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The environmental implications of tidal stream energy extraction need to be evaluated against the potential climate change impacts on the marine environment. Here we study how hypothetical very large tidal stream arrays and a business as usual future climate scenario can change the hydrodynamics of a seasonally stratified shelf sea. The Scottish Shelf Model, an unstructured grid three-dimensional ocean model, has been used to reproduce the present and the future state of the NW European continental shelf. Four scenarios have been modeled: present conditions and projected future climate in 2050, each with and without very large scale tidal stream arrays in Scottish Waters (UK). It is found that where tidal range is reduced a few centimeters by tidal stream energy extraction, it can help to counter extreme water levels associated with future sea level rise. Tidal velocities, and consequently tidal mixing, are also reduced overall by the action of the tidal turbine arrays. A key finding is that climate change and tidal energy extraction both act in the same direction, in terms of increasing stratification due to warming and reduced mixing; however, the effect of climate change is an order of magnitude larger.Plain Language Summary Tidal currents can turn underwater turbines, which can generate electricity. Tides in Scotland (United Kingdom, UK) can provide 10% of the present UK electricity demand. Does this come without side effects on the marine environment? The answer is no, but those side effects are going to be local and much smaller than the effects of climate change. We do not have a time machine, but we can use a numerical ocean model, a computer software that is able to predict the movement of ocean currents, among other things. We can simulate the present and future conditions in 2050 of the North and Irish Seas, with and without the ocean bottom being covered in some areas with underwater turbines. Our predictions tell us that the underwater turbines will slow down the ocean currents. This leads to a less vertical mixing of the ocean, reducing the exchange of water between the ocean bottom and the surface. However, the ocean model also anticipates that global warming will have the same effect, but 10 times larger. We discovered that the underwater turbines can also change the sea level, and, in some situations, this can help to protect the coast against future sea level rise.