  Irregularly sampled time series analysis is a common problem in various
disciplines. Since conventional methods are not directly applicable to
irregularly sampled time series, a common interpolation approach is used;
however, this causes data distortion and consequently biases further analyses.
We propose a method that yields a regularly sampled time series spectrum of
costs with minimum information loss. Each time series in this spectrum is a
stationary series and acts as a difference filter. The transformation costs
approach derives the differences between consecutive and arbitrarily sized
segments. After obtaining regular sampling, recurrence plot analysis is
performed to distinguish regime transitions. The approach is applied to a
prototypical model to validate its performance and to different palaeoclimate
proxy data sets located around Africa to identify critical climate transition
periods during the last 5 million years and their characteristic properties.
