Following up on @ColorStatistics excellent advice....
This post Variance inhomogeneity in time series when forecasting should be of interest to you . Note that the fact that the observed series may have increasing variability does not mean that the error variance from a reasonable model will also exhibit the characteristic. Take a look at https://autobox.com/cms/index.php/blog/entry/u-didnt-need-logs as to how a cursory examination about variability can often be flawed.