What's the relationship between environmental science and environmental sustainability?
These are two very closely related disciplines, with some slight differences.
Environmental science tends to focus on understanding the processes that operate in the natural environment. It usually also covers human impacts on the environment (land, air, water, ice) and current problems the environment is facing (e.g. global warming, acid raid, ozone hole, overfishing, etc).
Environmental sustainability tends to take a slightly broader look including studying sustainability concepts like; the environment, the economy, and society (or sometimes called people, planet and profit). This discipline usually also covers ways and solution that society can adopt to achieve environmental sustainability.
Having said the above, its not uncommon for environmental courses in most universities to cover both the science and the sustainability aspects in a single course.