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EGU2013: tree spotted in poster halls

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It's not often you see vegetation at the Austria Center Vienna, particularly inside the poster halls. But this year Rolf Hut of Delft University of Technology in the Netherlands positioned one of his research subjects, a tree, next to his poster display.

Whilst an unexpected encounter with plants can be pleasant for conference delegates, for those interested in measuring the moisture content in the top 5 or 6 cm of soil by satellite, vegetation can be a problem. The water it contains may be a source of noise in the radar backscatter signals they need, particularly as plants' water content tends to fluctuate during the day.

And that's where the tree comes in. Hut and colleagues are measuring the natural vibration frequency of trees in order to assess changes in their water content. Assuming the tree has a constant stiffness, any alteration in this frequency indicates a change in mass, and hence water content.

By 'plucking' the tree, which had accelerometers attached to its trunk, Hut was able to show delegates the principle of his technique. The oscillation data provided by the accelerometers enabled calculation of the tree's Eigenfunction, or natural frequency. Leaving the tree in the wind would also see it start to oscillate at its natural frequency, Hut said.

Hut's colleague Bouke Kooreman has been testing the approach in Ghana, where good satellite data are available.

Ultimately, understanding how vegetation water content changes during the day and its effect on radar backscatter could not only help remote soil moisture measurements but also provide a new technique for measuring plant water stress remotely.

Hut, whose work was featured in the session on 'Innovative techniques and unintended use of measurement equipment', has also used the Kinect motion detector for the Xbox 360, which incorporates a 3-D scanner, to improve the accuracy and efficiency of determining in situ soil moisture content.

EGU 2013: climate change hard to reverse

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It's early days, but scientists are developing techniques to remove carbon dioxide from the atmosphere, either directly through technologies such as artificial trees or, less directly, by biomass burning with carbon capture and storage. Even if these methods are implemented, however, the Earth will feel the temperature effects of climate change for centuries to come.

That's according to Andrew MacDougall of Canada's University of Victoria, who gave a press conference at the European Geosciences Union's General Assembly in Vienna. His simulations using the University of Victoria Earth-System Climate Model indicate that without any artificial carbon removal, and assuming that fossil fuels run out, around 60-75% of near-surface warming will remain 10,000 years into the future.

With a middle-of-the-road scenario for carbon dioxide removal, however, a 20th century-like climate could be restored by the late 24th or early 25th century, MacDougall found. But simulated surface air temperature would still be above the pre-industrial temperature by the end of the 30th century, even for the fastest carbon removal scenario he modelled, as oceans gradually release their stored heat.

Restoring climate will require removal of more carbon from the atmosphere than was originally emitted by man, MacDougall said. In some scenarios, 115-190% of anthropogenic emissions will need to be sequestered. Currently land and the oceans remove around half of the carbon dioxide man emits to the atmosphere each year. Once atmospheric carbon levels fall, this stored carbon will start to emerge. In addition, melting of permafrost as temperatures rise has released methane and carbon dioxide to the atmosphere. "There's no easy process to put this back in," said MacDougall.

The simulations indicate that it's much easier to return ocean pH levels to normal than temperatures. But sea-level rise from melting of the Greenland ice sheet seems largely irreversible - while atmospheric carbon levels of less than 350 ppm could stabilise the ice sheet, water from the oceans would only be refrozen into the ice very slowly.

MacDougall simulated carbon concentrations that followed the representative concentration pathways RCP 2.6, 4.5, 6.0 and 8.5 used in the IPCC's forthcoming fifth assessment until they reached their peak, in 2050, 2150 and 2250, respectively. Then he reduced carbon concentrations at the same rate that they had increased, as well as restoring pasture and croplands to their pre-industrial area.


EGU 2013: stormy times ahead

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Thunderstorms are getting stronger and more frequent, according to Eberhard Faust of Munich Re, speaking at a press conference at the European Geosciences Union General Assembly in Vienna.

In 2011 losses from thunderstorms east of the Rockies reached a record value of $47 billion, with two cities hit by outbreaks. For comparison, Hurricane Sandy caused losses of $60 billion.

Together with scientists from the German Aerospace Center (DLR), Faust examined data for severe US thunderstorm losses east of the Rockies from March to September each year from 1970 to 2009. Both the mean level of loss and the variability went up. Some have ascribed this rise to an increase in the value of building stock. But by correcting for socio-economic changes, Faust found that the change was due to altered thunderstorm activity.

