Prime
Meridian
(28) July 19, 2014
A newsletter following global environmental
issues alongside the cycle of the seasons in
Southern England
From desert to forest – the role of Sahara dust in fertilising the Amazon.
Above: Dust from the Sahara is carried across the Atlantic to South America on June 25, 2014. This is a
composite image from the Visible Infrared Imaging Radiometer Suite (VIRS) on the Suomi NPP satellite.
According to NASA's Earth Observatory website: “The dust flowed roughly parallel to a line of clouds in the
intertropical convergence zone, an area near the equator where the trade winds come together and rain and
clouds are common. Imagery captured by the Moderate Resolution Imaging Spectroradiometer (MODIS), the
dust appeared to be streaming from Mauritania, Senegal, and Western Sahara, though some of it may have
originated in countries farther to the east.”
At the close of March 2014, dust from the W. Sahara arrived
in Britain, adding to man-made air pollution and speckling
cars. Paul Hutcheon, Deputy Chief Forecaster at the UK's Met
Office stated: “We usually see this happen several times a
year, particularly in summer when you get these big dust
storms in the western Sahara and the southerly winds to
bring that dust here.” However, airborne Sahara dust is not
simply an occasional nuisance for car owners; it provides
essential nutrients to distant ecosystems. Left: A plume of
dust over 1,000 km long, blowing across Algeria on April 8,
2014. Credit: Jeff Schmaltz MODIS land Rapid Response Team,
NASA GSFC. Below: After rain fall, cars were speckled with
Sahara dust in the streets of South London. April 5, 2014.
May 2014 was the world's hottest May on record – see page 6
“A remarkable arrangement in nature”.
The invasion of Britain by dust from the Sahara
highlights the intriguing history of what is today
the world's largest desert.
In 2006, Ilan Koren of the Weizmann Institute,
Israel and co-workers published a paper in which
they concluded that: “About 40 million tons of
dust are transported annually from the Sahara to
the Amazon basin. Saharan dust has been proposed
to be the main mineral source that fertilizes the
Amazon basin, generating a dependence of the
health and productivity of the rain forest on dust
supply from the Sahara. Here we show that about
half of the annual dust supply to the Amazon basin
is emitted from a single source: the Bodélé
depression located northeast of Lake Chad,
approximately 0.5% of the size of the Amazon or
0.2% of the Sahara. Placed in a narrow path
between two mountain chains that direct and
accelerate the surface winds over the depression,
the Bodélé emits dust on 40% of the winter days,
averaging more than 0.7 million tons of dust per
day.” As they called it: “a remarkable arrangement
in nature”.
Right: A gap between the Tibesti Mountains and
Ennedi
Mountains serves to concentrate the
seasonal Harmattan wind onto the bed of a vast
dried-up lake, and the white colour of the dust
plumes is caused by the silica remnants “fustules”
of algae known as diatoms. The images are from
NASA and the lower shows a plume on Nov. 18,
2004, taken with the Moderate Resolution Imaging
Spectroradiometer aboard NASA's Aqua satellite.
The death of the Green Sahara.
The Bodélé Depression, today the dustiest place on Earth,
and Lake Chad are both part of a larger basin, which once
contained a major body of water of maybe 350,000 km 2 the Earth's most extensive lake. This has been dubbed
“Mega Lake Chad.” Today the area is arid, but back then
it supported savanna and woodland. Cave drawings,
discovered by the German explorer Heinrich Barth (18211865) in the middle of the 19 th Century, surprisingly
included representations of hippos, elephants and
giraffes, for whom the present climate is hostile. Left:
engraving of a sleeping antelope at Tin Taghirt, Tassili
N'Ajjer, Algeria (Linus Wolf. CC BY-SA 3.0).
It as accepted widely that the onset of wetter periods in the Sahara is essentially controlled by cyclic
changes in the eccentricity of the Earth's orbit, our planet's axial tilt and the direction in which its axis is
pointing in the sky. At those times when maximum solar energy is received in the N. Hemisphere, the
monsoon strengthens and extends northward. Several “mega-lakes,” the largest being Mega Lake Chad,
then fill. During the maximum of the last ice age, the Sahara was even more extensive and arid than
today, but early in our present interglacial, the Holocene (taken to have begun 11.8 thousand years ago),
it enjoyed the benefit of an African Humid Period, which peaked between 9,000 and 6,000 years ago. A
useful summary has been published by deMenocal & Tierney, J. E. (2012).
