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Discharge projections into the Hudson Bay Complex to 2070 are investigated for global mean temperature warming levels of 1.5 and 2.0 degrees C. Median precipitation increases from 1986-2005, ranging from 2% during summer to 19% during winter, are projected to increase discharge in all seasons except summer. The rise in discharge is greatest furthest north, into Foxe Basin, Ungava Bay, and Hudson Strait, exceeding 10% above historical annual means. A 2.0 degrees C warming results in higher discharge than 1.5 degrees C warming owing to greater precipitation (e.g., 6.5% greater spring discharge increase); however, summer discharge for 2.0 degrees C warming is lower due to enhanced evaporation and lower precipitation increase from historical (4.0% lower summer discharge increase). Extreme daily high flows are projected to be greater than historical, more so for 2.0 degrees C warming than 1.5 degrees C warming, and this is greatest in the eastern and northern regions. These projections suggest continued increasing river discharge into pan-Arctic coastal oceans.

Plain Language Summary In 2016, participants in the Paris climate agreement pledged to hold the global average temperature rise to less than 2.0 degrees C above preindustrial levels and to pursue limiting the rise to 1.5 degrees C. Northern regions, such as the 4 million-km(2) Hudson Bay Drainage Basin that discharges into the Hudson Bay Complex, are highly sensitive to climate change. We quantify changes in river discharge into the Hudson Bay Complex under future climate (2020-2070) and assess the impacts of 1.5 and 2.0 degrees C warming on discharge. The region is projected to warm at a rate greater than the global average, causing greater precipitation and discharge compared to historical. A 2.0 degrees C warming would increase discharge beyond that of 1.5 degrees C warming, but summer discharge may be lower due to higher evapotranspiration. Discharge increases are greatest furthest north, where there are no active streamflow gauges. Changing discharge has implications for the hydroelectric industry, with respect to managing possible increased spring flood risk but lower summer flows. Rising discharge impacts sea ice and the biological productivity of coastal ocean systems such as the Hudson Bay. Global carbon budgets may be impacted as coastal ocean systems are more active organic cycling sites than open oceans.