Earth's oldest ice: a history of polar ice-core research
1954
The isotope thermometer
Following a series of experiments including the impromptu collection of rainwater in beer bottles, pots and pans in his back yard, young Danish geochemist Willi Dansgaard makes a remarkable discovery. In a landmark paper, he suggests that stable oxygen isotopes laid down in layers of ice can be used as indicators of past air temperature. Water molecules in clouds are made of either light oxygen-16 isotopes or heavy oxygen-18 isotopes. The oxygen-18 molecules are the first to be lost as rain and snow, and as clouds gradually cool, less and less oxygen-18 is precipitated. By drilling ice cores and melting the ice for analysis, it is possible to reconstruct the climatic history of the Earth's poles. (See Dansgaard Geochim. Cosmochim. Acta 1954)
For International Geophysical Year 1957–1958, a global initiative involving 67 nations to better understand the Earth, the United States launches some of the first projects (PDF) to drill ice cores from glaciers at both poles — thanks to Henri Bader, who champions the value of the cores as chief scientist of the US Army's polar research unit.
Image: Byrd research station in Antarctica, established in 1956–1957 / US ARMY TRANSPORTATION MUSEUM
1966
First deep polar core hits bedrock
US military personnel led by Lyle Hansen are the first to drill through an ice sheet. The site is Camp Century, a military research station that literally evokes the Cold War with a veritable small village — complete with church, cinema, nuclear reactor, and a population of up to 250 men — dug down and hidden below the surface of the ice sheet. After hitting bedrock at 1,387 metres, they ship Hansen's drill to Antarctica, where a second deep ice core is being started at Byrd research station.
Using the drill Hansen designed for Camp Century, a US ice-core team at Byrd station becomes the first to reach bedrock in Antarctica at 2,164 metres below the surface. But as they pierce the sheet, meltwater collected at the Earth's warm surface floods into the hole, then refreezes and traps the drill irretrievably. The still-intact Antarctic core, however, offers more than 75,000 years of temperature data. (See Epstein et al. Science 1970)
Eager to be the first to examine the climatic record from Camp Century, Dansgaard proposes a collaboration with Chester Langway, the US Army scientist in charge of the core. Dansgaard and colleagues tentatively date the Greenland temperature profile back 100,000 years. In the upper core, oxygen isotope ratios differ between winter and summer ice, allowing the group to trace the build-up of ice layers year by year. But the flow of the Greenland glacier over time has distorted the lower layers, so the researchers use ice-flow theory to estimate dates for the older ice. (See Dansgaard et al. Science 1969)
The Byrd Station core proves more difficult to date than the one from Camp Century: because of light snowfall at the Antarctic site, the annual layers of oxygen isotopes aren't clear enough to count, and complicated ice-sheet conditions hamper other dating methods. Scientists know that both cores extend back about 100,000 years, but the exact timing of events at the two poles can't yet be compared. (See Johnsen et al. Nature 1972)
Image: Drilling hole in Antarctica / MALIN STENBERG DE SERVES
1981
Musketeers' mission
A Danish–Swiss–American group called the Greenland Ice Sheet Project (GISP) drills the second deep Arctic core, completing a mission set up in the 1970s by Langway and Dansgaard with Swiss climatologist Hans Oeschger. Although they had originally hoped to drill in central Greenland to obtain the thickest, least disturbed ice, it is deemed too risky to try the precious drill at such a remote site. Instead the core comes from Dye 3, a US radar station near the south coast. A 90,000-year comparison of the Camp Century and Dye 3 temperature profiles hints that the last glacial period was punctuated by a series of several warm spells. (See Dansgaard et al. Science 1982)
Image: Dansgaard, Langway and Oeschger were known as 'the three musketeers' during their work together on the Dye 3 core / W. DANSGAARD, FROZEN ANNALS
1985
Interglacial glimpse
A 150,000-year temperature record is recovered from east Antarctica, where the intensely cold, dry conditions preserve ancient ice much better than in snowy Greenland. The new core comes from Vostok station, a Soviet research post established during International Geophysical Year. After initial reports of the work are published in Russian, French glaciologist Claude Lorius organizes an international analysis of the ice. It is the first ice record to extend through the last glacial period and beyond, but its low resolution blurs changes on the timescale of less than a century. (See Lorius et al. Nature 1985)
Image: Vostok station, a Soviet research station in east Antarctica / NOAA PALEOCLIMATOLOGY PROGRAM, TODD SOWERS, COLUMBIA UNIVERSITY
1987
CO2 forces climate change
