Discussion

How to regard to these estimations of DOC, POC and TOC fluxes to the Arctic Ocean over next almost 100 year? It's clear that the most important forcing is an increase of global air temperature. "Warming of the climate system is in equivocal, as is now evident from observations of increases in global average air and ocean temperatures, widespread melting of snow and ice and rising global average sea level" (IPCC 2007, WGI 3.9). We feel an absence of any doubts in this position. However, there is an opposite view on the problem. Sorokhtin (2007)] and Sorokhtin et al. (2007) present the theory of the Earth's climate evolution based on universal chemical - physical laws of matter-energy transformation. They investigate the global forces of nature driving the Earth's climate.

The authors consider that the IPCC conclusions are erroneous. The main climate forcing, to their opinion, is the Sun activity. There is very good correlation between the temperature variations in the Northern Hemisphere and magnetic activity of the Sun (Fig. 7). The arising CO2 concentration in atmosphere is a result of climate warming but not vice versa. And at the highest CO2 concentration the climate will be cooler.

Fig. 7. Positive correlation between temperature variations in the Northern Hemisphere and magnetic activity of the Sun (the Wolf numbers). Left scale and the thick line - deviation of average surface temperature at the current 11-years smoothing, °C. Right scale and dotted line -averaged the sun-magnetic activity (Sorokhtin 2007).

Fig. 7. Positive correlation between temperature variations in the Northern Hemisphere and magnetic activity of the Sun (the Wolf numbers). Left scale and the thick line - deviation of average surface temperature at the current 11-years smoothing, °C. Right scale and dotted line -averaged the sun-magnetic activity (Sorokhtin 2007).

We are not able to prove the truth of one or another position. In this work we formally accept the IPCC position and assume the approach: If the global climate warming will be in reality, than the organic carbon fluxes to the Arctic Ocean will be increased with an acceptable level of probability.

Let us consider shortly the most weak points in our calculations.

The attempts to estimate the possible changes in the river DOC, POC and TOC fluxes to the Arctic Ocean face with a lot of influenced factors and we need to use the model simulations which it is not possible to verify at present time. Very important and poor investigated question is the feedback impact on temperature's increase the variations of atmospheric precipitations, river discharge and so on. Cox et al. (2000) write that the general circulation models have generally exclude the feedback between climate and the biosphere. Their fully coupled, three-dimensional carbon-climate model predicts that carbon-cycle feedbacks could significantly accelerate climate change in the 21st. By 2100, atmospheric CO2 concentrations on 250 ppm higher in their model than in uncoupled carbon models will result in a global-mean warming of 5.5 K, as compared to 4 K without the carbon-cycle feedback.

There are many uncertainties in the regularities of the delivery of organic carbon from the soils, especially such as the peatlands. The recent works demonstrate (Finlay et al. 2006; Frey et al. 2007a; McClelland et al. 2007; Holmes et al. 2008) that even the present-day assessments of the river DOC fluxes, which not always take into account the seasonal variations, appear to be not reliable. For example, Finley et al. (2006) show that the Kolyma River delivers DOC to the Arctic Ocean on 30% higher than it was accepted earlier.

The -2°C MAAT isotherm represents a critical temperature threshold, above which watersheds produce increasing DOC as a function of peatland abundance (Frey and Smith 2005). At present the MAAT's are ranging from -10°C to +2°C and permafrost is influencing ~55% of its area (northward of 60° N).

A warming Arctic climate may lead to increased release of currently sequestered peat carbon through permafrost degradation process. But this process in highly non-linear and we are not able to understand it in details. This makes our estimates to be quite rough.

Radiocarbon composition was measured for DOC, POC and soil organic carbon (SOC) from the Mackenzie, Yukon and Sagavanirktok River basins (Guo et al. 2007). The authors show that POC in these rivers is dominated by old SOC derived from permafrost thawing and riverbank erosion in contrast to DOC, which is more readily influenced by modern terrestrial biomass. So, melting will be manifest in the age and amount of POC in Arctic rivers. But we do not know if the POC content would be changed in this process or not.

Thawing will magnify the erosion process also. The importance of the different sources of terrigenous organic carbon in the Arctic Ocean was evaluated in (Grigoriev et al. 2004; Rachold et al. 2007). The evaluation shows that about 80% of the total terrigenous TOC flux (40.8 x106 t/year) is attributed to river discharge

(30 x 106 t/year). Coastal erosion is at present on the second place (6.7 x 106 t/year, or 15%). Sea ice input contributed about 2% only.

We didn't find any estimations of DOC, POC and TOC coastal erosion flux changes due to climate warming in literature. Their increase may be expected in 21st century.

So, we cannot even evaluate the reliability of our calculations of river organic carbon fluxes to the Arctic Ocean due to many factors of influence, which are not known enough at present.

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