Tree bark

This time of year in a deciduous forest in the northern hemisphere you get to see a great deal of tree bark - gray, brown, flaky, smooth, furrowed and fissured, sometimes with lichen or moss growing on it.

This is the bark on a dogwood tree growing near the Apex Reservoir in Cary, North Carolina. If you click on the image to see it full-sized, you can easily count layers along the descending sides of the fissures in the outer bark or cork. I have not found any confirmation that these layers represent annual growth increments of the outer bark, but if they do, I can count as many as 22 layers in this image.

This outer bark of a tree is a protective layer of dead cells meant to be partially shed as they shield the tree from physical impacts of the weather as well as the grazing of animals from deer, beavers, and birds to insects, and the biological attacks of bacteria, fungi, and viruses. In some trees, this outer bark layer contains materials that resist fire, allowing the tree to survive all but the most intense, canopy fires. A tree can also excrete waste products into the cells of the outer bark.

At or just below the base of the fissures seen in this photograph lies the cork cambium, a layer of actively dividing cells that produce the largely dead outer bark layers. Just beneath the cork cambium lies a layer of living cells called the phelloderm that can serve a variety of roles including photosynthesis, active disease defense, and storage.

Below the phelloderm lies the phloem, the inner bark layer filled with the vascular sieve tubes that carry the sugars produced during photosynthesis in the leaves down to the rest of the tree and its underground roots.

Dig just beneath the phloem and you hit the cambium, the layer of actively dividing cells that produces the thickening or radial growth of the tree trunk. This is the inner end of the bark of the tree, and also the outer beginning of the inside structure of the trunk.

The actively dividing cells of the cambium layer produce not only the bark of the tree, but also the entire inner trunk of the tree. This inner wood, called the xylem, includes a variety of tubes that carry water and dissolved minerals up from the roots to the stems and leaves. The xylem also contains stiff vertical tubes called fibers that support the heavy aboveground weight of the tree.

For an overview of bark found on trees around the world, take a look at ArtSylva's post on the biology of barks. This beautiful site created by photographer Cédric Pollet talks succinctly about the variety of barks, their function for the tree, and their uses for people. And the collection of pictures of bark of all colors and bark found on many different kinds of trees is amazing. If you have not seen a baobab tree, visit this site and find one in Cédric's "Photo Reports" link!

Sounds from a frozen lake!

Once in a while, a great while, down here in the south, it gets cold enough to freeze a lake. I think it's happened once or twice since the big freeze we had in January 2000. A few days ago the lake reservoir behind our house had developed a good bit of ice on its surface. Time to throw a rock across the ice and listen to the lake sing! It's a magical sound. A little web surfing found some recordings of ice "singing" on a frozen lake, and a few attempts to explain this interesting phenomenon. The sound seems to come from the ice itself vibrating like the skin stretched across a drum head. The best recording is Marc Namblard's "Chants of Frozen Lakes"! You can buy his CD, but scroll down a little to listen to some free audio extracts of his recording. Not quite the same as that stone skipping along the ice, but amazing nonetheless. Enjoy!

The "Saturation Myth"

William Lynch wrote in the News and Observer on Dec.17 that more than 300 ppm CO2 in the atmosphere would absorb all of the infrared re-radiation coming up from the Earth's surface. According to that logic, further increases in CO2 would not cause any more global warming. Unfortunately, Dr. Lynch repeated a very old mistake, first made around 1900. But Dr. Lynch has good company, as Knut Ångström was the first scientist to make this mistake.

Increasing carbon dioxide will continue to increase global warming. Satellite measurements show that as atmospheric CO2 increased between 1970 and 1997, from 320 to 365 ppm, there was greater absorption of the infrared wavelengths absorbed by the CO2 (see http://www.nature.com/nature/journal/v410/n6826/abs/410355a0.html). Two other articles extended those findings through 2006 when CO2 was up to 380 ppm. And the satellite observations indicate there is still plenty of infrared yet to be absorbed, so further increases in CO2 will cause further warming.

Ray Pierrehumbert and Spencer Weart give a more complete explanation and history at http://www.realclimate.org/index.php/archives/2007/06/a-saturated-gassy-argument-part-ii/. There are somewhat complicated reasons why the saturation hypothesis is a myth. The most important one may be that CO2 absorbs infrared at several wavelengths, and the energy at many of those wavelengths is not yet completely absorbed by CO2, and will not be even at double, triple, or four times the pre-industrial concentration of 280 ppm!

Is there some aspect of global warming that these climate scientists have not thought of already? Climate science, it's their passion AND their business!

We ignore history at our peril.