Air trapped in Vostok ice shows not only that CO2 levels fall and rise as the glacial period starts and ends, but that the size of the temperature swings can be explained only if the release of greenhouse gas amplifies warming caused by tiny changes in the Earth's orbit. Scientists find a strong natural feedback between rising temperatures and the release of CO2 from the land and sea. But because greenhouse gas concentrations are measured directly from air bubbles trapped in the ice, whereas temperature is inferred from other clues, attempts to align the records can err by many centuries. The chicken-and-egg problem of which came first in Antarctica remains unresolved. (See Barnola et al. Nature 1987, Jouzel et al. Nature 1987, Genthon et al. Nature 1987)
Image: Ancient air bubbles trapped in Antarctic ice / MALIN STENBERG DE SERVES
1992
Core highlights climate chaos
High-resolution ice at the Greenland summit is still being eyed by the coring communities on both sides of the Atlantic, generating two side-by-side projects in the 1990s. The European initiative known as the Greenland Ice Core Project (GRIP) discovers a remarkably turbulent climate during the last glacial period; temperatures oscillate up to 8 °C between glacial lows and highs, switching every few hundred to few thousand years. (See Johnsen et al. Nature 1992)
Earlier claims that the Earth's climate can flip between several different states — argued most forcefully by geoscientist Wally Broecker in the 1980s — are looking likely. Broecker soon names the spikes of warmth Dansgaard–Oeschger events, after the two scientists most involved in uncovering them. The question now is whether these flips happen only when ice covers much of the planet. (See Broecker et al. Nature 1985)
Image: GRIP headquarters in central Greenland / IVARS SILIS
1993
April–July - Fast flips and flickers
Early data from the US Greenland coring project — GISP2 — brings evidence that the three most recent periods of dramatic warming took place over just a few years. In the final warm-up about 11,700 years ago that ended the last glacial cold phase — the so-called Younger Dryas — snowfall appears to double in one to three years. (See Alley et al. Nature 1993)
In July, the European team working 28 kilometres away publishes their full core results ahead of the Americans. The longest yet at 3,029 metres, the core provides evidence of an erratic climate as far back as 115,000 years ago, during an interglacial period known as the Eemian. Average temperatures then were about 2 °C warmer than the present and appear to have fluctuated up to 10 °C in two decades or less. This leads to speculation that human-induced climate change might tip today's steady interglacial climate towards a similar state. (See GRIP members Nature 1993, Dansgaard et al. Nature 1993)
Image: Dryas octopetala, which flourished during past cold spells, gives the Younger Dryas its name / HANS-JÜRGEN BECKER, BODENHEIM
December - Two cores better than one
In the summer drilling season the US team hits bedrock 3,053 metres below the Greenland ice sheet summit. Their data clearly confirm the European finding that the glacial period 14,000 to 114,000 years ago was marked by an erratic climate. There is little agreement between the cores, however, on older time periods. Further analysis reveals that the Eemian sections of both cores are distorted and unreliable. (See Taylor et al. Nature 1993, Grootes et al. Nature 1993, Alley et al. Nature 1995)
Image: GISP2 drill dome at the summit of the Greenland ice sheet / NOAA PALEOCLIMATOLOGY PROGRAM, MICHAEL C. MORRISON, UNIVERSITY OF NEW HAMPSHIRE
1995
Linking the poles
Because atmospheric chemistry is similar at both poles, scientists are able to date lower-resolution Antarctic ice by comparing atmospheric oxygen isotopes from the cores with those from a higher-resolution Greenland ice core. Many of the dramatic warming periods in Greenland are found to have counterparts at the opposite pole. (See Sowers & Bender Science 1995, Bender et al. Nature 2002)
Image: GISP2 cores in storage at the US National Ice Core Laboratory in Denver, Colorado / NATIONAL ICE CORE LABORATORY, NSF, USGS
1996
A sticking point
At Dome Fuji, or Dome F — one of three high, flat peaks that top the East Antarctic Ice Sheet — a Japanese group headed by Okitsugu Watanabe sets out to drill some of the thickest and oldest ice on the continent. Unlike other coring efforts, work continues straight through the Antarctic winter, with a handful of personnel living in space-station-like conditions. But after extracting 2,503 metres of ice dating back 340,000 years, the drill clogs up with ice chips. Unable to recover the drill head, the team has to abandon the borehole. (See Watanabe et al. Ann. Glaciol. 1999)
Image: Ice from Dome F, Antarctica / HIDEAKI MOTOYAMA
1997
Recent cold spell spotted
Analysing the core taken by US researchers, veteran paleoclimatologist Richard Alley and his colleagues discover a marked cooling in Greenland about 8,000–8,400 years ago, together with changes in methane and windblown dust deposits that indicate widespread climate change. The Greenland temperature dip, which is given the less-than-colourful name 'the 8.2-kiloyear event', was about half that seen in the Younger Dryas. (See Alley et al. Geology 1997).
