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Atmospheric CO2: Principal Control Knob Governing Earth's Temperature

Authors:
  • NASA Goddard Institute for Space Studies,

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Ample physical evidence shows that carbon dioxide (CO2) is the single most important climate-relevant greenhouse gas in Earth’s atmosphere. This is because CO2, like ozone, N2O, CH4, and chlorofluorocarbons, does not condense and precipitate from the atmosphere at current climate temperatures, whereas water vapor can and does. Noncondensing greenhouse gases, which account for 25% of the total terrestrial greenhouse effect, thus serve to provide the stable temperature structure that sustains the current levels of atmospheric water vapor and clouds via feedback processes that account for the remaining 75% of the greenhouse effect. Without the radiative forcing supplied by CO2 and the other noncondensing greenhouse gases, the terrestrial greenhouse would collapse, plunging the global climate into an icebound Earth state.
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DOI: 10.1126/science.1190653
, 356 (2010); 330Science
et al.Andrew A. Lacis,
Governing Earth’s Temperature
: Principal Control Knob2Atmospheric CO
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measurements were very difficult because of the
buildup of a photothermal background over long
illumination times by the pump beam [the probe
had no noticeable effect on this buildup (figs. S6
to S8) (17)].
We co ul d no t ob se rv e si mu lt an eo us f lu or es -
cence and photothermal signals from the spots of
Fig. 2. Such simultaneous signals arose only from
abnormally bright spots, attributable to large ag-
gregates, and showed quick correlated decays of
both signals upon illumination (fig. S9) (17). On
the sample of Fig. 2, we also observed a few spots
with strong photothermal signals corresponding
to an absorption cross section of about 1 nm
2
,
which shows one-step bleaching. Possible inter-
pretations are aggregates of several molecules de-
sorbing simultaneously or, more probably, another
chemical species with a large absorption cross sec-
tion, possibly with excited-state absorption of the
intense probe beam. We only found very few ex-
amples of two-step photobleaching of the BHQ1-
10T-BHQ1 constructs. We believe that this absence
results from the low probability of bleaching,
joined with the low probability of finding two
favorable orientations in the same construct.
Our successful single molecule detection relied
here on favorable conditions, the use of glycerol
(with large n/Tand poor heat conduction) instead
of water, with a high heating power of 5.1 mW
focused into the diffraction-limited spot and with
an even higher probe power of more than 70 mW.
Although the conditions for single-molecule
absorption in a cell, for example, would be far
from these ideal ones, this result opens the way
for further optimization of the technique and to a
much broader variety of absorbing molecules.
Interesting candidates are natural absorbers, for
example, metal proteins such as hemoglobin,
which would be useful for applications in analyt-
ical biochemistry and medical assays (24).
An intrinsic limitation of the photothermal
method is the low transduction factor between
pump and probe because of the relatively weak
variation of refractive index with temperature. A
future search for more-efficient transduction, for
example, photomechanical or photoelectrical de-
tections using micro- and nano-optomechanical
systems (25), appears full of promise.
References and Notes
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Phys. Rev. B 73, 045424 (2006).
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021802 (2006).
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(2006).
14. G. Wrigge, J. Hwang, I. Gehardt, G. Zumofen,
V. Sandoghdar, Opt. Express 16, 17358 (2008).
15. P. Kukura, M. Celebrano, A. Renn, V. Sandoghdar, Nano
Lett. 9, 926 (2009).
16. W. Min et al., Nature 461, 1105 (2009).
17. Materials and methods are available as supporting
material on Science Online.
18. A. Gaiduk, P. V. Ruijgrok, M. Yorulmaz, M. Orrit, Chem.
Sci. 1, 343 (2010).
19. V. V. Didenko, Ed., Methods in Molecular Biology:
Fluorescent Energy Transfer Nucleic Acid Probes: Designs
and Protocols (Humana, Totowa, NJ, 2006).
20. S. A. E. Marras, F. R. Kramer, S. Tyagi, Nucleic Acids Res.
30, e122 (2002).
