A large-scale increase in the discharge of humic material in rivers, a
s well as a darkening of Swedish lakes during the last 15 years, are i
llustrated and discussed. Increased precipitation was responsible for
the greatest changes, but a marked part (10-40%) of the increase in ri
ver transport of coloured material was independent of discharge. Possi
ble explanations for this increase may be: increased ground water leve
l leading to increased runoff in superficial soil levels, a flow in di
rect contact with the organic soils; humus fractions in these layers m
ay also be rich in iron resulting in more coloured water; increased pr
imary production giving rise to increased amounts of litter and humic
material; reduced capacity of soils to absorb organic matter because o
f acidification; and effects of modern forest management. Models of gl
obal climate predict a doubling of atmospheric-CO2, an increase in mea
n annual temperature of up to 1.5 to 4.5-degrees-C, and in Fennoscandi
a possibly an increase in precipitation. The factors responsible for t
he observed large-scale change in humus flux may be strengthened by th
e increasing greenhouse impacts, and may result in more humic and colo
ured lakes in Fennoscandia during the next century.