As techniques for analysing ice records proliferate, an advance by post-doc researcher Jeffrey Severinghaus provides a sweeping view of the paleoclimate. He and colleagues report that rapid temperature changes leave a recognizable signature of nitrogen and argon isotopes in Greenland ice cores. A new method for tracing abrupt temperature change via these airborne chemicals allows climate and atmospheric greenhouse gases to be tracked side by side without an age gap. The researchers deduce that levels of methane — a potent greenhouse gas — rose about 30 years after the abrupt warming that ended the Younger Dryas cool spell and started the interglacial. Since it's thought that most methane is produced in wetlands far from the ice, the implication is that this sharp climate swing occurred almost simultaneously throughout the Northern Hemisphere, if not further afield. (See Severinghaus et al. Nature 1998, Severinghaus & Brook Science 1999)
An international team drilling at Vostok halts at a record depth of 3,623 metres. Biologists have convinced them to avoid penetrating a lake of meltwater below, which may contain a unique ecosystem trapped under the ice for millions of years. (See Life in the deep freeze)
But the core's 420,000-year history shows the ends of four glacial periods, each marked by almost doubled CO2 and methane levels and higher temperatures. The relative timing of warming and greenhouse gas release is debated, however, as temperature and gas records still can't be lined up precisely in time. (See Petit et al. Nature 1997, Petit et al. Nature 1999, Fischer et al. Science 1999).
Image: Drilling to the frozen surface of Lake Vostok / NATURE, ADAPTED FROM M. STUDINGER, R. E. BELL, LAMONT-DOHERTY EARTH OBSERVATORY
August - The 'see-saw' effect
A study of the last 50,000 years shows that warming and cooling spells in east Antarctica occur out of phase with corresponding events in Greenland. This supports an idea argued by Broecker, and also backed by models, that Arctic Dansgaard–Oeschger events represent one end of a climate 'see-saw' that is pushed up and down by changes in ocean currents, so that Antarctica cools as Greenland warms and vice versa. (See Blunier et al. Nature 1998, Broecker Paleoceanogr. 1998, Blunier & Brook Science 2001)
Image: Antarctic coast / MALIN STENBERG DE SERVES
2004
June - Amassing evidence
A core from Antarctica's Dome C almost doubles the length of any ice record, to 740,000 years — adding temperature data from four more glacial cycles. Trapped greenhouse gases from up to 650,000 years ago are soon analysed and show that present-day carbon dioxide and methane levels are through the roof by the standards of the last several hundred thousand years.
