Introduction

The problem of the global climate warming excites the apprehensions of mankind in view of the probable significant consequences for the ecosystems, especially in high latitudes. The expected warming may affect a whole spectrum of natural and human systems in the Arctic region (Serreze et al. 2000; McGuire et al. 2006 and other).

A comprehensive study of this problem is carrying out by the Intergovernmental Panel on Climate Change from the beginning of 90th. The IPCC Fourth Assessment Report was published recently (IPCC 2007). The Report includes the analysis of recent climatic variability based on the data of observations, paleoclimatic reconstructions and the results of modeling of present and future changes of climate.

Significant increase of average global air temperature was recorded for the last century (Fig. 1a). The 100-year linear trend (1906-2005) of 0.74°C is larger than the corresponding trend of 0.6°C (1901-2000). Eleven of the last 12 years (19952006) rank among the 12 warmest years in the instrumental record of global surface temperature (since 1850).

Fig. 1. Observed changes in (a) global average surface temperature; (b) global average sea level from tide gauge (white) and satellite (black) data; and (c) Northern Hemisphere snow cover for March-April (relative to corresponding averages for the period 1961-1990). Smoothed curves -decadal averaged values, circles - yearly values, shaded areas - uncertainty intervals (IPCC 2007).

Fig. 1. Observed changes in (a) global average surface temperature; (b) global average sea level from tide gauge (white) and satellite (black) data; and (c) Northern Hemisphere snow cover for March-April (relative to corresponding averages for the period 1961-1990). Smoothed curves -decadal averaged values, circles - yearly values, shaded areas - uncertainty intervals (IPCC 2007).

Satellite data show that annual average Arctic sea ice extent has shrunk by 2.7% per decade since 1978, with larger decreases in summer of 7.4% per decade. Very important information concerns the frozen ground in the Northern Hemisphere. The maximum area extent of seasonally frozen ground has decreased by about 7% since 1900, with decreases in spring up to 15%. Active layer thickness over permafrost in the Russian Arctic has increased from 1956 to 1990 by about 20 cm (31 stations of observation) while seasonal freeze depth has decreased by about 34 cm (211 stations of observation) (Frauenfeld et al. 2004) (Fig. 2).

The atmospheric concentrations of the most important anthropogenic greenhouse gases (GHGs) CO2 and CH4 in 2005 exceed by far the natural range over the last 650,000 years. The global atmospheric concentration of CO2 increased up to 379 ppm in 2005 (a pre-industrial level - 280 ppm), CH4 up to 1,732 ppb (from 715 ppb) and N2O up to 319 ppb (from 270 ppb).

Fig. 2. Variations in the thickness of the active layer over permafrost (a) and maximum soil freeze depth in non-permafrost area (b) in Russia from 1956 to 1990. The shaded area presents the 5-95% confidence interval from the mean for each year, and the dashed line is the zero reference (Frauenfeld et al. 2004).

Fig. 2. Variations in the thickness of the active layer over permafrost (a) and maximum soil freeze depth in non-permafrost area (b) in Russia from 1956 to 1990. The shaded area presents the 5-95% confidence interval from the mean for each year, and the dashed line is the zero reference (Frauenfeld et al. 2004).

All these findings look like the very conclusive proofs of the going climate warming. The multi-model global averages of surface air temperatures were evaluated for the different scenarios of anthropogenic impacts to the end of 21st century (relative to 1980-1999) (Fig. 3). Expected warming will be in a range between 1.8°C and 4.6°C. Decreasing of CO2 assimilation by the ocean and the land at climate warming may lead to additional increase of temperature on 1°C.

However, not all published data support the given point of view. Polaykov et al. (2002) consider that Arctic variability is dominated by multi-decadal fluctuations. The authors show that over 125-year record the periods were identified when arctic surface air temperature (SAT) trends were smaller or of opposite sign than Northern-Hemispheric trends. Their main conclusion is: "The data do not support the hypothesized polar amplification of global warming". At the same time, evidences of increasing runoff in the Arctic have been reported recently (Shiklomanov et al. 2000; Semiletov et al. 2000; Peterson et al. 2002). The analysis of the Roskomgidromet data shows (Peterson et al. 2002) that aggregate annual discharge of the six largest Eurasian rivers (Northern Dvina, Pechora, Ob, Yenisei, Lena and Kolyma) over the period of observations from 1936 to 1999 has increased at a mean annual rate of 2.0 ± 0.7 km3/year, so that mean annual discharge is now 128 km3/year greater than in the 1930s. This amounts to an increase of about 7%. The authors (Peterson et al. 2002) consider that the main mechanism is most likely due to increased precipitation as forecast by global climate models.

Fig. 3. Increase of global air surface temperature in 21st century. Solid are multi-model global averages of surface warming (relative to 1980-1999) for different scenarios shown as continuation of the 20th century simulations (IPCC 2007).

