Before revision:   In particular, we have detected two sudden jumps in the standard deviation, in the second quarter of 2017 and at the beginning of 2018, suggesting early warning signals of two majors price collapse that have happened in those periods.
After revision:  Therefore, while the autocorrelation does not display signals of critical slowing down, the standard deviation can be used to anticipate critical or stochastic transitions. In particular, we have detected two sudden jumps in the standard deviation, in the second quarter of 2017 and at the beginning of 2018, suggesting early warning signals of two majors price collapse that have happened in those periods.