21. M. A. van Dijk et al., Phys. Chem. Chem. Phys. 8, 3486
(2007).
22. C. F. Bohren, D. R. Huffman, Absorption and Scattering
of Light by Small Particles (Wiley, New York, 1998).
23. A. Arbouet et al., Phys. Rev. Lett. 93, 127401 (2004).
24. S. Lu, W. Min, S. Chong, G. R. Holtom, X. S. Xie, Appl.
Phys. Lett. 96, 113701 (2010).
25. S. W. Stahl, E. M. Puchner, H. E. Gaub, Rev. Sci. Instrum.
80, 073702 (2009).
26. We acknowledge financial support by European Research
Council (Advanced Grant SiMoSoMa). This work is a
part of the research program of the Stichting voor
Fundamenteel Onderzoek der Materie, which is
financially supported by the Netherlands Organization for
Scientific Research. We thank H. van der Meer for
precious help with the experimental setup.
Supporting Online Material
www.sciencemag.org/cgi/content/full/330/6002/353/DC1
Materials and Methods
Figs. S1 to S9
References
22 July 2010; accepted 16 September 2010
10.1126/science.1195475
Atmospheric CO
2
:PrincipalControlKnob
Governing EarthsTemperature
Andrew A. Lacis,*Gavin A. Schmidt, David Rind, Reto A. Ruedy
Ample physical evidence shows that carbon dioxide (CO
2
) is the single most important
climate-relevant greenhouse gas in Earths atmosphere. This is because CO
2
, like ozone, N
2
O, CH
4
,
and chlorofluorocarbons, does not condense and precipitate from the atmosphere at current
climate temperatures, whereas water vapor can and does. Noncondensing greenhouse gases,
which account for 25% of the total terrestrial greenhouse effect, thus serve to provide the stable
temperature structure that sustains the current levels of atmospheric water vapor and clouds via
feedback processes that account for the remaining 75% of the greenhouse effect. Without the
radiative forcing supplied by CO
2
and the other noncondensing greenhouse gases, the terrestrial
greenhouse would collapse, plunging the global climate into an icebound Earth state.
Itoftenisstatedthatwatervaporisthechief
greenhouse gas (GHG) in the atmosphere.
For example, it has been asserted that about
98% of the natural greenhouse effect is due to
water vapour and stratiform clouds with CO
2
contributing less than 2%(1). If true, this would
imply that changes in atmospheric CO
2
are not
important influences on the natural greenhouse
capacity of Earth, and that the continuing in-
crease in CO
2
due to human activity is therefore
not relevant to climate change. This misunder-
standing is resolved through simple examination
of the terrestrial greenhouse.
The difference between the nominal global
mean surface temperature (T
S
=288K)andthe
global mean effective temperature (T
E
=255K)
is a common measure of the terrestrial green-
house effect (G
T
=T
S
T
E
=33K).Assuming
global energy balance, T
E
is also the Planck ra-
diation equivalent of the 240 W/m
2
of global
mean solar radiation absorbed by Earth.
The Sun is the source of energy that heats
Earth. Besides direct solar heating of the ground,
there is also indirect longwave (LW) warming
arising from the thermal radiation that is emitted
by the ground, then absorbed locally within the
atmosphere, from which it is re-emitted in both
upward and downward directions, further heating
the ground and maintaining the temperature gra-
dient in the atmosphere. This radiative interaction
is the greenhouse effect, which was first dis-
covered by Joseph Fourier in 1824 (2), experi-
mentally verified by John Tyndall in 1863 (3),
and quantified by Svante Arrhenius in 1896 (4).
These studies established long ago that water
vapor and CO
2
are indeed the principal terrestrial
GHGs. Now, further consideration shows that
CO
2
is the one that controls climate change.
