Karsums debuted internationally at the junior level with Latvia at the 2001 IIHF World U18 Championships. At the Division I 2004 World Junior Championships, Karsums helped Latvia claim the bronze medal, leading the tournament in scoring with 13 points in 7 games. He later claimed the Gold medal and claimed promotion to the top tier at the 2005 World Junior Ice Hockey Championships. (D1-B)