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Geosciences Column: Do coastlines have memories?

do coastlines have memories

Did you know that the shape of coastlines is determined by the angle at which waves crash against the shoreline. It has long been thought that fluctuations in the wave incidence angle are rapidly felt by coastlines, which change the shapes of their shores quickly in response to shifting wave patterns.

Or do they?

Researchers at the British Geological Survey, Duke University (USA) and Woods Hole Oceanographic Institution in Massachusetts, have performed experiments which show that spits and capes hold ‘a memory’ of their former shapes and past wave climates, influencing their present geomorphology. The findings have recently been published in the EGU’s open access journal Earth Surface Dynamics.

Gradients in sediment distribution within wave-driven currents and shoreface depth play an important role in shaping coastlines. But the angle between an offshore wave crest and the shoreline is chief among the parameters which shape coasts worldwide.

Low-angle waves – those with approach the coast at an angle of 45° or less – have a smoothing effect on the coastline and keep its shape relatively steady. On the other hand, high-angle waves – those with slam against the shore at an angle of 45° or more – introduce instability and perturbations which shape the coast.

The figure shows the experimental set-up used in the study. It also nicely illustrates how coastlines are shaped by the angle of the incoming wave. The arrows indicatenet flux direction under waves incoming from the left; arrow lengths qualitatively indicate the flux. Sand is not transported through cells which are in shadow for a particular wave. From C. W. Thomas et al., 2016.

Alterations to the patterns of shorelines are caused by enhanced erosion and/or deposition, driven by changes in wave climate. Ultimately, coastline geomorphology evolves depending on the relative degree of high and low-angle waves in the wave climate, as well as the degree of irregularity in the wave angle distribution.

Climate change will alter the wave climate, particularly during storm events, so we can expect shorelines to shift globally. Predicting how coastlines will adapt to changing climatic conditions is hard, but more so if coastlines retain a memory of their past shapes when responding to changing wave regimes.

Flying spits (finger-like landforms which project out towards sea from relatively straight shoreline) and cuspate capes (a triangular shaped accumulation of sand and shingle which grows out towards sea) are particularly susceptible to climate change. They form when high angle waves approach the shore at a slant. Animal communities living within fragile marine and estuarine ecosystems largely depend on the protection they offer. They are also of socio-economic importance as many shelter coastal infrastructures. Understanding how they will be affected by a changing climate is vital to develop well-informed coastal management policies.

To understand how changing wave climates affect the evolution of flying spits and cuspate capes (from now on referred to as spits and capes), the team of researchers devised experiments which ran on a computer simulation.

They generated an initially straight shoreline and set the wave conditions for the next 250 years (which is the length of time it takes in nature) to allow the formation of spits and capes.

To test whether pre-existing coastal morphologies played a role in shaping coastlines under changing wave climates, over a period of 100 years (which is loosely the rate at which climate change is thought to be occurring under anthropogenic influences), the scientists gradually changed the angle at which waves approached the coast.  After the 100 year period the simulation was left to run a further 650 years under the new wave conditions.

The investigation revealed that when subjected to gradual changes in the angle at which waves approach the shoreline, capes take about 100 years to start displaying a new morphology. The tips of the capes are eroded away and so they slowly start to shrink.

Spits adjust to change much more slowly. Even after 750 years the experimental coastlines retain significant undulations, suggesting that sandy spits retain a long-term memory of their former shape.

Snapshots of simulated coastline morphologies evolved under changing wave climate. U is the fraction of waves which are approaching the shoreline at 45 degress or higher. Coastlines evolved for 250 years under initial conditions. (aii, bii)> The U values of the changed wave climate show the coastline morphologies evolved 200 and 500 years after the wave climate is changed at 250 years, and the morphologies evolved over 1000 years under static wave climates with the same U. From C. W. Thomas et al., 2016. See paper for full image caption. Click to enlarge.

The implications of the results are far reaching.

Be it implicitly or explicitly, many studies of coastal geomorphology assume that present coastal shape is exclusively a result of present wave climate. The new study shows that even with steady wave climate conditions at present, coastline shapes could still be responding to a past change in wave climate.

Reconstructions of ancient coastal geographies and paleo-wave climates might also be approached differently from now on. The researchers found that as spits adjust to changing wave climates they can leave behind a complex array of lagoons linked by beach bridges. Though there are a number of process which can lead to the formation of these coastal features, researchers must also consider alterations of coastlines as a response to changing wave climate from now on.

The findings of the study can also be applied to the management of sandy coastlines.

