New findings on a link between oceans and global climate look at ocean depths. Image: NOAA |
Earth’s deep oceans may
absorb enough heat at times to flatten the rate of global warming for periods
of as long as a decade—even in the midst of longer-term warming, according to a
new analysis led by scientists at the National Center for Atmospheric Research
(NCAR).
The study, based on
computer simulations of global climate, points to ocean layers deeper than
1,000 ft as the main location of the “missing heat” during periods
such as the past decade when global air temperatures showed little trend.
The findings also suggest
that several more intervals like this can be expected over the next century,
even as the trend toward overall warming continues.
“We will see global
warming go through hiatus periods in the future,” says NCAR’s Gerald
Meehl, lead author of the study.
“However, these
periods would likely last only about a decade or so, and warming would then
resume. This study illustrates one reason why global temperatures do not simply
rise in a straight line.”
The research, by
scientists at NCAR and the Bureau of Meteorology in Australia, was published online in Nature Climate Change.
Funding for the study came
from the National Science Foundation (NSF), NCAR’s sponsor.
“The research shows
that the natural variability of the climate system can produce periods of a
decade or more in which Earth’s temperature does not rise, despite an increase
in greenhouse gas concentrations,” says Eric DeWeaver, program director in
NSF’s Division of Atmospheric and Geospace Sciences.
“These scientists
make a compelling case that the excess energy entering the climate system due
to greenhouse gas increases may not be immediately realized as warmer surface
temperatures, as it can go into the deep ocean instead.”
The 2000s were Earth’s
warmest decade in more than a century of weather records.
However, the single-year
mark for warmest global temperature, which had been set in 1998, remained
unmatched until 2010.
Yet emissions of
greenhouse gases continued to climb during this period, and satellite
measurements showed that the discrepancy between incoming sunshine and outgoing
radiation from Earth actually increased.
This implied that heat was
building up somewhere on Earth, according to a 2010 study by NCAR researchers
Kevin Trenberth and John Fasullo.
The two scientists, who
are both coauthors on the new study, suggested that the oceans might be storing
some of the heat that would otherwise go toward other processes, such as
warming the atmosphere or land, or melting more ice and snow.
Observations from a global
network of buoys showed some warming in the upper ocean, but not enough to
account for the global build-up of heat.
Although scientists
suspected the deep oceans were playing a role, few measurements were available
to confirm that hypothesis.
To track where the heat
was going, Meehl and colleagues used a powerful software tool known as the
Community Climate System Model, which was developed by scientists at NCAR and
the Department of Energy with colleagues at other organizations.
Using the model’s ability
to portray complex interactions between the atmosphere, land, oceans, and sea
ice, they performed five simulations of global temperatures.
The simulations, which
were based on projections of future greenhouse gas emissions from human
activities, indicated that temperatures would rise by several degrees during
this century.
But each simulation also
showed periods in which temperatures would stabilize for about a decade before
climbing again.
For example, one
simulation showed the global average rising by about 2.5 F (1.4 C) between 2000
and 2100, but with two decade-long hiatus periods during the century.
During these hiatus
periods, simulations showed that extra energy entered the oceans, with deeper
layers absorbing a disproportionate amount of heat due to changes in oceanic
circulation.
The vast area of ocean
below about 1,000 ft (300 m) warmed by 18 to 19% more during hiatus periods
than at other times.
In contrast, the shallower
global ocean above 1,000 ft warmed by 60% less than during non-hiatus periods
in the simulation.
“This study suggests
the missing energy has indeed been buried in the ocean,” Trenberth says.
“The heat has not disappeared and so it cannot be ignored. It must have
consequences.”
The simulations also
indicated that the oceanic warming during hiatus periods has a regional
signature.
During a hiatus, average
sea-surface temperatures decrease across the tropical Pacific, while they tend
to increase at higher latitudes, especially in the Atlantic,
where surface waters converge to push heat into deeper oceanic layers.
These patterns are similar
to those observed during a La Niña event, according to Meehl.
He adds that El Niño and
La Niña events can be overlaid on top of a hiatus-related pattern.
Global temperatures tend
to drop slightly during La Niña, as cooler waters reach the surface of the
tropical Pacific, and they rise slightly during El Niño, when those waters are
warmer.
“The main hiatus in
observed warming has corresponded with La Niña conditions, which is consistent
with the simulations,” Trenberth says.
The simulations were part
of NCAR’s contribution to the Coupled Model Intercomparison Project Phase 5.
They were run on supercomputers at NCAR’s NSF-supported Climate Simulation
Laboratory and on supercomputers at Oak Ridge Leadership Computing Facility and
the National Energy Research
Scientific Computing
Center, both supported by
the U.S. Department of Energy.
