The first 15 years at KNMI he worked on ocean waves and storm surge prediction. Some of this work was done in the United Kingdom, Malaysia, Italy, Germany and Canada. Later, when he became Head of the Oceanographic Research Division, his interest shifted to air/sea interaction and climate. He gave invited lectures in more than 20 countries.

In 2001 he became Head and later Director of Climate Research and Seismology, and in 2004 deputy Director General. He also is emeritus professor of Climate Dynamics at Utrecht University. He was involved with many national and international panels and projects, published several books and numerous scientific papers. From 2001 until 2007 he was Dutch focal point for the Intergovernmental Panel on Climate Change. Until 2009 he was active as a member of the Earth and Life Science Board of the Netherlands Organisation for Scientific Research (NWO).

Currently, he is enjoying his retirement.