Two Records in Arctic Melt This Summer

August 11th, 2019

There is perhaps a bit too much media hype about July 2019 being the warmest month on record. If you go to the source — the European Copernicus Climate Service article,  the official statement is that “July 2019 was on a par with, and possibly marginally higher than, that of July 2016”.  This is important to note because NASA and NOAA have yet to provide their own summaries for July, and because of uncertainties in the surface temperature record and varying techniques for measuring global temperature among groups, it’s possible July 2016 will remain the record based on other scientific agencies. However, it’s still been a hot summer overall, with June 2019 very clearly the hottest June ever.

This warmth has been felt in the Arctic, as well. Surface melt on the Greenland Ice Sheet has been particularly high this summer, and it recently experienced its highest daily melt area ever recorded, based on data from the National Snow and Ice Data Center. Over 60% of the ice sheet was melting at the surface. In the world of sea ice, it is unlikely that we break the all-time record from 2012 for the annual daily minimum sea ice extent, as that coincided with the strongest storm ever recorded in the Arctic (which helped break up and melt the ice pack).  However, there has never been less sea ice in the Arctic Ocean in July than in 2019.

 

Shamrock Glacier, Neacola Mountains, Alaska

August 7th, 2019

While in the Neacola Mountains of Alaska last month, we flew over Shamrock Glacier. This first image is from the head of the glacier, where crevasses have been filled in with snow during the accumulation season.

Farther down the north-flowing glacier, we see the merging of the east and west branches and a fine example of a medial moraine.  However, also note on the far lower-right the recently exposed rock. This part of the glacier is shrinking in size, becoming a narrower ice stream.

The toe of Shamrock Glacier is just plain beautiful, ending at a small lake that is dammed from two larger lakes by a ring of moraines. Estimates in a blog post by Mauri Pelto are that the glacier extended all the way out to that moraine as recently as 1950.

In fact, back in 2015, Mauri showed a Landsat satellite image showing the retreat of Shamrock Glacier away from its moraine from 1987 to 2014.  Updating this with the most recent July 2019 imagery, you can see the continued retreat of Shamrock Glacier just in the past four years.  A big section on the left has calved off, and the glacier has slipped off a rise on the right.

Finally, taking a view from the ground, we can better see how the glacier has not only retreated back, but also thinned and narrowed over the past few years. (The 2015 line is based on a photo from Jerry Pillarelli It’s still a pretty glacier, and the sound of calving icebergs while eating lunch  is always welcome.  However, it won’t be long before it retreats upslope sufficiently to no longer calve off into the lake.

Warming at the Third Pole – A New Record of Climate Change from Kashmir, Northwest Himalaya

April 28th, 2019

The Wooster Tree Ring Lab collaborated on a publication describing the recent thermal history of the Lidder Valley, Northwest Himalaya. Dr. Santosh Shah, the lead author, is a multitalented paleoclimatologist at the Birbal Sahni Institute of Palaeosciences in Locknow, India. He and his colleagues led the study that appeared in Climate Dynamics and is titled: A winter temperature reconstruction for the Lidder Valley, Kashmir, Northwest Himalaya based on tree-rings of Pinus wallichiana. Here is the abstract from the study:

Abstract: A regional, 175 year long, tree-ring width chronology (spanning 1840–2014 C.E.) was developed for Pinus wallichiana A. B. Jacks. (Himalayan Blue pine) from the Lidder Valley, Kashmir, Northwest Himalaya. Simple and seasonal correlation analysis (SEASCORR) with monthly climate records demonstrates a significant direct positive relationship of tree growth with winter temperature. A linear regression model explains 64% of the total variance of the winter temperature and is used to reconstruct December–March temperatures back to 1855 C.E. The most noticeable feature of the reconstruction is a marked warming trend beginning in the late twentieth century and persisting through the present. This reconstruction was compared with instrumental records and other proxy based local and regional temperature reconstructions and generally agrees with the tree-ring records and is consistent with the marked loss of glacial ice over the last few decades. Spectral analysis reveals a periodicity likely associated with the Atlantic Multidecadal Oscillation and El Niño–Southern Oscillation. Spatial cor- relation patterns of sea surface temperatures with the observed and reconstructed winter temperatures are consistent with larger scale warming in the region.

