Before the advent of radar altimeters our understanding of the world's climate was based on a few instruments moored off the coats of Europe, Japan and North America and visual observations taken from merchant ships. This information was patchy and in many cases of poor quality. It was difficult to relate what was happening at one location with another. The advent of the radar altimeter has changed all that. We had a series of satellite instruments that made consistent measurements of significant wave height across the globe. Initial work concentrated on simply mapping the wave climate and investigating means and seasonal variation. However with longer records came the ability to look at inter-annual variability. It had been known since the late 70's that wave heights measured at a few sites around the North East Atlantic had shown a dramatic increase. It was only the combination of spatial and temporal sampling from the altimeter that allowed us to discover the extent of the changes and how they related to the North Atlantic Oscillation index. Of course it is not only the mean wave conditions that are important, extreme waves are of vital interest to naval architects and the designers of offshore structures. Being able to estimate extreme waves from altimeter data enables us for the first time to establish what extreme conditions might be in any part of the world. At present we do not exploit the spatial nature of the altimeter data in our estimation of the extremes but this is an active research area. Research into further uses of altimeter data for looking at the wave climate continues apace. Algorithms for new parameters such as wave period are being developed. Real time applications are appearing. For such applications space-time sampling is an issue and people are coming up with innovative ideas using constellations of cheap altimeters. In this paper we review the progress made in the study of wave climate using the radar altimeter. In addition we look into current directions of new research and where we might be going next.
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