|Date of birth||10 November 1986|
|Place of birth||Kiuruvesi, Finland|
|Height||1.85 m (6 ft 1 in)|
|Playing position||Central defender|
|* Senior club appearances and goals counted for the domestic league only and correct as of 12 September 2012.
‡ National team caps and goals correct as of 2 September 2006
Kärkkäinen started his career with Kiuruveden Palloilijat where he played, until joining PK-37 for the season 2003. In 2003, he went on trial at Molde FK. For the season 2004, he moved to Kuopio and KuPS. He signed for three years. KuPS was relegated in 2006 and Kärkkäinen left the club and signed with FC Lahti. At that time he was already a regular in the Finnish under-19 national team. FC Honka had Kärkkäinen at trial in 2006, but chose not to offer a contract and FC Lahti's coach Antti Muurinen signed him for the first time.
In November 2008, Kärkkäinen signed a two-year contract with the capital city club HJK. Also in this time Muurinen wanted to have the talented centre-back. He made his first appearance for HJK in the 5-1 home win against his old club KuPS on 18 April 2009. He also scored his first goal for HJK in the same match.
He was a part of the Finnish under-21 team, that qualified for the UEFA European Under-21 Football Championship final tournament in 2009. He has also played for U17's and U19's.