CO
2
is a well-mixed gas that does not con-
dense or precipitate from the atmosphere. Water
vapor and clouds, on the other hand, are highly
active components of the climate system that re-
spond rapidly to changes in temperature and air
pressure by evaporating, condensing, and precip-
itating. This identifies water vapor and clouds as
the fast feedback processes in the climate system.
Radiative forcing experiments assuming dou-
bled CO
2
and a 2% increase in solar irradiance
(5)showthatwatervaporprovidesthestrongest
climate feedback of any of the atmospheric GHGs,
but that it is not the cause (forcing) of global cli-
NASA Goddard Institute for Space Studies, 2880 Broadway,
New York, NY 10025, USA.
*To whom correspondence should be addressed. E-mail:
andrew.a.lacis@nasa.gov
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mate change. The response of the climate system
to an applied forcing is determined to be the sum
of the direct (no-feedback) response to the ap-
plied forcing and the induced radiative response
that is attributable to the feedback process con-
tributions. The ratio of the total climate response
to the no-feedback response is commonly known
as the feedback factor, which incorporates all the
complexities of the climate system feedback in-
teractions. For the doubled CO
2
and the 2% solar
irradiance forcings, for which the direct no-feedback
responses of the global surface temperature are
1.2° and 1.3°C, respectively, the ~4°C surface
warming implies respective feedback factors of
3.3 and 3.0 (5).
Because the solar-thermal energy balance of
Earth [at the top of the atmosphere (TOA)] is
maintained by radiative processes only, and be-
cause all the global net advective energy trans-
ports must equal zero, it follows that the global
average surface temperature must be determined
in full by the radiative fluxes arising from the
patterns of temperature and absorption of radia-
tion. This then is the basic underlying physics
that explains the close coupling that exists be-
tween TOA radiative fluxes, the greenhouse ef-
fect, and the global mean surface temperature.
An improved understanding of the relative
importance of the different contributors to the
greenhouse effect comes from radiative flux ex-
periments that we performed using Goddard
Institute for Space Studies (GISS) ModelE (6).
Figure 1 depicts the essence of these calculations,
including the separation of the greenhouse con-
tributors into feedback and forcing categories.
In round numbers, water vapor accounts for
about 50% of Earthsgreenhouseeffect,with
clouds contributing 25%, CO
2
20%, and the mi-
nor GHGs and aerosols accounting for the re-
maining 5%. Because CO
2
,O
3
,N
2
O, CH
4
,and
chlorofluorocarbons (CFCs) do not condense and
precipitate, noncondensing GHGs constitute the
key 25% of the radiative forcing that supports
and sustains the entire terrestrial greenhouse ef-
fect, the remaining 75% coming as fast feedback
contributions from water vapor and clouds.
We us ed the GISS 4 ° ×5° M od el E to cal-
culate changes in instantaneous LW TOA flux
(annual global averages) in experiments where
atmospheric constituents (including water vapor,
clouds, CO
2
,O
3
,N
2
O, CH
4
,CFCs,andaerosols)
were added to or subtracted from an equilibrium
atmosphere with a given global temperature struc-
ture, one constituent at a time for a 1-year period.
Decreases in outgoing TOA flux for each con-
stituent relative to the empty or the full-component
atmosphere define the bounds for the relative
impact on the total greenhouse effect. Had the
overlapping absorption been negligible, the sum
of the flux differences would have been equal to
the LW flux equivalent of the total greenhouse
effect (G
F
=sT
4
S
sT
4
E
=150W/m
2
), where s
is the Stefan-Boltzmann constant. We found the
single-addition flux differences to be overestimated
by a factor of 1.36, whereas in the single-subtraction
cases, the sum of the TOA flux differences was
underestimated by a factor of 0.734. By normal-
izing these fractional contributions to match the
full-atmosphere value of G
F
,weobtainedthefrac-
tional response contributions shown in Fig. 1.
Because of overlapping absorption, the frac-
tional attribution of the greenhouse effect is to
some extent qualitative (as shown by the dashed
and dotted extremum lines in Fig. 1), even though
the spectral integral is a full and accurate deter-
mination of the atmospheric greenhouse strength
for the specified global temperature structure.