Map showing the location of the study in the Lidder Valley in Kashmir, Northwest India.

The rivers of the Lidder Valley are fed by glaciers from the Himalaya, which are becoming increasingly impacted by climate change and population pressures. The people within the valley depends on the water from the rivers and managing the water in this rapidly warming region is an increasing challenge. The results in this work show the increasing pace of the recent warming (see figure below).

Temperature reconstructions (above) based on tree-rings for the Himalaya. The curve on the top is from the new publication. 

Dr. Shah is now working on using tree-rings to reconstruct river flow in the region. This is work that he presented last year at World Dendro in Bhutan and which we are are also collaborators. We are grateful to Dr . Shah for introducing us to climate change research in the Himalaya AND for his help to our former students of the Wooster Tree Ring Lab.

Jeff Gunderson,  who recently completed his masters thesis at The Ohio State University in Geography used tree-rings from the Peruvian Andes to reconstruct climate. Jeff collaborated with Dr. Shah who shared his computer code and guidance in calibrating his Peruvian tree-ring records.

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Sometimes a Cold Snap is Just a Cold Snap

February 7th, 2019

On Wednesday, January 30, 2019, The College Wooster closed due to cold. This cold snap was felt across much of the central and eastern USA. The message Wooster staff and faculty received included this statement:
“The National Weather Service is forecasting daytime temperatures tomorrow between -3 and -7 degrees Fahrenheit, and wind chills of -25 to -30. In these weather conditions, exposed skin can begin to show signs of frostbite in as little as 10 to 15 minutes outside.”

However, in the past week, I’ve heard diverging narratives from people across the eastern USA about their experience. Some lament that this was horrible weather, the worst ever — how can climate change be real? Others lament that it used to get much colder — we have it easy today because of global warming. So what’s the truth? 1) Was Wooster’s cold spell out of the ordinary? 2) Is winter not as harsh as (or harsher than) it used to be? 3) Is climate change to blame?

1) Last week’s cold was not out of the ordinary.

There’s two ways to think about how cold it was.  One is that the daily high was only 9.6°F, which is frigid — it never topped 10°F. Another is that the daily low was -6.0°F, and that occurred while students would have been walking to classes in the morning. I can’t speak much to the wind chill because the OARDC station is too far from campus to give an accurate assessment.  Wind varies a lot more temperature from place to place, so it’s hard to know exactly how bad the wind chill was for any random person walking with exposed skin outside.  For temperature, though, the weather forecast was spot on.

Figure 1: The distribution of the annual low temperature in Wooster (the lowest daily low) from 1900 to 2018, with 2019 marked.

That temperature, however, was not exceptional.  Funny enough, January 22, 2019 actually had a lower low of -7.2°F — it just wasn’t as windy. In Wooster, the average annual low temperature since 1900 is -7.6°F. The average lowest daily high is 11.0°F. Our 2019 is currently right in line with those numbers (although the winter is not yet over).

2) The coldest days might be getting less severe.

This is actually a tricky one to answer. If you look at the coldest temperature recorded each year at the OARDC, nearly every year before 1930 had at least one day in which the temperature fell below 0°F — but not so from 1930 to 1960.  The coldest cold was above 0°F about in about 25% of the years in that second period.  From 1960 to 1990, the reliable sub-zero temperatures returned.  Since 1990, the annual coldest day has been less severe again on average.  In other words, if you only look back to 1960, yes, the worst days have been getting less severe.  But if you look back to 1900, the last 120 years suggest that Wooster still gets plenty of cold.  So if you were born in the 1950s, no, the new generation doesn’t have it easier, but they may be more sensible about preventing frostbite. 

Figure 2: Time series of the the lowest daily low temperature (the coldest temperature each year) in Wooster from 1900 to 2019 (so far). Edit: With 99% confidence interval on the trend line.

3) There’s not a clear climate change signal here.

