What is bias adjustment?

The complex climate system, with many interdependent processes that are not always fully known or described in the climate model, inevitably leads to systematic deviations from observed values. The deviations can be of different magnitude for different variables and for different regions of the world. Most often, these deviations do not pose a problem for calculating climate indicators where the focus is on differences between a historical and a future scenario. However, some indicators are based on absolute limits, such as the number of days with frost, tropical nights, or precipitation above a certain level. A systematic deviation from observations, a bias, in the climate model can affect these indicators so that their interpretation is misleading. To remedy such issues, the variables are bias adjusted using a scientific developed method and compared to a global reference dataset.