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| Sunday, 25 September 2005 |
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Does climate change make hurricanes fiercer? As Hurricane Katrina steamed forward on Thursday, Aug. 25, residents of the southeastern U.S. shore breathed sighs of relief.
The storm passed Miami as a weak hurricane, rating as only a category 1 storm on a scale from 1 to 5. But within days, relief turned to alarm, amid warnings from forecasters that the worst might be yet to come. The storm sucked energy from the warm Gulf of Mexico waters as it moved west, swelling into a category 5 monster and then weakening only slightly before it slammed into the Mississippi shore as a category 4 hurricane. Abundant rain and a surge of ocean water overwhelmed flood-control measures and broke levees at Lake Pontchartrain, deluging New Orleans with up to 20 feet of water and plunging the city into mayhem. Katrina's ferocity left many people asking whether the monster storm came from mere chance or from something more long lasting-global warming. Although hurricane numbers and intensities are known to vary naturally, with some years producing many violent hurricanes and others hardly any, Hurricane Katrina isn't the only exceptionally destructive event in recent memory. In the tropical Atlantic, moreover, hurricane numbers have been on the uptick since 1995, according to the National Oceanic and Atmospheric Administration (NOAA). In 2004, Florida suffered its worst hurricane season in 118 years, with nine hurricanes, five of which were classified as major. For 2005, NOAA's forecast predicted yet another above-average hurricane season for the region. Scientists are divided on whether climate change, induced by industrial and automotive release of carbon dioxide and other greenhouse gases, is driving these statistics. Most climate scientists say that natural, cyclic phenomena that affect ocean currents and atmospheric temperature-such as El Nio in the Pacific Ocean and the North Atlantic Oscillation-yield decade-to-decade swings in total hurricane numbers that have nothing to do with global warming. Some researchers say that these phenomena are also responsible for all the observed changes in storm intensity. But many other climate scientists are now pointing to global warming as the culprit for increasingly ferocious hurricanes worldwide. Both scientific theory and computer modelling predict that as human activities heat the world, warmer sea-surface temperatures will fuel hurricanes, increasing wind speeds and rainfall. Now, several new studies suggest that climate change has already made hurricanes grow stronger. Many scientists predict that such an increase in storm violence will have consequences for coastal communities. Hurricanes gain their destructive power from ocean moisture and heat. As the sea and atmosphere warm, more water evaporates from the ocean surface. When that moisture reaches the cool upper atmosphere, it condenses, releasing the energy that originally went into evaporating it. This 'latent heat' powers the growing storm, says meteorologist Tom Knutson of NOAA's Geophysical Fluid Dynamics Laboratory in Princeton, N.J. How warm the sea surface gets and how high into the atmosphere the evaporated water climbs set a speed limit on hurricane winds, says Kerry Emanuel of the Massachusetts Institute of Technology in Cambridge. In 1987, Emanuel predicted that with global warming, this speed limit would rise and that hurricanes would rev up their engines. "If the climate warms, hurricanes have the potential to become substantially more intense," agrees Knutson. He and Robert E. Tuleya of Old Dominion University in Norfolk, Va., have used computer models to simulate how hurricanes would change in a warming world. If the atmospheric concentration of carbon dioxide, the greenhouse gas most responsible for global warming, doubles in the next 80 years, hurricanes' wind speeds will rise by about 5 percent, the researchers predicted in the Sept. 15, 2004 Journal of Climate. Moreover, with the increase in atmospheric moisture that accompanies global warming, hurricane rainfall will increase by about 18 percent, Knutson and Tuleya calculate. But in practice, changes in rainfall within a hurricane are hard to pick out. Hurricanes pour out rain in localized outbursts, but rain gauges tend to be widely dispersed and often miss the main downpour, Emanuel notes. Also, most hurricanes don't strike land, where rainfall can be tallied. "It's a hopeless measurement problem," he says. Pavel Groisman of the National Climatic Data Centre in Asheville, N.C., says that his work and that of others show no measurable change in the total rain dumped by hurricanes. "When we have very strong hurricanes, we do not see changes in intensity of precipitation," he reports. The increase of just 5 percent in hurricane intensity predicted by Knutson and Tuleya led many researchers to suggest that the variability attributable to El Nio or the North Atlantic Oscillation would dwarf any change resulting from global warming, at least for the next few decades. "What folks in the field thought was, we weren't going to see any global warming and hurricane association for decades to come," says Christopher Landsea of NOAA's Atlantic Oceanographic and Meteorological Laboratory in Miami. Researchers have recently discerned, however, storm-intensity trends that correlate with global warming. ############# Shocked metals are stronger Sudden shocks make you harder. At least, they can do if you're a metal. A team of researchers in the United States and Switzerland hope that this discovery could point the way to ultra-hard metals for engineering in extreme environments, such as nuclear fusion reactors. The team simulated the atomic-scale structure of a slab of copper, made up of a patchwork of grains about 20 nanometres (millionths of a millimetre) across. Their study showed that