[BLOGGER'S NOTE: This article was originally published online (Proceedings of the National Academy of Sciences), and all references can be found at this site:
http://www.pnas.org/cgi/content/full/104/16/6500 ]


Conventional wisdom and climate history

Steven M. Colman^*

Large Lakes Observatory and Department of Geological Sciences, University of Minnesota Duluth, Duluth, MN 55812

The Younger Dryas interval, a cold snap that chilled many parts of the world for 1,500 years or so in the midst of the last deglaciation (13,000–11,500 years ago), is perhaps the best known and most studied paleoclimate event of the last 2 million years. Only a few years ago, it was well accepted that a change in the drainage routing of the huge proglacial lake that fronted the North American ice sheet occurred at just about the same time as the beginning of the Younger Dryas cold period. This and other coincidences in timing, as well as considerations of the effects this event might have had on ocean circulation, led many to believe that the meltwater rerouting caused, or triggered, the Younger Dryas cold interval. Over the last few years, all of this conventional wisdom was thrown into turmoil by a few new observations and age determinations. Now, in this issue of PNAS, Carlson et al. (1) provide a new set of data about meltwater discharge at the start of the Younger Dryas, as well as new detail regarding events within this period. They also suggest that the conventional wisdom about the inception of the Younger Dryas may not be as flawed as has been suggested recently.

The Younger Dryas began and ended abruptly, at least as indicated in ice core records from Greenland, where temperature initially may have fallen by 15°C, with transitions no longer than a few decades (2); most of the final (warming) transition may have occurred in just a few years (3). At just about the time of the inception of the Younger Dryas, a major change in the routing of meltwater from the Laurentide Ice Sheet in North America seems to have occurred. The outlet of glacial Lake Agassiz, which fronted the Laurentide Ice Sheet across a vast section of the continental interior, appeared to have switched from Mississippi River drainage (and thence to the Gulf of Mexico) eastward to the Laurentian Great Lakes (and thence to the St. Lawrence River and the North Atlantic Ocean). The switch in outlets was accompanied by a major (>40 m) initial drawdown of Lake Agassiz during its Moorhead Phase, perhaps eventually reaching as much as 150 m below the southern outlet (4). The switch was also accompanied (within the uncertainties of radiocarbon dating) by an abrupt change in oxygen isotopes in the Gulf of Mexico (documented in many studies, most recently in ref. 5), interpreted as resulting from an increase in seawater salinity that accompanied the removal of Agassiz drainage down the Mississippi. Indeed, together with a plausible route for the eastern outflow, the evidence for an eastward switch in drainage seemed compelling. Ocean climate modeling studies (e.g., see references in ref. 6) suggested that the estimated increase in freshwater input to the North Atlantic would be sufficient to suppress Atlantic meridional overturning circulation, distinctly cooling the region where the Younger Dryas is best documented, as well as other areas. Oceanic proxy evidence for a slowdown or cessation of

The Younger Dryas began and ended abruptly with transitions no longer than a few decades.

Atlantic meridional overturning circulation (in the past loosely called "thermohaline circulation" or "the conveyor belt") during the Younger Dryas is relatively strong (1, 7).

Nevertheless, the inference that eastward routing of Lake Agassiz discharge was the cause of the Younger Dryas has not been without its problems. Isotopic evidence for a freshening of the North Atlantic off the mouth of the St. Lawrence has been equivocal at best (8, 9). In addition, the effect of Lake Agassiz inflow on the sediments of the Laurentian Great Lakes has not been clear; sedimentological and isotopic evidence from Lake Michigan was interpreted as a signal of Lake Agassiz inflow (10), whereas similar kinds of data from the Huron basin were interpreted differently (11).

Despite these difficulties, eastward routing of Lake Agassiz discharge as the trigger for the Younger Dryas was widely accepted. Until recently, the main issues were whether Younger Dryas meltwater rerouting provided an analog for mechanisms to explain earlier abrupt climate changes when the ice sheets were of intermediate size (12) and whether the changes in ocean circulation during the Younger Dryas were mostly due to the long-term increase in base discharge (12), the postulated initial catastrophic flood (13), or a combination of the two (6).