Currently, forecasts of future shoreline erosion and sediment deposition are made based on observations of how coasts have changed in recent decades. The new study highlights these short observation timescales may not be enough to fully appreciate how our beaches and coasts might be reshaped in the future.

This is especially true when it comes to climate change mitigation. Decisions on how to best protect the world’s shores based on their environmental and socio-economic importance will greatly benefit from long-term monitoring of coastal geomorphology.

But more work is needed too. The experiments performed by the team only consider two types of coastline morphology  (spits and capes) and only two types of wave climate. While the experiments provide a time-scale over which spits and capes might be expected to change, other factors not considered in the study (wave height, shoreface depth, etc…) will alter the predicted timescales. The time-scales given by the study should be used only as a guideline and highlight the need for more research in this area.

 

By Laura Roberts Artal, EGU Communications Officer

 

References

Thomas, C. W., Murray, A. B., Ashton, A. D., Hurst, M. D., Barkwith, A. K. A. P., and Ellis, M. A.: Complex coastlines responding to climate change: do shoreline shapes reflect present forcing or “remember” the distant past?, Earth Surf. Dynam., 4, 871-884, doi:10.5194/esurf-4-871-2016, 2016.

Geosciences Column: The complex links between shrinking sea ice and cloud cover

Sea ice breaking on the Chukchi Sea, Barrow, July 2014

The global climate system is complex. It is composed of, and governed by, a plethora of interconnect factors. Solar radiation, land surface, ice cover, the atmosphere and living things, as well as wind and ocean currents, play a crucial role in the climate system. These factors are intricately connected; changes to some can have significant effects on others, leading to overall consequences for the global climate.

Since the 1980s, sea ice has been decreasing gradually as a result of global warming. But the impact of retreating sea ice on the global climate system aren’ t yet fully understood. A new study published in the EGU’s open access journal, Atmospheric Chemistry and Physics, attempts to unravel the complex feedback systems between Arctic sea ice extent and cloud cover in the region.

The researchers, lead by Manabu Abe, of the Institute of Arctic Climate and Environmental Research in Japan, argue that shrinking sea ice extent in the Arctic is the cause for increased cloud cover in the region. This, in turn, further enhances the feedback processes of Arctic warming because it cause sea ice to retreat further.

Sea ice, is the ice that ‘grows’ as water in the poles is exposed to very low temperatures over long periods of time. Although some waters are covered by ice year round, most sea ice forms during the cold winter months and melts in the summer.

Global climate influences the annual growth of sea ice. This year, ocean waters in the Arctic are failing to freeze and sea ice isn’t forming as quickly as it normally would. Alarmingly, October 2016 registered the lowest sea ice extent since records began.

Scientists think that the unusually low amount of sea ice formed in the Arctic this year is the result of extraordinarily hot sea surface and air temperatures, which are essentially stopping the formation of ice on ocean waters.

But sea ice extent also influences global climate. Solar radiation is absorbed and reflected by the Earth’s atmosphere (including clouds) and surface. Ice is more reflective than water and land. So as ice cover across the globe decreases, so does the planet’s ability to reflect solar radiation, causing the Earth’s surface to warm further, which, in turn, causes more melting of ice. This is know as the ice-albedo feedback loop.

The effects of shrinking sea ice are not limited to surface warming. Ocean heat uptake and storage can be affected, as can be the formation of low-level cloud cover over the Arctic. While the surface of clouds reflect solar radiation, they also prevent heat from being lost from the Earth’s surface. That’s why, often, on overcast nights temperatures are higher than on clear nights.

A study back in 2012, proposed that increased cloud cover in the Arctic enhanced the radiation emitted by the atmosphere and clouds – known as longwave radiation (DLR) -, causing higher surface air temperatures in autumn. This would extend the sea ice melting season. But there is little data which measures radiation at the surface, making the claim controversial.

Other studies have used computer simulations of the global climate, to mimic the effects of reduced sea ice conditions on cloud cover. They show that the areas of open ocean created by the reduction in sea ice mean more moisture is transported from the ocean to the atmosphere, resulting in the formation of more clouds. But the simulations are not very good at representing polar clouds and so the results aren’t entirely reliable.

Now, Abe and his coworkers, used a new state-of-the-art climate simulation to try and shed light on the problem. They included data from as far back as 1850 in their study, as well as making it more robust by taking into account other factors, such as changing sea surface temperatures, greenhouse gases, aerosols and land use (from the 1980s to 2005), which might affect the formation of clouds.

