NATURE CLIMATE CHANGE | VOL 3 | JANUARY 2013 | www.nature.com/natureclimatechange 17

news & views

In some ways, northern Eurasia is the climate’s canary in a coal mine: the land surface at high latitudes has experienced

some of the largest warming rates globally during the past century1, and streamflow into the Arctic Ocean has been increasing since 19362. Previous studies have analysed temperature and precipitation to explain these trends, but they did not identify the cause of changes in precipitation. In this issue, Xiangdong Zhang and his colleagues identify atmospheric moisture transport (AMT) as a driver of the streamflow increases, including the record-high discharge of 20073.

According to the mass balance principle, what comes out (streamflow) must first go in (precipitation). This is also true for the atmosphere, where the output is net precipitation and the input is net AMT — that is, AMT for a closed river basin. Increases in AMT can result in more precipitation, which may lead to increases in streamflow. In northern Eurasia, precipitation measurements are geographically sparse, making it difficult to accurately estimate changes in precipitation across the entire region4. Large errors in the measurement of snow add to the struggle to produce reliable estimates5. However, looking at AMT allows the problems of precipitation measurement to be overcome.

Zhang and colleagues looked at the Ob, Yenisei and Lena rivers — the three largest drainage basins in northern Eurasia. After correcting for errors due to temporal changes in the observational system, they use the wind and specific-humidity fields from the NCEP–NCAR reanalysis6 — a gridded data set representing the state of the Earth’s atmosphere, incorporating observations and model output dating back to 1948. They found that net AMT was roughly equivalent to climatological streamflow, with the annual net AMT having a positive trend large enough to supply the water needed for higher streamflow. Furthermore, the two variables show similar temporal variability with net AMT, leading discharge by roughly a

season, indicating that net AMT explains a proportion of discharge variability. Zhang and co-workers3 conclude that net AMT into the region explains the streamflow increases, rather than other mechanisms.

In high latitutdes, 2007 was an anomalous year, with record high streamflow and record low sea-ice extent. Zhang et al. find that net AMT peaked in 2006 and early 2007. They attribute it to the same events that led to low sea-ice extent — namely a shift in atmospheric circulation patterns — resulting in increased AMT over the North Atlantic into Eurasia. This provided the water required for high discharge into the Arctic Ocean.

Although previous work hypothesized that increased AMT into the region is

the driver of streamflow trends7, there was a lack of evidence to support such an explanation until this study. Moreover, these findings highlight the seasonality of the hydrologic cycle in northern Eurasia, as AMT occurs predominantly during the cold season. Increased AMT during the cold season is responsible for greater winter precipitation, that is then stored as snow, resulting in high streamflow from melt during the warm season. This is in agreement with previous studies, which focused on the land surface rather than the atmosphere8,9.

Northern Eurasia is one of the regions where global projections of increased winter precipitation with warming temperatures are consistent. The work by Zhang and colleagues3 demonstrates that this impact of climate change is already

ATMOSPHERIC SCIENCE

Wetting the ArcticStreamflow from northern Eurasia into the Arctic Ocean has been increasing since the 1930s. Research shows that increased poleward moisture transport is responsible for additional water in the region.

Tara J. Troy

Figure 1 | The Lena River, Russia, pictured from Lena Pillars in September 2006. The very high water level is a result of heavy rainfall in the Lena River basin during the previous month. Image courtesy of Yoshihiro Iijima, Japan Agency for Marine–Earth Science & Technology.

© 2013 Macmillan Publishers Limited. All rights reserved

18 NATURE CLIMATE CHANGE | VOL 3 | JANUARY 2013 | www.nature.com/natureclimatechange

news & views

occurring and identifies the physical mechanism behind it. As of now, it is an open question about whether the change is simply due to more moisture being transported into the region by similar atmospheric circulation patterns, or if there is a change in storm tracks and moisture sources.

Zhang et al. quantitatively demonstrate that the Eurasian streamflow is increasing due to AMT, rather than changes in precipitation recycling within the region. This means the moisture driving the streamflow trends is being imported into northern Eurasia from other areas, such that the changes

seen in northern Eurasia are related to changes elsewhere.

The relationship between discharge trends and net AMT emphasizes the need to understand how different regions are connected globally, and how changes may propagate between them. By understanding the chain of physical processes that ultimately led to increasing Eurasian streamflow, we will have a better understanding of how the system might respond to future climate change, and why. This gap is not unique to northern Eurasia: the connection between AMT and land-surface hydrology is poorly quantified in many regions. ❐

Tara J. Troy is at Columbia Water Center, The Earth Institute at Columbia University, 842 S.W. Mudd Mailcode: 4711, 500 West 120th Street, New York, NewYork 10027, USA. e-mail: tarajtroy@gmail.com

References1. Chapin, F. et al. Science 310, 657 (2005).2. Peterson, B. J. et al. Science 298, 2171–2173 (2002).3. Zhang, X. et al. Nature Clim. Change 3, 47–51 (2013).4. Serreze, M. C. et al. Geophys. Res. Lett 30, 1110 (2003).5. Adam, J. C. & Lettenmaier, D. P. J. Geophys. Res. 108, 4257 (2003).6. Kistler, R. et al. Bull. Am. Meteorol. Soc. 82, 247–268 (2001).7. McClelland, J. W., Holmes, R. M., Peterson, B. J. & Stieglitz, M.

J. Geophys. Res. 109, D10182 (2004).8. Rawlins, M. A., Ye, H., Yang, D., Shiklomanov, A. &

Mcdonald, K. C. J. Geophys. Res. 114, D18119 (2009).9. Troy, T. J., Sheffield, J. & Wood, E. F. J. Geophys. Res.

117, D05131 (2012).

© 2013 Macmillan Publishers Limited. All rights reserved