Troubles with the conventional wisdom began with recent expeditions to the remote terrain postulated as the route of the catastrophic eastward drainage of Lake Agassiz at the beginning of the Younger Dryas. These observations failed to find geomorphic evidence of major flood channels or depositional features (14). In addition, new radiocarbon and cosmogenic radionuclide dating on glacial features south of the proposed outlet suggested that the outlet area was not deglaciated until after the start of the Younger Dryas (14). Finally, new ages related to the Moorhead Phase of Lake Agassiz have been interpreted to mean that this phase occurred significantly after the start of the Younger Dryas (15). On the other hand, complexities exist in the geomorphic interpretations, and virtually all of the ages on glacial features are minimum ages or have large uncertainties (16). The differences in interpretation have wide-ranging climatic and oceanographic implications (7).

Given the well documented, if poorly dated, drawdown of Lake Agassiz during its Moorhead Phase, the massive amounts of water involved had to go somewhere. Some researchers have suggested that the water discharged through the northwestern Clearwater Outlet to the Arctic Ocean, although there are chronological problems with this scenario (7, 16). The new data presented by Carlson et al. (1) strongly suggest that the water did indeed discharge eastward, eventually through the St. Lawrence estuary. They use three different geochemical signatures of ambient water, preserved as trace elements in the shells of foraminifera eventually buried in dated sediment. Each of the geochemical tracers indicates a source of water from the Canadian interior plains, i.e., the drainage basin of Lake Agassiz. The authors also attempt deconvolve complex stable-isotope proxies in a way that is consistent with the geochemical tracers. The isotope exercise is less convincing because the isotope ratios depend in a complex way on temperature and salinity, as well as on the isotopic composition of river and ocean water. The interpretation of each tracer or proxy has its complications, but taken together, they make a strong case for major Lake Agassiz discharge through the St. Lawrence. The chronology for their sediment cores is good enough for Carlton et al. to argue convincingly that a surge in Lake Agassiz discharge through the St. Lawrence estuary coincided with the start of the Younger Dryas. Moreover, the authors were able to use mixing models with the geochemical tracer data to quantitatively estimate the amount of the Lake Agassiz discharge and how it varied with time (see Fig. 1). They infer substantial variations in discharge within the Younger Dryas interval. Although other studies have suggested variation in climate, vegetation, and ocean processes during the Younger Dryas (e.g., ref. 17, as well as the references cited by Carlson et al.), this is the first estimation, independent of changes in the level of Lake Agassiz itself, of quantitative variation in the purported driver of climate change at the time: discharge of fresh water to the North Atlantic.

View larger version (39K): [in this window] [in a new window] Fig. 1. Comparison of reconstructed discharges down the St. Lawrence River around the time of the Younger Dryas (YD) cold interval. Items in blue are related to the article by Carlson et al. (1) discussed here. The thin blue line with symbols is one (U/Ca) of the five proxies used in their study. The thick blue line is their reconstructed discharge, calculated from an average of the different geochemical proxies and a water mixing model. Light blue shading indicates the time interval they consider as showing evidence for Lake Agassiz eastward discharge, and dark blue shading indicates their "intra-Younger Dryas routing event." The red line is a previous estimate of Lake Agassiz discharge down the St. Lawrence (12), based on reconstructions of lake levels and drainage areas for Lake Agassiz (19). The spike at 12.9 ka indicates possible catastrophic flood discharge (off-scale) of as much as 0.45 Sv (6, 13). The arrow at the top indicates the duration of the Younger Dryas (12.9–11.6 ka) in Greenland's GISP2 ice core (2). The arrow at lower left indicates the modern average annual discharge of the St. Lawrence River.

Many issues and questions remain. As always, chronological uncertainties obscure interpretations of history. Some of the dating conflicts that exist may arise from using minimum ages for glacial events, even if the ages are closely limiting minima. In addition, the Younger Dryas is a challenging interval for radiocarbon dating because it coincides with a change in the content of radiocarbon in the atmosphere. This change, itself partly the result of changes in oceanic circulation related to the Younger Dryas (18), makes age resolution of less than a few hundred years very difficult at this time.