Still, the fractional attribution is sufficiently pre-
cise to clearly differentiate the radiative flux con-
tributions due to the noncondensable GHGs from
those arising from the fast feedback processes.
This allows an empirical determination of the
climate feedback factor as the ratio of the total
global flux change to the flux change that is at-
tributable to the radiative forcing due to the
noncondensing GHGs. This empirical determi-
nation leads then to a climate feedback factor of
4, based on the noncondensing GHG forcing ac-
counting for 25% of the outgoing flux reduction
at the TOA for the full-constituent atmosphere.
This implies that Earthsclimatesystemoperates
with strong positive feedback that arises from
the forcing-induced changes in the condensable
species.
Adirectconsequenceofthiscombinationof
feedback by the condensable and forcing by the
noncondensable constituents of the atmospheric
greenhouse is that the terrestrial greenhouse ef-
fect would collapse were it not for the presence of
these noncondensing GHGs. If the global atmo-
spheric temperatures were to fall to as low as T
S
=
T
E
,theClausius-Clapeyronrelationwouldimply
that the sustainable amount of atmospheric water
vapor would become less than 10% of the current
atmospheric value. This would result in (radia-
tive) forcing reduced by ~30 W/m
2
,causingmuch
of the remaining water vapor to precipitate, thus
enhancing the snow/ice albedo to further dimin-
ish the absorbed solar radiation. Such a condi-
tion would inevitably lead to runaway glaciation,
producing an ice ball Earth.
Claims that removing all CO
2
from the at-
mosphere would lead to a 1°C decrease in
global warming(7), or by 3.53°C when 40%
cloud cover is assumed(8)arestillbeingheard.
Acleardemonstrationisneededtoshowthat
water vapor and clouds do indeed behave as fast
feedback processes and that their atmospheric
distributions are regulated by the sustained ra-
diative forcing due to the noncondensing GHGs.
To th is e nd, we perfor me d a si mp le climat e ex -
periment with the GISS 2° ×2.5° AR5 version of
ModelE, using the Q-flux ocean with a mixed-
layer depth of 250 m, zeroing out all the non-
condensing GHGs and aerosols.
The results, summarized in Fig. 2, show un-
equivocally that the radiative forcing by noncon-
densing GHGs is essential to sustain the atmospheric
temperatures that are needed for significant levels
of water vapor and cloud feedback. Without this
noncondensable GHG forcing, the physics of this
model send the climate of Earth plunging rapidly
and irrevocably to an icebound state, though per-
haps not to total ocean freezeover.
The scope of the climate impact becomes ap-
parent in just 10 years. During the first year
alone, global mean surface temperature falls by
4.6°C. After 50 years, the global temperature
stands at 21°C, a decrease of 34.8°C. Atmo-
spheric water vapor is at ~10% of the control cli-
mate value (22.6 to 2.2 mm). Global cloud cover
increases from its 58% control value to more than
75%, and the global sea ice fraction goes from
4.6% to 46.7%, causing the planetary albedo of
Earth to also increase from ~29% to 41.8%. This
has the effect of reducing the absorbed solar en-
ergy to further exacerbate the global cooling.
After 50 years, a third of the ocean surface
still remains ice-free, even though the global sur-
face temperature is colder than 21°C. At tropical
latitudes, incident solar radiation is sufficient to
keep the ocean from freezing. Although this ther-
mal oasis within an otherwise icebound Earth
Fig. 1. Attribution of the
contributions of individ-
ual atmospheric compo-
nents to the total terrestrial
greenhouse effect, sepa-
rated into feedback and
forcing categories. Horizon-
tal dotted and dashed lines
depict the fractional re-
sponse for single-addition
and single-subtraction of
individual gases to an
empty or full-component
reference atmosphere, re-
spectively. Horizontal solid
black lines are the scaled
averages of the dashed-
and dotted-line fractional
response results. The sum of the fractional responses adds up to the total greenhouse effect. The reference
atmosphere is for conditions in 1980.
www.sciencemag.org SCIENCE VOL 330 15 OCTOBER 2010 357
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appears to be stable, further calculations with an
interactive ocean would be needed to verify the
potential for long-term stability. The surface tem-
peratures in Fig. 3 are only marginally warmer
than 1°C within the remaining low-latitude heat
island.