The problem with evoking climate change is that weather extremes are by definition rare, so it’s hard to pinpoint the immediate cause of local-scale weather extremes to long-term, global-scale warming.  There is some evidence out there that the polar jet stream (a.k.a. the “polar vortex”) is becoming more erratic as the world warms, leading to more days like January 30 when the Arctic Ocean is warmer than Minnesota, but that is not settled science. Plus, there’s no clear trend with this particular measure. In other words, it’s premature to blame climate change for every weather event you don’t like.

Wooster Records its Third Wettest Year on Record

October 4th, 2018

If you live in Ohio and have felt wet and miserable the past year, you now have vindication. Based on the long-term record from the OARDC weather station, Wooster has just completed it’s third wettest year on record (i.e., since continuous record-keeping began at the OARDC in 1900).  I know, it’s the first week of October, but in the hydrology world, the “water year” typically begins on Oct 1.  This makes sense if you think about agriculture — water falling in Oct-Dec of 2018 isn’t all that helpful for most crops growing in 2018, but it can replenish surface reservoirs and/or  groundwater for 2019.  Therefore, the 2018 water year just ended Sunday.

Figure 1: Annual precipitation at the OARDC station in Wooster, Ohio by water year beginning Oct 1, scaled to a 365-day year. The record extends 118 years: 1901-2018. Linear (red) and cubic (green) fits to the dataset are also included. 

The total precipitation in 2018 was 50.17 in., which fell about an inch short of the record, set in 2004 (51.18 in.).  The only other year with over 50 in. was 1996 (50.81 in.). Both 2004 and 1996 were leap years, but even if you adjust precipitation to 365 days per year, 2018 still ranks third. Another interesting thing to note is that the annual precipitation in Wooster has been increasing over the past 118 years.   On average, the change is about 0.07 in./year. That might not sound like a lot, but over 118 years, it adds up.  In the past decade (2009-2018), Wooster has received 102.6 in. more precipitation than the period 1901-1910. In other words, we’re getting about 32% more precipitation in the 2010s than we did in the 1900s.  A linear trend is pretty good for explaining this long-term change, but you might notice that most of the change in annual precipitation takes place in the periods 1900 to 1925 and 1980 to today.  The green curve is a little better at fitting the data and better captures the mid-century stability and the rapid increase in wetness over the past few decades.

Figure 2: Histogram of Aug-Sep precipitation in Wooster with the extreme years of 2017 and 2018 indicated.

One more thing that’s interesting to note is just how different late summer was in Wooster this year compared to last.  The Aug-Sep period of 2017 was one of the driest on record in Wooster (even though the year overall was unexceptional). Only 2.42 in. fell in Wooster then. On the other hand, 2018 was one of the wettest years on record, with 11.36 in. falling. Only 3% of all Aug-Sep periods were drier than 2017, and only 4% were wetter than 2018. What a difference a year makes!

Climate Monday: The xkcd Earth Temperature Timeline

April 30th, 2018

It’s the final week of the semester, so it’s time for a little fun in the world of weather and climate visualizations.  One of the toughest things that Geologists have to deal with is conveying a sense of time scales.  It’s difficult for present-day humans to conceive of how long ago (or recent) the Roman Empire or Han Dynasty were, let alone 4.6 billion years of Earth history. We often use interesting comparisons, like how the time gap between Tyrannosaurus (68-66 million years ago) and humans is smaller than the gap between Tyrannosaurus and Stegosaurus (155-150 million years ago).  Sometimes we use analogies, like how an average human lifespan is 0.00000204% of all Earth’s history, which is about the same percentage of your life you just spent reading this paragraph.

With climate change, scientists often are approached with the question: “Climate has changed before, so why is this time worse?” An important is that this time it’s changing very fast, and rapid change is more problematic than gradual change. The faster the change, the harder it is for plants and animals (and humans) to adjust.  But conveying that sense of rapid change can be difficult when our time series are so long, stretching tens of thousands of years.  It rarely looks good on a single powerpoint slide or a single 8 1/2″ by 11″ piece of paper.  You either have to scrunch everything into a very condensed and crowded graph, use an inset box to zoom in on today, or use multiple slides/figures.  Or… you could use the tendency for modern webpages to scroll indefinitely to convey a sense of time.  This is the tactic of the webcomic xkcd.

