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The “Desert Maker”-
Climate Changesin
History
Phil Gersmehl
Michigan Geographic Alliance
To understand how climatecould play an important role in history,
students should be aware of three things:
1. the system that makes tropical deserts,
2. the system that causes natural climate cycles,
3. the difference between weather and climate.
1. What makes deserts?Typical textbook maps have a lot of info.
Fun to look at, almost impossible to remember.
Let’s remove the clutter and focus.“Describe the pattern of dry climates.”
The cause of tropical deserts israin near the equator.
Sun-heated air rises and makes rainover the equator . . .
AND
it has to come down
somewhere.
With the earth’s size, rotation speed, distance from sun, etc.“down” is about 25 degrees of latitude from where it went up.
And where the air comes down, it makes a desert.
With the earth’s size, rotation speed, distance from sun, etc.“down” is about 25 degrees of latitude from where it went up.
How would you describe the geographic pattern of deserts?How would you describe the geographic pattern of deserts?“Start 1000 miles west of each continent on the Tropic line.
Fan out to about 20 latitude degrees by the time you hit land.”
How does this relate to history?
This is a simple version of the temperature graph.
Agriculturestarts
Irrigationneeded
South Asiaflourishes
Anatoliais rainy Greece
is rainy
Rome expandsinto Europe
Barbariansinvade Rome, Han dynasty
collapses
Vikings, Mongols
Mingdynastycollapses
Hunters thrive
Uplandfarmersdo well
To help us remember these facts,let’s put them on a world map of deserts.
Remember, the great Tropical desertsform where air that rose up at the equatoris pushed back down toward the surface
This downward air movement(called the Subsidence)
will be farther from the equatorif warmer ground pushes air
more strongly upwardnear the Equator.
When the global average temperature is HIGH,the “desert-maker” shifts toward the poles.
Which places are likely to be hurt?
When the global average temperature is HIGH,the “desert-maker” shifts toward the poles.
Which places are likely to be helped?
? ?
When the global average temperature is HIGH,the “desert-maker” shifts toward the poles.
Which places are likely to be helped?
Plus some large cold areas that might get warm enough for farming.
When the global average temperature is LOW,the “desert-maker” shifts toward the equator.
Which places are likely to be helped?
When the global average temperature is LOW,the “desert-maker” shifts toward the equator.
Which places are likely to be hurt?
When the global average temperature is LOW,the “desert-maker” shifts toward the equator.
Which places are likely to be hurt?
Plus some large areas that still have plenty of rain but are colder.
Agriculturestarts
Irrigationneeded
South Asiaflourishes
Anatoliais rainy Greece
is rainy
Rome expandsinto Europe
Barbariansinvade Rome, Han dynasty
collapses
Vikings, Mongols
Mingdynastycollapses
Hunters thrive
Uplandfarmersdo well
You’ll hear this again:
Climate changes hada big impact on history.
Global warming is bad for some countries
and good for others.
6000 to 4500
3900 to 3200
2800 to 2300
Whatabout
today?
The name is covered, becausethis is about ideas, not personalities
S U N S H I N E
C A R B O N D I O X I D E
T E M P E R A T U R E
Let’s look atthree graphs
together,to see how
they compare.
One conclusionis beyond doubt:
temperatureshave been higher
in the pastthan they are
today.
Temperatures were much higherin the distant past
and a little higherquite recently.
PRESENTAVERAGE
BUT, there is a
big problemwith this graph.
the most recentmeasurements
do not fiton the scale.
When we adjustthe vertical scaleso the numbers
all can fit,
you can see thatcarbon dioxide
today isWAY above its
“normal” range.
In justa few hundred years,
humans have “pushed”the climate system
far out of linewith “normal” cycles.
In 2014, 397
In 2013.
the Pacific Ocean hadvery high temperatures
in Autumn. One result wasone of the strongest
hurricanes ever measured near the Philippines.
Weeks later,this hurricane carriedenormous amounts
of energy northward.
Excessive heatmoving toward the pole
can push cold airtoward the equator
Excessive heatmoving toward the pole
This map shows world temperature two months later.When you cite 2014 as an unusually cold winter in Michigan,
you must consider the average for the globe as a whole.
5 areas “much cooler than average”
0 areas “record coldest”
19 areas “much warmer than average”
9 areas “record warmest”
And if you look at the total for the entire year,the imbalance becomes even more obvious.
5 small areas “cooler than average”
0 areas “record coldest”
14 large areas “much warmer than average”
16 areas “record warmest”
As one Blogger asked:
Can global warmingbe real
if it is unusually cold in the United States?
As one Blogger asked:
Can global warmingbe real
if it is unusually cold in the United States?
Eh
Si
Ja
Ndiyo
Haa’n
Oui
Na’am
Hai
OoSim
Da
Evet
Tak
Aye
Ya
Ee
Han
AapHwa’a
‘Ae
To
Shi
AeChai
Tamo
Yin
A
EniE
IaDi
Si
Ngee Yao
Agriculturestarts
Irrigationneeded
South Asiaflourishes
Anatoliais rainy Greece
is rainy
Rome expandsinto Europe
Barbariansinvade Rome, Han dynasty
collapses
Vikings, Mongols
Mingdynastycollapses
Hunters thrive
Uplandfarmersdo well
You’ll hear this again:
Climate changes hada big impact on history.
Global warming is bad for some countries
and good for others.
Copyright 2015, Phil Gersmehl
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