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Ancient Climate

For the first time, glaciologists have combined and compared sets of ancient climate records trapped in ice cores from the South American Andes and the Asian Himalayas to paint a picture of how climate has changed – and is still changing – in the tropics.

Their conclusions mark a massive climate shift to a cooler regime that occurred just over 5,000 years ago, and a more recent reversal to a much warmer world within the last 50 years.
The evidence also suggests that most of the high-altitude glaciers in the planet's tropical regions will disappear in the near future. The paper is included in the current issue of the journal Proceedings of the National Academy of Science.
Lastly, the research shows that in most of the world, glaciers and ice caps are rapidly retreating, even in areas where precipitation increases are documented. This implicates increasing temperatures and not decreasing precipitation as the most likely culprit.
The researchers from Ohio State University's Byrd Polar Research Centre and three other universities combined the chronological climate records retrieved from seven remote locations north and south of the equator. Cores drilled through ice caps and glaciers there have captured a climate history of each region, in some cases, providing annual records and in others decadal averages.

"Approximately 70 percent of the world's population now lives in the tropics so when climate changes there, the impacts are likely to be enormous" - Lonnie Thompson, professor of geological sciences at Ohio State.

For the last three decades, Thompson has led nearly 50 expeditions to remote ice caps and glaciers to drill cores through them and retrieve climate records. This study includes cores taken from the Huascaran and Quelccaya ice caps in Peru; the Sajama ice cap in Bolivia; the Dunde, Guliya, Puruogangri and Dasuopu ice caps in China.
For each of these cores, the team -- including research partner Ellen Mosley-Thompson, professor of geography at Ohio State – extracted chronological measurements of the ratio of two oxygen isotopes -- O18 and O16 -- whose ratio serves as an indicator of air temperature at the time the ice was formed. All seven cores provided clear annual records of the isotope ratios for the last 400 years and decadally averaged records dating back 2000 years.

"We have a record going back 2,000 years and when you plot it out, you can see the Medieval Warm Period (MWP) and the Little Ice Age (LIA)" - Lonnie Thompson.

During the MWP, 700 to 1000 years ago, the climate warmed in some parts of the world. The MWP was followed by the LIA, a sudden onset of colder temperatures marked by advancing glaciers in Europe and North America .

"And in that same record, you can clearly see the 20th Century and the thing that stands out – whether you look at individual cores or the composite of all seven – is how unusually warm the last 50 years have been. There hasn't been anything in the record like it – not even the MWP. The fact that the isotope values in the last 50 years have been so unusual means that things are dramatically changing. That's the real story here" - Lonnie Thompson.

While the isotope evidence is clear throughout all of the cores, Thompson says that the more dramatic evidence is the emergence of unfossilised wetland plants around the margin of the Quelccaya ice cap, uncovered as the ice retreated in recent years.
First discovered in 2002, the researchers have since identified 28 separate sites near the margin of the ice cap where these ancient plants have been exposed. Carbon-dating revealed that the plants range in age from 5,000 to 6,500 years old.

"This means that the climate at the ice cap hasn't been warmer than it is today in the last 5,000 years or more. If it had been, then the plants would have decayed" - Lonnie Thompson

The researchers say a major climate shift around 5,000 years ago in the tropics had to have cooled the region since the ice cap quickly expanded and covered the plants. The fact that they are now being exposed indicates that the opposite has occurred – the region has warmed dramatically, causing the ice cap to quickly melt.

The role of precipitation in the global retreat of alpine glaciers may have been clarified by this study. Some researchers, convinced that a reduction in local precipitation is causing their retreat, have been sceptical about the role of rising temperatures.

"While all the glaciers we have measured throughout the tropics are retreating, the local precipitation at all of these sites but one, has increased over the last century. That means that the retreat of the ice is driven mainly by rising temperatures" - Lonnie Thompson

Changes in the oxygen isotope ratios over the past 100 years have also pointed to temperature, rather than precipitation, as the engine driving glacial retreat.

"Tropical glaciers are the ‘canaries in the coal mine' for our global climate system, as they integrate and respond to most of the key climatological variables – temperature, precipitation, cloudiness, humidity and radiation" - Lonnie Thompson

The evidence arising from the tropics is particularly important.

"The uniformity of the climate in the tropics makes these kinds of records so critical since they tell us what is happening to global temperatures. What this is really telling us is that our climate system is sensitive, it can change abruptly due to either natural or to human forces. If what happened 5,000 years ago were to happen today, it would have far-reaching social and economic implications for the entire planet. The take-home message is that global climate can change abruptly, and with 6.5 billion people inhabiting the planet, that's serious" - Lonnie Thompson.