No, seriously. The main reason for reading the comic is to laugh at the little bits of humor slipped in, but Randall Munroe at xkcd is diligent about scientific research.  The temperature data for the visualization are based on a combination of  HADCRUT4 (from the UK Meteorology Office), the Intergovernmental Panel on Climate Change (IPCC, funded by the UN), and peer-reviewed journal articles in the journals Nature (Shakun et al. 2012), Science (Marcott et al. 2013), and Climate of the Past (Annan and Hargreaves 2013). Those last three are all paleoclimate reconstructions.

Climate Monday: Repeat Photography

April 15th, 2018

The semester is winding down, so we only have a few more of these climate visualization posts to go. Today, I want to highlight repeat photography. Taking a picture of the same place several or many years apart can be a striking demonstration of change and capture the imagination better than a sterile graph or abstract map.

The flashiest examples applying the idea to climate change come from productions like Chasing Ice by the Extreme Ice Survey or Chasing Coral by Exposure Labs. (Chasing Coral was recently featured here at the College of Wooster as part of the “Great Decisions” series.) The products can be truly fascinating, especially when many photographs are combined in a time lapse video.  For example, the video of the Extreme Ice Survey’s repeat photography of Mendenhall Glacier, embedded below, gives a better impression of glaciers as flowing masses of ice than any single photograph or model simulation.  It also shows the decline in mass at the toe of the glacier between May 2007 and August 2011.

Rules of Climate Change Repeat Photography

Showing climate change with repeat photography requires a few special considerations:

First, you need to have not only the same location, but the same angle for your shot, showing the same context around the feature of interest in each and every photograph being compared.  This is why the Extreme Ice Survey set automated stations with the cameras well-mounted rather than using hand-held cameras.  Much of the Chasing Ice documentary is about building and installing the equipment necessary to achieve this fundamental “rule” of repeat photography. The Chasing Coral team tried similar techniques, only with the added complication of being under water.  Needless to say, it was harder for the Chasing Coral team.

Second, any repeat photography of environmental phenomena had better avoid making natural seasonal cycles look like climate change.  The two pictures of Sub Lake in Rocky Mountain National Park above were taken in June 2015 and January 2013.  The difference between the two isn’t climate change; it’s winter. Another example: The Mendenhall Glacier video shown above is labeled as “May 2007 to August 2011”.  A red outline of the glacier in the first frame is compared to the outline of the glacier in the final frame, and that’s a little deceptive.  Just like snow cover and sea ice, the Menhendall Glacier has a greater extent and thickness at the end of winter than the end of summer — especially at the toe. So part of that difference you’re seeing is just the fact that a) over 80 inches of snow typically fall on the toe of the Mendenhall from October to March and b) the average high temperature is over 60°F in June, July and August in southeast Alaska.  It’s likely to look more robust in May than August, so the time lapse would be better starting and ending in the same month.

Image result

Cherry blossoms in Washington, D.C. (from the National Park Service)

Third, climate change is not the only factor that determines whether one year is cooler or warmer or wetter or drier than the last.  Climate change is not the only factor that determines whether coral bleaching will occur or whether a glacier will retreat.  For example, in Chasing Coral, a coral bleaching event in Australia is highlighted and attributed to climate change.  At the same time, though, an El Niño event was occurring. El Niño is a natural part of the climate system, but it can also lead to warming and coral bleaching. Was global warming a factor in this bleaching event? Absolutely, but the devastation depicted in Chasing Coral may have been less overwhelming in a La Niña or normal year. As another example, if you were trying to take pictures in Washington DC to show how the date of cherry trees blooming was coming earlier each year, you might be disappointed.   Although blooming is now occurring on average about a week earlier than in the 1970s, peak bloom was actually slightly later than average this year. The best way to get around this issue is to have several decades between the start and end of the repeat photography pair or sequence.

The Repeat Photography Project at Glacier National Park

With all these rules in mind, the United States Geological Survey (USGS) is currently undertaking a repeat photography project for the failing glaciers of Glacier National Park. They’re also soliciting help from visitors in a crowd-sourcing effort. The time spans exceed 50 or even 100 years for these photos, which is enough time to see some truly remarkable changes to Glacier National Park’s namesakes. It’s even long enough to avoid the seasonal issues discussed above. As an example of the output, below is repeat photography of Boulder Glacier (1932 to 2005) from the Northern Rocky Mountain Science Center (NOROCK).

