Sunday, January 31, 2010

Sun on Snow

You know how the sunlight shining on fresh snow at a sharp angle can make the snow appear to sparkle? That's what you're looking at in the picture above, believe it or not. If you click on the image to look at a larger version, you should be able to make out different colors, probably a result of diffraction of the sunlight as it passes through tiny ice crystals. Our "snow" actually was sleet, three or four inches of it.

Saturday, January 30, 2010

The Tiniest Air Raid Sirens

These little guys are members of the Globigerinoides, planktonic foraminifera that have lived in the surface waters of the ocean for a very long time. The picture of their shell on the left (about the size of a grain of sand) and the living organism on the right, was found at Oceanus, the online magazine of the Woods Hole Oceanographic Institution. The composition of their shells when they lived millions of years ago contains a warning for us today.

But first, what is a foraminifera? Well, living things can be divided into the prokaryotes and the eukaryotes. The prokaryotes include the bacteria and other single-celled organisms without internal membranes, while the more complex eukaryotes have internal membranes around their nucleus and other organelles. The eukaryotes include multicellular plants and animals, the fungi, and a fourth group of single-celled organisms called the protists which are neither animals nor plants. Foraminifera are protists with shells. Their name derives from the tiny holes (foramina) that perforate their shells.

The shells of the foraminifera consist of calcium carbonate, and the changing ratio of boron to calcium in these shells indicates the concentration of carbon dioxide when they formed. The proportion of a particular isotope of oxygen (δ18O) indicates the temperature when they formed. Aradhna Tripati and her colleagues at UCLA, in work published in Science magazine on December 4th, 2009, measured the boron:calcium ratio and δ18O ratio in Globigerinoides shells over a 20 million year span of time, extending all the way back to the Miocene Epoch.

Up until now, the 800,000 year Vostok ice core record in Antarctica held the oldest measurements of atmospheric carbon dioxide and temperature, using air trapped in bubbles in the ice. This ice core record shows carbon dioxide and temperature closely tracking each other over 800,000 years, powerful evidence that CO2 influences climate. But now, Tripati, using the tiny shells of foraminifera dug up in layers of sediment at the bottom of the ocean, extends that correlation between CO2 and temperature 25 times further back in history, to 20 million years before the present.

The significance of this longer and older record of temperature tracking CO2 levels lies with the ice sheets of the Miocene and Late Pliocene. Starting 20 million years ago and continuing for five million years, the globe was warmer, no massive ice sheets covered Greenland and Antarctica, and sea levels may have been 25 to 40 meters higher than today. Atmospheric carbon dioxide during this time increased from around 375 ppm to 425 ppm as climate continued to warm.

Then, 14 million years ago, CO2 levels began a steady decrease over a five million year span of time from over 400 ppm down to 250 ppm. The climate cooled, closely tracking the carbon dioxide decrease, as ice sheets grew and sea levels likely dropped as much as 40 meters.

The take home lesson - massive ice sheets may not survive on planet Earth when atmospheric CO2 levels exceed 350 ppm for an extended period of time. Carbon dioxide reached 350 ppm back in the mid-1980s, peaked at 390 ppm in 2009, continues to increase more than 1.5 ppm every year, and that rate of increase is growing.

Without massive ice sheets, global sea levels can rise as much as 40 meters. That will not likely happen in this century, but the last time CO2 levels rose from 350 ppm to over 400 ppm, it took a million years to do so, and it happened in the Miocene Epoch at least 12 million years ago. We will see CO2 concentrations reach 400 ppm by 2015, just 30 years after CO2 passed 350 ppm.

A sea level rise of just a couple of meters in the next 100 years would constitute a major worldwide catastrophe.

The chemical make-up of ancient foraminifera shells suggests we may be headed to or may have already reached a level of carbon dioxide in our atmosphere that cannot maintain the large ice sheets now covering Greenland and Antarctica, making the Globigerinoides the tiniest air raid sirens in history. Listen to them.

Sunday, January 17, 2010

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!

Monday, January 11, 2010

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!