1) All kidding aside 2) Are you kidding me 3) Badly 4) Earnestly 5) Grimly 6) In earnest 7) Severely 8) Sincerely 9) Solemnly 10) Sombrely 11) Toheart 12) Very 1) Critically 2) Earnestly 3) Gravely 4) Hopelessly 5) In a serious manner 6) In earnest 7) To a severe or serious degree 
[adv] - in a serious manner
seriously 1. In a serious manner: 'He shook his head seriously.' 2. To an alarmingly grave extent: 'He is seriously ill.' 3. With genuine, earnest intent; sincerely: 'Seriously, Jimmy, we have to get home before dark.'
earnestly 2 in earnest adverb in a serious manner; `talking earnestly with his son`; `she started studying snakes in earnest`; `a play dealing seriously with the question of divorce`