Working along with Thompson and Mosley-Thompson on the project were Henry Brecher, Mary Davis, Ping-Nan Lin and Tracy Mashiotta, all with the Byrd Centre; Blanca Leon of the University of Texas; Don Les of the University of Connecticut, and Keith Mountain of the University of Louisville.




Posts: 131433
RE: Iceage melt

New Model Suggests Antarctic More Dynamic Than Previously Believed

Through dated geological records scientists have known for decades that variations in the Earth's orbit around the sun – subtle changes in the distance between the two – control ice ages. But, for the first 2 million years of the Northern Hemisphere Ice Age there has always been a mismatch between the timing of ice sheet changes and the Earth's orbital parameters.

A new model of ice volume change developed by Boston University researchers Maureen Raymo and Lorraine Lisiecki proposes a reason for this discrepancy. Like other models, it is consistent with traditional Milankovitch theory – which holds that the three cyclical changes in the Earth's orbit around the Sun (obliquity, precession, and eccentricity) influence the severity of seasons and high latitude temperatures over time. However, the new model differs from earlier ones in that it allows for a much more dynamic Antarctic ice sheet.
According to the researchers, from 3 million years ago to about 0.8 million years ago, Northern Hemisphere ice volume appears to have varied mostly with the 41,000 year period of obliquity – the periodic shift in the direction or tilt of Earth's axis. However, summer insolation (incoming solar radiation), which is widely believed to be the major influence on high-latitude climate and ice volume change, is typically dominated by the 23,000 year precessional period – the slow "wobble" of the Earth on its axis.

"Because summer insolation is controlled by precession, and summer heating controls ice sheet mass balance, it is difficult to understand why the ice volume record is dominated by the obliquity frequency. It's not a complete mismatch, but the precession frequency we think should be strong in geological records is not" - Dr. Maureen Raymo.

The new model proposes that during this time, ice volume changes occurred in both the Northern Hemisphere and Antarctica, each controlled by different amounts of local summer insolation paced by precession.

"The reason the frequency is not observable in records is because ice volume change occurred at both poles, but out of phase with each other. When ice was growing in the Northern Hemisphere, it was melting in the Southern" - Dr. Maureen Raymo.

The team believes scientists have been operating under the assumption that Antarctica has been exceptionally stable for 3 million years and very difficult to change climatically.

"We don't tend to think of ice volume in that region as varying significantly, even on geologic time scales. However, only a modest change in Antarctic ice mass is required to "cancel" a much larger Northern ice volume signal" - Dr. Maureen Raymo.

Records used to measure the ice volume, such as sea levels, integrate the whole world. According to Raymo, the new model demonstrates that while the precession frequency is actually strong in ice volume changes at each pole, in geologic records Northern and Southern hemisphere ice volume trends act to cancel each other out at this frequency.
The paper, "Plio-Pleistocene Ice Volume, Antarctic Climate, and the Global δ18O Record," will appear in an upcoming issue of the journal Science, proposes that the Antarctic ice sheet is more dynamic and far more capable of change than previously believed.

"If our theory holds true, it is a cause for concern with regard to climate changes not associated with orbital patterns as well" - Dr. Maureen Raymo.

Source Boston University



Posts: 131433
last Ice Age

The end of the recurring, 100,000-year glacial cycles is one of the most prominent and readily identifiable features in records of the Earth's recent climate history. Yet one of the most puzzling questions in climate science has been why different parts of the world, most notably Greenland, appear to have warmed at different times and at different rates after the end of the last Ice Age.

However, a new study appearing in the upcoming issue of the journal Science suggests that, except for regions of the North Atlantic, most of the Earth did, in fact, begin warming at the same time roughly 17,500 years ago. In addition, scientists suggest that ice core records from Greenland, which show that average temperatures there did not warm appreciably until about 15,000 years ago, may have remained in a hyper-cold state largely as a result of events triggered by warming elsewhere.
The research, led by Joerg Schaefer from the Lamont-Doherty Earth Observatory, a member of The Earth Institute at Columbia University, and George Denton at the University of Maine, relied on a method known as cosmogenic or surface-exposure dating, which enabled the scientists to determine how long rock surfaces have been exposed since the glaciers retreated. As cosmic rays penetrating the Earth's atmosphere strike the scoured rock, they form an isotope of the element beryllium, 10Be, at a known rate. By measuring the minute amounts of 10Be in rock samples from glacial moraines in California and New Zealand and comparing these data to previously published results from Wyoming, Oregon, Montana Argentina, Australia and Switzerland, Schaefer and his colleagues were able to narrow down when glaciers around the world began to retreat. They found that almost everywhere they looked the glaciers began to pull back approximately 17,500 years ago. Additional studies from tropical South America southern Tibet have also produced similar results.

