Article: Qinghua Ding et al.Tropical forcing of the recent rapid Arctic warming in northeastern Canada and Greenland. Nature. 509, 209-212 (2014). doi:10.1038/nature13260
Speaking of climate change, you can easily picture a scene in which big ice sheets are melting and poor polar bears standing on the last piece of ice finding nowhere to live. To some extent this is true, since impact from global warming can be amplified in Arctic region. As shown in Fig 1, the Arctic is the place where the largest atmospheric warming occurs and this is especially true in the winter. The phenomenon has several causes.
Imagining a warming Arctic in the summer, there will be vast areas with no ice. The ocean without ice cover absorbs more radiation and thus stores more heat than when it is covered with ice; therefore, the ocean with more open water stores more warmth during the summer. When winter comes, the heat from the warm water takes a longer time to cool meaning thinner ice will be formed. This in turn prevents thick ice cover formation for next summer and introduces a positive feedback that enhances the heating process in the Arctic. The process of ice reflecting radiation back is called the albedo feedback mechanism which is the main driven force of Arctic amplification. Other factors such as cloud cover and water vapor, black carbon aerosols, and ocean circulation also have impacts on Arctic atmosphere. Cloud cover in lower latitudes has a cooling effect whereas in the Arctic it is the opposite. Black carbon aerosols, mainly from emissions from Asia, absorb solar radiation so they have a warming effect. Changes in ocean currents influence the heat transported to the Arctic by water. Increased water flow from lower latitude brings heat to the Arctic.
Questions , Explorations and Conclusions
The retreat of sea ice and warming will continue in the Arctic accompanying future green house gases and aerosol emissions. Northeastern Canada and Greenland are the places where the most prominent annual mean surface and troposphere warming happens. Questions arise as to whether anthropogenic impact is solely the reason, or do natural influences contribute to this phenomenon at all?
Scientists used observational analyses and modeling to tell the relative contribution from anthropogenic forcing and natural variability. It is suggested that about half of the observed warming in northeastern Canada and Greenland is due to a uniform warming of the entire Arctic (directly attributable to anthropogenic forcing). The other half is surprisingly forced by trends in the tropical Pacific SST (Sea Surface Temperature). When the tropical Pacific warms up, it forms a Rossby wave train that travels towards the Arctic and then returns to the tropics. This wave train warms up northeastern Canada and Greenland as shown in Fig 2.
Why do we study the warming trend in the Arctic?
Conditions in the Arctic have great impacts on our everyday life. Still remember the unusually cold European winter in 2009? At the same time, Canada reported an abnormally warm winter. The probability of cold winters in Central Europe rises when the Arctic is covered by less sea ice in summer. This is because that when temperature rises it alternates the patterns of the atmosphere pressure. Cold air from the Arctic is forced into Europe in the winter and extreme cold weather appears. (See Fig 3.)
Therefore we want to know the causes of the warming trend in the Arctic. Is it caused by us? How much percent? What can we do to alleviate this warming up? According to Ding, we are responsible for half of the warming. But because it is a complex ocean-atmosphere interactive system, even the other half is driven by natural variability in the tropical Pacific we contributed to the warming up in the tropical Pacific and so we need to take responsibilities for this part as well. Furthermore as the Arctic is an area particularlt sensitive to small perturbations, the human impacts are amplified, in turn damaging our own habitats.
References and further readings:
 Mark C. Serreze, Roger G. Barry. Processes and impacts of Arctic ampliﬁcation: A research synthesis. Global and Planetary Change 77 (2011) 85–96.
 Wikipedia: North Atlantic Oscillation
 Katja Matthes. Atmospheric science: Solar cycle and climate predictions. Nature Geoscience 4, 735–736