Faust sees this increase in thunderstorm activity as due to changes in climate, which have boosted humidity at low levels of the atmosphere and increased seasonal aggregated potential convection energy. These storm changes are consistent with the modelled effects from manmade climate change, he said, but he currently can't make a call on whether they are down to natural climate variability or to man.

To come up with the results, which are published in Weather, Climate and Society, Faust and colleagues looked at thunderstorm severity potential, a measure of the potential energy in the atmosphere available for convection, and the strength and direction of the wind between ground level and 6 km. The team counted occurrences of thunderstorm severity potential of more than 3000 J per kg. The mean value of occurrences increased by a factor of two between1970-1989 and 1990-2009, while standard deviation, a measure of variability, changed by a factor of 1.5.


EGU 2013: Texan wind farms raise temperatures

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When Liming Zhou of SUNY at Albany, US, and colleagues found a link between Texan wind farms and warmer temperatures during summer nights, many argued that the effect was simply because the wind farms were sited on top of mountain ridges. But now, by comparing temperatures above wind farms with those for similar wind-farm-free ridges nearby, Zhou is confident that the raised temperatures he found are caused by operation of the wind turbines.

Speaking at the European Geosciences Union General Assembly in Vienna, Zhou explained how he and his colleagues looked at an area in West-Central Texas containing four of the world's largest wind farms between 2003 and 2011. The average temperature increase about 1.1 km above the wind turbines at night in summer was up to 1 °C, as measured by MODIS kit onboard satellites. During the day, the presence of wind turbines did not seem to affect temperatures. In winter, when the wind turbines were generally operating at lower speeds, the night-time warming effect was less pronounced.

Zhou believes that at night-time the turbulence from wind turbine operation brings warmer air higher in the atmosphere to lower levels. During the day, the atmosphere is much less stable so the wind turbines do not have as great an effect.

Zhou stressed that the temperature effects of the wind farms are small and local, compared to the global temperature changes being caused by burning of fossil fuels. Zhou's latest results, which check out the mountain ridge effect, are in review for publication.

EGU Abstract: Assessing Possible Climatic Impacts of Large Wind Farms Using Satellite Data

EGU 2013: a bumpy ride for transatlantic flights

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If, like me, you're a nervous air passenger, the news from today's European Geosciences Union General Assembly wasn't good. Speaking at a press conference just 15 minutes after the publication of his paper, Paul Williams of the University of Reading, UK, revealed how climate change is likely to bring stronger and more widespread clear air turbulence for transatlantic flights.

A doubling of carbon dioxide concentrations, which could well occur by the 2050s, would increase the average strength of clear air turbulence by 10-40%, Williams and colleague Manoj Joshi of the University of East Anglia, UK, calculated. The amount of airspace containing significant turbulence would also increase by 40-170%; Williams said the most likely figure would be 100%.

To come up with these results, the pair employed the GFDL-CM2.1 climate model to simulate 20 years' worth of data for pre-industrial and doubled carbon dioxide concentrations, using 21 separate measures of turbulence. They analysed clear air turbulence during the winter, when it's at its most intense, along the North Atlantic flight corridor, one of the busiest in the world, with 300 flights in each direction each day.

This turbulence isn't just a problem for scaredy-cats. It can injure, or even kill, passengers and aircrew, it can damage planes, for example breaking off engines or parts of the wing, and it currently costs society about $150 million each year in injuries, damages and investigations. As a result of the additional turbulence, airlines may have to reroute flights, boosting fuel consumption, increasing air pollution, and potentially causing delays and increasing ticket costs.

So why the increase? As climate changes, the atmosphere is warming above our heads as well as at ground level, explained Williams, leading to higher wind speeds. Or, to put it another way, a stronger jetstream is destabilising the atmosphere and the random fluctuations in upwards and downwards winds push against aircraft wings.

The atmosphere strikes back

Worse still, unlike the turbulence caused by clouds or storms, clear air turbulence is hard to detect. You can't see it, and satellites or aircraft electronic systems don't pick it up. Some ground-based radar systems can detect it, but only the very powerful ones, said Williams, as could radiosondes on balloons, which measure the amount of wobble along a single trajectory.

In their calculation of the amount of airspace containing significant turbulence, Williams and Joshi used the industry definition of moderate-or-greater turbulence, which produces an acceleration in the plane of 5 m/s2 or more, a force equivalent to half a G. This is enough to bounce the aircraft around, make it hard to walk and knock over drinks, said Williams.

"Aviation is partly responsible for changing the climate in the first place," he said, in a University of Reading press release. "It is ironic that the climate looks set to exact its revenge by creating a more turbulent atmosphere for flying."

The study is published in Nature Climate Change.

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