Left top: The Amazon rainforest near Manuas (Neil
Palmer/CIAT CC BY-SA 2.0). Left centre: Lake
Chad in 1968 from Apollo 7. Left lower: The
shrinking of Lake Chad (NASA).
According to P. G. C. Amaral of Aix-Marseille
Université et al., the pollen-containing
sequence from Lake Chad, the African Humid
Period did not end abruptly and vegetation
changed gradually between 6,700 and 5,000
years ago, but with century-scale climate
fluctuations superimposed on the general
trend. The geological record indicates that
many such humid periods have occurred over
millions of years. J. C. Larrasoaña of the
University of Southampton and colleagues
have traced the alternation between dry and
humid episodes over the last 3 million years
in dust deposits in the E. Mediterranean.
During humid spells the amount of mineral
nutrients reaching the Amazon must be
significantly reduced and the implications for
the rainforest require further research.
The Mega Lake Chad basin has no connection
with the sea and today's Lake Chad
(occupying areas of Chad, Cameroon, Nigeria
and Niger, is its modest remnant. It has
always been vulnerable to fluctuations in
climate and the last time that the water
level was high was in the period 1600 to 1700
(2010 presentation by Ibrahim Baba Goni of
Maiduguri University, Nigeria et al.). It stood
near its present level in the mid 19 th C and
around 1900. However, during the last half
century, human contributions to the
vanishing waters have included: “the
damming of rivers discharging to the Lake,
population growth and its attendant
increase in water demands and agricultural
practices that demand huge volumes of
fresh water. The consequences of the
present shrinkage are the increased
competition over natural resources and its
associated
conflict
amongst
farmers,
graziers and fishermen.” O. A. Ojoina of the
Kogi State University, Nigeria described how
“The reckless uses of waters for irrigation
and construction of dams are destroying the
environment”. At the same time, “over 20
million people who reside within the basin
and depend solely on the renewable natural
resources of the basin for agriculture,
fisheries, recreation and other socioeconomic activities.”
In 1963, the area of the lake was around
25,000 km2, shrinking to 2,000 km 2 in the
1990s. It re-expanded to 3,000 km2 in 2004,
but had fallen to 1,350 km2 by 2011.
References: Amaral, P. G. C. et al. (2013). Clim. Past 9: 223–241. deMenocal, P. B. & Tierney, J. E. (2012). Green Sahara: African Humid Periods
Paced by Earth's Orbital Changes. Nature Education Knowledge 3(10): 12. Goni, I. B. et al. (2010). Historical changes in climate and Lake Chad surface
area in the Chad basin. ICID+18, 08/2010. Koren, I. et al. (2006). Environ. Res. Lett. 1 (October–December 2006) 014005 doi:10.1088/17489326/1/1/014005. Larrasoaña, J. C. et al. (2003). Climate Dynamics 21: 689–698. Ojoina, O. A. (2011). Journal of Environmental Sciences and
Resource Management 3: 85-93.
Seasons in South East England
May, 2014
Above: Rain storms deluge areas of London on May 12, 2014.
Looking roughly north from Sydenham Hill, South London.
A wet, but warm May, with average sunshine.
The Met Office reported that for the UK as a whole, the
mean temperature was 11.2oC. This was 0.9oC higher than
the average for the interval 1981-2010. The south enjoyed
near average sunshine totals, but the UK as a whole saw just
82% average sunshine. Their monthly report noted that:
“This was largely due to there being many mild nights,
whereas daytime maximum temperatures were only a little
above average. Rainfall was above average for many, with
totals well above average in eastern counties of England”
May began with scattered showers (although with episodes
of bright weather in the south). On May 2, London saw light
showers in the afternoon. This day also saw the UK's, lowest
temperature of -0.5oC at Cromdale (Moray). The weather
became more unsettled with showers after May 5. On May 9,
E areas saw scattered showers in the afternoon.
Thunderstorms broke out in Norfolk on May 9 and May 10.
Left: Looking slightly west of north over Dulwich and
Sydenham Hill Wood, S. London on, May 3, 2014.
Britain
from the NOAA-18 satellite on May 3, 2014 at 15:59 GMT.
Courtesy Geoff Hamilton.
Below: May 3. Around West Kingsdown, Kent, deciduous
woods and hedgerows were well into the greening process.
Above: Common hawthorn (Crataegus
monogyna) flowering along woodland
margins near West Kingsdown. May 3.
