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When climate change is not fully responsible for warming

It is a well known fact that over the past few years the Arctic Ocean has experienced a drastic melting of sea ice attributed mainly to climate change caused by anthropogenic activities. In the Arctic region, north eastern Canada and Greenland have experienced the greatest warming in the past 30 years. While it may seem that this is due to the warming caused by greenhouse gases in the atmosphere, a study published today (May 8) in Nature by Qinghua Ding, Research Scientist, Department of Atmospheric Sciences, University of Washington, and colleagues argues that roughly half the warming has been caused not by greenhouse gases but by natural variability of atmospheric temperatures.


Analysing atmospheric pressure and temperature data for the period 1979-2012, the study states that a Rossby train of waves originating in the central tropical Pacific Ocean caused by abnormally high sea surface temperatures may be the cause for part of the warming in Greenland and north eastern Canada. The atmospheric Rossby wave train is an atmospheric phenomenon, a system of meandering winds that travels thousands of kilometres and greatly influences climate and weather patterns in its path. In the case of warming of Greenland and north eastern Canada, the Rossby waves carry warm air from the tropics to those regions.


The higher temperatures in the upper troposphere of Greenland and north eastern Canada were spotted by satellites which measured the changes in geopotential height in those regions. When the air in the upper troposphere warms and rises it increases the height of the air column. As a result, a particular atmospheric pressure (say 200 hecta pascals) now occurs at greater height. This new height is sensed by satellite and indicates an increase in temperature associated with warming. Thus the geopotential height is the height in the upper troposphere that varies with temperature. Another factor for the anomalous warming is the North Atlantic Oscillation (NAO). NAO is the leading mode of the circulation variability in the North Atlantic region. In the last 30 years, the NAO has had a negative trend.


A positive trend is when there is a low pressure in the north (Greenland) and high pressure in midlatitude of the North Atlantic so that a large pressure gradient exists. On the other hand when both regions have low pressure and the gradient flattens out it is called a negative trend. There is no widely accepted view for the cause of its recent trend. Mr. Ding notes in an email: “We argued that its trend is caused by remote forcing from the tropical Pacific. The negative trend of the NAO will also warm Greenland through changing the circulation in the Arctic.”


Hindu, Delhi, 8th May 2014