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As acid rain fades, climate steers water browning

DOC rose at 383 of 426 northern sites since 1990; early increases tracked falling sulfate, while later increases are nearly half climate-driven.

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How widespread was DOC increase in 1990–2016?

383 of 426 sites had positive slopes; 269 were significant.

What dominated early DOC change?

Falling sulfate (and some chloride): about 90% of explained variance.

What rose in importance later?

Climate trends, mainly precipitation, at 48% of explained variance.

How much did basin DOC export increase?

About 27%, or 2.3 Tg C per year, in under three decades.

Which continent contributed more extra DOC?

Europe (+35%, +1.7 Tg C/yr) versus North America (+16%, +0.7 Tg C/yr).

Did proportional %DOC rates keep rising?

Median absolute ΔDOC was higher later, but median %ΔDOC was not.

How were relative DOC trends computed?

Theil–Sen slopes ×100 divided by the site median concentration.

What chemistry besides DOC was tracked?

SO4, Cl, pH, nitrate, and divalent base cations (Ca+Mg).

How much variance did the best early-type model explain?

67% of ΔDOC variance (lowest AIC seven-variable model).

Why might drought complicate the carbon story?

Lower DOC transport in a warming world could pair with higher soil CO2 emissions.