the material becomes harder and stronger after a shock wave has passed through. An explosion could produce such a shock, suggest the team, led by Eduardo Bringa at the Lawrence Livermore Laboratory in California. Ultra-hard metals are needed not just for military armour but for applications such as nuclear fusion. Researchers are looking into initiating fusion reactions using laser blasts, and very strong materials are needed to contain these reactions. Metals are a patchwork of grains stuck together. These materials bend and deform when misalignments of rows of atoms, called dislocations, slip through a grain. This allows the material to adapt itself to different shapes when under stress, making the metal soft. Smaller grains make for harder metals, because dislocations tend to get stuck when they reach the edge of a grain. So in grains just a few tens of nanometres across, dislocations can move travel a very small distance, limiting how much the material can change shape. But there is a limit to the strengthening effects of shrinking crystalline grains. If stressed far enough, the grains themselves may slip and slide against each other, deforming the material. Bringa and colleagues sought to stop these slips by investigating what happens when one subjects a metal with nanoscale grains to sharp shocks. A shock wave creates a very high pressure over a very narrow region as it travels through a material. As the region of stress is on the same size scale as a grain itself, the pressure can't force grains to slide over each other. Instead, the material can only accommodate the deformation by dislocations appearing within the grains. But this time, that hardens the metal, they report in Science1. The dislocations produce kinks on the grain edges that knit them together, providing extra strength. "The dislocations hook the grain boundaries a little bit and stop them sliding," says team member James McNaney of the Lawrence Livermore. "That way, we can take nanocrystalline metals even further along the hardening path than people thought we could." The researchers have preliminary evidence that the same thing happens in real life, as well as simulations. A piece of nanocrystalline nickel, after being subjected to an explosive shock, becomes peppered with dislocations inside the grains. "Normally you never see that, because the grains slide first," says McNaney. The team hasn't tested the strength of their shocked nickel, because such measurements are tricky with such tiny amounts of material. ############# Cassini probe spies spokes in Saturn's rings The Cassini spacecraft orbiting Saturn has finally spotted spokes cutting across the planet's rings, a phenomenon astronomers have long hoped their plucky orbiter might find. While flying past the dark side of Saturn's B ring, Cassini's camera eye photographed the spokes - which appear as radial markings - in a series of three images taken over about 27 minutes. The find is a gem of sorts for mission imaging scientists, who have been hunting for the ring spokes since Cassini arrived at Saturn. "We've been on the lookout for them since February, 2004," said Carolyn Porco, Cassini imaging team leader at the Space Science Institute in Boulder, CO, of the spokes. "Spokes are one of those Saturn-system phenomena that we are keenly interested in understanding." Saturn's odd ring spokes were photographed during NASA's Voyager mission, which swung passed the planet in the 1980s, and later observed by astronomers using the Hubble Space Telescope. But spokes were noticeably absent when Cassini made its final approach toward Saturn in February 2004, and are a prime target for astronomers because of their role and formation within the planet's rings are not fully understood. "These are among the things we hope to learn," said Porco, who participated in the Voyager mission as well. "[The spokes] are obviously related to a host of processes...and may point to some important effects in understanding the magnetic field and the planet's magnetosphere, and how these systems interact with the rings and atmosphere." Porco and her imaging team did not initially expect to observe ring spokes until about 2007, when certain models predicted spoke formation and visibility. "Well, in some sense we should have expected, if the recent models are correct, to see them on the dark side where the photoelectron abundance is low," Porco said of the spokes. "So, I was surprised to see them. But once they showed up, I realized we should have expected them there all along." While the images were released on Sept. 13, Cassini actually photographed the ring spokes on Sept. 5, 2005, using clear filters and its wide-angle camera from a distance of about 198,000 miles (318,000 kilometers) from Saturn. The spokes themselves are fairly faint, and are about 60 miles (100 kilometers) wide and 2,200 miles (3,500 kilometers) long, researchers said. Unlike Voyager or Hubble, Cassini is in a unique position to study ring spoke phenomena at Saturn, Porco said. "Remember, Voyager was just a flyby, Cassini is in orbit," Porco said, adding that Cassini is a vastly superior observation platform when compared to Voyager. "We have the opportunity for monitoring them and their behavior, their comings and goings, how they evolve, when they appear and disappear." By observing the spokes on the dark side of Saturn's rings, Cassini recreated a bit of space exploration history. Its predecessor, Voyager, also first observed the ring spoke phenomena while photographing the unilluminated side of the Saturn's rings. "It felt like the old days, when we first saw the spokes," Porco said. "They are one weird phenomena and it was a joy to see them again...especially since we hadn't seen them yet and were eager to know why." ########### 'Better' DNA out of fossil bones Improved technologies for extracting genetic material from fossils may help us find out more about our ancient ancestors.