The question of the route of the discharge from Lake Agassiz to Lake Superior at the beginning of the Younger Dryas is still problematic. Seismic stratigraphic studies beneath Lake Superior are underway to help address this issue. The classical story of the switch from southward to eastward drainage of Lake Agassiz involved an initial massive and catastrophic flood (13). Carlson et al. (1) make no mention of this flood and instead explain their record entirely on the basis of increases and decreases in base flow down the St. Lawrence. Therein may lie an explanation for the lack of spectacular flood canyons on the drainage route to the east, such as those that are related to a younger, post-Younger Dryas drainage event. If the drawdown during the Moorhead Phase took place gradually (over decades to a couple of centuries, not resolvable in the records of Carlson et al.), the discharge may not have formed pronounced geomorphic features (16). If the discharge occurred near the ice margin but under (or over) the ice (7), bedrock erosion and large-scale depositional forms would also be minimized.

Whatever the issues that remain, the article by Carlson et al. (1) is a major contribution to the controversy over the Younger Dryas cold interval, and it is sure to stimulate a great deal of follow-up research. As for the recent differences of opinion that have arisen from reexamining the accepted history of the Younger Dryas, they have generated much new research, including the article by Carlson et al. Other creative theories, including the possibility of an extraterrestrial impact at this time, are looming on the horizon. There is something to be said for challenging conventional wisdom.

Footnotes

Author contributions: S.M.C. wrote the paper.

The author declares no conflict of interest.

See companion article on page 6556. Click on the page number to go to the original article by Carlson et al.

The Zeppelin station was among those showing a significant rise

Higher atmospheric levels of the greenhouse gas methane noted last year are probably related to emissions from wetlands, especially around the Arctic.

Scientists have found indications that extra amounts of the gas in the Arctic region are of biological origin.

Global levels of methane had been roughly stable for almost a decade.

Rising levels in the Arctic could mean that some of the methane stored away in permafrost is being released, which would have major climatic implications.

The gas is about 25 times more potent than carbon dioxide as a greenhouse gas, though it survives for a shorter time in the atmosphere before being broken down by natural chemical processes.

Northern lights

Indications that methane levels might be rising after almost a decade of stability came last month, when the US National Oceanic and Atmospheric Administration (NOAA) released a preliminary analysis of readings taken at monitoring stations worldwide.

NOAA suggested that 2007 had seen a global rise of about 0.5%.

Some stations around the Arctic showed rises of more than double that amount.

One is the station at Mount Zeppelin in Svalbard, north of Scandinavia.

In addition to the long-term monitoring carried out there by Norway and Sweden, a British team has recently started gathering samples and analysing them in a way that could reveal where the methane is coming from.

Methane produced by bacteria contains a high proportion of molecules with the lighter form (isotope) of carbon, carbon-12, rather than the heavier form, carbon-13.

I think 2007 is probably down to wetland emissions Ed Dlugokencky, NOAA

"Anything where bacteria form methane, you get depletion in C-13 because methanogens (the bacteria) preferentially use C-12," said Rebecca Fisher from Royal Holloway, University of London, who has been running the Svalbard experiments.

"The results we have so far imply a predominantly biogenic source," she told BBC News.

The researchers also match methane levels with wind direction, so they can see where the gas is being produced. This analysis also implies a source in the Arctic regions, rather than one further afield such as the additional output from Asia's rapid industrialisation.

Warm and wet

Ed Dlugokencky, the scientist at NOAA's Earth System Research Laboratory (ESRL) who collates and analyses data from atmospheric monitoring stations, agrees that the 2007 rise has a biological cause.

"We're pretty sure it's not biomass burning; and I think 2007 is probably down to wetland emissions," he said.

"In boreal regions it was warmer and wetter than usual, and microbes there produce methane faster at higher temperatures."

Dr Dlugokencky also suggested that the drastic reduction in summer sea ice around the Arctic between 2006 and 2007 could have increased release of methane from seawater into the atmosphere.

Companies are looking to exploit the energy in methane hydrates

A further possibility is that the gas is being released in increasing amounts from permafrost as temperatures rise.

Researchers will be keeping a close eye on this year's data which will indicate whether 2007 was just a blip or the beginning of a sustained rise.

Methane concentrations had been more or less stable since about 1999 following years of rapid increases, with industrial reform in the former Soviet bloc, changes to rice farming methods and the capture of methane from landfill sites all contributing to the levelling off.

In the recent past, concentrations have risen during El Nino events, whereas the world is currently amid the opposite climatic pattern, La Nina.

Solid evidence

An upturn in methane concentrations emissions could have significant implications for the Earth's climatic future.

