Sunday, August 9, 2009 7:22:32 AM
geology, sea level, glaciers
At the so-called Last Glacial Maximum (LGM) the ice sheets grew to their maximum positions between 33,000 and 26,500 years ago due to decreases in northern summer insolation, tropical Pacific sea surface temperatures, and atmospheric CO
2. Nearly all ice sheets were at their maximum extent from 26,500 to some 19,000-20,000 years ago.
Some key questions that remain widely debated are what initiated the last deglaciation of the global ice sheets and what was their subsequent role during deglaciation in climate change, questions may be best assessed from the record of individual ice sheets rather than the integrated record.
In a study published on 7 August 2009 in the journal
Science Clark et al. compiled and analyzed more than 5000 dated ages in order to develop a record of maximum regional ice extent around the time of the Last Glacial Maximum.
There is considerable regional variability in the timing of when ice sheets (and various sectors of ice sheets) first reached their local last glacial maxima. mountain glaciers were near or at their maximum extent by around 30,000 years ago. The onset of Northern Hemisphere deglaciation 19,000 to 20,000 years ago was induced by an increase in northern summer insolation, providing the source for an abrupt rise in sea level.
The authors have shown that the duration of the LGM sea-level lowstand (26.500 to 19,000 years ago) is in excellent agreement with the duration of maximum extent of most of the global ice sheets, suggesting that most of the global ice sheets were in near-equilibrium with climate during this 7500-year interval.
Another paper published on 8 August 2009 in
Palaeocanography on the Last Glacial maximum concerns the “Impact of strong deep ocean stratification on the glacial carbon cycle”. During the Last Glacial Maximum, the climate was substantially colder and the carbon cycle was clearly different from the late Holocene (the Holocene is the geological epoch after the Ice Age, which means that it began approximately 11 700 years ago).
Friday, March 13, 2009 3:36:22 PM
glaciers, sea level, climate change
Small glaciers occupy only 0.5% of the total glaciated area in the world, yet their contribution to sea level rise is 70%. Amplified melting of small glaciers in the Arctic has the largest impact on changing global sea level. Glaciers in Svalbard and Alaska melt much quicker than glaciers in other Arctic regions. In fact Svalbard/Spitsbergen glaciers melt twice as fast as the ones in Canadian Arctic do. Arctic glaciers are particularly sensitive to even small change in air temperature and they react quickly. Due to climate warming as much as 13 km
3 of meltwater from Svalbard glaciers is delivered every year to the ocean. Meltwater from Svalbard glaciers alone rise sea level by 0.035 mm each year.
The following pie diagrams show the distribution on Antarctica, Greenland, and other glaciers of ice volume, their present contribution to sea level rise, and the precipitation on the glaciers.
This is the present situation, but if all ice on Antarctica melted the sea level would rise 61 m, while melting of all other glaciers and ice fields in the world would only rise the sea level 0.5 m. It is therefore surprising to see that seventy per cent of the present melting glacier induced sea level rise is due to the small glaciers. Today the sea level rises about 3 mm per year. Half of this is due to melting of glaciers, and the other half due to thermal expansion of the sea water.
The figures are based on a Svalbard Science Forum news article and a research article (in Norwegian) from the University of Oslo.
•
http://www.ssf.npolar.no/pages/news277.htm In
Norwegian only:
•
http://www.apollon.uio.no/vis/art/2009_1/artikler/isbre
Monday, March 9, 2009 8:22:25 PM
geology, sea level, biogeography, climate change
An ancient seaway between the Gulf of Thailand and the Andaman Sea was once thought have split the peninsula in two, allowing separated populations of animals to diverge. But a recent revision of the history of sea levels reveals that the ocean hasn’t cut through the peninsula in the past 40 million years. More than 58 rapid sea level rises of more than 40 m in the last 5 million years could account for the shift by crowding out species, particularly at the narrowest part of the peninsula called the Isthmus of Kra.
For most of the past few million years, the shallow ocean shelf surrounding the peninsula and islands of Malaysia and Indonesia has been exposed, creating a land area about the size of Europe. That habitat shrank dramatically each time sea levels rose.
The map shows the situation sixteen thousand years ago, when surrounding seas were 110 meters lower than today. Massive ice sheets covered parts of North America, northern Europe, and several other regions during the last ice age. This huge volume of ice lowered global sea level by around 120 meters as compared to today. After the ice sheets began to melt and retreat, sea level rose rapidly, with several periods of even faster spurts.
The figure shows a generalised curve of sea level rise since the last ice age.
•
http://www3.interscience.wiley.com/journal/122217747/abstract •
http://www-biology.ucsd.edu/news/article_030509.html •
http://www.giss.nasa.gov/research/briefs/gornitz_09/ The
closing of the “Indonesian Seaway” (east of Borneo/Kalimantan) has by the way been seen as triggering the Ice Ages.
Friday, March 7, 2008 4:26:50 PM
sea level, plate tectonics
Kim at
All my Faults are Stress Related has already written a
post on a new paper in
Science on estimates of global sea level during the Cretaceous period and what National Geographic got out of it. It is good, so just go and read it, and don’t forget to read the comments as well.
