focus of additional scrutiny. Magnetic activity
that impedes the local convective velocity in solar-type stars is invariably accompanied by a dark
starspot, so the low contrast of spots in the ARs
of Barnard’s star and GJ 581 may be a feature
unique to low-mass stars. More robust theoretical modeling of magnetohydrodynamics in the
atmospheres of old, low-mass stars is required
to fully understand this phenomenon.
GJ 581d and (the now less widely believed to
exist) GJ 581 g were considered to be among
the first exoplanets likely to host habitable environments if they were rocky (7, 15). Given the
small number of habitable-zone (HZ) (25) planets
discovered by Doppler surveys around M dwarfs,
the removal of GJ 581d affects the RV-based estimate of h⊕ (the fraction of stars hosting low-mass planets in their HZs) around M stars.
This has been estimated as h⊕ = 0:41þ0:54 −0:13 by the
HARPS M dwarf survey (26). The exclusion of
GJ 581d reduces the rate to 33%, still within the
stated error limits. More precise estimates of h⊕
for M stars from Kepler exist [e.g., (5)], but the
various HZ limits used by these estimates prevent
direct comparison. Although GJ 581 may still be
dynamically capable of accommodating terrestrial-mass planets in its HZ, we see no evidence at this
time for additional planets in the activity-corrected
residuals around our three-planet model.
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ACKNOWLEDGMENTS
We acknowledge support from NSF grants AST-1006676,
AST-1126413, AST-1310885, The Penn State Astrobiology Research
Center, and the NASA Astrobiology Institute (NNA09DA76A) in our
pursuit of precise RVs in the near infrared. This work was
supported by funding from the Center for Exoplanets and Habitable
Worlds. The Center for Exoplanets and Habitable Worlds is
supported by the Pennsylvania State University, the Eberly College
of Science, and the Pennsylvania Space Grant Consortium. M.E. is
supported by NASA through the Origins of Solar Systems Program
grant NNX09AB30G and grant AST-1313075 from the NSF. This
research has made use of the Keck Observatory Archive (KOA),
which is operated by the W. M. Keck Observatory and the NASA
Exoplanet Science Institute (NExScI), under contract with the
National Aeronautics and Space Administration. We thank C.
Bender for assistance in studying telluric absorption in the vicinity
of Ha. The data produced for this study are included in table S3 of
the supplementary materials.
SUPPLEMENTARY MATERIALS
www.sciencemag.org/content/345/6195/440/suppl/DC1
Materials and Methods
Figs. S1 to S10
Tables S1 to S3
References (27–32)
12 March 2014; accepted 17 June 2014
Published online 3 July 2014;
10.1126/science.1253253
PALEOCLIMATE
Synchronization of North Pacific and
Greenland climates preceded abrupt
deglacial warming
Summer K. Praetorius* and Alan C. Mix
Some proposed mechanisms for transmission of major climate change events between the
North Pacific and North Atlantic predict opposing patterns of variations; others suggest
synchronization. Resolving this conflict has implications for regulation of poleward heat
transport and global climate change. New multidecadal-resolution foraminiferal oxygen
isotope records from the Gulf of Alaska (GOA) reveal sudden shifts between intervals of
synchroneity and asynchroneity with the North Greenland Ice Core Project (NGRIP) d18O
record over the past 18,000 years. Synchronization of these regions occurred 15,500 to
11,000 years ago, just prior to and throughout the most abrupt climate transitions of the
last 20,000 years, suggesting that dynamic coupling of North Pacific and North Atlantic
climates may lead to critical transitions in Earth’s climate system.
Abrupt climate transitions observed during the last deglaciation (1, 2) and within the last glacial interval (3) demonstrate that internal climate feedbacks can amplify the ffects of relatively weak external climate
forcing. Understanding the mechanisms involved
in generating past abrupt transitions, which have
led to regional warming events of ~10°C within 3
to 60 years (2), will help to assess the dynamic
nature of climate tipping points.
Fluctuations in the Atlantic Meridional Overturning Circulation (AMOC) are often invoked to
explain millennial-scale climate changes in the
North Atlantic region (4–6), as well as the so-called bipolar seesaw, which reflects changes in
net oceanic heat transport between the southern
and northern hemispheres (7, 8). An interocean
seesaw also has been proposed to operate between the North Atlantic and North Pacific, such
that poleward heat transport and/or deep-water
formation increases in the North Pacific during
times of weakened AMOC strength (9–11). This
remains uncertain, however, because models
show conflicting responses for the North Pacific
(10–13). Paleoclimate reconstructions are similarly
in conflict; some support an interocean seesaw
(9, 11, 14), whereas others suggest in-phase behavior
between the North Atlantic and North Pacific
(15, 16), and still other studies suggest a blend of
northern (atmospheric) and southern (oceanic)
influences (17, 18). If an Atlantic-Pacific seesaw
exists, low northward heat transport in one ocean
might be partly compensated by high northward
heat transport in the other. Conversely, synchronous variations in the two oceans would tend
to amplify climate changes in the high northern
latitudes by either enhancing or diminishing
meridional heat transport.
Changes in the AMOC and Arctic sea ice have
been identified as “tipping elements” in the climate system (19); both are influenced by poleward heat transport and have the potential for
rapid transitions (10 to 100 years) to a new climate state, accompanied by climate and ecosystem
effects that are to some degree irreversible (19).
Several diagnostic signs of approach to a tipping
point have been proposed, including enhanced
spatial correlation (i.e., interconnection or dynamic coupling), increase in short-term variability
(i.e., flickering), and critical slowing down (i.e.,
increased autocorrelation) (20–22). Some paleoclimate records document flickering and enhanced
variance preceding the abrupt onset of Holocene
and Bølling warmth (23, 24), but evidence for
increased autocorrelation before these transitions has been mixed (20, 24, 25), leading to debate
as to whether these transitions are true climate
bifurcations (24). No paleoclimate records have yet
shown symptoms of dynamic coupling (see supplementary materials for illustrative model). Here,
we document the onset of enhanced correlation
between North Pacific and North Atlantic climate variability that shortly precedes the most
abrupt warming events of the last deglaciation.
444 25 JULY 2014 • VOL 345 ISSUE 6195
sciencemag.org SCIENCE
College of Earth, Ocean, and Atmospheric Sciences, Oregon
State University, Corvallis, OR 97331, USA.
*Corresponding author: E-mail: spraetor@coas.oregonstate.edu