Scientists in Israel have just developed a new technique to retrieve better quality, less contaminated DNA from very old remains, including human bones. It could aid the study of the evolution and migration of early modern humans, as well as extinct populations such as our close relatives, the Neanderthals. Many researchers would dearly love to get their hands on DNA samples from hominids further back in time-from those that lived 100,000 years ago or more-to find out how they were related to people alive today. But fossil studies this far back in time have long been hindered by contamination with foreign genetic material and the problem of recovering long, intact DNA sequences. The new method provides hope, however. "DNA gets everywhere. So when we're dealing with a sample and you find it's got human DNA in it - is that DNA from the fossil, or is it actually DNA from the person who unearthed it?" says Professor Chris Stringer, the head of human origins at the Natural History Museum in London, UK. Also, DNA falls apart over the course of time. "It breaks up into very small fragments so it is quite technically complicated to put it all back together again," explains Dr Robert Foley, the director of the Leverhulme Centre for Human Evolutionary Studies at the University of Cambridge, UK. An improved technique for retrieving DNA from fossil bone, just published in the journal Proceedings of the National Academy of Sciences (PNAS), may help. Dr. Michal Salamon, from the Weizmann Institute of Science in Rehovot, Israel, and colleagues, showed that "crystal aggregates", small mineral pockets formed during fossilisation, can preserve DNA better than the rest of the bone. They compared DNA extracted from these crystal aggregates with genetic material taken from untreated, whole-bone powder. The samples were taken from eight different modern and fossil bones. They found better preserved, less contaminated DNA could be recovered from the isolated crystals. This approach, significantly improves the chances of obtaining authentic ancient DNA sequences, especially from human bones. Commenting on the latest research, Dr. Michael Hofreiter, from the Max Planck Institute for Evolutionary Anthropology in Leipzig, Germany, who helped decode 40,000-year-old nuclear DNA from a cave bear earlier this year, said: "It's possible; but there need to be more studies on more samples, and they need to show that you don't get human contamination of animal bones. Then I would believe that it is a breakthrough for ancient DNA research. Scientists are hopeful the new technique will help them get at the DNA in the chromosomes of a cell - the nuclear DNA. Ancient DNA research has so far mainly focused on mitochondria, the tiny "power-stations" of the cell. These exist outside of the nucleus and have their own DNA. And, although this information is very useful, it is more limited in its scope than that which could be obtained from nuclear DNA. It is partly a question of sensitivity. "There's about 1,000 times more mitochondrial DNA than nuclear DNA in our cells, so it's much easier to pick up," explains Professor Stringer. The mitochondrial DNA is inherited only through the egg - through females. This means it is a useful marker for tracing a line back into the past, as it has never been mixed with DNA from males. "One of the most important discoveries from studying ancient mitochondrial DNA is the estimate of when humans diverged in evolution from the Neanderthals - around half a million years ago," according to Dr. Foley. Professor Stringer adds: "We've now got about 10 Neanderthal specimens of around 40-50,000 years old that have yielded DNA that is clearly distinct from anyone alive today." This means scientists can be sure that it is ancient, not just modern DNA from contamination. It has also given them a measure of how different Neanderthals were from modern people. ############# Inchworm-like robot smallest ever Researchers have built an inchworm-like robot so small you need a microscope just to see it. In fact about 200 hundred of them could line up and do the conga across a plain M&M. The tiny bot measures about 60 micrometers wide (about the width of a human hair) by 250 micrometers long, making it the smallest untethered, controllable microrobot ever. "It's tens of times smaller in length, and thousands of times smaller in mass than previous untethered microrobots that are controllable," said designer Bruce Donald of Dartmouth University. "When we say 'controllable,' it means it's like a car; you can steer it anywhere on a flat surface, and drive it wherever you want to go. It doesn't drive on wheels, but crawls like a silicon inchworm, making tens of thousands of 10-nanometer steps every second. It turns by putting a silicon 'foot' out and pivoting like a motorcyclist skidding around a tight turn." Because it makes use of this innovative bending movement and is untethered, it can move freely across a surface without the wires or rails that restricted the mobility of previously developed microrobots. The caterpillar strategy also helped the researchers avoid a common problem in microrobotics. Machines this small tend to stick to everything they touch, the way sand sticks to your feet after a day at the beach," said Craig McGray of the National Institute of Standards and Technology. "So we built these microrobots without any wheels or hinged joints, which must slide smoothly on their bearings. Instead, these robots move by bending their bodies like caterpillars. At very small scales, this machine is surprisingly fast." To get around, the robot makes use of two independent microactuators - the robot's "muscles." One is for forward motion and the other for turning. It doesn't have pre-programmed directions. Instead, it reacts to electric changes in the grid of electrodes it moves on. This grid also supplies the microrobot with the power needed to make these movements. This microrobot and similar versions that could be developed might eventually ensure information security, inspect and make repairs to integrated circuits, explore hazardous environments, or even manipulate human cells or tissues. |
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