Image: The drill at Dome C, Antarctica / JOSEPH KIPFSTUHL, ALFRED WEGNER INSTITUTE
September - Breaking the ice
The North Greenland Ice Core Project (NGRIP), a Northern cousin of EPICA involving Europe, the United States and Japan, drills a site where they hoped to find undisturbed bedrock ice from the Eemian interglacial. The team has a rocky start, including the loss of a drill stuck in the ice in 1997 and friction among the scientists leading to the resignation of their steering committee chair. Having weathered the setbacks, the project comes up short in 2003. At a depth of 3,085 metres, water starts flooding the borehole. But the melting lessens the layer-thinning effects of glacial flow, so the core contains distinct annual layers as old as 123,000 years. It portrays climatic swings during the last glaciation in greater detail than was once thought possible and uncovers a previously unrecognized early warming event. The Eemian's final throes in Greenland appear in the record as a stable climate about 5 °C hotter than today. (See NGRIP members Nature 2004 and Ice core reveals gentle start to last ice age)
After a month-long, 1,200-kilometre trek from the nearest research station, a Chinese expedition becomes the first to reach the summit of Dome Argus, also called Dome A — the tallest of Antarctica's three ice domes and possible site of a future core. (See Polar Research: Buried treasure)
Image: A supply convoy traverses Dome A, Antarctica, in 2008 / JEAN DE POMEREU, INTERNATIONAL POLAR FOUNDATION
November - 'See-saw' strengthened
A core from EPICA's second drilling site, at Dronning Maud Land, Antarctica, gives a detailed view of the last glacial period that is unprecedented in Antarctic cores. Lined up against Greenland records, the temperature profile strongly supports an out-of-phase 'see-saw' relationship between climate fluctuations in the Northern and Southern Hemispheres. (See EPICA community members Nature 2006)
Image: EPICA at work at Kohnen station in Dronning Maud Land, Antarctica / HANS OERTER, ALFRED WEGNER INSTITUTE
2007
January - More drilling at Dome F
A new core is drilled through Dome F's thick ice. At first the scientists working on the core believed its ice might be even older than Dome C's, but a preliminary analysis pegs the earliest usable ice near the bottom of the core at about 720,000 years old. (See Motoyama Scientif. Drill. 2007 - PDF)
Image: Dome F, Antarctica / HIDEAKI MOTOYAMA
November - Towards a hi-res Antarctic core
In a project of the International Polar Year 2007–2008, a US group led by GISP2 alumni begins drilling a deep core on the West Antarctic Ice Sheet (WAIS) divide.
The second in a planned pair, the WAIS divide core follows up ill-fated work in the 1990s at an inland site, Siple Dome. There, a combination of inexperienced drilling contractors, a poorly prepared drill and exceptionally brittle ice produced a low-quality core with some sections delivered in finger-sized shatters.
Having sussed out a west Antarctic site with plenty of snowfall, the group now expects to extract the first Antarctic core whose clarity can measure up to cores taken in Greenland, with about 40,000 countable annual layers and a narrow time gap between the trapped air and the temperature record. This should provide a high-resolution, though short, greenhouse gas timeline, covering only one glacial cycle.
Image: Galley at the West Antarctic Ice Sheet divide / SPRUCE SCHOENEMANN
2008
May - The oldest ice yet
The deepest ice in the Dome C core from EPICA — 3,270 metres down and aged at about 800,000 years — is analysed, yielding the oldest air samples yet. Modern-day greenhouse gas concentrations remain unparalleled through those many millennia, with CO2 levels now 28 per cent higher, and methane 134 per cent higher, than at any other point in the record. Abrupt climate changes punctuated previous glacial periods, much like the last one. (See Jouzel et al. Science 2007, Lüthi et al. Nature 2008, Loulergue et al. Nature 2008 and Ice cores reveal climate secrets)
Meanwhile in the Arctic, drilling begins in May on a new core in northwest Greenland as part of International Polar Year 2007–2008. No Arctic ice core yet has provided an undistorted record of the entire Eemian interglacial period, but the North Greenland Eemian (NEEM) ice-drilling project aims to fill this gap.
NGRIP researchers zoom in on some of the most abrupt changes spied in ice. The core's clear layers make it possible to pinpoint the timing of rapid climatic shifts, including the start and finish of the Younger Dryas as well as the beginning of a preceding warm period, known as Bølling-Allerød. The team concludes that during these events atmospheric circulation patterns shifted dramatically in just one to three years. (See Steffensen et al. Science 2008)
Spurred by International Polar Year, the hunt is on for the oldest ice. Researchers seek the holy grail of polar paleoclimatology — a frozen trace of the major transition 1 million years ago when Earth's glacial cycles went into a stately 100,000-year rhythm, slowing down from a previous 40,000-year cycle. A Chinese team begins setting up a research station at Dome A, one promising site for drilling in the next several years. (See China builds inland Antarctic base)
More than 40 years after the first deep core, important secrets still lie buried under ice, and drilling projects at the poles are expected to yield up new climatic clues well into the future.
