The
previous post described how changes in the Arctic were causing changes in the
weather and climate. These changes
extended down in the mid-latitudes where the impacts included cold, snowy
winters. However, the impacts of a
warming earth do not end there. Research
shows that mankind is shifting the entire global atmospheric circulation.
Background
Meteorologists
have developed an idealized view of the global atmospheric circulation over the
past century. It has long been
recognized that there is too much heat at the equator and too little at the
poles. The atmosphere acts like “the
great equalizer “ by transporting heat from the tropics to the poles. If the earth were not rotating, this would be
a simple straight-forward transfer.
The
situation is complicated by the fact that the earth does rotate on its
axis. This has given rise to the
three-cell global circulation model. The
three cells from the tropics to the poles are named: Hadley, Ferrel, and Polar.
This model is pictured below with the
resulting wind patterns.
 |
| The three-cell general circulation model of the atmosphere. This is an idealized model based on observations. Image credit: NASA. |
The
Hadley cell (also known as the tropical cell) extends from the equator to 30
degrees latitude. Many of the world’s
major deserts lie at the poleward extent of the Hadley cell where the air
descends from upper levels. This
produces a persistent dry climate with little rainfall. The North American Desert, Sahara, Australian
Desert, and Kalahari Desert all are products of this circulation.
Observations
Recent
studies show that the Hadley cell has expanded poleward while the polar cell
has retreated toward the pole in both hemispheres. The subtropical jet stream generally marks
the boundary between the Hadley and Ferrel cells.
 |
| A cross-section of the troposphere from the Equator to the North Pole. The jet streams are at the intersection of the upper boundaries of the different circulation cells. Image Credit: NOAA. |
A study published just over a month ago
found that between 1979 and 2010 the subtropical jet stream moved northward 3.7
+/- 0.3 degrees latitude in the Northern Hemisphere and moved southward 6.5 +/-
0.2 degrees in the Southern Hemisphere.
The author sums up the importance this way:
“The
observed poleward movement in both hemispheres over the past thirty years
represents a significant change in the position of the sub-tropical jet
streams, which should lead to significant latitudinal shifts in the global weather
patterns and the hydrologic cycle.”
Research
The
study above looked at a number of potential causes of such a shift. A linear regression analysis was used to try
to identify the primary cause. It was
found to be the direct radiative forcing from greenhouse gases.
In
other words as greenhouse gases have increased over the past 30 years, the
earth has warmed. This warming of the
lowest layer of this atmosphere has led to an expansion of the Hadley cell
which in turn has led to a poleward shift in the storm track. Thus, global warming is leading to a shift in
weather and climate patterns.
Could
there be other influences that might impact this shift? The answer is yes.
Another study published in May
found that black carbon aerosols and tropospheric ozone played a dominant role
in the expansion of the Hadley cell in the Northern Hemisphere. Their study concluded that these were
noticeably better at driving the expansion than greenhouse gases.
The
authors used a climate model with detailed aerosol physics. The model still underestimated the expansion,
but the knowledge the true aerosol forcing is poorly understood. Dr. James Hansen of NASA has been pushing for
better understanding of the aerosol forcing for decades. This is one way to make the climate models
better.
Bottom-line
All
of the studies recently have shown a poleward expansion of the Hadley cell and
a shrinking polar cell. This has
resulted in a poleward shift of storm tracks.
Thus, the belts of deserts around 30 degrees latitude are moving
poleward and shifting climate zones.
In
addition, the rise of greenhouse gases and the presence of black carbon
aerosols and tropospheric ozone are products of burning fossil fuels and bio
fuels. This is leading to a warming of
the atmosphere which is in turn driving the expansion.
Thus,
climate change is happening now and will continue as long as the earth
continues to warm. Only a vast reduction
in the emission of greenhouse gases and tropospheric aerosols can halt the
ongoing change.
