Peatlands occupy approximately 15% of boreal and sub-arctic regions, contain approximately one third of the world's soil carbon pool, and supply most of the dissolved organic carbon (DOC) entering boreal lakes and rivers and the Arctic Ocean. The high latitudes occupied by these peatlands are expected to see the greatest amount of climatic warming in the next several decades. In addition to increasing temperatures, climatic change could also affect the position of the water-table level and discharge from these peatlands. Changes in temperature, water tables, and discharge could affect delivery of DOC to downstream ecosystems where it exerts significant control over productivity, biogeochemical cycles, and attenuation of visible and UV radiation. We experimentally warmed and controlled water tables while measuring discharge in a factorial experiment in large mesocosms containing peat monoliths and intact plant communities from a bog and fen to determine the effects of climate change on DOC budgets. We show that the DOC budget is controlled largely by changes in discharge rather than by any effect of warming or position of the water-table level on DOC concentrations. Furthermore, we identify a critical discharge rate in bogs and fens for which the DOC budget switches from net export to net retention. We also demonstrate an exponential increase in trace gas CO2–C and CH4–C emissions coincident with increased retention of dissolved organic carbon from boreal peatlands.