Image: Dome A team at the new Antarctic research station / JEAN DE POMEREU, INTERNATIONAL POLAR FOUNDATION
Following a series of experiments including the impromptu collection of rainwater in beer bottles, pots and pans in his back yard, young Danish geochemist Willi Dansgaard makes a remarkable discovery. In a landmark paper, he suggests that stable oxygen isotopes laid down in layers of ice can be used as indicators of past air temperature. Water molecules in clouds are made of either light oxygen-16 isotopes or heavy oxygen-18 isotopes. The oxygen-18 molecules are the first to be lost as rain and snow, and as clouds gradually cool, less and less oxygen-18 is precipitated. By drilling ice cores and melting the ice for analysis, it is possible to reconstruct the climatic history of the Earth's poles. (See Dansgaard Geochim. Cosmochim. Acta 1954)
For International Geophysical Year 1957–1958, a global initiative involving 67 nations to better understand the Earth, the United States launches some of the first projects (PDF) to drill ice cores from glaciers at both poles — thanks to Henri Bader, who champions the value of the cores as chief scientist of the US Army's polar research unit.
US military personnel led by Lyle Hansen are the first to drill through an ice sheet. The site is Camp Century, a military research station that literally evokes the Cold War with a veritable small village — complete with church, cinema, nuclear reactor, and a population of up to 250 men — dug down and hidden below the surface of the ice sheet. After hitting bedrock at 1,387 metres, they ship Hansen's drill to Antarctica, where a second deep ice core is being started at Byrd research station.
Using the drill Hansen designed for Camp Century, a US ice-core team at Byrd station becomes the first to reach bedrock in Antarctica at 2,164 metres below the surface. But as they pierce the sheet, meltwater collected at the Earth's warm surface floods into the hole, then refreezes and traps the drill irretrievably. The still-intact Antarctic core, however, offers more than 75,000 years of temperature data. (See Epstein et al. Science 1970)
Eager to be the first to examine the climatic record from Camp Century, Dansgaard proposes a collaboration with Chester Langway, the US Army scientist in charge of the core. Dansgaard and colleagues tentatively date the Greenland temperature profile back 100,000 years. In the upper core, oxygen isotope ratios differ between winter and summer ice, allowing the group to trace the build-up of ice layers year by year. But the flow of the Greenland glacier over time has distorted the lower layers, so the researchers use ice-flow theory to estimate dates for the older ice. (See Dansgaard et al. Science 1969)
The Byrd Station core proves more difficult to date than the one from Camp Century: because of light snowfall at the Antarctic site, the annual layers of oxygen isotopes aren't clear enough to count, and complicated ice-sheet conditions hamper other dating methods. Scientists know that both cores extend back about 100,000 years, but the exact timing of events at the two poles can't yet be compared. (See Johnsen et al. Nature 1972)
A Danish–Swiss–American group called the Greenland Ice Sheet Project (GISP) drills the second deep Arctic core, completing a mission set up in the 1970s by Langway and Dansgaard with Swiss climatologist Hans Oeschger. Although they had originally hoped to drill in central Greenland to obtain the thickest, least disturbed ice, it is deemed too risky to try the precious drill at such a remote site. Instead the core comes from Dye 3, a US radar station near the south coast. A 90,000-year comparison of the Camp Century and Dye 3 temperature profiles hints that the last glacial period was punctuated by a series of several warm spells. (See Dansgaard et al. Science 1982)
