Marine data sets and the shift of shipping routes

Shipping routes of merchant ships have been changing since 1854, when the first observations over the ocean were organized. In certain areas where longitudinal and/or latitudinal gradients of surface parameters exist, spurious climatic signals due to the shift in the shipping routes can appear even when space averaging is made over a large domain. This problem is documented, and two methods to correct the time series are presented. Fields resulting from an objective analysis of the observations take into account the shift of shipping routes.