A sustained release from Arctic regions or tropical wetlands could drive a feedback mechanism, whereby higher temperatures liberate more of the greenhouse gas which in turn forces temperatures still higher.

A particularly pertinent question is whether methane is being released from hydrates on the ocean floor.

These solids are formed from water and methane under high pressure, and may begin to give off methane as water temperatures rise.

The amount of the gas held in oceanic hydrates is thought to be larger than the Earth's remaining reserves of natural gas.

In collaboration with other British institutions, Dr Fisher's team will begin work this summer sampling water near hydrate deposits to look for indications of gas emerging.

Richard.Black-INTERNET@bbc.co.uk

Link to article: http://news.bbc.co.uk/2/hi/science/nature/7408808.stm

I booked flights the other day. Ouch. Since I travel by air four or five times a year, I watch the industry a little. Its struggling. Its fleet is aging, fuel prices have a big impact on bottom line, and nearly everything about airlines is regulated or unionized. Business models are old. A few carriers, like Southwest, are doing sort of okay, and the others are doing less than okay.

We think that we need more fuel efficient cars? We do. And airlines need better jets.

Yet access to new planes is controlled by a worldwide duopoly: Airbus and Boeing. Boeing is late on its popular (but still not flying) Dreamliner, and just announced a probably multi-year delay in completing design for 737 replacement. Airbus isn't doing well enough to take up that slack, and may be adding to the shortage of anything modern or useful available for sale. So the existing airplane suppliers are in trouble, but the demand for airplanes is huge. Airplanes and air flight, and even better, pleasant air flight like we once enjoyed (How long has it been since you looked forward to getting on a plane?).

I bet someone out there is assembling a design team to beat Boeing and Airbus to the punch. Yes, it's a high barrier to entry. Yes it's an expensive and risky and regulated process. But look how much good it could do the world. And whoever makes an agile and green plane first, might also see a pretty good reward.

by Larry O'Hanlon, Discovery News, May 8, 2008

The idea that Greenland's ice melts sluggishly in response to global warming has long been one hedge against rapid global sea level rise -- but the idea may be wrong, say researchers.

New geologic evidence from the seafloor off the southern tip of Greenland shows that during the two past periods of global warming, the melting of Greenland glaciers was right in synch with rising global temperatures -- rather than lagging behind as models have predicted.

In other words, the ice is very sensitive to recent losses of ice there could be the beginning of a much larger melt than expected.

"People had thought that there was this thermal lag," said Anders Carlson of the University of Wisconsin in Madison.

But offshore sediments that record the debris that is washed out from valleys when glaciers recede tell another story. As Carlson said, "As soon as it starts warming, it starts to melt."

The evidence, which Carlson and his colleagues present in the May issue of the journal Geology, comes in the form of titanium and iron found in cores of sediments offshore at what's called Eirik Drift.

The only sources for the two elements are land-based rocks which wash out of glacial valleys and into the sea once glaciers melt back and expose the ground to the weather. That process, the scientists figure, is a pretty good sign of a warmer climate.

Those two warmer periods were about 120,000 to 132,000 years ago and 10,000 to nearly 15,000 years ago.

"The last very big change was the one that ended the last ice age," explained Kurt Cuffey, a professor in the Geography and Earth and Planetary Sciences departments at the University of California at Berkeley. "What they found pretty clearly is that the melt…began immediately."

By immediately, Cuffey explained, he means geologically speaking -- within a few centuries or less.

The new method fills in a void of information caused by the glaciers themselves. Each new glacial advance tends to obliterate the evidence of past surges and retreats, unless it's left piled up at what was once the end of the glacier.

In the case of Greenland, however, the glaciers end in the sea, which complicates the job of locating and studying them.

As for how it all relates to the recent accelerated melt and net loss of ice mass over much of Greenland, Carlson said their data could herald a lot more of the same.

"What we can say is that (the Greenland Ice Sheet) responded very fast," said Carlson. "Our data suggest that the current negative mass balance is the beginning of a long-term trend."

