Has the gravitational field of the earth changed since the 1980's ?
Actually, time (especially non-geological recent time) really has nothing to do with it. Changes and variations in the Earth's gravitational field are not really time dependent but due to the fact that the Earth is not a perfect sphere and its orbit is not a perfect circle. If the Earth were a perfect sphere, then the gravitational field would be uniform across the entire planet. But the Earth is actually an oblate spheroid with bulging around the equatorial region. Changes in topographical height, varying density, and the centripital force from the rotation of the planet all affect gravitational variations seen across the planet. But the changes over time are due to the fact that the proximity of the Earth to the Sun and the relative position of the Moon are not constant. Depending on how these three bodies are situated, the Earth's gravitational field is affected.
The gravitational field of the Earth has most definitely changed since the 1980s. In fact, there have been many occurrences only just recently. According to NASA's satellite data in 1998, there was a large bulge at the equator due to gravitational force. Prior to that event, including many after, the Earth's gravitational field has been changing so much, in fact, that the Earth's field is not quite the spheroid, or sphere, that we would imagine it to be. It is actually an oblate spheroid because of all of the bulges and formations along the surface of Earth, seen via the planet's geoid (a topographical map of Earth's surface).