A low pressure area arrived from May
10 to 11, with a gust on May 10 of
around 108 km per hour at the Needles
Old Battery (Isle of Wight). The east
saw showers on May 11 and May 12 and
13 saw thunderstorms across areas of
central and eastern England.
Thanks to a high pressure area, it was
generally warm and dry from May 14 to
May 19. Heathrow (Greater London)
saw
the
UK's
maximum
May
temperature of 26.3oC. Showers spread
from the SW, however and May 20 was
cooler and saw not only showers but
also thunderstorms, which heralded the
onset of more unsettled weather. On
May 24, Liscombe in Somerset (SW
England) saw 32 mm of rain. East Anglia
and other eastern counties saw
particularly wet conditions from May 26
to 27. On May 27, an easterly wind
brought rain to eastern and southern
areas, with areas in Suffolk and Norfolk
receiving 30 mm of rain. The final two
days of May enjoyed sunnier conditions.
Left upper: May 12. Showers of rain
falling on London. The views are
looking roughly northwest (upper) and
to the north (lower) across the woods
at Dulwich, South London. Left lower:
May 17. Sunset fell over St. Peter's and
St. Paul's Church, Ash, Kent and an
adjacent hedgerow.
SE and central S England, mean max.
temp.: 16.8oC (0.1oC); mean min.
temp.: 8.1oC (0.9oC). Hours of sunshine:
205.2 (102%). Rain: 72.9 mm (133%).
Anomalies re. 1981-2010 norm in
brackets.
Source: online Met Office data.
Above: Roses bloom in a hedgerow, whilst in the background the inky
blue clouds of a rain storm. Ruxley Manor, Kent. May 24.
The world has seen the hottest May in a
record extending back to 1880.
Following an April which tied with that of 2010 as the hottest on
record, May 2014 actually broke the record. According to the
regular monthly summary from the National Oceanic and
Atmospheric Administration (USA), planet Earth's global average
temperature for May 2014, taking together land and ocean
surfaces, was 0.74 ± 0.07oC above the 20th Century average of
14.8oC.
For land surfaces across the world, the temperature was 1.13 ±
0.13oC warmer than the average, the 4th warmest May on record
(with 2012 as the warmest). For the oceans, taken as a whole, the
temperature was 0.59 ± 0.04oC above the average, which made
this the warmest May on record. For the Northern Hemisphere, the
combined result for land and ocean was 0.79 ± 0.08 oC, the 3rd
warmest with 2012 as the hottest on record. Land in the Northern
Hemisphere was overall 1.10 ± 0.14 oC above the norm (6th
warmest, the warmest was May 2012), however, the ocean, 0.60 ±
0.04oC above the average was the hottest seen in the record.
Left: Poppy (Papaver rhoeas) provides a splash of red in a field near Ash, Kent
(May, 17). Yellow flag (Iris pseudacorus) flowers in wetland area of Belair Park,
South London (May 21). Elder flowers (Sambucus nigra) from a hedgerow near
West Kingsdown (May 31). Below. Overcast on May 20 from the margins of a field
near Ash and on May 26, rain drenched a country lane winding between woodland
and a field at Hartley Bottom, Kent.
Above: Ox-eye daises (Leucanthemum vulgare) around the margin of a field near
West Kingsdown, Kent (May 20, 2014). Left: Footpath down to Hartley Bottom,
Kent crosses a field of wheat (May 20). Below: Cutting fodder in a field near
West Kingsdown (May 31).
In the Southern Hemisphere, the combined land and ocean
temperature was + 0.68 ± 0.06, which made it the joint hottest
May, together with 1998. The ocean, at 0.58 ± 0.04 oC above the
average, was the 2nd warmest on record, behind 1998. Southern
land masses notched up their warmest recorded May at 1.21 ±
0.23oC above the mean.
The report noted that May 2014; “also ties with June 1998,
October 2003, and July 2009 as the highest departure from
average for any month on record.”
Source: NOAA National Climatic Data Center, State of the Climate: Global
Analysis for May, 2014. Published online. Data provisional.
At the time of publication, the Global Analysis for June, 2014 was
not available. It will be interesting to see whether we are at the
start of a trend for record-breaking months.
Prime Meridian is published as part the outreach programme of
the Ecospheres Project – Earth Campaign.
[email protected]
http://www.ecospheresproject.wordpress.com/
Images in Prime Meridian are from M.J. Heath (unless otherwise
specified). © M. J. Heath, 2014. Editorial assistance, Penelope Stanford.
This issue may be copied and distributed freely.