As climate changes are progressing so is the need for further knowledge surrounding them and their consequences. This includes the ocean’s role in the climate system. Previous studies indicate a general lack of research in students’ understanding of the ocean, which this thesis aims to fill a part of. This thesis has investigated upper secondary school students’ basic knowledges of the ocean’s natural processes and chemistry, as well as the students’ understanding of the consequences of the ongoing ocean warming. The methods used have been a combination of participant observation and a quiz, considering students in an upper secondary school in Stockholm. The results of the quiz show that two thirds of the students have a basic understanding of the ocean’s processes and chemistry. The participant observation supports this conclusion. It additionally showed that most students displayed superficial knowledge of oceanic processes, while having a poorer understanding of more complex processes and the concurrence between processes. Some misunderstandings and alternative understandings surfaced amongst the students’ answers. These, along with systematic geography and its preconditions, are discussed in the discussion section. While the present study was too limited to draw definite conclusions, this study indicated preliminary that students with focus on natural science display a deeper knowledge in the ocean’s chemistry and natural processes than students with focus on social sciences, which would be interesting to investigate further in future research.