Climate change attribution in the river runoff using model-based approach

The main goal of this study is to obtain new results of a physical assessment of the modern hydrological consequences of the natural and anthropogenic components of climate change based on the synthesis of detailed process-based models of river runoff formation using the ECOMAG software and an ensemble of the Earth system models (ESMs) within the large river basins of Eastern Siberia. This approach allows calculating the river flow using the ESMs-based data over the observation period under two scenarios: (1) taking into account the anthropogenic impact of increasing greenhouse gas emissions and (2) taking into account only internal fluctuations of the climate system and natural external forcing.

In the first stage, the long-term annual and seasonal runoff of the Lena and Selenga rivers over the period of instrumental observations was evaluated using the ESMs-based data as boundary conditions in spatially distributed hydrological models. The ensemble of 4 ESMs (GFDL-ESM2M, HadGEM2-ES, IPSL-CM5A-LR, MIROC5) satisfactorily reproduces annual and seasonal values of both air temperature and precipitation and river runoff calculated using hydrological models over the observation period. The PBIAS in the calculation of annual runoff was 4% and 5% for the Lena and Selenga rivers, respectively.

In the second stage, the attribution of natural and anthropogenic climate-driven runoff change over the historical period was evaluated by runoff formation models and ESMs, including before streamflow measurements on the Lena and Selenga rivers. According to the results of numerical experiments, it was found that the anthropogenic component of climate change in the dynamics of the Lena runoff is weak, i.e. during the observation period the Lena River flow statistically significantly increases, but it occurs mainly due to natural climate variability. The changes in the Selenga runoff are intensely influenced by the anthropogenic component of climate change. This has been particularly noted since the 1970s, when in the case of natural climatic conditions the flow would have increased, but taking into account anthropogenic greenhouse gas emissions, it has decreased especially since the mid-1980s due to a decrease in summer precipitation, which caused the last low-water period of 1996-2017 in the Selenga basin.