A 150,000-year temperature record is recovered from east Antarctica, where the intensely cold, dry conditions preserve ancient ice much better than in snowy Greenland. The new core comes from Vostok station, a Soviet research post established during International Geophysical Year. After initial reports of the work are published in Russian, French glaciologist Claude Lorius organizes an international analysis of the ice. It is the first ice record to extend through the last glacial period and beyond, but its low resolution blurs changes on the timescale of less than a century. (See Lorius et al. Nature 1985)
Air trapped in Vostok ice shows not only that CO2 levels fall and rise as the glacial period starts and ends, but that the size of the temperature swings can be explained only if the release of greenhouse gas amplifies warming caused by tiny changes in the Earth's orbit. Scientists find a strong natural feedback between rising temperatures and the release of CO2 from the land and sea. But because greenhouse gas concentrations are measured directly from air bubbles trapped in the ice, whereas temperature is inferred from other clues, attempts to align the records can err by many centuries. The chicken-and-egg problem of which came first in Antarctica remains unresolved. (See Barnola et al. Nature 1987, Jouzel et al. Nature 1987, Genthon et al. Nature 1987)
High-resolution ice at the Greenland summit is still being eyed by the coring communities on both sides of the Atlantic, generating two side-by-side projects in the 1990s. The European initiative known as the Greenland Ice Core Project (GRIP) discovers a remarkably turbulent climate during the last glacial period; temperatures oscillate up to 8 °C between glacial lows and highs, switching every few hundred to few thousand years. (See Johnsen et al. Nature 1992)
Early data from the US Greenland coring project — GISP2 — brings evidence that the three most recent periods of dramatic warming took place over just a few years. In the final warm-up about 11,700 years ago that ended the last glacial cold phase — the so-called Younger Dryas — snowfall appears to double in one to three years. (See Alley et al. Nature 1993)
In the summer drilling season the US team hits bedrock 3,053 metres below the Greenland ice sheet summit. Their data clearly confirm the European finding that the glacial period 14,000 to 114,000 years ago was marked by an erratic climate. There is little agreement between the cores, however, on older time periods. Further analysis reveals that the Eemian sections of both cores are distorted and unreliable. (See Taylor et al. Nature 1993, Grootes et al. Nature 1993, Alley et al. Nature 1995)
Because atmospheric chemistry is similar at both poles, scientists are able to date lower-resolution Antarctic ice by comparing atmospheric oxygen isotopes from the cores with those from a higher-resolution Greenland ice core. Many of the dramatic warming periods in Greenland are found to have counterparts at the opposite pole. (See Sowers & Bender Science 1995, Bender et al. Nature 2002)
At Dome Fuji, or Dome F — one of three high, flat peaks that top the East Antarctic Ice Sheet — a Japanese group headed by Okitsugu Watanabe sets out to drill some of the thickest and oldest ice on the continent. Unlike other coring efforts, work continues straight through the Antarctic winter, with a handful of personnel living in space-station-like conditions. But after extracting 2,503 metres of ice dating back 340,000 years, the drill clogs up with ice chips. Unable to recover the drill head, the team has to abandon the borehole. (See Watanabe et al. Ann. Glaciol. 1999)
Analysing the core taken by US researchers, veteran paleoclimatologist Richard Alley and his colleagues discover a marked cooling in Greenland about 8,000–8,400 years ago, together with changes in methane and windblown dust deposits that indicate widespread climate change. The Greenland temperature dip, which is given the less-than-colourful name 'the 8.2-kiloyear event', was about half that seen in the Younger Dryas. (See Alley et al. Geology 1997).