From the foregoing, it is clear that CO
2
is the
key atmospheric gas that exerts principal control
over the strength of the terrestrial greenhouse
effect. Water vapor and clouds are fast-acting
feedback effects, and as such are controlled by
the radiative forcings supplied by the noncon-
densing GHGs. There is telling evidence that at-
mospheric CO
2
also governs the temperature of
Earth on geological time scales, suggesting the
related question of what the geological processes
that control atmospheric CO
2
are. The geological
evidence of glaciation at tropical latitudes from
650 to 750 million years ago supports the snow-
ball Earth hypothesis (9), and by inference, that
escape from the snowball Earth condition is also
achievable.
On million-year time scales, volcanoes are the
principal source of atmospheric CO
2
,androck
weathering is the principal sink, with the bio-
sphere acting as both source and sink (10). Because
the CO
2
sources and sinks operate independently,
the atmospheric level of CO
2
can fluctuate. If the
atmospheric CO
2
level were to fall below its crit-
ical value, snowball Earth conditions can result.
Antarctic and Greenland ice core data show
atmospheric CO
2
fluctuations between 180 to
300 parts per million (ppm) over the glacial-
interglacial cycles during the past 650,000 years
(11). The relevant physical processes that turn the
CO
2
control knob on thousand-year time scales
between glacial and interglacial extremes are not
fully understood, but appear to involve both the
biosphere and the ocean chemistry, including a
significant role for Milankovitch variations of the
Earth-orbital parameters.
Besides CO
2
,methaneisanotherpotentgreen-
house control knob, being implicated in the
Paleocene-Eocene thermal maximummass ex-
tinction 55 million years ago, when global warm-
ing by up to 5°C (12)occurredbecauseofa
massive release of methane from the disinte-
gration of seafloor clathrates (13,14). Methane is
the second most important noncondensing GHG
after CO
2
.Ofthe2.9W/m
2
of GHG radiative
forcing from 1750 to 2000, CO
2
contributed
1.5 W/m
2
,methane0.55W/m
2
,andCFCs0.3
W/m
2
,withtherestcomingfromN
2
Oandozone
(15). All of these increasesin noncondensing GHG
forcing are attributable to human activity (16).
Climate control knobs on the solar side of the
energy balance ledger include the steady growth
in luminosity since the beginning of the Solar
System (from about 70% of present luminosity,
depending on the postulated early solar mass
loss), as hydrogen is consumed in nuclear re-
actions in the solar interior (17,18). Milankovitch
variations of the Earth-orbital parameters, which
alter the relative seasonal distribution as well as
the intensity of incident solar radiation within
the polar regions, are another important solar
energy control knob that is intimately associated
with glacial-interglacial cycles of climate change.
For solar irradiance changes over the past several
centuries, an increase by about 0.1 W/m
2
is in-
ferred since the time of the Maunder minimum,
based on trends in sunspot activity and other
proxies (19).
Of the climate control knobs relevant to cur-
rent climate, those on the solar side of the energy
balance ledger show only negligible impact. Sev-
eral decades of solar irradiance monitoring have
not detected any long-term trends in solar ir-
radiance beyond the 11-year oscillation associ-
ated with the solar sunspot cycle. Large volcanic
Fig. 2. Time evolution of global surface temperature, TOA net flux, column water vapor, planetary
albedo, sea ice cover, and cloud cover, after the zeroing out of the noncondensing GHGs. The model used
in the experiment is the GISS 2°×2.5° AR5 version ofModelE,withtheQ-fluxoceanandamixed-layer
depth of 250 m. Model initial conditions are for a preindustrial atmosphere. Surface temperature and TOA
net flux use the lefthand scale.
