28 FEBRUARY 2014 VOL 343 SCIENCE www.sciencemag.org 976
Average temperatures at Earth’s sur- face are now higher than they were in the mid-19th century, but the rate
of warming has not been steady. A pause in
surface warming in the mid-20th century
coincided with increases in the atmospheric
concentrations of sulfate aerosols, which are
generally understood to cool the planet. Sur-
face warming resumed in the 1970s, when
strong pollution controls were implemented
in developed countries. Thus, a balance of
warming by greenhouse gases and cooling
by aerosols may explain the variable rates
of surface warming in the past century. A
pause in global warming since 2000—a
global warming “hiatus”—has opened up
new questions about natural and human
effects on global mean trends
in surface temperature. Recent
studies point to the importance
of the tropical Pacific in driving
A range of factors may have
contributed to the current pause
in global warming, including
changes in stratospheric water
vapor, aerosol concentrations
(1), and reductions in the Sun’s
output (2). The quantitative
influence of these factors is still
uncertain. However, what is striking about
the current hiatus is that while many regions
of the globe have continued to warm, the
tropical Pacific has been colder than it was
during the latter part of the 20th century.
In a recent study, Kosaka and Xie (3)
The Tropical Pacific Ocean—
showed that by prescribing the cold tem-
peratures in this region (which represents
less that 10% of Earth’s surface),
their model can simulate the pause
in global mean temperature since
2000, even when greenhouse gases
have been increasing. In another
climate model study, Meehl et al.
found that a cold tropical Pacific
increases the heat stored below the
ocean surface, thus partially off-
setting the warming at the surface
(4). In the latter model, such hiatus
periods arise as a result of natural
variations in the climate system,
Will these results hold up in other models?
The answer depends on the Pacific’s natural
Back in the Driver’s Seat?
Amy Clement1 and Pedro DiNezio2
Persistent cool conditions in the eastern
tropical Pacific may explain the current
global warming “hiatus.”
1Rosenstiel School of Marine and Atmospheric Science, University of Miami, 4600 Rickenbacker Causeway, Miami, FL
33149, USA. 2School of Ocean and Earth Science and Technology, University of Hawaii at Manoa, Honolulu, HI 96822,
USA. E-mail: firstname.lastname@example.org
ing the evolution of the surface morphology
of a specimen and then analyzing the result
by means of the Navier-Stokes equation (5,
10) or an equivalent model of fluid flow (7).
The basic idea is that any nonflat surface
structure (artificially or spontaneously created) produces pressure gradients that then
drive the specimen to flow. In the lubrication approximation (usually applicable to
thin-film specimens with thickness less than
~100 nm), the flow is planar and on average
parallel to the pressure gradient. The current
(or flow of fluid) per unit width is proportional to the pressure gradient and the film
mobility, which can be used to determine the
In one example study, the dynamics for
the Brownian height fluctuations of an equil-
ibrated film was monitored and modeled
against that of overdamped surface capillary
waves (10). In two others, surface structures,
either shorter (5) or taller (7) than equilib-
rium, were created and the dissipative dynam-
ics toward equilibrium (equivalent to that of
the former example by the fluctuation-dissi-
pation theorem) was monitored. To discern
any anomalous surface mobility, the Navier-
Stokes equation was solved for a bilayer film
comprising a mobile layer on top of a bulk-
like layer. The solution predicts that a cross-
over from bulk flow to surface flow can occur
by either decreasing the thickness or lowering
the temperature. The former has been verified
by systematically decreasing the thickness
from 86 to 2 nm (5).
Chai et al. measured the flattening
dynamics of a step edge created on the surface of polymer films with an average thickness around 100 nm. Upon cooling the films,
they observed an analogous flow transition
at Tg. A previous experiment (7) studying the
flattening of surface gratings imprinted on
micrometer-thick films of an organic glass
also observed a transition from bulk diffusion [a mechanism only feasible in thick
films (2)] to surface diffusion at Tg + 12 K
upon cooling the films. All these findings
reinforce the conclusion that surface diffusion is directly tied to the phenomenon
of enhanced surface mobility of glasses.
Indeed, it becomes the dominant transport
process upon lowering the temperature or
thinning the specimen.
It remains unknown whether surface dif-
fusion is possible for long-chain polymers,
particularly for those with radii of gyration
exceeding several nanometers [the thickness
of the surface mobile region as derived from
surface relaxation time studies (4, 6), which
can reveal local motions besides surface
flow]. Efforts to understand the dynamics of
these materials will have to incorporate mate-
rial viscoelasticity in the data analysis, which
has hitherto been treated sparingly (11–13).
References and Notes
1. R. Gomer, Rep. Prog. Phys. 53, 917 (1990).
2. W. W. Mullins, J. Appl. Phys. 30, 77 (1959).
3. Y. Chai et al., Science 343, 994 (2014).
4. Z. Fakhraai, J. A. Forrest, Science 319, 600 (2008).
5. Z. Yang et al., Science 328, 1676 (2010).
6. K. Paeng et al., J. Am. Chem. Soc. 133, 8444 (2011).
7. L. Zhu et al., Phys. Rev. Lett. 106, 256103 (2011).
8. J. L. Keddie, R. A. L. Jones, R. A. Cory, Europhys. Lett. 27,
9. J. Baschnagel, F. Varnik, J. Phys. Condens. Matter 17,
10. H. Kim et al., Phys. Rev. Lett. 90, 068302 (2003).
11. S. A. Hutcheson, G. B. McKenna, Phys. Rev. Lett. 94,
12. M. Hamdorf, D. Johannsmann, J. Chem. Phys. 112, 4262
13. C.-H. Lam, O. K. C. Tsui, D. Peng, Langmuir 28, 10217
Acknowledgments: O.K. C. T. is supported by NSF through
projects DMR-1004648 and DMR-1310536.