Browning
of surface waters because of increasing terrestrial dissolved
organic carbon (OC) concentrations is a concern for drinking water
providers and can impact land carbon storage. We show that positive
trends in OC in 474 streams, lakes, and rivers in boreal and subarctic
ecosystems in Norway, Sweden, and Finland between 1990 and 2013 are
surprisingly constant across climatic gradients and catchment sizes
(median, +1.4% year–1; interquartile range, +0.8–2.0%
year–1), implying that water bodies across the entire
landscape are browning. The largest trends (median, +1.7% year–1) were found in regions impacted by strong reductions
in sulfur deposition, while subarctic regions showed the least browning
(median, +0.8% year–1). In dry regions, precipitation
was a strong and positive driver of OC concentrations, declining in
strength moving toward high rainfall sites. We estimate that a 10%
increase in precipitation will increase mobilization of OC from soils
to freshwaters by at least 30%, demonstrating the importance of climate
wetting for the carbon cycle. We conclude that upon future increases
in precipitation, current browning trends will continue across the
entire aquatic continuum, requiring expensive adaptations in drinking
water plants, increasing land to sea export of carbon, and impacting
aquatic productivity and greenhouse gas emissions.