英语流利说 Level 5 Unit 3-4 Magnetic
Scientists now believe that the Earth's magnetic field is about to reverse.
This is called magnetic field reversal.
Every 200,000 to 300,000 years, the Earth's magnetic field reverses.
In other words, the North and South magnetic poles switch places.
This has happened many times in Earth's history, but it has been happening more lately.
Recent research provides evidence that there have been five magnetic reversals during the past 500,000 years.
The most recent event happened approximately 41,000 years ago.
Recent studies have found that the Earth's magnetic field has weakened about 5% per decade.
At its current rate, the next reversal could happen within the next century.
Some people worry about the effects of a magnetic field reversal.
They think there is a possibility that it could cause many problems.
For example, without a magnetic field, communications could be disrupted.
Birds and other forms of life could become confused and get lost.
Another effect would be an increase in some forms of cancer, especially skin cancer.
This would be because of an increase in the amount of radiation absorbed by our bodies.
The possibility that a field reversal would be an extinction event is very low.
There is very little historical evidence to show that previous reversals have caused extinctions.
Humans have been on Earth for millions of years, during which there have been many reversals.
Therefore, scientists believe that any changes would be temporary.
Another theory is that the Earth's outer layer could rotate around the liquid metal outer core that makes up most of the earth.
So in the next reversal continents could shift positions, from north to south.
Imagine, if that happened, Australia would be in the northern hemisphere!
With so many theories, how can we know what to believe?
This is important because some events require us to act well in advance.
Otherwise it will be too late to prevent a catastrophe.
This underlines the importance of getting reliable information, backed by strong evidence.
（Any increase in radiation causes some forms of cancer.）