Durgerov and Carter (2004) confirm that an increase in freshwater inflow to the Arctic Ocean is evident. But they conclude that the arctic mountain and subpolar glaciers are the main source of increased freshwater inflow to the Arctic Ocean over the 1961-1998 and the glacier input will continue to rise.

Wu et al. (2005) report that their climate model predicts an increase of total river discharge into the Arctic Ocean by an annual rate of 8.73 km3 since the 1960s. Similar results were obtained by Manabe et al. (2004). Their coupled ocean-atmosphere-land model predicts an increase of the Ob and Mackenzie water discharge on 20% to 2050 relative to pre-industrial period level and more than 40% in a few next centuries.

There are the predictions of an increase of delivery into the Kara Sea and the Arctic Ocean of inorganic dissolved salts up to 60% when all permafrost will be degradated (Frey et al. 2007a), and also increase of organic nutrients (on 30-50% of DON, TDN and TDP) by 2100 to the Kara Sea (McClelland et al. 2006; Frey et al., 2007b) and to global ocean (Beusen et al. 2005; Harrison et al. 2005).

A critical issue is the carbon cycling and storage in the Arctic region. Organic carbon in high-latitude soils and peatlands account for up to 50% of global soil carbon (Dixon et al. 1994). Climate warming could mobilize a substantial fraction of this carbon to rivers and streams creating a positive feedback on global warming (Freeman et al. 2004).

A fate of organic carbon in the Arctic rivers and ocean was investigated in several works (Benner et al. 2004; Frey and Smith 2005; Cooper et al. 2005; Neff et al. 2006; Raymond et al. 2007). Frey and Smith (2005) show on a base of measurements of stream and river DOC concentration from 96 watersheds distributed throughout West Siberia that cold, permafrost influenced watersheds release little DOC to streams, regardless of the extend of peat lands cover, and much higher concentrations in warm, permafrost-free watersheds, rising sharply as a function of peat land cover (Fig. 4). The author's climate model predicts a northward advance of the -2°C mean annual air temperature isotherm by 2100 nearly doubling the land surface with air temperature exceeding this threshold. This will lead to up to about 700% increases in stream DOC concentrations and 29-46% increases in DOC flux to the Arctic Ocean.

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Fig. 4. Dependence of DOC concentration on the present peatland cover (P%) with the sampled watershed. Concentrations in cold permafrost influenced (CPI) watersheds are uniformly low, with a mean value of 10.29 mg^l-1 and no statistically significant correlation. Concentrations in warm permafrost-free (WPF) watersheds rise significantly with P% (Frey and Smith 2005).

These predictions were obtained at assuming that no changes in either river water discharge, or in-channel processes. They are in fact conservative. But, as we have seen above, Siberian precipitation and river discharge continue to increase (Frey and Smith 2003; Peterson et al. 2002), so even larger increases are likely. And again, there are the works the conclusions of which are contradicted to these results. Striegel et al. (2005) indicate that in the Yukon River basin water discharge corrected DOC export significantly decreased during the growing season from 1978-1980 to 2001-2003. Counter to current predictions, the authors argue that continued warming could result in decreased DOC export to the Bering Sea and the Arctic Ocean by the rivers, due to increased respiration of organic carbon on land.

The Arctic is a particularly sensitive area in relation to river sediment discharge (Syvitski 2003). An analysis of the sediment loads of 145 rivers in the world with records of more than 25 years including the Siberian rivers with records of up to 62 years (Bobrovitskaya et al. 2003) indicates that 70 rivers show a decrease, due to mainly to dams, and only 7 rivers show evidence of an increase in sediment load (Walling and Fang 2003).

A new stochastic sediment transport model by Morehead et al. (2003) was applied to the Arctic rivers to estimate the sediment load increase as a result of climate warming. In the paper (Gordeev 2006) the data by Peterson et al. (2002)

0 20 40 60 80 100 Percent peatland cover

Fig. 4. Dependence of DOC concentration on the present peatland cover (P%) with the sampled watershed. Concentrations in cold permafrost influenced (CPI) watersheds are uniformly low, with a mean value of 10.29 mg^l-1 and no statistically significant correlation. Concentrations in warm permafrost-free (WPF) watersheds rise significantly with P% (Frey and Smith 2005).

on six Siberian rivers water discharge and this model were used to estimate the sediment load increase of the same Arctic rivers by 2100. The assessments show that probable increase of the sediment flux of six rivers will be in a range from 30% to 122%.

The aim of this work is an attempt to predict an increase of river DOC and POC input to the Arctic Ocean due to global climate change to the end of this century. These predictions are based on our data on the current Arctic river sediment discharge and DOC, POC and TOC concentrations and fluxes in the Siberian rivers and also on the IPCC predictions of global air temperature increase in 21st century, measured increase of water discharge in the Siberian rivers (Peterson et al. 2002) and model simulation of sediment load due to warming (Morehead et al. 2003).

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