Geographical map of the simulated linear trend in the total cloud cover (shaded) and sea ice concentration (contours) in (a) September, (b) October, and (c) November during the period 1976–2005. The units are decade. From M.Abe at al., 2016

Geographical map of the simulated linear trend in the total cloud cover (shaded) and sea ice concentration (contours) in (a) September, (b) October, and (c) November during the period 1976–2005. The units are decade. From M.Abe et al., 2016

The new simulation found that between 1976 and 2005, Arctic sea ice decreased through the summer and autumn months (which is corroborated by satellite observations). Meanwhile, cloud cover increased throughout autumn, winter and spring, reaching its peak in October.

The researchers argue that the link between the two trends is not coincidental. Reduced sea ice extent in the autumn months,coupled with a decrease in atmospheric temperatures, means more heat is exchanged from the oceans to the atmosphere, which fuels the formation of clouds. More clouds mean downwards longwave radiation (DLR) in October is increased by as much as 40 to 60% (compared with clear autumn skies). With less heat being reflected off the surface of the Earth, sea ice extent decreases further due to melting and so a feedback loop (not dissimilar to the ice-albedo loop) is established.

The results reinforce the findings of previous studies, but some questions remain unanswered. The scientists point out that, it is not only important to understand how much cloud cover increases by as a result of shrinking sea ice extent. In a warming climate, how increases in air temperature and humidity affect the vertical structure of clouds will play an important role in the sea ice-cloud feedback loop. The vertical profile of a cloud also strongly influences how and how much DLR is reflected back on the Earth’s surface, so there is a need for a better understanding of the feedback processes related to clouds too.

By Laura Roberts Artal, EGU Communications Officer

References

Abe, M., Nozawa, T., Ogura, T., and Takata, K.: Effect of retreating sea ice on Arctic cloud cover in simulated recent global warming, Atmos. Chem. Phys., 16, 14343-14356, doi:10.5194/acp-16-14343-2016, 2016.

Wu, D.L., and Lee, J.N.:Arctic low cloud changes as observed by MISR and CALIOP: Implication for the enhanced autumnal warming and sea ice loss, J. Geophys. Res.-Atmos., 117, D07107, doi:10.1029/2011JD017050, 2012

GeoSciences Column: The ‘dirty weather’ diaries of Reverend Richard Davis

GeoSciences Column: The ‘dirty weather’ diaries of Reverend Richard Davis

Researching the Earth’s climate of the past, helps scientists make better predictions about how the climate and our environment will continue to be affected by, change and adapt to rising temperatures.

One of the most invaluable sources of data, when it comes to understanding the Earth’s past climate, are historical meteorological records.

Accounts of weather and climate conditions for the Southern Hemisphere, prior to the 1850s, are particularly sparse. This makes the recently discovered, painstakingly detailed and richly descriptive weather diaries of a 19th Century missionary in New Zealand, incredibly valuable.

Researchers from the National Institute of Water and Atmospheric Research, In Auckland (New Zealand), poured over the contents of the diaries, which provide an eyewitness account to the end of the Little Ice Age (between 1300 and the 1870s winter temperatures – particularly in the Norther Hemisphere- were lower than those experienced throughout the 20th Century). The journals reveal that 19th century New Zealand experienced cooler winter temperatures and more dominant southerly winds when compared to the present day climatic conditions. The researchers present these and other findings in the open access journal of the EGU, Climate of the Past.

Print of a photomechanical portrait of Reverend Richard Davis taken ca. 1860, from the file print collection, Box 16. Ref: PAColl-7344-97, Alexander Turnbull Library, Wellington, New Zealand, sourced from http://natlib.govt.nz/records/23073407 (From A. M. Lorrey et al., 2016).

Print of a photomechanical portrait of Reverend Richard Davis taken ca. 1860, from the file print collection, Box 16. Ref:
PAColl-7344-97, Alexander Turnbull Library, Wellington, New Zealand, sourced from http://natlib.govt.nz/records/23073407 (From A. M. Lorrey et al., 2016).

The diaries were kept by Reverend Richard Davis, born in Dorset (England) in 1790. The Reverend was associated to the Church Mission Society (CMS) of England; an connection which lead him to settle in the blustery Northland Peninsula in the far north of New Zealand, back in 1831.

From 1839 to 1844, and then again from 1848 to 1851, Davis collected over 13,000 meteorological measurements and made detailed notes about the condition of the local environment.

The Reverend’s collection of data is remarkable, not only for its detail, but also because it is the earliest record of land-based meteorological measurements from New Zealand found to date.

