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SoGE Open Day 13 July 16
Deserts, dust and climate change: Why Physical Geography matters
David Thomas
Professor of Geography
University of Oxford
@kalaharidave
Deserts and drylands
What are the big dryland environmental issues?
• Urbanisation?
SW Kalahari
What are the big dryland environmental issues?
• Desertification?
What are the big dryland environmental issues?
• Climate change impacts?
What are the big dryland environmental issues?
• Urbanisation?
• Desertification?
• Climate change impacts?
Two‐way human‐environment interactions in a fragile environmental context. Need to know the science
to better understand the issues
• Silt‐sized sediment particles (3‐63 microns).
• Formed by rock weathering, glacial grinding, aeolian collisions, etc.
• Fine (low mass and density): once picked up, carried in suspension in water and in atmosphere. • Can be transported very long distances.
2006 Beijing dust storms
• 10 major dust storms by April 2006.
• 14‐18 April 2006: est. 300,000 tonnes of dust fell on Beijing.
• Caused health concerns for 2008 Olympics.
• ‘Most of dust comes from Inner Mongolia and places abroad’ (Head of China Met. Office)
Texas 1935
Melbourne, Australia, 1958
London, April 2015
Mali 1977
Melbourne again…1988
Dust hazards
1960s California
1930s South Dakota
Iraq 2008
1990s: West
Africa
8 reasons why desert dust is very important
and why we need to know more about it
A critical part of the linked land‐sea‐air global system

Reduces shortwave and absorbs long wave radiation: it potentially affects global warming

Interacts with cloud microphysics

Suppresses rainfall e.g. in Sahel

Modifies tropical storm and cyclone intensities

Transports nutrients very long distances‐ affects plant growth

Critical source of ocean nutrients: draws down CO2

Impacts on human health and land use

Linked to soil erosion and productivity
Imagery & specialist satellites have allowed patterns of particle movement in the atmosphere to be identified as never before.
Most major dust sources are natural (desert dry lakes and valleys)
Key source: Bodélé Depression, Chad
Annual mean TOMS Aerosol Index (AI)
The Aral Sea – the world’s greatest natural disaster?
1989
2008
From productive lake to major dust source…
36,000km2 of lake bed exposed, 43‐75 mill tonnes dust emitted annually.
25 years of TOMS data reveals major persistent dust sources
Location
Mean AI Value Bodélé Depression Sahara 3.0
West Sahara (Mali)
2.4
Arabia (Oman/Saudi border) 2.1
East Sahara (Libya)
1.5
SW Asia (Makran coast) 1.2
Taklamakan Basin (China) 1.1
Etosha Pan (Namibia)
1.1
Lake Eyre Basin (Australia) 1.1
Makgadikgadi (Botswana) 0.8
Salar de Uyuni (Bolivia) 0.7
Great Basin (USA)
0.5
But identifying precise sources, precise sinks, and the mix of conditions causing dust entrainment, are poorly understood.
NEED NEW DATA!!
Dust Observations for Models (DO4 Models) Project
4 years, 4 universities, UK funded.
To generate a new data set which characterises and quantifies surface erodibility and erosivity in dust source areas from remote sensing and fieldwork. Data to be used to improve dust component in climate models. Basically, to understand precisely what controls the generation of dust form the surface of the land and its movement into the atmosphere
Makgadikgadi Pans, Botswana
Etosha, Namibia
Namibian desert valleys
Grid Characterisation – Sua Pan, Makgadikgadi
• 2010 pilot survey in field and RS
• 12 km x 12 km grid with range of soil characteristics and erodibilities
Spot image 2010
A‐G clean well developed crust, stable
B‐C re‐worked, degraded, semi‐stable
D‐H‐E clean, new crust, emissive
F‐I shallow groundwater, stable (too wet)
MODIS 10 Year Surface Water
“The Grid”
Measurements:
24th July to 14th Oct 2011
25th July to 14th Oct 2012
1
MET sites
2
AWS sites
3
4
5
6
7
8
9
10
11
12
A B C D E F G H I J K L
Horizontal flux, c. 14 day sampling at 0.25 m, 0.47 m, 0.89 m, 1.68 m
Dust concentration, 2 min sampling at 3.2 m of PM1, PM2.5, Respirable, PM10, Total
AOT Cimel
Depositional flux, c. 1 month sampling at 1.68 m
Climate & erodibility measurements
‐ Net or inc solar radiation (link to changing roughness and albedo)
‐ Precipitation
‐ Soil moisture (2cm & 6cm)
‐ Saltation (Sensit‐ piezoelectric sensor)
Spatial variability in erodibility: surface characteristics
surface moisture (g/g, 2 cm depth)
sub‐surface moisture (g/g, 6 cm depth)
penetrometer strength (kg/m2)
shear vane strength (kg/m2) Cumulative daily TSP dust concentrations (mg/m3/day) DO4 Models – Field Campaign 2011
6. Resulting dust emissions (day 275)
DustTrak total PMTSP – PM2.5 (mg/m3/day)
Windspeed (m/s)
20
10
8
15
6
10
4
5
2
0
0
260
262
264
266
268
270
Julian Day (2011)
272
274
276
Dust Conc. (mg/m^3)
DustTrak total PMTSP (mg/m3/day)
Windspeed 0.25 m
Windspeed 6.00 m
Dust Conc. (2 min avg)
Namibian desert valleys
Sept 2013
Deserts, dust and climate change: Why Physical Geography matters
Drylands are extensive, variable and complex, supporting a growing % of world population.
Atmospheric dust comes from deserts‐
and affects dryland people and global environmental systems.
There are many gaps in dust science‐ and new data are needed from fieldwork and remote sensing.
Physical geography embraces and links the areas of science needed to answer the big dust questions.