Fire and Rain in Africa: Does Burning Across the Sahel Hold Back the Rains?

In 1968, nine-year-old Charles Ichoku was a refugee in southern Nigeria. Civil war had driven his family from Zaria in the north to his parents' home village of Nawfia, and for three years they lived in schools turned into refugee camps, with food almost always scarce. Amid the war, he found comfort in nature: "I was attracted to the order I found in nature." Nearly 50 years later, as a senior scientist at NASA's Goddard Space Flight Center, he used satellites to study the region where he grew up, where more fires burn per square kilometer than almost anywhere on Earth.

Fires (red) detected by the Suomi NPP satellite across Africa on January 30, 2016. NASA Earth Observatory map by Joshua Stevens, using VIIRS data.

Drought in the Sahel

Ichoku's project covered the band of Africa south of the Sahara and north of the equator, from Senegal and Liberia to Eritrea and Somalia. Its northern half, the Sahel, is dry most of the year, with nearly all its rain in May and June. Rainfall climbs steeply southward, from under 200 millimeters (8 inches) a year in the north to as much as 2,000 millimeters (79 inches) farther south.

Central Africa is soaked while the north and south stay dry. NASA Earth Observatory map by Joshua Stevens, using African Flood and Drought Monitor data.

About 58 million people lived in the Sahel, with upwards of 200 million expected by 2050, mostly subsistence farmers on poor soil who depend entirely on rain. When the rains grew less reliable from the 1960s, a few dry years could mean famine. Between 1968 and 1972 hundreds of thousands starved and tens of millions were displaced; in the 1980s millions died in droughts in Kenya and Ethiopia; and in 2012 a drought left 15 million people in the Sahel malnourished. Since 1990, unusually dry and wet years have swung the region between feast and famine.

Two suspected causes

• The ocean. About every three decades, the North Atlantic flips between a warm and a cool phase, the Atlantic Multi-decadal Oscillation. Warm phases bring rain to the Sahel; in cool phases the rain stays to the south. The worst droughts, 1960 to 1980, came in a cool phase.

The Atlantic Multi-decadal Oscillation, rising and falling even as global sea temperatures climb. NASA Earth Observatory chart by Joshua Stevens, using NOAA data.

• The land. In 1975 Jules Charney of MIT, using a NASA climate model, argued that overgrazing made dry years drier. Bare soil holds less moisture and reflects more sunlight, which steadies the air and suppresses the rising heat that builds storms, so less rain falls, less grows, and the cycle feeds itself.

A band of fire

Ichoku suspected a third factor: fire. From November to April, satellites see an enormous band of fire across the region, most lit by people clearing fields, fertilizing soil, growing fodder or driving game. His interest began in the late 1990s, when Lake Chad, a key water source for millions in Niger, Nigeria, Chad and Cameroon, was shrinking from about 25,000 square kilometers in 1960 to 1,500 by 1980. Fire, like grazing, strips vegetation and dries the surface. And research showed that smoke and dust aerosols seed so many cloud droplets that the moisture spreads too thin for them to grow into raindrops.

Smoke from numerous fires over western Africa, December 10, 2015. NASA Earth Observatory image by Joshua Stevens, using VIIRS data from Suomi NPP.

Aerosols change the size and number of cloud droplets, and how clouds reflect light. NASA Earth Observatory illustration by Robert Simmon.

What the satellites showed

In 2009 Ichoku assembled a team of hydrologists, atmospheric scientists, modelers, climatologists and fire experts from Goddard, the University of Missouri, the University of Maryland, Howard University, the University of Nebraska and the Desert Research Institute, with partners in Africa. Comparing ten years of satellite records, he found that after dry seasons with heavy burning, soil moisture, evaporation and green vegetation, all of which feed rain, dipped in the following wet season. Colleagues studied how black carbon from fires warms the air and burns off clouds, delaying or preventing afternoon rain.

The share of each map cell detected as burned. NASA Earth Observatory map by Joshua Stevens, using MODIS fire data from the Global Fire Emissions Database.

The picture was not simple. Fires across the region fell by 2 to 7 percent a year from 2006 to 2013 without a matching rise in rain, a reminder that fire is only one factor. But burning increased in the wetlands around Lake Chad, an important source of moisture. "It may be that the location of the fires … are as important as the overall number of fires," Ichoku said.

Trouble in the field

To check the satellites against the ground, the team planned sensors around Lake Chad, but the region had become a war zone, with Boko Haram fighting governments there since 2009. For years U.S. authorities refused permission to travel, also because of Ebola. When Ichoku reached N'Djamena in 2015, he could not leave the city; colleagues installed much of the equipment, about half of it failed in the harsh climate, and one sensor was stolen. Still, some stations delivered data.

Fires from a Boko Haram attack on the Nigerian village of Doro Gowon in early 2015, before and after, in shortwave infrared. NASA Earth Observatory images by Joshua Stevens, using Landsat data from the USGS.

As his project wound down, Ichoku planned a workshop in Ghana for 35 West African scientists, supported by WASCAL, to teach them to use free satellite fire data from MODIS and VIIRS, and worked on tools to help governments manage what remains of Lake Chad. "Considering where I started," he said, "I never would have imagined I would be blessed and fortunate enough to end up at NASA."

Sources

• NASA Earth Observatory, "A Tale of Fire and Water: A NASA Scientist's Quest to Understand the Rain in Africa"; rewritten in hubnx's own words and shortened. Maps, charts and images from NASA.