He took twice daily temperature measurements – one at 9 a.m. and one at 12 noon – as well as noon pressure measurements. Qualitative observations included information about wind direction and strength, as well as detailed cloud cover descriptions, and notes on the occurrence of hail, frost, rainfall, snowfall, thunderstorms, lightning, sunsets and behavior of wildlife.

The journals also reveal that the Reverend was not keen on particularly gloomy days, when the winds were strong and blustery, cloud cover hung low and was often accompanied by rain.  On 67 separate occasions, Davis’ used the term “dirty weather” to described days like this.

It is important to assess the reliability of the measurements taken by Davis before drawing comparisons between 19th and 20th Century weather patterns for the island of the long white cloud; and especially if the data are to be integrated within past climate and weather reconstructions for New Zealand and the Southern Hemisphere.

To do so, the Auckland based researchers, compared the Reverend’s pressure measurements with observations made by ships travelling through New Zealand waters or stationed on the island (usually completing military operations), during the same time interval. The Reverend’s daily pressure observations are regularly lower (on average by -0:64 ± 0:10 inches of mercury) than those taken on board the ships. The offset is consistent with the change in altitude between the ships anchored in harbour versus the land-based measurements made by Davis; meaning the Reverend’s pressure measurements are robust.

Reverend Richard Davis pressure observation vs. expedition measurements (leader noted in parentheses) from USS Vincennes (Wilkes), the corvettes Astrolabe and Zelee (d’Urville) and the HMS Erebus (Ross). There are 29 pairs of daily observations and so the x axis simply shows the comparisons of Davis’ record to the three ships in a sequence with the specific intervals noted. (From A. M. Lorrey et al., 2016).

Reverend Richard Davis pressure observation vs. expedition measurements (leader noted in parentheses) from USS Vincennes (Wilkes), the corvettes Astrolabe and Zelee (d’Urville) and the HMS Erebus (Ross). There are 29 pairs of daily observations and so the x axis simply shows the comparisons of Davis’ record to the three ships in a sequence with the specific intervals noted. (From A. M. Lorrey et al., 2016).

There is no similar test which would verify the accuracy of Davis’ temperature measurements. However, the researchers argue that the (expected) annual cycles evident in his measurements, as well as the reliability of his other records mean that, at least some, of his readings are faithful to the local conditions. Not only that, Davis’ mean winter temperature anomalies are comparable to the temperatures reconstructed from tree rings and can be used by the researchers to gather information about the local atmospheric circulation at the time.

When compared to modern-day temperature measurements (from the Virtual Climate Station Network, VCSN), the journal data reveals that mid 1800s winters, at the far north of the island, were cooler. At present, atmospheric circulation over Northland means winds from the southwest are common, especially during the winter and spring. During the summer, easterly winds become dominant. There is a higher frequency of records of south and southwesterly winds in Davis’ diaries. Reconstructions of atmospheric flow over New Zealand in the 1800s, made with proxy tree-ring and coral data, also point towards more frequent south and southwesterly winds and cooler temperatures.

Not only that, the timing of monthly and seasonal climate anomalies, recorded both in tree-ring and the Davis diary data suggest that El Niño-Southern Oscillation (ENSO)-like conditions existed, in New Zealand, during the 1839-1851 time period. However, more work (and data) is needed in Australasia to corroborate the findings and define the extent of the ENSO conditions at the time.

With more data, better reconstructions of the atmospheric conditions in the southwest Pacific and Southern Hemisphere can be made. Combined with the newly found Davis’ records, these will make an important impact to the understanding of past weather and climate in the region.

By Laura Roberts Artal, EGU Communications Officer

The Waimate North mission house in the Far North of New Zealand where Davis lived (From: A. M. Lorrey et al., 2016).

The Waimate North mission house in the Far North of New Zealand where Davis lived (From: A. M. Lorrey et al., 2016).

References

Lorrey, A. M. and Chappell, P. R.: The “dirty weather” diaries of Reverend Richard Davis: insights about early colonial-era meteorology and climate variability for northern New Zealand, 1839–1851, Clim. Past, 12, 553-573, doi:10.5194/cp-12-553-2016, 2016.

Geosciences column: Making aurora photos taken by ISS astronauts useful for research

Geosciences column: Making aurora photos taken by ISS astronauts useful for research

It’s a clear night, much like any other, except that billions of kilometers away the Sun has gone into overdrive and (hours earlier) hurled a mass of charged particles, including protons, electrons and atoms towards the Earth.  As the electrons slam into the upper reaches of the atmosphere, the night sky explodes into a spectacular display of dancing lights: aurora.