Boulder Glacier - 1932 Boulder Glacier - 2005 color

 

Climate Monday: Climate Change Hot Spots

April 2nd, 2018

It’s no secret that global warming does not simply mean more warm days and fewer cold ones. Warming is uneven, with some regions (like the Arctic) warming faster than others. Additionally, warming of the atmosphere and oceans has a cascading effect on other parts of the Earth system, from the amount of ice stored in Greenland to the variability of global wind patterns, to the extent of various habitats. The world is complex, and it the impacts of climate change myriad. With so many changes happening, what places or changes should humans focus adaptation and mitigation efforts? Enter the concept of “climate change hot spots”. Let’s examine three frameworks and how they’re visualized.

Example #1: One of the simplest frameworks for talking about climate change hot spots is to consider places where various physical aspects of the climate are projected to change the most (Kerr 2008). This was the tactic used by a group of climate modelers from the National Center for Atmospheric Research back in 2008.  They ran detailed, regional-scale climate models into the future and looked for a) places with the most change in average temperature and precipitation, and b) places with the most change in the variability of temperature and precipitation (in other words, heat waves, cold snaps, floods, and droughts).  The result was a relative index from low change to high change:

Figure 1: Map of the “relative responsiveness” of the USA and northern Mexico to climate change based on projected changes in temperature and precipitation under a suite of climate models. (Kerr 2008)

The nice thing about this measure is that it’s objective and gives a value of overall impact for everywhere in the lower 48.  It’s limited in it’s utility, though.  For one thing, it only measures temperature and precipitation, omitting related concepts like sea level rise and wildfire frequency/intensity.  It also is a projection of the future, which is problematic both because there’s less certainty about the future and because there are changes already happening that might be more pressing to address.

Example #2: That in mind, another way to define “climate hot spot” is a location that has already changed substantially. The Union of Concerned Scientists (2011) has compiled a map of locations that have “well-documented” changes already occurring. Here’s a snapshot, but the visualization is meant to be an interactive map, not a static image, which is certainly inviting.  The “well-documented” claim is supported by reference lists and descriptions for each event. In other words, these have been researched substantially.  Another interesting point is that the map shows a much broader view of “climate change” than the earlier climate model studies.  Sure, there’s “extreme wet” and “air temperature”, but there’s also “ecosystem” sections and “health” and “food” for people. This is definitely better suited for a broader audience and broader concerns.

Figure 2: Snapshot example of climate hot spots by the Union of Concerned Scientists (2011).

Still, the above example may seem lacking with regard to two elements (and maybe others): First, it is clearly focused on the USA.  There is a data bias, of course — the Union of Concerned Scientists has many American scientists, and many of them study the USA. But it may give the false impression that the USA has more dire situations than the rest of the world.  Second, there is still little sense of risk versus vulnerability.

If we think of climate change as a natural hazard, just like a volcanic eruption or an earthquake or a hurricane, we can talk about both risk and vulnerability of populations.  For example, both the Netherlands and Florida are at great risk of sea level rise, but the Dutch are bettered prepared to adapt to rising seas because of past experience and current cultural, political, and physical infrastructure. The same risk can lead to more or less hardship depending on how vulnerable a place is — and assuming sea level rises about the same in both locations, Florida is likely to have more hardship from sea level rise than the Netherlands.

Example #3: This added concept of vulnerability is used to define “climate hot spots” in yet another way: as locations where “strong physical and ecological effects of climate change come together with large numbers of vulnerable and poor people and communities” (Neumann and Szabo 2016). Their map is still really a measure of risk, not vulnerability, but they use it to help highlight areas with high risk that also have special vulnerability (originally identified by De Souza et al. 2015):

  1. Deltas in Africa and South Asia that have large populations of poorer people. Groundwater extraction and other human activities that make deltas sink can exacerbate the effects of sea level rise.
  2. Semi-arid regions in parts of Africa, South Asia, and Central Asia that may become drier. Again, the lower economic resources in these regions make them more vulnerable.
  3. River basins dependent on glaciers and snowpacks as a water source, especially in the Himalaya, where there are large populations of poorer people.

