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Global Warming & Droughts

April 2, 2014

By Paul Homewood

 

 

 

Probably the biggest scare story, that is regularly wheeled out to stoke up global warming alarmism, is drought. And particularly drought in Africa and Asia.

This is strange, because HH Lamb recognised back in the 1970’s that a cooling world, and in particular a cooling Arctic, was responsible for an expansion of drought zones. For instance, the Sahara desert pushed much further south, creating the catastrophic famines in the Sahel. Furthermore, this dry belt extended across to Central Asia, where monsoon rains regularly failed to reach Northern India.

Lamb, and many others, were clear on the reason – colder Arctic air was pushing the rain belts further and further south, with the result that there was a greater concentration of rain near the Equator.

(For more background on this, see here and here).

 

Which all rather got me thinking. It is known that between the end of the last Ice Age and about 5000 years ago:

1) Temperatures were much higher than now in the Arctic and large parts of the Northern Hemisphere. There is also evidence, albeit much sparser, that the same was true in the Southern Hemisphere, for instance here and here.

2) The Sahara was not the desert we know today, but was covered in vegetation and lakes. (Evidently, the same applied to desert regions in Central Asia).

 

But were the two things connected? The answer is yes.

A paper by deMenocal & Tierney in 2012, “Green Sahara: African Humid Periods Paced by Earth’s Orbital Changes”, explains very well how this connection worked.

 

……….we now know that these images document a dramatic climate change across North Africa from the hyperarid desert it is today to a nearly completely vegetated landscape dotted with large and small lakes during the early and middle Holocene epoch. This event is commonly called the "African Humid Period (AHP)". The AHP was a direct result of African monsoonal climate responses to periodic variations in the Earth’s orbit around the Sun that recur roughly every 20,000 years. Impressively, the AHP is just the most recent of hundreds of earlier humid events spanning as far back as the Miocene (9 million years ago) and likely much earlier.

The Earth’s axial rotation is perturbed by gravitational interactions with the moon and the more massive planets that together induce periodic changes in the Earth’s orbit, including a 100,000 year cycle in the shape of the orbit (eccentricity), a 41,000 year cycle in the tilt of the Earth’s axis (obliquity) and a 20,000-year cycle in the "wobble" — much like a top wobbles — of the Earth’s axis (precession). All three of orbital cycles — called Milankovitch cycles — impact African climate on long geologic timescales, but the cycle with the most influence on the rains in Africa is the "wobble" cycle, precession. The main climatic effect of precession is to shift the season when the Earth has its closest pass to the Sun (perihelion) — the so-called precession of the equinoxes. Today, perihelion occurs in northern hemisphere winter but at 10,000 years ago (half of a precession cycle) it occurred in northern hemisphere summer, and summer radiation over North Africa was about 7% higher than it is today.

Orbital precession greatly influences North African climate because it controls the strength and northward penetration of the monsoonal rains (Kutzbach, 1981; Liu et al., 2006). Strengthening summer-season solar radiation causes the North African landmass to heat up relative to the adjacent Atlantic Ocean due the lower thermal inertia of the land surface relative to the upper ocean mixed layer. The warmer land mass creates a broad low pressure zone, driving the inflow of moist air from the tropical Atlantic. The resulting summer monsoonal rains nourish the landscape. During winter, the land cools relative to the ocean and the winds reverse (one definition of a monsoon), returning dry conditions across North Africa. Since precession controls summer insolation, it effectively controls the amount and northward penetration of the monsoonal rains into North Africa. Simple atmosphere-only climate models have shown that a 7% increase in summer radiation, similar to what occurred during the AHP, results in at least a 17% increase in African monsoonal rainfall, and up to 50% if ocean feedbacks are included.

 

Distribution map of reconstructed lake levels across Africa, 9,000 years ago relative to today.

 http://www.nature.com/scitable/knowledge/library/green-sahara-african-humid-periods-paced-by-82884405

 

 

 

I must point out that there is nothing new in this analysis, which has been well known for sometime.

 

Regardless of the cause, a warmer landmass in North Africa is seen to  lead to greater rainfall, not less. Combined with the fertilisation effect of extra CO2 in the atmosphere, this will bring about a greening of Africa, and not the opposite.

But I doubt if you will hear any of this from Rowan Williams.

3 Comments
  1. mkelly permalink
    April 2, 2014 2:57 pm

    James Burke in the ’70’s PBS series put forth the idea that the Sahara came into existence as the glaciers melted taking the rains of a temperate zone northward. So the 10000 year ago precession fits.

    Also there was another science program that looked at the sediment in the sea floor off the west coast of Africa to show a 20000 year cycle of greening in the Sahara.

  2. Paul permalink
    April 3, 2014 7:49 am

    It is extraordinary the extent to which our climate fluctuates. Its clear though that with the gradual accumulation of scientific papers like this one we will soon have a comprehensive understanding of the forces responsible for our daily and annual weather.
    Maybe, one day, the Royal Society, the Met Office, even the BBC too will acknowledge our climate is not fixed and unchanging and that man is not responsible for the small variations one can experience within a single lifetime.

  3. April 4, 2014 2:35 am

    in the 70, Hubert Lamb was lying and scaring the people that: ”by year 2000 will be an ice age” otherwise, colder or warmer polar caps have nothing to do with the amount of rain or snow, BUT the warmth in the ocean is most important, to produce more, or less water vapor; which is the raw material for rain and snow.

    b] the more dry heat Sahara and Australian deserts are producing -> the more water vapor in the atmosphere is destroyed denying enough rain and snow for tera firma: and the Arctic ocean http://globalwarmingdenier.wordpress.com/midi-ice-age-can-be-avoided/

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