Tuesday, October 8, 2013

My View: A Brief Account of CO2


==========  Update 8 October 2013  ==========

Ocean acidity rises and falls with atmospheric CO2 concentration.  A major article in Science (March 2012) concludes the rate of ocean ocean acidification today, due to increasing atmospheric CO2, is at least ten times faster than any event in the fossil record. Study details here: New York Times article; Science abstract; Science full text.

==========  Original Post 25 November 2011  ==========

Fear makes the wolf bigger than he is. --- German Proverb.

[Note: A version of this blog entry appeared in The Leading Edge (Feb, 2009)] 1st Update

I had a lot of work to do on arrival at the University of Houston in January 2008: Planning classes, finding a house, managing a move from Saudi Arabia, and much more. You would think there could be a transition period to ramp up research, but that was a luxury cut short by a stranded Department of Energy (DOE) project -- original Principal Investigator (PI) gone, interim PI wanting out, subcontractors performing erratically, matching funds not paid, and most of the money spent with a full year remaining. In short, it was a rescue operation. I stepped in to PI the project that centered on something I knew nothing about, geologic CO2 sequestration.

Now two years later, the project is in great shape. The nucleus of a good CO2 team is in place, including a geology research professor, a flow simulation consultant, one masters (MS) student slated for May 2010 graduation, two more MS student projects underway, two excellent PhD candidates, and a recently awarded follow-on DOE award of $300K. Your stimulus money at work.

For me a byproduct has been two years immersion in the strange topic of CO2. My goal here is to offer some broad comments, a 'view from a height' so to speak. The topic brings together several aspects of my background (and that doubtless frames my viewpoint) -- my BS Geology degree along with courses I have developed in Environmental Geophysics, Physical Geology, and The Earth in Space. So here we go.

Carbon Dioxide. CO2 is a naturally occurring component of the atmosphere. It's ultimate origin on this planet is outgassing of the rocky mantle as the earth cooled. Currently CO2 represents about 385 parts per million (ppm) -- a puny 0.0385% -- of the atmosphere, up some 30% from a preindustrial level of 300 ppm. But over the course of geologic time vast quantities of CO2 have been introduced into the atmosphere by natural processes. We need look no further than our twin planet Venus to see direct evidence of that. A bit too close to the sun for liquid water to form on a large scale, Venus was robbed of a key mechanism that pulls CO2 out of the atmosphere. The result is an atmosphere 97% CO2 and a runaway greenhouse effect driving surface temperature to nearly 900 degrees F, even though the intensity of sunlight that falls on Venus is only about 60% higher than on Earth.

How has our planet avoided this fate? Fortuitously located farther from the sun, liquid oceans formed early in Earth's history. Water can hold dissolved CO2 in solution to form a weak acid that in turn reacts with silicate rocks. Through a series of inorganic chemical reactions, the CO2 gets tied up and precipitates as lime mud and, eventually, limestone. The immense quantity of carbonate rock is the prime repository of the Earth's original atmospheric CO2 budget. The amount of CO2 bound up carbonate rock is approximately equal to that in the current atmosphere of Venus.

Not all carbonate is of inorganic origin, which brings us to the second major mechanism extracting CO2 from the Earth's atmosphere: Life. The earliest forms of life arose in a rich CO2 atmosphere. From a geologic time perspective, mantle outgassing to load CO2 in the atmosphere is a rapid event, much faster than mineralization and sedimentation. So even after a couple of billion years, CO2 still dominated the atmosphere. Thus early life developed to take in CO2 and emit the (to them) poisonous gas oxygen. An early crisis on our planet occurred when the efficiency of these early life forms modified the atmosphere with so much oxygen that life was threatened with extinction. From this crisis came the kind of life we are know -- oxygen in, carbon dioxide out. Free oxygen, however, is highly reactive and could bind up carbon from CO2 into organic compounds in living things that eventually died and were buried to form organic rich rocks (coal, shale, and more limestone). Life had utilized the reactive power of free oxygen in a slow way to build up organic compounds. For five billion years, first inorganic then organic processes scrubbed CO2 out of the atmosphere, permanently sequestering it in geologic formations. Permanently, that is, until an intelligent life form arrived and figured out that free oxygen could also release energy through a fast reaction with live and fossil organic material -- it could burn.

Sea level. Human civilization developed in a world with a certain sea level. But as any geologist knows, sea level has fluctuated significantly over geologic time with various tectonic processes and glacial cycles. When the first fixed settlements of the Tigris-Euphrates valley were built about 7500 years ago, world sea level was just creeping up after a dramatic rise. A mere 18000 years ago, sea level was 120 m (360 ft) lower than today. At the low water mark someone could have walked from Bahrain to Iran, easily crossing a wide river in what is now the Arabian Gulf and never getting a glimpse of the ocean beach at what we call the Straights of Hormuz.

All of this occurred without human intervention, completely due to natural processes. Since those first mud brick cities were built 7500 years ago in Mesopotamia, sea level has risen 5.8 m, or about 1 inch every 30 years. In the natural cycle of things, we are in a maximum interglacial period of sea level rise associated with melting of glacial ice. We are on the curve of a natural sea level rise, so it is hard to find good evidence linking future rise to our CO2 emissions. Almost by definition, we will never know what sea level would have done without our CO2 release.

Sea level rise over the last 24,000 years. Inset calculation shows most rapid rise is 4.7 ft/century (1.43 cm/ry), about the same rate at which tectonic plates move. The red 70 m bar indicates the maximum potential sea level rise if all ice on earth were melted.