Figure 3: Climate risks based on three factors: snow-dependence, semi-arid climate, and river deltas. (Neumann and Szabo 2016).

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Works Cited

De Souza, K., Kituyi, E., Harvey, B. et al.  (2015). Vulnerability to climate change in three hot spots in Africa and Asia: key issues for policy-relevant adaptation and resilience-building research. Reg Environ Change, 15: 747. https://doi.org/10.1007/s10113-015-0755-8

Kerr, R. (2008). Climate Change Hot Spots Mapped Across the United States. Science, 31: 909. http://science.sciencemag.org/content/sci/321/5891/909.full.pdf

Neumann, B. and Szabo, S. (2016). Climate change ‘hotspots’: why they matter and why we should invest in them. The Conversation. Accessed 2 Apr 2018. http://theconversation.com/climate-change-hotspots-why-they-matter-and-why-we-should-invest-in-them-68770

Union of Concerned Scientists (2011). Climate Hot Map. Accessed 2 Apr 2018. http://www.climatehotmap.org

Wooster Geology at AGU 2017

December 18th, 2017

The Mississippi River in New Orleans, Louisiana. Photo: Dr. Karen Alley

Three Wooster Geologists (Dr. Karen Alley, Dr. Alex Crawford, and senior Geology major Cole Jimerson) descended on New Orleans last week to attend the Fall Meeting of the American Geophysical Union. With 20 to 25 thousand attendees each year, this is the largest Earth and space science meeting in the world.

On Wednesday, Dr. Crawford gave a talk about his research in seasonal sea ice prediction. As the Arctic continues to warm, seasonal sea ice melt is occurring progressively earlier each year. Although almost unheard of 20 years ago, commercial shipping along the Russian coastline (the “Northern Sea Route”) is now a routine summer operation. However, the seasonal timing of when the sea ice melts enough for normal shipping is highly variable from year to year. Dr. Crawford and his collaborators are investigating various ways of improving our ability to predict that variability. Better predictions can aid shipping companies in planning their summer routes.

The Northern Sea Route through the Arctic Ocean and the “last retreat day” (LRD), which means the last day of the year on which sea ice concentration is above 50% (left) or 15% (right). Adapted from Stroeve, Crawford, & Stammerjohn (2016); 10.1002/(ISSN)1944-8007.

On Thursday, Dr. Alley gave an invited talk about a new data product she and her collaborators have developed for researchers studying the Antarctic ice sheet. Using ice velocity derived from satellites and sophisticated mathematics and computer coding, they calculated strain rates for Antarctica’s ice sheet and ice shelves. These strain rates are a measure of how fast the ice deforms by stretching and compressing as it moves. They are a fundamental parameter to know for anybody trying to understand how Antarctica’s ice is responding to climate change.

Strain rates on the Filchner Ice Shelf, Antarctica. From Dr. Karen Alley.

Finally, Cole Jimerson presented a poster on Friday overviewing some of the research he and other students performed through a Keck Geology project concerning erosion rates on the Caribbean Island of Dominica. Dominica is a volcanic island prone to explosive ash eruptions. Many of the rocks and sediments on the island are quickly eroded by rivers and chemical weathering in the hot, wet tropical climate. These and other factors lead to landslide risks, and better understanding erosion rates can improve hazard mitigation strategies.

Cole Jimerson presents his poster at the Fall Meeting of the American Geophysical Union.

Wooster’s Fossils of the Week: Sponge and clam borings that revealed an ancient climate event (Upper Pleistocene of The Bahamas)

April 28th, 2017

This week’s fossils celebrate the publication today of a paper in Nature Geoscience that has been 20 years in the making. The title is: “Sea-level oscillations during the Last Interglacial highstand recorded by Bahamas coral”, and the senior author is the geochronological wizard Bill Thompson (Woods Hole Oceanographic Institution). The junior authors are my Smith College geologist friends Al Curran and Brian White and me.

