We thought he was joking at first, but then we realized that he was 'in earnest'. [=he was being serious]
It's hard to tell if he's making this proposal 'in earnest'.
It began to rain 'in earnest' [='heavily'] in the evening.
(..) We bought ourselves some time. Volunteer forces are delivering the vaccine to areas that have been cordoned off. Distribution efforts began in earnest on April th.. The importance of getting the information out to those still alive within the quarantine.. But it's given us a chance. (..)2013 World War Z