Wednesday, November 2, 2011

Study Of Earth's Crust for CLIMATE INSIGHTS

Is not it surprising to know that the latest evidence of the dominant role humans play in changing Earth's climate comes from observations of deep molten core rather than from Earth's ocean, atmosphere, or land surface.

A NASA/university study of data on Earth's rotation, movements in Earth's molten core and global surface air temperatures has uncovered interesting correlations. 

The length of an Earth day, as known by the scientists fluctuates around 24 hours. Over the course of a year, there is a variation of 1 millisecond in the length of a day, getting longer in the winter and shorter in the summer.
The exchange of energy between the solid earth and the fluid motions of Earth's atmosphere (blowing winds and changes in atmospheric pressure) and its oceans drives these seasonal changes in Earth's length of day. These small changes can be measured using astronomical observations and very precise geodetic techniques by the scientists.


But the fluctuation in the length of the day varies over much longer scales, such as interannual(two to ten years), decadal(approximately ten years), or those lasting multiple decades or even longer. A change of 4 millisecond in the length of the day was observed at the beginning of the twentieth century which was a dominant longer timescale mode ranging from 65 to 80 years. These longer fluctuations are too large to be explained by the motions of Earth's atmosphere and oceans. Instead, they are due to the flow of liquid iron within Earth's outer core, the place of origination of Earth's magnetic field. Thus Earth's rotation is affected by the interaction of this fluid with Earth's mantle. The movements of these liquids cannot be directly observed by the scientists but can be inferred logically 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 motions that lasts for decades with timescales that correspond closely to long duration variations in Earth's length of day.

Thus Jean Dickey and Steven Marcus of NASA's Jet Propulsion Laboratory, Pasadena, California, and colleague Olivier di Viron of the Universite Paris Diderot and Institute de Physique du Globe de Paris in France, set out to discover in a first-of-its-kind study on how might Earth's rotation, movements in Earth's core and global surface air temperature be related?

By combining measurements of Earth's magnetic field from stations on land and ships at sea with satellite data, scientists were able to isolate six regularly occurring waves of motion taking place deep within Earth's liquid core, with varying timescales.

The scientists mapped existing data from a fluid movements within Earth's core and data on yearly averaged length of day 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 UK's Met Office that extends back to 1860. Since total air temperature is composed of two components -
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 uncorrected data on temperature was found to be strongly correlated with the data on movement of Earth's core and Earth's length of day untill about 1930 by the researchers. 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 gas.

"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.

Since scientists know air temperature can't affect movements of Earth's core of Earth's length of day to the extent observed, one possibility is the movements of the Earth's core might disturb Earth's magnetic shielding of charged-particle fluxes that have been hypothesized to affect the formation of clouds. This cloud affect how much of 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 on climate, or that an external process affects the core and climate simultaneously.

Time series of Earth's surface air temperature (black line) and time series corrected for the influence of human activities (red line), Earth's length of day (green line) and Earth's core angular momentum (blue line).

So what mechanism is under these correlations? Scientists aren't sure yet but some hypotheses like the ones mentioned above is put forth by them.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 affects 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.

 

[via : nasa.gov.in]














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