Link to article: http://dsc.discovery.com/news/2008/05/08/greenland-ice-melt.html

This post is to provide a link to the abstracts of the studies presented at the Arctic Forum:

http://www.arcus.org/annual_meetings/2008/abstracts.html

Here is a sample of one of the "softer" articles:

Public Perceptions of Climate Tipping Point Information:

The Case of Santa Barbara

Bruce Caron^1
1The New Media Studio, New Media Research Institute, 417 Samarkand Drive, Santa Barbara, CA, 93105, USA, Phone 805-568-0115, Fax 805-966-1100, bruce@tnms.org

This talk will outline lessons learned from a public-art-based climate-change awareness/education project. In the summer of 2006 a group of Santa Barbara, California residents decided to catalyze public awareness of the future impacts of human-induced climate change (HICC) by marking these future impacts on the built environment. Marking the future impacts of HICC on the local scene would provide a daily reminder of the future impacts of current energy use choices. The hope was that awareness would lead to personal motivations to help stop HICC. The group decided that the best story to tell would be the tipping point for the melting of the ice sheet on Greenland. After a year of public meetings, the City Council approved the "lightblueline" project. Teams of volunteers would paint the seven-meter elevation contour (the amount of sea-level rise held in the Greenland ice sheet) on city streets. An anti-lightblueline effort, led by the local daily newspaper, succeeded in stalling the installation of these markings by feeding public fears about declining property values while fostering uncertainty about the science of climate change. The idea of anthropogenic climate tipping points was termed a "belief." Arguments that climate change was still being debated by scientists and that current climate models are inadequate sidelined the main conversation. Timescale factors played a part: any effect beyond, say, fifty years acquired insignificance in the face of current problems. Opponents spent twenty times the resources of the group to delay the project.

Jessica Marshall, Discovery News, May 22, 2008

Greenland's caribou are arriving at their spring breeding grounds to find a food supply far past its prime. Caribou in West Greenland migrate inland in spring, to the western edge of the country's inland ice sheet, where they give birth to their calves. There, they feed on freshly emerged plants, which provide the best nutrition.

But two new studies by Eric Post of the Pennsylvania State University and colleagues show that global warming has thrown this system out of whack. Plants are emerging earlier, and all at once across the landscape, so the caribou are arriving to find the plants they rely on are past their prime.

"Because of warming, food is becoming available earlier in the year for caribou. That might sound like a good thing, because caribou come out of the long Arctic winter hungry," Post told Discovery News by e-mail from Greenland, where he is awaiting the spring calving season.

"But we also found that caribou are not adjusting their birth season to help keep up with changes in plant growth. As a consequence, their food is in a sense being taken off the table again before caribou have had a chance to get what they need."

Post documented that over a period of six seasons -- 1993, and 2002-2006 -- the average spring temperature rose by more than 8 degrees Fahrenheit and plants emerged about two weeks earlier, while calf mortality increased fourfold and calf production declined by a factor of seven. These results were published online earlier this month in Philosophical Transactions of the Royal Society B.

In the second study, appearing online today in the Proceedings of the Royal Society B, Post's team showed that warming is also causing plants across the landscape to mature all at the same time. Normally, caribou would start out in the valley where plants emerge first and move uphill, following new plants as the season progresses.

"As it gets warmer, there's less and less difference between the plants in the valley and those on the hilltop. So now, when the caribou goes uphill, counting on finding a good meal, it's out of luck," Post said. "The plants are past their peak."

"Overall, this means that the caribou isn't getting the best diet, and isn't able to provide its newborn calf with the best nutrition," he added. "We think that makes it harder for the calves to survive the first few days of life."

"While we were collecting data on plant growth during those seven years, we also made observations on caribou calving, and found that in years when plant growth started almost simultaneously across the landscape, caribou had fewer calves," Post said.

The mismatch arises because caribou use changing day length in spring to cue their reproduction, while plants rely on temperature.

With global warming, "temperatures rise but day length stays exactly the same," said Marcel Visser at the Netherlands Institute of Technology in Heteren.

This is not the first example where climate change has skewed food supply and demand. Visser was the first to discover this relationship in great tits, whose caterpillar prey emerged earlier because of warming, before the birds' eggs hatched.

But "it's the first time people have shown this in mammals," Visser said.

For the animals to adapt, "they need genetic variation in the population that will push back the earliest date for reproducing," he added. "But that is a slow process. One of the worries is that climate change will occur faster than these changes can occur."

Link to article: http://dsc.discovery.com/news/2008/05/22/caribou-climate-change.html