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Mars lost its magnetic field 4.5 billion years ago according to this article:

Mars lost its global magnetic field during the time of this heightened activity. After 500 million years, the Martian atmosphere is substantially destroyed, all due to the disappearance of Mars global magnetic field.

When will the Earth lose its magnetic field and atmosphere?

I am wondering: how much time do we have to find another planet to live on?

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    $\begingroup$ My guess is that losing the magnetic field is a lesser worry. We have only a billion years at most before the Sun becomes too bright for photosynthesis to continue (the air warms and carbon dioxide erodes rocks faster, thus starving green plants). We're outta here, one way or another, before the much slower process of core solidification and loss of magnetic dynamo come into play. $\endgroup$ Jun 25 '21 at 20:35
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Measurements have been made of the Earth's magnetic field more or less continuously since about 1840. Some measurements even go back to the 1500s, for example at Greenwich in London. If we look at the trend in the strength of the magnetic field over this time (for example the so-called 'dipole moment' shown in the graph below) we can see a downward trend. Indeed projecting this forward in time would suggest zero dipole moment in about 1500-1600 years time. This is one reason why some people believe the field may be in the early stages of a reversal. We also know from studies of the magnetisation of minerals in ancient clay pots that the Earth's magnetic field was approximately twice as strong in Roman times as it is now.

(from http://www.geomag.bgs.ac.uk/education/reversals.html)

If you look into magnetic field calculators (IGRF), you will see that they are not valid for predicting total field strength even for a few years from now

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