The “Desert Maker” - Climate Changes in History Phil Gersmehl Michigan Geographic Alliance [email protected].

The “Desert Maker”-

Climate Changesin

History

Phil Gersmehl

Michigan Geographic Alliance

[email protected]

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?


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.






Plus some large areas that still have plenty of rain but are colder.

Agriculturestarts

Irrigationneeded



is rainy



collapses

Vikings, Mongols


Hunters thrive


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



is rainy



collapses

Vikings, Mongols


Hunters thrive


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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