Engineering knowledge forms an essential part of our planetary fight against climate change. Traditionally, engineering curricula emphasizes the importance of technical knowledge and encourages to specialize in niche areas, where engineers develop themselves into experts. However, it is important to be able to reflect on complex societal challenges from a variety of perspectives to produce not only innovative, but also long-lasting and inclusive solutions for the greater good. This paper aims to understand the extent of systems thinking abilities of engineers by differentiating experts from novices. The study traces sustainability connections made by professional engineers and master's level engineering students when solving engineering design problems. This qualitative study highlights seven recurring themes that relate to the global sustainability discourse and describe a problem-centred approach through a real-life case that focuses on paper and pulp production, through a thematic analysis of 59 responses. The results portray system-level differences in how novice and expert engineers approach sustainability questions and how these differences shape their solution spaces.