Probable increase of DOC concentration and flux by 2100

At the starting point of near 2000 we have the multi-annual assessments of mean concentrations of DOC, POC and TOC and water, SPM and carbon fluxes of the six main arctic rivers into the Arctic Ocean (Table).

There are few data available on seasonal variations of DOC, POC and TOC concentrations and fluxes. Maltseva et al. (1987) indicated that 58-78% of TOC was discharged during freshet and that the portion of TOC transported to the Arctic seas during this period increased from west to east.

Seasonal variations in TOC concentration and fluxes in the lower reaches of the Lena River are given in Fig. 6a. Minimum TOC concentrations are observed in winter (3.1-4.8 mg C/l) while maximum concentrations occur during freshet in June-July (9.6-14.4 mg C/l).

This pattern is even more pronounced for the TOC fluxes to the Laptev Sea (freshet accounts for more than 50% of the annual TOC discharge).

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Nov-Apr May June July Aug Sep Oct

Nov-Apr May June July Aug Sep Oct

Nov-Apr May June July Aug Sep Oct

Fig. 6. Seasonal variations of TOC concentrations (Cauwet and Sidorov 1996) and TOC fluxes in the lower reach of the Lena River (a), and seasonal variations of DOC concentrations of and fluxes of the Ob and Yenisei Rivers (b, c) (Kohler et al. 2003).

A similar pattern has been observed for the DOC transport of the Ob and Yenisey rivers (Fig. 6b, c).

Let us estimate a probable increase of DOC river fluxes to the Arctic Ocean by 2100.

If to assume the correctness of the long-term trends in water discharge of the six largest Eurasian rivers that an increase will be at a mean annual rate of 2.0 + 0.7 km3/year up to 2100 and the conservativity of the present mean concentrations of DOC, than it is easy to calculate a probable increase of DOC flux by 2100.

2000: concentration of DOC = 8.3 mg/l, Q (water discharge) = 1,910 km3/year, DOC flux = 15.82 x 106 km3/year;

2100: concentration of DOC = 8.3 mg/l, Q = 2,110 km3/year, DOC flux = 17.5 x 106 km3/year, or 10.7% increase.

Wu et al. (2005) predict an increase of total river discharge into the Arctic Ocean by an annual rate of 8.73 km3/year (since the 1960th). In this case an increase of six rivers discharge by 2100 would be about 2,780 km3/year and DOC flux about 23.07 x 106 t/year, or 46% increase.

The IPCC projects a global surface air temperature increase of between 1.4 and 5.8°C by 2100. Peterson et al. (2002) consider that on this basis the discharge of the six Arctic rivers would increase by 18-70% or 315-1,260 km3/year by 2100.

2000: concentration of DOC = 8.3 mg/l, Q = 1,910 km3/year, DOC flux = 15.82 x 106 t/year.

2100: concentration of DOC = 8.3 mg/l, Q = 2,225-3,170 km3/year, DOC flux = (18.5-26.3) x 106 t/year, or 17-66% increase.

These estimates are in fact conservative.

Several works show that the release of DOC from peatlands and permafrost into rivers will occur under a warming climate in the Arctic (Freeman et al. 2001; Frey and Smith 2005; Guo et al. 2007).

Frey and Smith (2005) have studied 96 cold permafrost influenced (CPI) and warm permafrost-free (WPF) catchments throughout West Siberia in 1999-2001. Measured DOC concentrations in streams and rivers revealed a remarkable contrast between CPI and WPF watersheds (Fig. 5): (1) low DOC concentrations in CPI watersheds; (2) a positive correlation between DOC and peatland cover in WPF watersheds.

The authors find a sharp rise in DOC release to streams where mean annual air temperature (MAAT) exceeds -2°C. Climate model simulations, as we mentioned above, predict a doubling the West Siberian land surface with MAAT exceeding -2°C isoterm by 2100. Based on these empirical relationships and modelled land surface areas the averaged stream DOC concentration will rise from its current value of 29-46%. For CPI watersheds containing extensive peatland cover, the predicted DOC increase is expected to produce up to a 670% for watersheds with 100% peat cover.

The Nadym and Pur rivers with 50% covered by peatlands will show a ~ 400% increase in DOC concentration (from ~10 to ~40 mg/l) in the next century.

It is understandable that this DOC concentration will be rising with time.

If we assume on a base of these findings than the DOC concentration in the six Arctic rivers, which include the Ob and Yenisey, the rivers with the most extensive peatlands in the World, will increase up to 25 mg/l (this is a half of increase for the Pur and Taz, the peatland cover of which among the highest), than DOC flux will rise by 2100 due to this reason on (5.3-21.4) x 106 t/year, or 33-135%.

2000: DOC = 8.3 mg/l, Q = 1,910 km3/year, DOC flux = 15.8 x 106 t/year.

2100: DOC = 25 mg/l, Q = 2,225-3,170 km3/year, DOC flux = (55.6-79.2) x 106 t/ year.

Total DOC flux increase due to both of water discharge increase (Peterson et al. 2002) and DOC concentration increase as a result of DOC release from peatlands during climate warming would be estimated as 350-500% by 2100.

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