The paper’s thesis is best told with an explanation of this 2006 image:
This photograph was taken on the island of Great Inagua along the coast. The flat dark surface in the foreground is the top of a fossil coral reef (“Reef I”) formed during the Last Interglacial (LIG) about 123,000 years ago. It was eroded down to this flat surface when sea-level dropped, exposing the reef to waves and eventually terrestrial weathering. The student sitting on this surface is Emily Ann Griffin (’07), one of three I.S. students who helped with parts of this project. (The others were Allison Cornett (’00) and Ann Steward (’07).) Behind Emily Ann is a coral accumulation of a reef (“Reef II”) that grew on the eroded surface after sea-level rose again about 119,000 years ago. These two reefs show, then, that sea-level dropped for about 4000 years, eroding the first reef, and then rose again to its previous level, allowing the second reef to grow. (You can see an unlabeled version of the photograph here.) The photograph at the top of this post is a small version of the same surface.

The significance of this set of reefs is that the erosion surface separating them can be seen throughout the world as evidence of a rapid global sea-level event during the Last Interglacial. Because the LIG had warm climatic conditions similar to what we will likely experience in the near future, it is crucial to know how something as important as sea-level may respond. The only way sea-level can fluctuate like this is if glacial ice volume changes, meaning there must have been an interval of global cooling (producing greater glacial ice volume) that lowered sea-level about 123,000 years ago, and then global warming (melting the ice) that raised it again within 4000 years. As we write in the paper, “This is of great scientific and societal interest because the LIG has often been cited as an analogue for future sea-level change. Estimates of LIG sea-level change, which took place in a world warmer than that of today, are crucial for estimates of future rates of rise under IPCC warming scenarios.” With our evidence we can show a magnitude and timing of an ancient sea-level fluctuation due to climate change.

Much of the paper concerns the dating techniques and issues (which is why Bill Thompson, the essential geochronologist, is the primary author). It is the dating of the corals that makes the story globally useful and significant. Here, though, I want to tell how the surface was discovered in the first place. It is a paleontological tale.

In the summer of 1991 I worked with Al Curran and Brian White on San Salvador Island in The Bahamas. They were concentrating on watery tasks that involved scuba diving, boats and the like, while I stayed on dry land (my preferred environment by far). I explored a famous fossil coral exposure called the Cockburntown Reef (Upper Pleistocene, Eemian) that Brian and Al had carefully mapped out over the past decade. The Bahamian government had recently authorized a new harbor on that part of the coastline and a large section of the fossil reef was dynamited away. The Cockburntown Reef now had a very fresh exposure in the new excavation quite different from the blackened part of the old reef we were used to. Immediately visible was a horizontal surface running through the reef marked by large clam borings called Gastrochaenolites (see below) and small borings (Entobia) made by clionaid sponges (see the image at the top of this post).
Inside the borings were long narrow bivalve shells belonging to the species Coralliophaga coralliophaga (which means “coral eater”; see below) and remnants of an ancient terrestrial soil (a paleosol). This surface was clearly a wave-cut platform later buried under a tropical soil.


My colleagues and I could trace this surface into the old, undynamited part of the Cockburntown Reef, then to other Eemian reefs on San Salvador, and then to other Bahamian islands like Great Inagua in the far south. Eventually this proved to be a global erosion surface described or at least mentioned in many papers, but its significance as an indicator of rapid eustatic sea-level fall and rise was heretofore unrecognized. Finally getting uranium-thorium radioactive dates on the corals above and below the erosion surface placed this surface in a time framework and ultimately as part of the history of global climate change.

This project began 25 years ago with the discovery of small holes left in an eroded surface by humble sponges and clams. Another example of the practical value of paleontology.

References:

Thompson, W.G., Curran, H.A., Wilson, M.A. and White, B. 2011. Sea-level oscillations during the Last Interglacial highstand recorded by Bahamas coral. Nature Geoscience (DOI: 10.1038/NGEO1253).

White, B.H., Curran, H.A. and Wilson, M.A. 1998. Bahamian coral reefs yield evidence of a brief sea-level lowstand during the last interglacial. Carbonates and Evaporites 13: 10-22.

Wilson, M.A., Curran, H.A. and White, B. 1998. Paleontological evidence of a brief global sea-level event during the last interglacial. Lethaia 31: 241-250.

[Originally posted September 11, 2011. Some updates and editing.]

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