As part of global environmental monitoring, scientists have been tracking the total volume of an alpine glacier in South America. The table below shows the estimated volume of the glacier, in cubic kilometres (km3\text{km}^3km3), over a thirty-year period.
| Year | 1990 | 2005 | 2020 |
|---|---|---|---|
| Glacier volume (km3\text{km}^3km3) | 9.6 | 8.0 | 4.5 |
Calculate the percentage decrease in the volume of the glacier between 1990 and 2020. Give your answer to 2 significant figures.
Which of the following is the best description for how the volume of the glacier changed between 1990 and 2020?