Lake Pukaki, New Zealand
Latitude 44° 5'2.42"S Longitude 170°10'42.25"E

"It's amazing everything fits so well and that every moraine record of the last termination seems to match with rising temperature in the Antarctic and CO2 in the atmosphere. It's especially surprising because Antarctica was classically thought to be too remote and climatically isolated to respond in a synchronous manner with the rest of the planet" - Joerg Schaefer, Doherty Associate Research Scientist.

The only place that does not fit the observed warming pattern is Greenland, which did not begin to emerge from the last Ice Age until roughly 15,000 years ago. The authors believe that this anomaly may be because the North Atlantic experienced continued, hyper-cold winters during the intervening 2,500 years that prevented the region from warming on average.
Glaciers, they write, are highly sensitive to summer temperatures. Instead of responding to the rise in global summer temperatures that occurred around 17,500 years ago, however, Greenland may have experienced a continued ice age climate caused by massive armadas of icebergs from the melting continental ice sheets on North America and Northern Europe spreading across the North Atlantic. The freshwater from the melting icebergs likely caused an ocean current known as the meridonal overturning circulation, which transports heat northward from the equator, to almost cease and prevent Greenland and the North Atlantic from warming for millennia after the rest of the planet had begun to do so.

"The spreading sea ice would have also brought the circum-polar winds farther south. This would have interfered with temperature and precipitation patterns in the northern mid-latitudes and put the North Atlantic in the deep freeze for 2,500 years" - Joerg Schaefer.




Posts: 131433
Iceage melt

The sudden deep freeze of the northern hemisphere that occurred 13,000 years ago has been traced to events originating in northern Canada, according to University of Toronto research. The findings could shed light on the future of climate change due to greenhouse gases.

The study, published in the June 2 issue of Nature, pinpoints the exact location where freshwater generated by the melting of the massive Canada-wide Laurentide ice sheet entered the global ocean and caused the Younger Dryas cold reversal, a frigid period where the planet temporarily plunged into ice age conditions.
Contrary to previous thinking, the study shows that this meltwater entered the Arctic Ocean rather than the Atlantic and the point of entry was through the MacKenzie River.

As the freshwater - lighter due to its lack of salt content - flowed into the ocean it was transported across the pole into the North Atlantic where it shut down the process whereby heavy surface water sinks into the abyss and leads to a warming of the northern hemisphere.

While the Younger Dryas cold reversal occurred just as the Earth was emerging from the most recent ice age, a rapid meltback of the Greenland ice sheet - another large accumulation of land ice adjacent to the North Atlantic Ocean - could theoretically contribute to another such shutdown.

"Greenland contains enough ice to raise sea level by about seven metres if it were all to melt. If it were to melt very quickly we could easily have a similar event, so the question is just how Greenland will react to the ongoing warming due to the increasing concentration of atmospheric greenhouses gases. How probable this is remains an open question." - Professor Richard Peltier, co-author University of Torontos Department of Physics.

To pinpoint the location of where the Younger Dryas event occurred, Peltier and his co-author, physic's research associate Lev Tarasov, used the University of Toronto Glacial Systems Model (GSM) - a model that produces a three-dimensional view of the evolving ice-sheet as it expands and contracts over the North American continent in response to climate variations.

The model also analyses how the shape of the Earth is affected by the evolution of the heavy ice loads. As the continental ice melted, a huge amount of deglaciation derived freshwater was added to the oceans. At the time of Younger Dryas onset the routing of this meltwater was into the Arctic Ocean.

"In considering the issue of climate change, many people imagine that this could only happen very gradually. This event shows that our climate could change extremely rapidly and with very dramatic effect." - Richard Peltier.

Peltier stresses that climate changes, such as a massive Greenland melt, are very difficult to predict as Earth's climate system is highly non-linear, involving the interactions between a number of distinct and individually complex components such as sea ice and land surface processes as well as the atmosphere and oceans.

"These systems are capable of responding in a way that is out of proportion to the stimulus. You can push them just a little bit and cause them to cross a threshold, such that the response is extremely surprising. From a physics standpoint, the climate system of the planet is a beautiful example of such non-linear systems." - Richard Peltier.

The research was supported by the Natural Sciences and Engineering Research Council of Canada and by the Canadian Foundation for Climate and Atmospheric Science through a new collaborative research network called Polar Climate Stability which is led by Peltier.
The network involves researchers from seven different Canadian universities.

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