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.
Source
Cleaner air reveals growing influence of climate on dissolved organic carbon trends in northern headwaters
Study at a glance
- Design
- Other — Theil–Sen DOC trends at headwater sites with attribution to sulfate vs climate
- N
- N=426 · 426 headwater monitoring sites (1990–2016)
- Population
- European and North American headwater catchments
- Outcome
- Dissolved organic carbon trends and drivers after air-quality improvement
Structured fields used in claim comparison tables when every cited study has a complete layer.
What they did
Authors computed Theil–Sen DOC trends for 1990–2016 at 426 headwater sites, split early vs late periods, and used AIC model attribution among ΔSO4, ΔCl, climate, and base cations, then scaled fluxes to European and North American basins.
What they found
383 sites had positive DOC slopes (269 significant). Early on, ~90% of explained ΔDOC variance was sulfate (and some chloride). Later, climate—mainly precipitation—accounted for 48%. Estimated DOC export rose 27% (2.3 Tg C/yr), more in Europe (+35%) than North America (+16%).
The limits
What it doesn't show
Site networks are biased toward well-monitored northern lakes/streams. Flux scaling uses median trends times runoff, not full hydrologic models. Drought could still cut DOC transport while raising soil CO2.
Key terms
- DOC / browning
- Dissolved organic carbon; rising DOC stains surface waters brown.
- Sulfate deposition
- Acid-rain SO4 that historically lowered DOC solubility; its decline lets soils release more organic carbon.
- Theil–Sen slope
- A robust nonparametric trend used for annual median DOC.
- lmg variance share
- A metric of how much each driver contributes to explained ΔDOC variance.
- DOC export
- Mass of dissolved organic carbon leaving a basin to downstream waters.
Flashcards
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Quiz yourself
DOC rose at how many of 426 sites?
Common questions
Has browning stopped now that air is cleaner?
No. Positive DOC slopes still dominate 2002–2016.
What replaced sulfate as a driver?
Climate, especially precipitation, rose to 48% of explained late-period variance.
Where did export rise most?
Europe (+1.7 Tg C/yr, +35%) versus North America (+0.7 Tg C/yr, +16%).
Is this a global ocean carbon budget?
No. It is northern headwaters in Europe and North America over <30 years.
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