Autocorrelation refers to the correlation of a time series with its own past and future values.
Autocorrelation is also sometimes called “lagged correlation” or “serial correlation”, which
refers to the correlation between members of a series of numbers arranged in time. Positive
autocorrelation might be considered a specific form of “persistence”, a tendency for a system to
remain in the same state from one observation to the next. For example, the likelihood of
tomorrow being rainy is greater if today is rainy than if today is dry. Geophysical time series are
frequently autocorrelated because of inertia or carryover processes in the physical system. For
example, the slowly evolving and moving low pressure systems in the atmosphere might impart
persistence to daily rainfall. Or the slow drainage of groundwater reserves might impart
correlation to successive annual flows of a river. Or stored photosynthates might impart
correlation to successive annual values of tree-ring indices. Autocorrelation complicates the
application of statistical tests by reducing the number of independent observations.
Autocorrelation can also complicate the identification of significant covariance or correlation
between time series (e.g., precipitation with a tree-ring series). Autocorrelation can be exploited
for predictions: an autocorrelated time series is predictable, probabilistically, because future
values depend on current and past values. Three tools for assessing the autocorrelation of a time
series are (1) the time series plot, (2) the lagged scatterplot, and (3) the autocorrelation function.