Snow: a reliable indicator for global warming in the future?
The cryosphere consists of water in the solid form at the Earth's surface and includes, among others, snow, sea ice, glaciers and ice sheets. Since the 1990s the cryosphere and its components have often been considered as indicators of global warming because rising temperatures can enhance the melting of solid water (e.g. Barry et al 1993, Goodison and Walker 1993, Armstrong and Brun 2008). Changes in the cryosphere are often easier to recognize than a global temperature rise of a couple of degrees: many locals and tourists have hands-on experience in changes in the extent of glaciers or the duration of winter snow cover on the Eurasian and North American continents. On a more scientific basis, the last IPCC report left no doubt: the amount of snow and ice on Earth is decreasing (Lemke et al 2007). Available data showed clearly decreasing trends in the sea ice and frozen ground extent of the Northern Hemisphere (NH) and the global glacier mass balance. However, the trend in the snow cover extent (SCE) of the NH was much more ambiguous; a result that has since been confirmed by the online available up-to-date analysis of the SCE performed by the Rutgers University Global Snow Lab (climate.rutgers.edu/snowcover/). The behavior of snow is not the result of a simple cause-and-effect relationship between air temperature and snow. It is instead related to a rather complex interplay between external meteorological parameters and internal processes in the snowpack. While air temperature is of course a crucial parameter for snow and its melting, precipitation and radiation are also important. Further physical properties like snow grain size and the amount of absorbing impurities in the snow determine the fraction of absorbed radiation. While all these parameters affect the energy budget of the snowpack, each of these variables can dominate depending on the season or, more generally, on environmental conditions. As a result, the reduction in SCE in spring and summer in the NH was attributed to faster melting because of higher air temperatures, while the winter months (December to February) saw an increase in the SCE due to increased precipitation (Lemke et al >2007). Cohen et al (2012) confirmed these opposing effects in the SCE and showed that on the Eurasian continent the average SCE in October has increased by approximately 3 × 106 km2 in the last two decades; a growth of almost 40%, corresponding to roughly 1.5 times the area of Greenland. For the same period, Cohen et al (2012) found a negligible trend in the average temperatures above the continents of the NH for the winter months despite a significant increase in the annual mean temperature for the same regions. Cohen et al (2012) propose the following link between temperatures and snow: the reduced sea ice cover of the Arctic Ocean and the enhanced air temperatures in fall cause higher evaporation from the Arctic Ocean, leading to increased tropospheric moisture in the Arctic. More moisture results in more snowfall over the Eurasian continent, increasing the SCE. The increased snow cover strengthens the Siberian High, a strong anticyclonic system generally persistent between October and April. This system is strong enough to affect weather patterns in large parts of the NH, resulting in changes in the large-scale circulation of the NH (Panagiotopoulos et al 2005). As a result, outbreaks of cold Arctic air masses into the mid-latitudes are more frequent, leading to low temperatures over the eastern part of North America and Northern Eurasia. According to Cohen et al (2012), these are exactly the same regions that have experienced a cooling trend in the winter temperature over the past twenty years. While this chain of events is plausible (and some are confirmed by observations), existing climate models are not yet capable of reproducing these processes. On the contrary, Cohen et al (2012) showed that they predict a slightly decreasing SCE in October for Eurasia and an increase in winter temperatures over the continents in the NH. This is not surprising because the simulation of snow and its interactions with the atmosphere in global models is imperfect (Armstrong and Brun 2008). Most models have difficulty in simulating successfully the complex behavior of snow cover. A better representation of snow in the models is vital in order to understand the possible far-reaching consequences of changes in the SCE and its effects on the local climate and on large-scale circulations in the atmosphere to utilize snow as a reliable indicator for a changing climate. However, the SCE is only one of many possible snow parameters that can be used (Goodison and Walker 1993). Although omni-present in many regions and during many seasons, there is still much to be learned about snow and how it is linked to the global climate system. References Armstrong R L and Brun E 2008 Snow and Climate: Physical Processes, Surface Energy Exchange and Modeling (Cambridge: Cambridge University Press) Barry R G, Goodison B E and LeDrew E F (ed) 1993 Snow watch '92—detection strategies for snow and ice Glaciological Data Report GD-25 (Boulder, CO: World Data Center A: Glaciology (Snow and Ice)) p 273 Cohen J L, Furtado J C, Barlow M A, Alexeev V A and Cherry J E 2012 Arctic warming, increasing snow cover and widespread boreal winter cooling Environ. Res. Lett. 7 014007 Goodison B E and Walker A E 1993 Use of snow cover derived from satellite passive microwave data as indicator for climate change Ann. Glaciol. 17 137–42 Lemke P et al 2007 Observations: changes in snow, ice and frozen ground Climate Change 2007: The Physical Science Basis. Contribution of Working Group I to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change (Cambridge: Cambridge University Press) Panagiotopoulos F, Shahgedanova M, Hannachi A and Stephenson D B 2005 Observed trends and teleconnections of the Siberian high: a recently declining center of action J. Clim. 18 1411–22