October 19, 2013 — andyextance

Having dodged pirates and extremists, and slogged for two years to interpret the record collected, US scientists have shown how abruptly rainy climates in East Africa come and go. Jessica Tierney puzzled out a rainfall record back to the last ice age from mud collected in one of the last research cruises to brave the Horn of Africa. “The region goes from being pretty humid to very arid in hundreds of years,” Jessica, who works at Woods Hole Oceanographic Institution (WHOI) in Massachusetts, told me. “That’s important because there’s a threshold behaviour in its rainfall. We need to better understand what drives those thresholds, and when we’d expect to be pushed over one, as it has huge implications for predicting drought and famine in the region.”

Long interested in ancient East African climate, Jessica wanted to study the Horn of Africa area, which includes Ethiopia and Somalia, because the climate there is very sensitive and variable. But its dry conditions rule out many options scientists use to build historical records from ice, cave deposits, sediments from lake beds or tree rings. So in 2010, she started working with Peter deMenocal at Lamont-Doherty Earth Observatory in New York, who collected sea bed sediments from the area in April and May 2001.

“We boarded ship in Dar Es Salaam in Tanzania and our cruise was to end in Port Said, in Egypt,” Peter told me. That took the team down the Somali coast and into the Gulf of Aden, where a few months earlier suicide bombers killed 17 sailors aboard the USS Cole. Though the scientists were worried, the captain of their Dutch research ship, R/V Pelagia was vigilant. “He had ordered radio silence, and we actually turned off all our lights on the ship at night, even navigation lights,” Peter recalled. “He had also put in orders for us to train on what to do in case we were boarded.”

A changing world

Their work, collecting 10-metre long cylinders of mud from the seafloor, put them at particular risk. “We assembled our piston coring device on the deck of the ship,” Peter explained. “It’s a one or two tonne lead weight on top, and hanging from that is a long steel tube. Just to prepare takes a couple of hours. It then takes an hour or two to lower to the seafloor, before we trigger a device that lets it freefall like a dart into sediment at high speed. Then it has a little piston inside the tube that works like a hypodermic, drawing the sediment in as we were coring it. We’re typically on station for between 2-4 hours, just sitting there. That’s when we were at our most vulnerable.”

Nonetheless, Peter and his teammates took a line of cores along the Gulf of Aden, closely checking reports of attacks by pirates from Somalia, or extremists from Yemen. “In a few cases the attacks happened right on top of where we were planning on taking our samples,” he said. “We still ended up taking those samples. We figured that they’d done their business.” Their wariness, and perhaps their Dutch flag, kept them out of trouble, but before long the area was shut for research business. In August 2001, Lamont’s research vessel Maurice Ewing was attacked – unsuccessfully – by pirates toting AK-47s and rocket-propelled grenades. And then in September, New York’s World Trade Center was attacked. “I’d largely forgotten the concern we’d had because it had been eclipsed by 9-11,” Peter admitted. “But looking back it was really just a taste of what the world became.”

Though Jessica had gained access to one of these rare valuable cores in 2010, it came from the sea, and she wanted to look at rainfall on land. But she realised it would contain waxes washed off plant leaves, which researchers were increasingly using to track past rainfall. “These waxes are really well preserved in sediment cores,” she underlined. “They don’t degrade very rapidly.” The balance between the most common form of hydrogen and a heavier form, deuterium, in these waxes tracks rainfall, with more deuterium indicating a drier period. She could then use carbon-dating to work out when the waxes were formed, to create a historical record.

The definition of painstaking

Yet taking these measurements from a 10-metre long cylinder, with sediments dating back 40,000 years, is itself a big challenge. This method was previously rarely used in such ‘paleoclimate’ research because it took such a long time. Efforts by Jessica and others to speed it up made it possible to look at African rainfall, but even then you could hardly call the process fast. “It took about two years in the lab to generate the data,” she said. “It’s an exercise in patience, which is true for all paleoclimate work.”

While global changes over those 40,000 years are now well-known, driven in part by changes in Earth’s orbit, Jessica found that East African rainfall hasn’t followed them smoothly. In a paper published in leading journal Science last week, she and Peter found that the area could switch from humid to arid in just a few centuries. One particular example was the ‘African Humid Period’ from around 11,500-6000 years ago, when today’s Sahara Desert was dotted with lakes, grasslands, forests and shrubs. The new record suggests that the area dried out over just 280-490 years. Modern shifts on that timescale would have definite impacts on life in Africa.

“It’s very clear from our data that rainfall can change rapidly,” Jessica stressed. Having also shown this year that parts of East Africa were much wetter than at present just 300 years ago, she wants to know more about the region’s rainfall. “I’m not sure we really know the reasons for that,” she said. “We definitely need more research with good climate model simulations to understand what could push the region into a regime that’s either persistently wetter or drier. Both would be important to understand.”

Journal references:

Jessica E. Tierney, Jason E. Smerdon, Kevin J. Anchukaitis, Richard Seager (2013). Multidecadal variability in East African hydroclimate controlled by the Indian Ocean Nature DOI: 10.1038/nature11785

Jessica E. Tierney, Peter B. deMenocal (2013). Abrupt Shifts in Horn of Africa Hydroclimate Since the Last Glacial Maximum Science DOI: 10.1126/science.1240411