Horn of Africa faces more extreme future - a tale from the lake

By Martin Leggett - 04 Aug 2011 18:0:0 GMT
Horn of Africa faces more extreme future - a tale from the lake

The woefully familiar pictures coming from the Horn of Africa over the last few weeks, with famine once again stalking the lives of people there, bring with them the common refrain of our times; is climate change to blame?

That's a question that may have no simple answer; cruel droughts, and times of sufficiency, have beaten out their alternating rhythms in eastern Africa for millennium. But could global warming be changing the pattern of that beat?

A timely paper, published in Science today, suggests a tentative 'yes' to that question. Its authors have been looking to muddy lake sediments in Kenya, for climate clues from the past. But perversely, the preliminary results paint a future picture with both more rain, and more drought, for a part of the world that is living on the edge.

Rains at whim of Pacific

The countries of Ethiopia, Somalia and Kenya lie on a vulnerable climate fault-line, depending for their rains - and so their harvests - on the wanderings of a band of storms, known as the Inter-tropical Convergence Zone (ITCZ). This meandering front wraps itself around the planet near the equator, and marks the boundary between winds coming from the northern and southern halves of the globe.

At the start of each year, the rain-laden storms of the ITCZ lie to the south of the African Horn countries. As the year progresses, they sweep north, providing much-needed rain during March and April. For the next few months, the ITCZ lies out-of-range again, now up to the north, pulling behind it strong dry winds from the south-east - usually until November. Then the winds drop and the ITCZ wanders back south, to douse the whole area with a second helping of crop-nourishing rain.

But this cycle is far from secure. It is affected by events on the other side of the planet, in the vast expanse of the Pacific Ocean. There, one of the main drivers of global weather patterns -– the El Nino/ La Nina cycle - shows itself in pulses of warmer (El Nino) and cooler (La Nina) sea-waters. And when La Nina is in charge, the east of Africa suffers, with the late season rains often failing.

'Potentially severe drought' warned of

Last year was a particularly strong La Nina year - something which flagged a warning signal from climate scientists. Co-author of the Science paper, Axel Timmermann, from the University of Hawaii, said ''the drought was actually predicted already last year. With the emerging consensus that a major La Nina event would develop, climate modeling centers had already warned in June 2010 of potentially severe drought conditions at the Horn of Africa.''

And the signal of that La Nina event were also being lain in the Lake Challa muds that the team - which included Christian Wolff at the University of Potsdam in Germany - have analyzed for their study. He explained ''During La Nina, rainfall is sparse and the winds over Lake Challa are strong. The winds enhance upwelling of nutrients, intensifying the seasonal blooms of algae. After dying and sinking, they form thick layers of light-colored sediments. During El Nino events, on the other hand, rainfall is frequent and the winds are weak, resulting in thinner white layers in the sediment.''

Delving into the ice age

What is useful about these sediments is their depth. They lie at least 60 feet (20m) thick under the lake, and cover a slice of time running well into the last ice age, around 25,000 years ago. Because the muds, called varves, seem to have a strong signal for the pulse of La Nina events, the team realized that cores taken from them could offer insights into the climate's past - and maybe its future - for this part of the world.

In order to test this, two sections of core were taken; one down to 10 feet (3m), covering the last three thousand years; and another from 45 to 60 feet (14 to 18m), that took in much of the last ice age, from 18,000 to 21,000 years ago. The thickness of the lighter diatom-rich layer was then measured, and plotted against the sea-temperatures in the Pacific Ocean over the last century; these show whether El Nino and La Nina was holding sway there.

Amazingly, these two different measures, from parts of the globe thousands of miles apart, showed strong ties - implying that El Nino and La Nina swings could be measured from these Kenyan lake sediments, going all the way back to the last ice age. Comparisons between the two cores showed that the cooler climate of the last glaciation, some 20,000 years ago, made a significant difference to east Africa's rainfall patterns.

More rain, less predictable

The climate then seems to have been both drier and more stable than today. ''Compared with this coldest time, the last 3,000 years have been wetter, but more variable with severe century-long droughts sprinkled throughout,'' said Timmermann. That may be something that holds a portent for the future in this water-challenged area.

As the globe warms, climate models show that eastern Africa is likely to get more rain - but less predictably. That could mean a dangerous mixing of floods and droughts. This new research confirms that such a future is a real possibility. By tying down in some detail how the local climate has varied over the past few thousand years, it will also help climate modelers to improve their forecasts at the regional level.

Timmermann said that the research could be seen as ''a first step to test the performance of climate models in this vulnerable part of the world... Our sediment core section may definitely provide a benchmark.'' But there is more work to do on the muddy varves, he concluded. ''Repeated moderate La Nina conditions occurring during the short rainy season could generate an accumulated stress on soils and agriculture. This may exacerbate the effects of reduced rainfall on agriculture and water availability. This scenario can be tested using our varve record.''

Interviewed and written by Martin Leggett

Top Image Caption: Lake Challa with Mt. Kilimanjaro in the background. The white speck on the lake is the sediment-core drilling platform. Credit: Photo courtesy Stephan Opitz