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Greenland deglaciation puzzlesCLIMATE

Summer melt water on glacial ice, George VI Sound, Antarctica.

PERSPECTIVES

Nitrogen isotope data help to resolve puzzling observations during the last deglaciation

About 23,000 years ago, the southern

margins of the great Northern Hemi-

sphere ice sheets across Europe and

North America began to melt. The

melt rate accelerated ~20,000 years

ago, and global sea level eventually

rose by ~130 m as meltwater flowed into

the oceans. Ice cores from the Greenland

and Antarctic ice sheets show the rise in

atmospheric CO2 concentrations that ac-

companied this shift in global ice volume

and climate. However, discrepancies in the

temperature reconstructions from these

cores have raised questions about the long-

term relationship between atmospheric CO2

concentrations and Arctic temperature. On

page 1177 of this issue, Buizert et al. ( 1) re-

port temperature reconstructions from

three locations on the Greenland ice sheet

that directly address these problems.

The relative amount of heavy to light wa-

ter isotopes in snow mostly depends on how

cold it is when the snow falls. For this rea-

son, ratios of light to heavy water isotopes

By Louise Claire Sime from ice cores (see the figure) have long

been used to reconstruct past temperatures

in Antarctica and Greenland ( 2). These

temperature reconstructions, alongside ice

core CO2 records, have been crucial for ad-

vancing understanding of past climate, but

they also have puzzling features.

The first puzzle is the timing of North-

ern Hemisphere warming. Once a certain

mass of Northern Hemisphere ice has accu-

mulated, changes in Northern Hemisphere

summer insolation—that is, the amount of

solar irradiation received at Earth’s surface PH

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5 SEPTEMBER 2014 • VOL 345 ISSUE 6201 1117SCIENCE sciencemag.org

during summer—are expected to initiate

melting. Northern Hemisphere warming

would thus be expected to precede or ac-

company Southern Hemisphere warming.

Yet, although most Northern Hemisphere

ice sheets were in retreat by ~19,000 years

ago ( 3), the Greenland isotopic record does

not begin to rise until ~14,700 years ago (see

the figure). Denton et al. ( 4) used the term

“mystery interval” to describe the Oldest

Dryas period (~18 thousand to 15 thousand

years ago), when Northern Hemisphere

summer insolation, Northern Hemisphere

ice melt, atmospheric CO2 concentrations,

and Southern Hemisphere temperatures

rose, yet Greenland water isotope ratios do

not record a similar rise in Northern Hemi-

sphere temperatures.

Using a general circulation model (GCM),

He et al. ( 5) have shown that allowing North-

ern Hemisphere ice sheet meltwater to flow

into the ocean generates a bipolar seesaw ef-

fect, with the Southern Hemisphere warming

at the expense of the Northern Hemisphere.

This effect is large enough to explain most

of the north-south onset discrepancy. In the

North Atlantic, surface freshwater from ice

melt inhibits the formation of dense salty

deep water, leading to extensive winter sea

ice in the Northern Hemisphere.

Buizert et al. further this analysis by ex-

ploiting the fact that water isotopes are not

the only way to track temperature in ice

cores. When snow becomes denser after it

has been deposited, the nitrogen isotope ra-

tio, δ15N, changes as a function of tempera-

ture, thus providing information that does

not depend on water isotope data. The au-

thors use δ15N to reconstruct temperatures

from three locations in Greenland. They

show that previous temperature recon-

structions based on water isotopes masked

a small temperature rise during the mystery

interval. Furthermore, the geographical pat-

tern of their results supports the idea that

the timing of the deglacial warming onset

strongly depended on heat transported by

the Atlantic Ocean ( 6). Thus, variations in

meltwater flowing into the North Atlantic,

via ocean circulation, largely controlled

both the timing and magnitude of the

onset of the Greenland deglacial warming

( 5, 6).