What I would like to stress here is the difference it makes, whether we are talking about long term or short term changes, and the importance of the little word
“global”.
Kim states that
The high sea levels of the Cretaceous are hardly big news to geologists, and the role of mid-ocean ridges in controlling global sea levels was accepted by the 1980's, at least.
For the non-geologists among you I would like however to rehearse the possible causes for global sea-level change:
1. On a
long term geological scale (i.e. millions of years):
-
Plate tectonics (changes in sea-floor spreading rates, collision of continents and changes in ocean ridge lengths).
- Emplacement of oceanic plateaus, so-called large igneous provinces (by many people believed to be caused by so-called Superplumes).
These factors are governing oceanic area and average oceanic depth. They are the ocean basin dynamics treated in the Science paper. These changes, although important, relevant and interesting, are totally irrelevant for the current discussion about global warming or climate change.2. On a
short term scale
- Global temperatures, e.g. through thermal expansion of ocean water and melting of ice cover.
- Variations in water volume of groundwater and lakes.
Global is about global average, which means that some places will have values below the global average and other places have values above the global average. During global warming some places will most probably become colder. I just read about a non-important, but telling, little detail. Over the last 35 years the sea level at Esbjerg, a town at the southern west coast of Denmark, has risen 14 cm. The world ocean average rise, however, was only 12 cm. This small difference of 2 cm is supposed to be due to more frequent westerlies in the North Atlantic Area since 1972. (This higher frequency is probably due to global warming). In the US they are by the way arguing that they have had a winter much colder than normal - well I can assure you that where I live our winter has been extremely mild, with temperatures far above average and nearly as warm as the year before. What we must look at however is not single local exceptions but
global trends.
Calculating or estimating the global sea level 65 million years ago is a tricky business. It is dangerous to base the calculations on one specific location (like New Jersey). It is no surprise that the estimates differ enormously (from ca. 40 to ca. 250 metres above the present level).
The burning problem today is that the (short term) change is extremely fast. The long term fluctuations won’t help our children and grand children next century.
•
http://shearsensibility.blogspot.com/2008/03/misleading-headline-of-day-sea-level-is.html •
http://news.nationalgeographic.com/news/2008/03/080306-sea-levels.html •
http://www.sciencemag.org/cgi/content/short/319/5868/1357 •
http://www.planetark.com/dailynewsstory.cfm/newsid/47376/story.htm •
http://www.abc.net.au/science/articles/2008/03/07/2183231.htm
Thursday, February 14, 2008 3:28:29 PM
geology, sea level, ice ages, palaeoenvironment
Unfortunately there is not many earth science articles to be found on PLoS ONE
http://www.plosone.org/home.action, the interactive open-access journal for the communication of all peer-reviewed scientific and medical research. On 13 January 2008 they published an anthropological paper that at least had a teeny bit of geological relevance, or maybe rather the other way round - it illustrates the importance of geology for the history of mankind.
In the paper titled
A Three-Stage Colonization Model for the Peopling of the Americas there is of course a lot of genetic DNA-discussion, but here I shall keep to the geological and palaeoecological constraints on the timing.
My map is not very precise, and please don’t take the time too seriously (it is no better than something around 20,000 to 18,000 years ago), but I hope it can serve my purpose.
During the last Ice Age, when much of the northern hemisphere was covered with ice, the sea level was much lower than now. During the last glacial maximum sea level was lowered by ~125m. That is the opposite of the present situation with global warming. A landbridge connected America and Asia, where the Bering Strait now separates the two continents. The area called Beringia was a continuous landmass that connected Asia and North America for around 60 thousand years until around 11 or 10 thousand years ago. Because of its aridity, much of Beringia remained unglaciated during the ice ages, the snowfall was extremely light due to the southwesterly winds from the Pacific Ocean having lost their moisture over the fully glaciated Alaska Range. Fossils indicate productive, dry grassland ecosystems and demonstrates that large mammals roamed Beringia. In fact it was fit for human population. Immigration from East Central Asia may have taken place in the period from 43 to 36 thousand years ago. To the east they would however have been stopped by the barrier formed by the North American Ice Sheets. This isolation from continental North America lasted until at least 14 thousand years ago when an intracontinental ice-free corridor opened up between the Laurentide and Cordilleran Ice Sheets. Migration could by then also take place along the ice free Pacific coast.
After 11–10 thousand years ago sea levels rose sufficiently to re-inundate Beringia, creating the Bering strait that now separates Alaska from Siberia by at least 100 km of open frigid water.
* A Three-Stage Colonization Model for the Peopling of the Americas -
http://www.plosone.org/article/info%3Adoi%2F10.1371%2Fjournal.pone.0001596 * Beringian Standstill and Spread of Native American Founders -
http://www.plosone.org/article/info%3Adoi%2F10.1371%2Fjournal.pone.0000829Other land bridges around the world have been created and re-flooded in the same way: approximately 14 thousand years ago, mainland Australia was linked both to New Guinea and to Tasmania, the British Isles were an extension of continental Europe via the English Channel, and the dry basin of the South China Sea linked Sumatra, Java and Borneo to the Asian mainland.
My Opera