Aurora remain shrouded in mystery, even to the scientists who’ve dedicate their lives to studying them. Photographs provide an invaluable source of data which can help understand the science behind them. But, for aurora images to be of scientific value researchers need to know when they were taken and, more importantly, where.

You’ve got to be in the right place at the right time to catch a glimpse of the elusive phenomenon. In the Northern Hemisphere, aurora season peaks in autumn through to winter. Geographically, the best chance of seeing them is at latitudes between 65 and 72 degrees – think the Nordic countries.

That is unless you are an astronaut on the International Space Station (ISS), in which case, you’ve got the best seat in the house!

The orbit of the ISS means it skims past the point at which aurora intensity is at its peak, which also happens to be the point at which they look their most spectacular. Its orbital speed means it can get an almost global-scale snapshot of an aurora, passing over the dancing lights in just under 5 minutes.

Not as much is known about Aurora Australis (those which occur in the southern hemisphere) as we do about the Northern Lights (visible in the northern hemisphere), because there are far less ground-based auroral imagers south of the equator. The ISS orbit means that astronauts photograph Aurora Australis almost as frequently as Aurora Borealis, helping to fill the gap.

Testament to the privileged viewpoint is the hoard of photographs ISS astronauts have amassed over time – perfect for scientists who study aurora to use in their research.

Time-lapse shot from the International Space Station, showing both the Aurora Borealis and Aurora Australis phenomena. Credit: NASA

Except that, until recently, the ISS photographs were of little scientific value because they aren’t georeferenced. The images are captured by astronauts in their spare time using commercial digital single lenses reflector cameras (DSLRs), which can’t pinpoint the location at which the photographs were taken – they were never intended to be used in research.

Now, researchers at the European Space Agency (ESA) have developed a method which overcomes the problem. By mapping the stars captured in each of the photographs and the timestamp on the image (as determined by the camera used to take the photograph), the team are now able to geolocated the images, giving them accurate orientation, scale and timestamp information.

Despite the success, it’s not a straightforward thing to do. One of the main problems is that the timestamps aren’t always accurate. Internal clocks in DSLRs have a tendency to drift. Over the period of a week they can be out by as much as a minute, making it difficult to establish the location of the ISS when the image was captured. This has implications when creating the star map, as the location of the station is used as a starting point.

To resolve the issue, aurora images which also include city lights can be aligned to geographical maps using reference city markers to get a timestamps accurate to within one second or less. In the absence of city lights, images which also capture the Earth’s horizon are aligned with its expected position instead. The correction works best if both city lights and the horizon can be used.

Errors are also introduced when the star maps can’t be fully resolved (due to the original image being noisy, for example) and because the method assumes that auroras originate from a single height, which isn’t true either.

detailed comparison between the ISS image plotted in Fig. 11 (b) and the contemporaneous image acquired by the SNKQ THEMIS ASI (a) . The original ISS image is plotted in (c) . Red and blue symbols trace the locations of the j shaped arc and northern edge of the main auroral arc, respectively, derived from their locations in the THEMIS image. The features are marked with the same coloured arrows in (c) . The magenta arrows point out a vertical feature projected very differently in (a) and (b) .

A detailed comparison between an ISS image of aurora (a) plotted and (b) the contemporaneous image acquired by the SNK THEMIS ASI [ground-based]. The original ISS image (a) is plotted in (c). For more detail see Riechert, et al., 2016.

Comparing images of an aurora on 4 February 2012, captured both by the ISS crew and a ground-based instrument, has allowed the researchers to test the accuracy of their method. Overall, the results show good agreement, but highlight that the projection of the ISS images has to be taken into account when interpreting the results.

Now, a trove of thousands of Aurora Borealis and Australis photographs can be used by researchers to decipher the secrets of one the planet Earth’s most awe-inspiring phenomenon.

By Laura Roberts Artal, EGU Communications Officer

 

References:

Riechert, M., Walsh, A. P., Gerst, A., and Taylor, M. G. G. T.: Automatic georeferencing of astronaut auroral photography, Geosci. Instrum. Method. Data Syst., 5, 289-304, doi:10.5194/gi-5-289-2016, 2016.

Automatic georeferencing of astronaut auroral photography: http://www.cosmos.esa.int/web/arrrgh

The research was accomplished using only free and open-source software. All the images processed to date are made freely available at htttp://cosmos.esa.int/arrgh, as is the software needed to produce them.

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