NASA Study Goes to Earth's Core for Climate Insights

MEDIA RELATIONS OFFICE
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Alan Buis 818-354-0474
Jet Propulsion Laboratory, Pasadena, Calif.
Alan.buis@jpl.nasa.gov

Feature: 2011-074 March 9, 2011

NASA Study Goes to Earth's Core for Climate Insights

The full version of this story with accompanying images is at:
http://www.jpl.nasa.gov/news/news.cfm?release=2011-074&cid=release_2011-074

The latest evidence of the dominant role humans play in changing Earth's climate comes
not from observations of Earth's ocean, atmosphere or land surface, but from deep within
its molten core.

Scientists have long known that the length of an Earth day — the time it takes for Earth to
make one full rotation — fluctuates around a 24-hour average. Over the course of a year,
the length of a day varies by about 1 millisecond, getting longer in the winter and shorter in
the summer. These seasonal changes in Earth's length of day are driven by exchanges of
energy between the solid Earth and fluid motions of Earth's atmosphere (blowing winds
and changes in atmospheric pressure) and its ocean. Scientists can measure these small
changes in Earth's rotation using astronomical observations and very precise geodetic
techniques.

But the length of an Earth day also fluctuates over much longer timescales, such as
interannual (two to 10 years), decadal (approximately 10 years), or those lasting multiple
decades or even longer. A dominant longer timescale mode that ranges from 65 to 80
years was observed to change the length of day by approximately 4 milliseconds at the
beginning of the 20th century.

These longer fluctuations are too large to be explained by the motions of Earth's
atmosphere and ocean. Instead, they're due to the flow of liquid iron within Earth's outer
core, where Earth's magnetic field originates. This fluid interacts with Earth's mantle to
affect Earth's rotation. While scientists cannot observe these flows directly, they can
deduce their movements by observing Earth's magnetic field at the surface. Previous
studies have shown that this flow of liquid iron in Earth's outer core oscillates, in waves of
motion that last for decades with timescales that correspond closely to long-duration
variations in Earth's length of day.

Still other studies have observed a link between the long-duration variations in Earth's
length of day and fluctuations of up to 0.2 degrees Celsius (0.4 degree Fahrenheit) in
Earth's long-term global average surface air temperature.

So how might all three of these variables — Earth's rotation, movements in Earth's core
(formally known as the core angular momentum) and global surface air temperature — be
related? That's what researchers Jean Dickey and Steven Marcus of NASA's Jet
Propulsion Laboratory, Pasadena, Calif., and colleague Olivier de Viron of the Universite
Paris Diderot and Institut de Physique du Globe de Paris in France, set out to discover in a
first-of-its-kind study.

The scientists mapped existing data from a model of fluid movements within Earth's core
and data on yearly averaged length-of-day observations against two time series of
observed annual global average surface temperature: one from NASA's Goddard Institute
of Space Studies in New York that extends back to 1880, and another from the United
Kingdom's Met Office that extends back to 1860. Since total air temperature is composed of
two components — temperature changes that occur naturally and those caused by human
activities — the researchers used results from computer climate models of Earth's
atmosphere and ocean to account for temperature changes due to human activities. These
human-produced temperature changes were then subtracted from the total observed
temperature records to generate corrected temperature records.

The researchers found that the uncorrected temperature data correlated strongly with data
on movements of Earth's core and Earth's length of day until about 1930. They then began
to diverge substantially: that is, global surface air temperatures continued to increase, but
without corresponding changes in Earth's length of day or movements of Earth's core. This
divergence corresponds with a well-documented, robust global warming trend that has
been widely attributed to increased levels of human-produced greenhouse gases.

But an examination of the corrected temperature record yielded a different result: the
corrected temperature record remained strongly correlated with both Earth's length of day
and movements of Earth's core throughout the entire temperature data series. The
researchers performed robust tests to confirm the statistical significance of their results.

"Our research demonstrates that, for the past 160 years, decadal and longer-period
changes in atmospheric temperature correspond to changes in Earth's length of day if we
remove the very significant effect of atmospheric warming attributed to the buildup of
greenhouse gases due to mankind's enterprise," said Dickey. "Our study implies that
human influences on climate during the past 80 years mask the natural balance that exists
among Earth's rotation, the core angular momentum and the temperature at Earth's
surface."

So what mechanism is driving these correlations? Dickey said scientists aren't sure yet,
but she offered some hypotheses.

Since scientists know air temperature can't affect movements of Earth's core or Earth's
length of day to the extent observed, one possibility is the movements of Earth's core might
disturb Earth's magnetic shielding of charged-particle (i.e., cosmic ray) fluxes that have
been hypothesized to affect the formation of clouds. This could affect how much of the
sun's energy is reflected back to space and how much is absorbed by our planet. Other
possibilities are that some other core process could be having a more indirect effect on
climate, or that an external (e.g. solar) process affects the core and climate simultaneously.

Regardless of the eventual connections to be established between the solid Earth and
climate, Dickey said the solid Earth's impacts on climate are still dwarfed by the much
larger effects of human-produced greenhouse gases. "The solid Earth plays a role, but the
ultimate solution to addressing climate change remains in our hands," she concluded.

Study results were published recently in the Journal of Climate.

For more information, see: http://www.jpl.nasa.gov/news/features.cfm?feature=2420 and
http://www.jpl.nasa.gov/news/features.cfm?feature=15 .

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