CLIMATE warming, acid deposition and increasing exposure to ultraviolet radiation are all regarded as widespread problems in boreal ecosystems. Here we report observations from twenty years of whole-lake acidification experiments, which show that these three problems are intimately linked, In our study area in northwestern Ontario, both climate warming and lake acidification led to declines in the dissolved organic carbon content of lake waters, allowing increased penetration of solar radiation, We suggest that some of the changes in aquatic ecosystems that have been attributed to lake acidification may in fact have involved increased exposure to ultraviolet light. Moreover, it seems that-particularly in clear, shallow lakes and streams-climate warming and/or acidification can be more effective than stratospheric ozone depletion in increasing the exposure of aquatic organisms to biologically effective UV-B radiation.