The geomagnetic field has an unusual weak spot over South America and the South Atlantic Ocean, called South Atlantic Anomaly (SAA). The magnetospheric particles trapped in this field penetrate deep into the atmosphere over the SAA resulting in lower inner boundary of the radiation belt. Over the past 400 years, the magnetic field in the SAA region has decreased consistently. This study shows that the weakened geomagnetic field has a bearing on the position of the inner boundary of the radiation belt. The present simulation revealed that the inner boundary of the radiation belt over the SAA region is moving earthward at the rate of 4.1$\pm$0.1 km/year and that earthward penetration of energetic particles has increased by $\approx$480 km during period 1900-2020. If the geomagnetic field in the SAA region continues to decrease, the resulting deepening of the radiation belt will pose increased risks to our satellites, life, and climate.