It could be the beginning of a joke: two bicyclists walk into a bar and ask the bartender what’s the correlation between Earth's core, Earth’s rotation and global surface air temperature?
The bartender, a geophysicist during the day, takes out an issue of the Journal of Climate and says, "I have just what you need."
Physicists Jean Dickey and Steven Marcus of NASA's Jet Propulsion Laboratory in Pasadena, Calif., working with French colleague geophysicist Olivier de Viron of the Institute of Planetary Physics in Paris, took the three variables and spliced out the natural components of surface temperature change that correspond to movements in Earth’s core and changes in rotation. They found, as announced in a NASA online features story yesterday, that what remained was a distinct trend of surface temperature warming beginning in 1930 that deviates from the natural oscillations.
Earth’s 24-hour rotation around its axis is similar to a biker maintaining a 24 mile-per-hour pace. Slight variations in speed are hallmarks of larger processes taking place. An Iron Man competitor desperately trying to maintain a bike speed of 24 miles per hour still travels faster downhill and slower up hills.
Each year Earth’s rotation takes a millisecond longer to spin during the Northern Hemisphere’s winter but then quickens by a millisecond in the summer. The difference, as explained in the NASA article, is associated with the seasonal interactions that occur between Earth’s atmosphere, ocean and land. In this way, the winter months are akin to hills in the rotational terrain.
But on a longer timescale, Earth’s rotation can change by as much as 4 milliseconds as it did at the beginning of the 20th century. During these longer time-scale variations in rotation the Earth’s global average surface air temperature also changes by up to 0.4 degree Fahrenheit – though no one really knows why.
Dickey and her team wanted to know how much of the warming occurring now could be affiliated with these natural oscillations or if they could see any signal that the warming was human-induced.
In 2009 Dickey and de Viron helped confirm that deep wave oscillations of liquid iron were taking place in the Earth’s liquid outer core, by matching the oscillations to changes in the magnetic field at the surface. These deep waves of motion occurred in periods of 85, 50, 35 and 28 years and are at the root of the long-term rotational variations as well.
In the recent study the team compared the liquid iron outer core oscillations with Earth’s yearly-averaged rotational speed and two annual global averaged surface air temperature observations dating back to 1880 and 1860. The team found that the trends correlated closely until 1930 – at that time, the surface air temperatures continued to increase, but without corresponding to changes in Earth's length of day or to movements of Earth's core.
But if they corrected for human-induced warming, as estimated by computer climate models of Earth's atmosphere and ocean, the trend returned.
"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," Dickey reported on the NASA website. "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."