It is useful to put a frame of reference on the discussion. No one is suggesting the possbility, but if all the Earth's ice were melted sea level would rise an additional 70 m (210 ft). To reinforce the idea that our civilization is tied to a specific and narrow range of sea level, we note that a 210 ft sea level rise would put many of the world's great cities in the drink: New York, Bejing, London, and Houston to name a few. To check your favorite city, use Google Earth by placing the cursor over a site and read off the elevation. As a final comment, we note the complete sea level stroke, so to speak, from maximum glacial to maximum interglacial is about 190 m (620 ft). The current thinking on sea level rise due to human CO2 emissions (could it be proved) is on the order of 1 ft per century.

Climate Change. For most people the face of the CO2 concern is global warming. It is unfortunate that one aspect of the problem dominates, particularly this one. Each of us has a lifetime of experience, and opinion, about temperature. Atmospheric temperature varies with latitude, longitude, altitude, and time. One number, global mean temperature, is usually quoted as a proxy for this highly variable quantity. Even if global mean temperature for a given year is the highest on record, we know a significant portion of the globe at ground level could be cooler than usual. This is due to the definition of 'mean', which is a kind of average. People in those colder regions (who are not climate scientists) will tend to disbelieve global warming since they experience cooler than normal temperatures that year. If mean temperature goes up year after year, this will happen every time. Before long almost everyone disbelieves these global warming pronouncements. It is human nature we are up against here.

The connection to CO2 is the greenhouse effect. There is no point debating or taking an opinion poll about the greenhouse effect, it can be (and is) easily verified in a high school physics class. On the planetary scale we can describe it this way. Sunlight, which is electromagnetic (EM) energy, passes through the atmosphere to heat the Earth's surface. Due to a phenomenon called black body radiation, any object whose temperature is above absolute zero will emit EM radiation according to a fixed law. Earth's surface temperature is such that the black body radiation is just the right wavelength to set greenhouse gas molecules (CO2, methane, water vapor) vibrating. This transfer of energy increases the atmospheric temperature because, at the atomic level, temperature is nothing more than molecular motion. Without greenhouse gasses, the black body waves would radiate into space leaving Earth an ice planet.

So there is danger on each end of the CO2 issue. Too much means a hot planet, too little means a cold planet. One could argue that such an important thing as atmospheric CO2 is not something we should tinker with. But in fact we already have, by burning fossil fuels as a high-density energy source for the last century and a half. The question is this: Will we continue this unmanaged CO2 release, or work to reduce it?

Atmospheric CO2 change over the last half-century (main plot) with measurements (red line) showing seasonal variation, along with running average (black line). In 2005, 13 ppm (27 Gt) of CO2 were emitted into the atmosphere by human activity, yet the observed CO2 level only increased by 1.7 ppm. This means that 87% of the emitted CO2 returned to the earth and ocean through natural sinks.

The most significant real impact of higher atmospheric CO2 is an unwelcome change in the chemistry of seawater. This could well reduce the ocean's already dwindling ability to produce food for a growing world population. For this reason alone, it is worth the effort to reduce human emissions of CO2.

We should also understand that atmospheric CO2 has been much higher in the relatively recent geologic past. If we take 300 ppm as the preindustrial benchmark of CO2, then we now stand at about 1.3 relative CO2 (RCO) concentration. In the Carboniferous period 220 million years ago, the RCO was between 2 and 5, perhaps as high as 10. In the very early ages of the Earth, CO2 completely dominated the atmosphere. A unsteady progression of natural processes brought the RCO value down to 1. From this planetary experiment we know a runaway greenhouse effect, like that on Venus, is not possible on Earth. If it were possible, it would have occurred long ago. Leave the catastrophic scenarios for Hollywood, our CO2 worries are about a foot or two of sea level rise and a few degrees of mean temperature increase. Important for ocean chemistry, food chains, ecosystems, and coastal cities, but hardly disaster movie material.

Deep time atmospheric CO2 (relative to recent pre-industrial 300 ppm level) estimated from various geological proxies.

Energy. Now that we have an understanding of CO2 and natural sequestration, the connection to human energy use is clear. Over the last 150 years we have been extracting and burning fossil fuels (oil, gas, coal) as our primary energy source. These fossil organic materials slowly pulled CO2 from the atmosphere over tens to hundreds of millions of years. Burning them simply releases the CO2 back into the atmosphere.

Put this way, it makes one wonder: "Were we insane? Why would anyone do that?" But the fact is that concentrated energy is a rare and precious commodity. Note the word 'concentrated', it is important. Sunlight is a marvelous energy source and the ultimate one driving Nature's CO2 removal engine. But sunlight is low-density energy. If we think about using energy to generate, say, electricity we then have a kind of common currency. Higher energy density means the ability to generate more electricity per unit of the energy resource. It turns out there are very few high-density energy sources, perhaps only three viable ones: Fossil fuels, nuclear, and regionally important hydropower.

As I see it today, something very close to half of the accessible fossil fuels have been used. With population and per capita energy growth, the other half will be lucky to last 100 years, perhaps much less. But we have no viable alternative -- nuclear fission is plagued with waste and weapons issues, nuclear fusion may (sadly) never work, and all of the renewables together cannot possibly scale up to replace fossil fuels. In my opinion, we will eventually have no choice except a wholesale push to nuclear fission despite the concerns.

World population and oil production have a positive correlation, with occasional reversals representing severe recessions.

I welcome CO2 worries, well-founded or not, if they can slow down the fossil fuel juggernaut. Efficiency gains, development of renewables, and fully-burdened fossil fuel costs are not just welcome, they are badly needed. The CO2 problem is important on it's own, but more important as a wake up call for the real issue: energy.

2 comments:

Anonymous said...

300 ppm would be 0.03%.

Gerard Wieggerink said...

Thank you, Prof Liner, for explaining these complex issues in easy-to-read blog posts. For non-techies like myself who are working in the energy business, it is really great.