The resulting temperature calibrations are accurate, in the best cases, to about 1 degree C. More problematic, the calibration error, or measurement bias, tends to drift slowly over a period of years. A successor instrument may have a bias that differs by several tenths, even a full degree C. While this is not a big issue for short-term weather forecasting, it is a fundamental problem for tracking climate change. Long-term average temperature changes are on the order of a few tenths of a degree C per decade. With our sensors drifting by that much or more, it becomes impossible to distinguish climate change from instrumental drift. NASA has noted that, "Perhaps the greatest roadblock to fundamental advances in our understanding of climate variability and climate change is the lack of robust and unbiased long-term global observations."