1977 - 1988 Oceanographer (Royal Netherlands Meteorological Institute, KNMI).
As a guest he has worked at the Max-Planck Institute for Meteorology in Hamburg and at the Institute of Oceanographic Sciences in Wormley, UK. He took an active part in the international coordination of wave research. As chairman of the Wave Modelling (WAM) group he helped with the development and implementation of a third generation ocean wave model. He also chaired working group 83 (wave modelling) of the Scientific Committee of Oceanic Research (SCOR). In his studies of the energy balance in growing surface waves - from huge ocean gravity waves to tiny capillary wavelets - he addressed theoretical questions, numerical applications and the practical issue of making marine forecasts. Later, his interest shifted towards the role of air/sea interaction in the climate system.

1988 - 2001 Head of the Department of Oceanographic Research (KNMI).
Responsible for oceanic aspects of climate research and (until 1992) for the development of marine forecasting tools. Marine forecasting comprises the development of numerical sea wave, storm surge and circulation models, their validation and the use of data assimilation techniques. Climate research focused on the improvement of general circulation models of the ocean and their use in coupled atmosphere/ocean models for the study of natural and anthropogenic climate variations. Member of various national and international committees and involved with several European projects. The WASA project studied variations in the storm, wave and storm surge climatology of the North Sea. Decadal fluctuations in the 20th century were found to be much larger than the centennial trend.
1994 - 1997 Netherlands focal point for IPCC working group 1
1995 - 1998 Euroclivar-coordinator

2000 - 2005 Professor of Climate Dynamics, Utrecht University
2001 - 2006 Director of Climate Research and Seismology (KNMI)
While responsible for the Climate Research and Seismology department he still continued research in air/sea interaction and ocean waves. This culminated in the derivation of a global 40-year wave climatology, in which decadal variability could be detected.He also helped develop and implement a number of large research programmes, and was active in the communication about climate variability and climate change.

Climate research at KNMI was carried out as part of the international research effort in this field. There were more than a hundred staff members working in different divisions: Climate Variability Research, Atmospheric Research, Oceanography, Atmospheric Chemistry and Climate Analysis. An overarching aim was observing, understanding and predicting changes in the climate system. Research attempted to answer questions raised by society: 1. How is our climate changing? 2. What is the cause? 3. What will our climate be like in the future? This let to the following activities:

Observations and monitoring
The analysis of meteorological observations, research and development in the field of satellite observations of clouds and the atmospheric composition, a contribution to ocean observations (Argo floats), and dedicated measurement campaigns using the tower in Cabauw and Meetpost Noordwijk.

Understanding of the climate system
This involved study of the causes of climate variations and variability of climate, through global and regional atmospheric and oceanic modelling, the study of extremes in the present climate and the study of key processes such as clouds, the atmospheric boundary layer, patterns of natural climate variability (La Nino and the North Atlantic Oscillation).

Predictability
Study of patterns of natural and of anthropogenic variations of the regional (West-European) climate and its extremes. This was done by analysing observations and GCM simulations made elsewhere plus downscaling with a regional GCM (Racmo) and statistical techniques. Among the results are the KNMI'2006 climate scenario's, outlining possible states of the Dutch climate in 2050.

At the Institute for Marine and Atmospheric Research (IMAU) he lectured on "Integration in Climate Research" with emphasis on the complexity of the climate system and the need to combine observations and models. He supervised the PhD work of Hans Bonekamp, Hein Zelle and Albert Klein Tank. Earlier he was closely involved, as project leader, with the PhDs of Klaartje van Gastel, Nanne Weber, Aart Voorrips and Kees Mastenbroek.

2001 - 2004 Co-coordinator of the PRogramme for Integrated Earth System Modelling
2002 - 2006 Member of the Scientific Advisory Board of SRON
2003 - 2006 Member of the Board of ENSEMBLES
2003 - 2006 Member of the Programme Committee of Klimaat voor Ruimte
2003 - 2006 Member and co-chair of PCCC
2004 - 2006 Deputy Director General of KNMI
2001 - 2007 Netherlands focal point for IPCC
2003 - 2009 Member of the NWO Earth and Life Sciences (ALW) Board
2007 - 2009 Chair of the Board supervising the implementation of the NWO 'Sustainable Earth' research theme (until July 2009).