A second puzzle lies in the temperature

relationship between the Oldest Dryas and

the Younger Dryas (~12.8 thousand to 11.5

thousand years ago). Atmospheric CO2 con-

centrations rose by about 50 parts per mil-

lion (ppm) between these intervals. The

ocean likely delivered a similar amount of

heat to Greenland during each interval. Yet,

Greenland ice core temperature reconstruc-

tions based on water isotopes imply that the

Younger Dryas was colder than the Oldest

Dryas ( 7). Scientists have suggested that the

Younger Dryas was an outlier, triggered ei-

ther by a single catastrophic flood discharge

or a comet impact, but these ideas have lost

traction due to a lack of evidence ( 8). Climate

models predict that rising CO2 will cause a

rise in Greenland temperature. The colder

temperatures of the Younger Dryas thus rep-

resent the second puzzle.

Liu et al. ( 9) performed GCM simulations

of the whole period and also modeled water

isotope behavior during selected intervals.

Despite a simulated ~5°C Greenland warm-

ing, their modeled water isotope results

match the observed isotopic drop in the

Younger Dryas. Buizert et al.’s temperature

reconstructions confirm this warming. The

misleading water isotope results may have

come about because reduction in the North-

ern Hemisphere ice sheet height by 2 km

between the Oldest and the Younger Dryas

modified atmospheric circulation, causing a

relative increase in moisture advected from

the North Pacific region ( 9, 10) that reduced

water isotope ratios.

Buizert et al.’s successful Greenland tem-

perature reconstructions based on δ15N imply

that similar Antarctic core reconstructions

may be possible. Alongside GCM-based pa-

leoclimate modeling tools (5, 9), new Ant-

arctic temperature information could help

to clarify the relationship between Antarctic

climate and ice sheet changes. ■

REFERENCES AND NOTES

1. C. Buizert et al., Science 345, 1177 (2014). 2. W. Dansgaard, Tellus 16, 436 (1964). 3. A. E. Carlson, K. Winsor, Nat. Geosci. 5, 607 (2012). 4. G. Denton, W. Broecker, R. Alley, PAGES News 14, 14

(2006). 5. F. He et al., Nature 494, 81 (2013). 6. J. D. Shakun, A. E. Carlson, Quat. Sci. Rev. 29, 1801 (2010). 7. K. M. Cuffey et al., Science 270, 455 (1995). 8. W. Broecker et al., Quat. Sci. Rev. 29, 1078 (2010). 9. Z. Liu et al., Proc. Natl. Acad. Sci. U.S.A. 10.1073/pnas.

1202183109 (2012). 10. P. Kindler et al., Climate Past Discuss. 9, 4099 (2013). 11. B. Lemieux-Dudon et al., Quat. Sci. Rev. 29, 8 (2010). 12. D. Lüthi et al., Nature 453, 379 (2008). 13. The Antarctic ice core record is an average of the Dome

C, EPICA (European Project for Ice Coring in Antarctica) Dronning Maud Land, and Vostock water isotopes. The Greenland record is an average of the North Greenland Ice Core Project (NGRIP), GRIP, Greenland Ice Sheet Project 2, and Renland ice cores. Envelopes show the maximum and minimum values from each set of cores. Each ice core record is shown as an anomaly relative to the past 3000 years. All cores are on the Lemieux-Dudon et al. ( 11) time scale and are low-pass filtered at 3000 years for clarity. The summer Northern Hemisphere insolation record is solstice insolation at 60°N (W m−1). The CO

2 (ppmv)

record is from Lüthi et al. ( 12).

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10.1126/science.1257842

Glacial termination puzzles. Ice core water isotope records suggest that Greenland warmed suddenly and much

later than Antarctica, even though summer Northern Hemisphere insolation and atmospheric CO2 levels rose in

step with Antarctic warming. Buizert et al. report temperature reconstructions that help to resolve this puzzling

observation. Envelopes show the maximum and minimum values for each set of ice cores. See ( 13) for more details.

British Antarctic Survey, High Cross, Cambridge, CB23 7PP, UK. E-mail: lsim@bas.ac.uk

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