As techniques for analysing ice records proliferate, an advance by post-doc researcher Jeffrey Severinghaus provides a sweeping view of the paleoclimate. He and colleagues report that rapid temperature changes leave a recognizable signature of nitrogen and argon isotopes in Greenland ice cores. A new method for tracing abrupt temperature change via these airborne chemicals allows climate and atmospheric greenhouse gases to be tracked side by side without an age gap. The researchers deduce that levels of methane — a potent greenhouse gas — rose about 30 years after the abrupt warming that ended the Younger Dryas cool spell and started the interglacial. Since it's thought that most methane is produced in wetlands far from the ice, the implication is that this sharp climate swing occurred almost simultaneously throughout the Northern Hemisphere, if not further afield. (See Severinghaus et al. Nature 1998, Severinghaus & Brook Science 1999)
An international team drilling at Vostok halts at a record depth of 3,623 metres. Biologists have convinced them to avoid penetrating a lake of meltwater below, which may contain a unique ecosystem trapped under the ice for millions of years. (See Life in the deep freeze)
A study of the last 50,000 years shows that warming and cooling spells in east Antarctica occur out of phase with corresponding events in Greenland. This supports an idea argued by Broecker, and also backed by models, that Arctic Dansgaard–Oeschger events represent one end of a climate 'see-saw' that is pushed up and down by changes in ocean currents, so that Antarctica cools as Greenland warms and vice versa. (See Blunier et al. Nature 1998, Broecker Paleoceanogr. 1998, Blunier & Brook Science 2001)
A core from Antarctica's Dome C almost doubles the length of any ice record, to 740,000 years — adding temperature data from four more glacial cycles. Trapped greenhouse gases from up to 650,000 years ago are soon analysed and show that present-day carbon dioxide and methane levels are through the roof by the standards of the last several hundred thousand years.
The North Greenland Ice Core Project (NGRIP), a Northern cousin of EPICA involving Europe, the United States and Japan, drills a site where they hoped to find undisturbed bedrock ice from the Eemian interglacial. The team has a rocky start, including the loss of a drill stuck in the ice in 1997 and friction among the scientists leading to the resignation of their steering committee chair. Having weathered the setbacks, the project comes up short in 2003. At a depth of 3,085 metres, water starts flooding the borehole. But the melting lessens the layer-thinning effects of glacial flow, so the core contains distinct annual layers as old as 123,000 years. It portrays climatic swings during the last glaciation in greater detail than was once thought possible and uncovers a previously unrecognized early warming event. The Eemian's final throes in Greenland appear in the record as a stable climate about 5 °C hotter than today. (See NGRIP members Nature 2004 and Ice core reveals gentle start to last ice age)
After a month-long, 1,200-kilometre trek from the nearest research station, a Chinese expedition becomes the first to reach the summit of Dome Argus, also called Dome A — the tallest of Antarctica's three ice domes and possible site of a future core. (See Polar Research: Buried treasure)
A core from EPICA's second drilling site, at Dronning Maud Land, Antarctica, gives a detailed view of the last glacial period that is unprecedented in Antarctic cores. Lined up against Greenland records, the temperature profile strongly supports an out-of-phase 'see-saw' relationship between climate fluctuations in the Northern and Southern Hemispheres. (See EPICA community members Nature 2006)
A new core is drilled through Dome F's thick ice. At first the scientists working on the core believed its ice might be even older than Dome C's, but a preliminary analysis pegs the earliest usable ice near the bottom of the core at about 720,000 years old. (See Motoyama Scientif. Drill. 2007 - PDF)
In a project of the International Polar Year 2007–2008, a US group led by GISP2 alumni begins drilling a deep core on the West Antarctic Ice Sheet (WAIS) divide.
The deepest ice in the Dome C core from EPICA — 3,270 metres down and aged at about 800,000 years — is analysed, yielding the oldest air samples yet. Modern-day greenhouse gas concentrations remain unparalleled through those many millennia, with CO2 levels now 28 per cent higher, and methane 134 per cent higher, than at any other point in the record. Abrupt climate changes punctuated previous glacial periods, much like the last one. (See Jouzel et al. Science 2007, Lüthi et al. Nature 2008, Loulergue et al. Nature 2008 and Ice cores reveal climate secrets)
NGRIP researchers zoom in on some of the most abrupt changes spied in ice. The core's clear layers make it possible to pinpoint the timing of rapid climatic shifts, including the start and finish of the Younger Dryas as well as the beginning of a preceding warm period, known as Bølling-Allerød. The team concludes that during these events atmospheric circulation patterns shifted dramatically in just one to three years. (See Steffensen et al. Science 2008)
Spurred by International Polar Year, the hunt is on for the oldest ice. Researchers seek the holy grail of polar paleoclimatology — a frozen trace of the major transition 1 million years ago when Earth's glacial cycles went into a stately 100,000-year rhythm, slowing down from a previous 40,000-year cycle. A Chinese team begins setting up a research station at Dome A, one promising site for drilling in the next several years. (See China builds inland Antarctic base)