Fig. 3. Zonally averaged
annual mean surface tem-
perature change after the
zeroing out of noncon-
densing GHGs.
Table 1. Planetary greenhouse parameters.
Parameter Mars Earth Venus
T
S
(K) 215 288 730
T
E
(K) 210 255 230
sT
S4
(W/m
2
) 121 390 16,100
sT
E4
(W/m
2
) 111 240 157
G
T
(K) 533 500
G
F
(W/m
2
) 10 150 ~16,000
P
S
(bar) 0.01 1 100
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eruptions can happen at any time, but no sub-
stantial eruptions have occurred since the erup-
tion of Mt. Pinatubo in the Philippines in 1991.
In a broader perspective, CO
2
greenhouses
also operate on Mars and Venus, because both plan-
ets possess atmospheres with substantial amounts
of CO
2
.Theatmosphericgreenhouseeffectre-
quires that a substantial fraction of the incident
solar radiation must be absorbed at the ground in
order to make the indirect greenhouse heating of
the ground surface possible. Greenhouse param-
eters and relative surface pressure (P
S
)forMars,
Earth, and Venus are summarized in Table 1.
Earth is unique among terrestrial planets in
having a greenhouse effect in which water vapor
provides strong amplification of the heat-trapping
action of the CO
2
greenhouse. Also, N
2
and O
2
,
although possessing no substantial absorption
bands of their own, are actually important con-
tributors to the total greenhouse effect because of
pressure-broadening of CO
2
absorption lines, as
well as by providing the physical structure within
which the absorbing gases can interact with the
radiation field.
The anthropogenic radiative forcings that fuel
the growing terrestrial greenhouse effect continue
unabated. The continuing high rate of atmospher-
ic CO
2
increase is particularly worrisome, be-
cause the present CO
2
level of 390 ppm is far in
excess of the 280 ppm that is more typical for the
interglacial maximum, and still the atmospheric
CO
2
control knob is now being turned faster than
at any time in the geological record (20). The con-
cern is that we are well past even the 300- to
350-ppm target level for atmospheric CO
2
,
beyond which dangerous anthropogenic interfer-
ence in the climate system would exceed the 25%
risk tolerance for impending degradation of land
and ocean ecosystems, sea-level rise, and inevi-
table disruption of socioeconomic and food-
producing infrastructure (21,22). Furthermore,
the atmospheric residence time of CO
2
is exceed-
ingly long, being measured in thousands of years
(23). This makes the reduction and control of at-
mospheric CO
2
aseriousandpressingissue,
worthy of real-time attention.
References and Notes
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17. I. J. Sackmann, A. I. Boothroyd, K. E. Kraemer,
Astrophys. J. 418, 457 (1993).
18. I. J. Sackmann, A. I. Boothroyd, Astrophys. J. 583,
1024 (2003).
19. P. Forster et al., in Climate Change 2007: The Physical
Science Basis. Contribution of Working Group I to the
Fourth Assessment Report of the Intergovernmental Panel
on Climate Change, S. Solomon et al., Eds. (Cambridge
Univ. Press, Cambridge, 2007), pp. 433497.
20. D. Archer et al., Annu. Rev. Earth Planet. Sci. 37,
117 (2009).
21. L. D. D. Harvey, Clim. Change 82, 1 (2007).
22. J. Hansen et al., Open Atmos. Sci. J. 2, 217 (2008).
23. F. Joos, R. Spahni, Proc. Natl. Acad. Sci. U.S.A. 105,
1425 (2008).
