Attributing global warming to extreme weather events
Ten years ago, from a scientific perspective on climate, trying to attribute to global warming the production of a particular Category 3 hurricane in the North Atlantic (such as Hurricane Sandy that hit New York City in 2012) would have been considered risky.
Today, it is well understood that for the last century Earth’s seas have been absorbing solar energy that has not returned to the atmosphere. As a result, the average temperature of the seas is increasing. Climate scientists are now investigating how global warming is affecting what would be considered extreme weather events in certain geographic areas. Swanson (2013) summarizes the idea of the relationship between the greenhouse effect, global warming and extreme weather events, noting that the probability of these extreme weather events is increasingly linked to man-made global warming: too much dioxide is emitted carbon in the environment. This is no longer an abstract idea. It is felt on all levels. [1]
Global warming and melting Arctic ice
As a result of global warming, temperatures in the Arctic Ocean have increased. This higher temperature is extending the melting of the Arctic ice in the summer well into the fall and winter. Arctic areas that were previously covered in reflective white snow have now been replaced by dark land and sea areas that do not reflect sunlight. In estimating the amount of glacier mass lost in Greenland, Jay Zwall of NASA’s Goddard Space Flight Center noted that each year “20 percent more” ice comes out of the snowfall. Biello (2006) [2] Since this sunlight is no longer reflected, this energy is now absorbed by the ocean, causing further warming of the sea and reinforcing the melting effect of the Arctic ice shelf.
This melting is causing significant changes in the jet stream (narrow, fast-moving westerly winds flowing west to east over northern parts of the US, Canada, Europe, and Asia), which is the gradient of pressure (difference) in the atmosphere. affecting the winter climate of the northern mid-latitudes.
Fundamental changes in the jet stream are caused by global warming
The jet stream can be seen as waves in ridges and valleys moving around the center of the Northern Hemisphere and gently sloping north and south. The temperature gradient (difference) between Arctic latitudes and North Atlantic latitude narrows in autumn, when the Arctic Ocean releases additional solar energy absorbed by the progressive melting of ice produced by global warming. So the difference in air pressure between the two pressure fields is also reduced, and the speed of the winds from west to east of Jet Stream is also reduced.
Two pressure fields are present in the northern hemisphere. The Arctic Oscillation, or AO, a field of positive or negative pressure that covers from latitude 70 ° north to the North Pole, and the North Atlantic Oscillation, or NAO, a field of positive or negative pressure that covers from parallel 70 ° north latitude to the subtropics. Now, since the NAO pressure field affected by global warming is more likely to be negative in fall and winter, the jet stream is more likely to be disturbed in winter.
Extremely hot US winters and cold European winters 2011-2012
As mentioned above, global warming has an effect on the extent of the Arctic thaw. The sea absorbs more energy from the sun in the summer, and then this heat is released by the Arctic Sea in the fall, reducing the pressure gradient (difference) between the pressure field of the Arctic Oscillation and the pressure field of the North Atlantic Oscillation, and the Jet Stream slows down. The pressure gradient between the AO and the NAO becomes brittle, facilitating a larger Jet Stream curve that extends further north or south.
Winter 2011-2012
During the winter 2011-2012, extremely warm temperatures developed in the northeastern United States. The jet stream leaned further north than usual over the northeastern central states of the US, allowing warm semi-tropical air to reach as far north as the US-Canada border, and remained there for a long time. In addition, La Niña (which means the pressure oscillation originating in the South Pacific) was also present. This weather phenomenon tends to deflect the jet stream north over the northeastern US.
Simultaneously, in Eastern Europe, the coldest winter in 25 years occurred in 2011-2012. The pressure gradient (difference) between NA and NAO was weak due to the additional heat released by the sea in autumn from the melting of the Arctic caused by global warming. The jet stream spread further south, causing Arctic air to reach locations in Eastern Europe and stay there longer than usual, producing a massive freeze. Fischetti (2012) summarizes all of this, stating that as more Arctic ice melts in summer, this will result in longer Jet Stream curves and longer stay in place, making winters warmer or colder than usual. habitual. [3]
There are no normal winters anymore
Scientists are investigating how the Arctic ice mass has been melting as a result of global warming. The question now is: Why is the Arctic ice melting faster than computer models can predict from global warming?
As global warming continues, the Arctic ice melting process and its effect on pressure fields in the Northern Hemisphere are likely to continue to push the Jet Stream pattern further north or south in large curves. Normal, regular winters will no longer be the norm.
By Alfonso de Garay
May 2014
References:
[1] Swanson, Jeanene. 2013. “Cloudy, Typhoon Possible,” Storm Warnings: Climate Change and Extreme Weather, Scientific American Publishers, 11/12/13 e-book
[2] Biello, David. 2006. “Greenland glaciers go, go …”
Scientific American online, October 19, 2006
[3] Fischetti, Mark. 2012. “The northern hemisphere could have extreme winters”,
Scientific American online, October 30, 2012
