Historical Ocean Surface Temperatures: Accuracy, Characterisation and Evaluation

The surface temperature of the land and sea is the main measure of recent climate change and sea surface temperature (SST) is its ocean component. Measurements of SST have been made for more than 200 years, first on sailing ships, now on a mixture of ships and buoys. We hope HOSTACE will improve our understanding of how the dramatic change in technology we have seen over the observational record has affected the consistency of the measured SSTs. We will also produce a new dataset of monthly mean SST fields covering at least the past 160 years. We expect this "SeaTemp" dataset to have better estimates of uncertainty than existing datasets.

The main problem we face is the lack of detailed information about how the measurements were made. The available evidence tells us that the method used to make the observation can lead to systematic biases in the measurements. These biases are expected to vary with environmental conditions, and also among platforms, for example from ship-to-ship. We will therefore look at the data from every ship and buoy separately, to understand the biases and make adjustments to account for them. The wealth of SST data available from satellites in recent decades will be used to estimate the expected SST variability, in terms of both ocean basin-scale modes and with local environmental parameters.

Once we have adjusted for biases in the SST observations, we will construct the SeaTemp dataset, covering the ice-free global oceans. The further we go back in time, the sparser the observations and so generating a gap-filled dataset will be extremely challenging. Methods exist to fill the gaps based on the large-scale modes of variability seen in modern data and HOSTACE will further develop these methods. The final steps will be to use the dataset in a detection and attribution study, and in collaboration with other SST dataset developers to make a detailed comparison of available SST data products.

HOSTACE has been funded by a grant from the UK Natural Environment Research Council and builds on previous research funded by NERC, the European Space Agency and the Met Office [link to related projects].