text: Hansen is best known for his research in the field of climatology , his testimony on climate change to congressional committees in 1988 that helped raise broad awareness of global warming , and his advocacy of action to limit the impacts of climate change .
simplified: Hansen is best known for his research in the field of climatology , his testimony on climate change to congressional committees in the 1980s that helped raise broad awareness of the global warming issue , and his continuing advocacy of action to limit the impacts of climate change .