24. We thank B. Carlson, A. Del Genio, J. Hansen, G. Russell,
R. Stothers, and L. Travis for comments and the NASA
Earth Science Research Division managed by J. Kaye and
D. Considine for support.
Supporting Online Material
www.sciencemag.org/cgi/content/full/330/6002/356/DC1
SOM Text
Figs. S1 and S2
Table S1
References
8 April 2010; accepted 10 September 2010
10.1126/science.1190653
The Structure of Iron in
Earths Inner Core
Shigehiko Tateno,
1,2
*Kei Hirose,
1,2
*Yasuo Ohishi,
3
Yoshiyuki Tatsumi
2
Earthssolidinnercoreismainlycomposedofiron(Fe).Becausetherelevantultrahighpressure
and temperature conditions are difficult to produce experimentally, the preferred crystal structure
of Fe at the inner core remains uncertain. Static compression experiments showed that the
hexagonal close-packed (hcp) structure of Fe is stable up to 377 gigapascals and 5700 kelvin,
corresponding to inner core conditions. The observed weak temperature dependence of the c/a
axial ratio suggests that hcp Fe is elastically anisotropic at core temperatures. Preferred orientation
of the hcp phase may explain previously observed inner core seismic anisotropy.
Determining the crystal structure of iron
(Fe) under ultrahigh pressure and tem-
perature (P-T )conditionsisakeypiece
of information required to decipher the complex
seismic structures observed in Earthsinnercore
(13). Fe adopts body-centered cubic (bcc) struc-
ture at ambient conditions and transforms into
the hexagonal close-packed (hcp) phase above
15 GPa. Although hcp Fe can persist under core
pressures at 300 K (4,5), a phase transition at
elevated temperature is a possibility. Both theory
and experiments have proposed different forms
of Fe at simultaneously high P-T conditions,
which include bcc (6,7), face-centered-cubic
(fcc) (8), and hcp structures (5,9). The structure
of Fe has never been examined experimental-
ly at the inner core P-T conditions (>330 GPa
and 5000 K), because the techniques previous-
ly used to produce such extreme conditions
dynamical shock-wave experimentsimpede the
ability to make simultaneous structure measure-
ments on the order of a microsecond.
Based on a combination of static compression
experiments in a laser-heated diamond-anvil cell
(DAC) and synchrotron x-ray diffraction (XRD)
measurements (Fig. 1), we determined the struc-
ture of Fe up to 377 GPa and 5700 K (10). A
temperature gradient was relatively large when
the sample was heated to more than 5000 K at
>300 GPa (Fig. 2A); nevertheless, the variations
were less than T10% in the 6-mmregionacross
the hot spot, which corresponds to the x-ray beam
size at full width at half maximum, considering
the fluctuations in temperature with time (Fig.
2B). We calculated the sample temperature by
averaging the variation in the 6-mmareaprobed
by x-rays. Pressure was determined from the unit-
cell volume of hcp Fe, using its P-V-T (where V
is volume) equation of state (11). The T10%
temperature variation leads to about T2% un-
certainty in pressure (377 T8.5 GPa at 5700 K).
The pressure gradient in the sample was <5 GPa
at ~300 GPa in a 10-mmareaafterheating.
To construct the phase diagram of Fe at in-
ner core conditions, we conducted six separate
sets of experiments (Fig. 3). The first exper-
iment at 303 GPa and room temperature re-
sulted in an XRD pattern that included peaks
from hcp Fe and Re (the gasket material) (fig.
S1A) (10). Subsequently, we heated this sam-
1
Department of Earth and Planetary Sciences, Tokyo Institute
of Technology, Ookayama, Meguro, Tokyo 152-8551, Japan.
2
Institute for Research on Earth Evolution, Japan Agency for
Marine-Earth Science and Technology, Yokosuka, Kanagawa
237-0061, Japan.
3
Japan Synchrotron Radiation Research In-
stitute, Sayo, Hyogo 679-5198, Japan.
*To whom correspondence should be addressed. E-mail:
tateno.s.aa@m.titech.ac.jp (S.T.); kei@geo.titech.ac.jp (K.H.)
www.sciencemag.org SCIENCE VOL 330 15 OCTOBER 2010 359
REPORTS
on October 15, 2010 www.sciencemag.orgDownloaded from
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