The region studied includes the Laurentian Great Lakes and a diversity of smaller glacial lakes, streams and wetlands south of permanent permafrost and towards the southern extent of Wisconsin glaciation. We emphasize lakes and quantitative implications. The region is warmer and wetter than it has been over most of the last 12 000 years. Since 1911 observed air temperatures have increased by about 0. 118C per decade in spring and 0. 068C in winter; annual precipitation has increased by about 2. 1% per decade. Ice thaw phenologies since the 1850s indicate a late winter warming of about 2. 58C. In future scenarios for a doubled CO 2 climate, air temperature increases in summer and winter and precipitation decreases (summer) in western Ontario but increases (winter) in western Ontario, northern Minnesota, Wisconsin and Michigan. Such changes in climate have altered and would further alter hydrological and other physical features of lakes. Warmer climates, i.e. 2  CO 2 climates, would lower net basin water supplies, stream ¯ows and water levels owing to increased evaporation in excess of precipitation. Water levels have been responsive to drought and future scenarios for the Great Lakes simulate levels 0. 2 to 2. 5 m lower. Human adaptation to such changes is expensive. Warmer climates would decrease the spatial extent of ice cover on the Great Lakes; small lakes, especially to the south, would no longer freeze over every year. Temperature simulations for strati®ed lakes are 1±78C warmer for surface waters, and 68C cooler to 88C warmer for deep waters. Thermocline depth would change (4 m shallower to 3. 5 m deeper) with warmer climates alone; deepening owing to increases in light penetration would occur with reduced input of dissolved organic carbon (DOC) from dryer catchments. Dissolved oxygen would decrease below the thermocline. These physical changes would in turn a€ect the phytoplankton, zooplankton, benthos and ®shes. Annual phytoplankton production may increase but many complex reactions of the phytoplankton community to altered temperatures, thermocline depths, light penetrations and nutrient inputs would be expected. Zooplankton biomass would increase, but, again, many complex interactions are expected. Generally, the thermal habitat for warm-, cool-and even cold-water ®shes would increase in size in deep strati®ed lakes, but would decrease in shallow unstrati®ed lakes and in streams. Less dissolved oxygen below the thermocline of lakes would further degrade strati®ed lakes for cold water ®shes. Growth and production would increase for ®shes that are now in thermal environments cooler than their optimum but decrease for those that are at or above their optimum, provided they cannot move to a deeper or headwater thermal refuge. The zoogeographical boundary for ®sh species could move north by 500±600 km; invasions of warmer water ®shes and extirpations of colder water ®shes should increase. Aquatic ecosystems across the region do not necessarily exhibit coherent responses to climate changes and variability, ...