Carbon Dioxide Lessons

With the Copenhagen meeting in the news, I wanted to give my talented students an up-to-date picture of climate change science. The best available evidence makes deciding what to do more rationale and less emotional.

Examining climate change means understanding the greenhouse effect, and the atmosphere's key greenhouse gas, carbon dioxide (CO2). A greenhouse gas has two essential characteristics. First, it lets short-wavelength sunlight pass through the atmosphere.

This sunlight heats up the Earth's surface, which then sends out its own light energy, in the form of longer wavelength infrared light.

Greenhouse gas characteristic #2 - it absorbs the infrared light from the Earth, preventing this heat energy from passing through the atmosphere, away from the planet. When carbon dioxide absorbs this infrared light, it heats up and emits its own infrared light, most of which heads back towards the Earth. Infrared heat bounces back and forth between the surface and the atmosphere, warming the planet.

Infrared light eventually leaks through the atmosphere back to space. But the more greenhouse gases there are, the longer this takes and the warmer we get.

We depend on the greenhouse effect to keep Earth's temperatures livable. Take away all greenhouse gases and the average temperature would be 60°F colder. But it is the increased greenhouse effect caused by higher than usual carbon dioxide levels and other greenhouse gases that drives our climate change concerns.

Why is CO2 increasing, and by how much? When we burn coal, oil, gasoline, natural gas and even wood, carbon dioxide is a by-product.

Before people started burning fossil fuels there were about 280 parts of CO2 per million parts of other gases in the atmosphere (280 ppm). Today the CO2 concentration is around 387 ppm, higher than it has been for at least 650,000 years, and it continues to rise at an ever-increasing rate, now nearly 2 ppm per year.
The Keeling Curve, showing the record of atmospheric carbon dioxide measurements from the observatory atop the Mauna Loa volcano in Hawaii

Do we know that this atmospheric CO2 comes from burning fossil fuels? Yes. As John Rennie recently pointed out in Scientific American (11/30/2009), human activities now release over 8 billion tons of CO2 each year, while the nearest "natural" source, volcanoes, average less than one-third of a billion tons per year. Moreover, the amounts of different isotopes of carbon atoms found in the atmosphere confirm that the vast majority of this CO2 comes from burning fossil fuels.

Carbon dioxide is not the only greenhouse gas. Water vapor is also transparent to sunlight and absorbs infrared light. As some climate skeptics point out, there is more water vapor in the atmosphere than CO2, so why worry about CO2?

Carbon dioxide absorbs a portion of Earth's infrared light that water vapor cannot absorb, and water vapor cycles in and out of the atmosphere in days, while CO2 molecules can stick around for 100 years. Also, as temperature rises due to carbon dioxide's greenhouse impact, more water evaporates into the air, further increasing the greenhouse effect. In this way water evaporation makes carbon dioxide's global warming impact even greater.

There are still other greenhouse gases - including methane and ozone air pollution. To compare the relative warming caused by the human-generated portion of these gases, imagine Earth covered with football fields, each with an overhead array of light bulb sockets filled with 100 Watt lightbulbs. Carbon dioxide's warming effect would be like turning on 74 such lightbulbs over each football field. Methane adds 21, nitrous oxides and hydrocarbons add 22, and ozone air pollution adds 16.

Accumulated over the planet, the energy increases produced by adding all these 100 Watt lightbulbs warms the Earth significantly more than the background greenhouse effect we depend on for a comfortable life.

How much warmer?

Long-frozen mountain glaciers and the giant ice sheets atop Greenland and Antarctica are melting at increasingly faster rates. Since 1974, 95% of 829 documented physical changes on the planet have been in directions consistent with warming (see Nature, http://www.nature.com/nature/journal/v453/n7193/abs/nature06937.html).

The same study found that in 90% of over 29,500 cases of plants or animals altering the timing of their seasonal cycles or shifting their home ranges, the changes were consistent with warming.

This article appeared in the Raleigh News and Observer on Thursday, December 17, 2009.

Backyard Hawk

Never one to make a definitive statement when there is any doubt, this is likely an immature red-shouldered hawk. He has shown up two mornings in a row now. Yesterday he dared Connie to drive past as he perched proudly on the top of our mailbox post at the head of the driveway. This morning he found the top of the bluebird nestbox at the far end of our backyard. I got the binoculars so Connie could confirm it was the same fellow she saw yesterday. He decided to get a closer look, and flew towards the house, landing on a branch in a large red maple, about 25 feet from our bedroom window. I decided it was time to get the camera and telephoto lens. He (or she?) waited patiently. This is the first of about a dozen shots I got off before it was time to fly. Mr. Hawk seemed to notice me in the window, and may have decided it was not wise to stay in one place too long with a human watching.

