英语听力：自然百科 地球力场 Earth's Force Field—10

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To determine where the magnetic pole / is, we can't rely on one single observation. What I try to do is surround the estimated position of the pole, taking as many observations as possible. 

 

This wandering is / symptomatic of fluctuations¹ in the geodynamo. In recent decades, scientists have noticed that the pole has been moving faster. 

 

Over much of the past a hundred years, it has been around 10 kilometers per year. But since about 1970, it started to accelerate and now it's moving along at about 40 kilometers per year.  

 

In around 50 year's time, it might twitch² Siberia. So does the wandering pole's acceleration³ have any connection with the Earth's weakening magnetic field? 

 

Professor Jeremy Bloxham is searching for an answer. He uses the records of / early sailors to chart the magnetic north movement over the past 300 years. 

 

Because of the importance of the magnetic field to navigation, people on trading ships and voyages of exploration back in the 17th century or the 18th century were making very careful systematic⁴ measurements of the magnetic field.

 

Bloxham feeds the historical data into a computer which creates an impression of the Earth's magnetic history. 

 

What we have here is an animation⁵ that would show us how the field has changed over the 300 years or also since 1690. And in the shades of red and orange, were showing the strength of the magnetic field as it comes out of the core. And in the shades of blue, the strength of the magnetic field as it goes back into the core. 

 

Over the last 150 years, scientists have measured a 10% decline in the overall strength of the Earth's magnetic field. It's fading 10 times faster and if the geodynamo suddenly stops, crucially, parts of the field are behaving differently. 

 

Now, as we get into the beginning of the twenty of century, we begin to see the emergence⁶ of this blue patch here, beneath Southern Africa which then drifts to the west and joins up with this other patch, making a large region of the core-mantle boundary with the field is going in the opposite direction from what we would expect.

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