predicts that the global average temperature could be 4 deg C (7 deg F) higher than today (note: 4.7 deg C higher than preindustrial revolution times) by 2100, using a “high-end emissions scenario”.  The level in our atmospheric “bathtub” is foreseen as rising continuously into the future, with no pattern currently in view that eases the increase, or the rate of increase, of the bathtub’s level.

We Must Level Off the Bathtub As Soon As Possible.  Our imagined bathtub shows clearly that the world’s peoples have to stop the level of atmospheric CO2 from rising any further as soon as possible, because each increase in the bathtub’s level correlates with additional global temperature rise.  This simple picture makes it very clear that it is not sufficient simply to slow the rate of burning fossil fuels, because all this will do is slow the rate of rising of the bathtub’s level.  What is really needed is to approach the situation imagined by Davis and coworkers, i.e., not building any new fossil fuel-burning facilities right away.  As described above, even this drastic requirement still results in having the level in our bathtub continue rising for several decades before leveling off.  In our model the bathtub is equipped with both a faucet and a drain.  The scenarios of Davis and coworkers, and of the Met Office, are concerned primarily with the faucet.  In fact, there is little technology currently available, other than reforestation, that has the effect of “opening” the drain more than it is now.  It is for this reason that global warming scientists stress the critical need for reducing the burning of fossil fuels to zero as soon as possible.

Note 2.  Abstract available online free, or the full article for a fee or through personal or institutional subscription.  Many public libraries, and university libraries open to the public, receive the journal.

1 comment:

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