Skeptics Spin Doubt, Scientists Study Data

My foot doctor suggested I participate in an experiment that might help relieve my heel pain. He thought my scientific background would lead me to appreciate the opportunity. What it caused me to do was get a second opinion, and politely decline. You see, experiments often do not turn out as hoped. From the scientific standpoint, unexpected, even undesired results increase our knowledge, always a good thing. But it was my foot and my mobility, and I was not interested in taking chances on losing many years of hiking through wonderful woods with my lovely wife.

We are conducting a giant experiment on this planet today. And undoubtedly we will learn much whichever way that experiment turns out. But Earth is, after all, our only home. No other planet helps us grow food, find clean water, or breathe oxygen. We may not be close to losing Earth's life-supporting environment, but the great climate change experiment might not turn out as anyone expects. That's the tricky thing about experiments.

While skeptics spin their tales of doubt, scientists continue to decipher the atmosphere's riddles. The scientists' goal is understanding. The skeptics' goal is uncertainty. Whom would you trust to advise you on the health of your planet?

Recent spins point to temperature data that, if you turn sideways and peer through one eye from an obtuse angle, could look like a global temperature decrease over the past several years. A temperature decrease does not fit with global warming does it? Trouble is, if you turn the other way, peer through the other eye, the decrease goes away (see S. Borenstein, News and Observer, 10/27).

While those profiting from our current energy dependence on fossil fuels would be happy to keep us confused just enough to maintain the status quo a while longer, there exists a host of data that clears away the confusion. This data focuses on the responses of plants and animals involved in this giant experiment.

Ecologists call these plants and animals bioindicators, species whose presence or absence indicates the quality of the environment. The valuable thing about bioindicators is that they give a much better reading of environmental quality than a hundred or a thousand or a million thermometers. Those thermometers can only tell us the temperature at specific locations and specific times. The plants and animals live across an entire range of the environment 24-7. If you want to know what's happening to climate, ask the plants and wildlife.

Scientists have been doing that for years. Skeptics have not thought of that yet. They are too busy talking to you and me and bunches of important folks in Congress.

Birds arrive at their usual time in the spring, but their food source has already come and gone. Trees drop leaves earlier than usual, or their flowers open up weeks earlier than before. Rosenzweig et al. (see Nature magazine, May 15, 2008, Vol.453, pages 353-358) reviewed changes in the observed timing of various life cycle stages over the past 34 years in species ranging from weeds to trees and from molluscs to mammals. They found that 90% of 28,800 measured shifts in timing matched what would be expected if the climate was warming. It appears that plants and animals have not been swayed by the skeptics, and have reached a broad consensus on climate change.

It is time to listen to these plants and animals and the scientists studying climate change instead of to self-appointed critics quick to highlight any detail that might support their case. Time to decide if using Earth in a big experiment is good for our long-term health.

It cannot happen soon enough. The world is meeting in Copenhagen to take the next critical steps tackling climate change. Without firm commitments from both the President and Congress of the United States to lead in solving a problem we did more than our fair share to create, the developing countries of the world can hardly be expected to do their crucial part.

If ever you wanted to help protect this one planet you call home, now is the time, and the U.S. Senate is the place. Sit back and watch the experiment unfold, or step up and do something for the future health of Earth, and everyone who lives on it or will. Contact your Senator, indeed every Senator not sure of the science and let them know that the plants and animals on the planet do not doubt the evidence of global warming. Giant unplanned experiments on the only planet we know of that supports life might not be a good idea.

Anthocyanins in Quercus rubra

Northern Red Oak leaves on 11/14/09 in Cary, North Carolina

As deciduous trees prepare for the winter season, they get ready to drop their leaves. Frost and wind would damage the leaves, and a large number of damaged leaves would rob the tree of water and open it up to infection by bacteria, fungi, and viruses. So as trees prepare to drop their leaves, they produce changes at the base of the petiole where the leaf attaches to the stem. These changes occur in an abscission layer of cells which are easily altered to allow the petiole to detach from the stem and separate the leaf from the tree. The tree also thickens cells along this abscission layer to prevent water loss and pathogen entry through the soon-to-open wound. Once these changes cut off the flow of water to the leaf, the leaf can no longer conduct photosynthesis, and the green pigment chlorophyll, integral to photosynthesis and the green color of leaves, breaks down. As the green chlorophyll dissolves away, the remaining pigments in the leaf may be seen, and in northern red oak, that means anthocyanins. Guess what colors anthocyanins are!