Nasa shocker: Africa's getting wetter
... except where it's getting drier
South-eastern Africa is getting drier, with a six percent decrease in rainfall predicted this century. But the Okavango delta is “remarkably” wet compared with the beginning of the century.
These are two of the key African findings from a 14-year study by the National Aeronautics and Space Administration in the US, which combines satellite observations of Earth with data on human activities to map locations where freshwater is changing — and to determine why.
The global study, published on Thursday in the journal Nature, finds that wet land areas are getting wetter and dry areas are getting drier due to factors including human water management, climate change and natural cycles.A team led by Matt Rodell, head of the hydrological sciences laboratory at Nasa’s Goddard Space Flight Centre in Maryland, used observations from the Gravity Recovery and Climate Experiment (Grace) spacecraft mission to track freshwater in 34 regions around the world.
They combined the observations with other satellite imagery as well as data about rainfall, irrigation, agriculture, mining and reservoirs, and Rodell said: “This is the first time that we’ve used observations from multiple satellites in a thorough assessment of how freshwater availability is changing, everywhere on Earth.
“A key goal was to distinguish shifts in terrestrial water storage caused by natural variability — wet periods and dry periods associated with El Niño and La Niña, for example — from trends related to climate change or human impacts, like pumping groundwater out of an aquifer faster than it is replenished.”
The research confirmed the severe drought in south-eastern Africa which has caused severe food shortages. “Rainfall was four percent below average during the Grace period, including annual accumulations that were below normal in five of the last eight years and barely above normal in the other three,” it said.“Water levels in Lake Malawi, which is in the centre of the region, are well correlated with regional terrestrial water storage. The lake declined at a mean rate of 78mm per year during the period, accounting for 2.3 gigatons per year of the observed [storage] trend.” A gigaton of water is the equivalent of 400,000 Olympic swimming pools.
“What we are witnessing is major hydrologic change,” said Jay Famiglietti of Nasa’s Jet Propulsion Laboratory in Pasadena, California.“We see a distinctive pattern of the wet land areas of the world getting wetter — those are the high latitudes and the tropics — and the dry areas in between getting dryer. Embedded within the dry areas we see multiple hotspots resulting from groundwater depletion.”
He said it would take longer to conclude whether the changes in Africa were related to global warming. “The pattern of wet-getting-wetter, dry-getting-drier during the rest of the 21st century is predicted by the Intergovernmental Panel on Climate Change models, but we’ll need a much longer dataset to be able to definitively say whether climate change is responsible for the emergence of any similar pattern in the Grace data,” he said.
“The human fingerprint is all over what we see in the map. There are implications in that map for food security, for water security and for human security in terms of things like conflict and climate refugees.”The twin Grace satellites precisely measured the distance between the two spacecraft — they orbited about 220km apart — to detect changes in Earth’s gravity field caused by movements of mass on the planet.
A microwave link between the two satellites allowed scientists to precisely monitor minuscule changes in their separation down to a distance of 10 microns, or about one-tenth the width of a human hair.
The orbiting “scale” measured slight regional variations in Earth’s gravitational pull. Many are due to geological features, such as mountain ranges, but by taking measurements over 14 years it also detected changes caused by the movement of massive amounts of water at or near Earth’s surface.
Changes in California’s Central Valley may hold lessons for the Western Cape, where groundwater is increasingly being seen as the answer to the water shortage.Famiglietti said significant groundwater depletion was observed because it was not fully replenished by rain and snowfall, while agriculture used more.
A downward freshwater trend in Saudi Arabia also reflected agricultural pressures. From 2002 to 2016, the region lost 6.1 gigatons per year of stored groundwater. At the same time, satellite imagery shows an explosive growth of irrigated farmland in the arid landscape since 1987.
The Nasa scientists also identified large, decade-long trends that do not appear to be directly related to human activities. Natural cycles of high or low rainfall can cause a trend that is unlikely to persist, Rodell said.An example is Africa's western Zambezi basin and Okavango Delta, a vital watering hole for wildlife in northern Botswana. In this region, water storage increased at an average rate of 29 gigatons per year from 2002 to 2016. This wet period during the Grace mission followed at least two decades of dryness. Rodell believes it is a case of natural variability that occurs over decades in this region of Africa.
The Grace satellites burned up as their orbit decayed in March. A new mission, Grace Follow-On, is due to be launched on Tuesday from Vandenberg Air Force Base in California.Six key African trends1. A powerful wetting trend was observed in the western Zambezi basin, the Okavango delta and areas west of the coast. This region experienced a remarkable change in its hydroclimate. The area-averaged annual rainfall was less than 970mm in every year from 1979 to 2005. That threshold was exceeded five times from 2006 to 2011. It appears the region may have endured a prolonged drought from the late 1970s to the early 2000s. Thus, we attribute the Grace-period trend to natural variability. Although terrestrial water storage appears to have peaked in 2012, considering that the previous wet and dry periods lasted upwards of 25 years, it is plausible that the wetting trend could resume.2. Lakes Tanganyika and Victoria experienced minimum water levels in 2006 and that their annual mean levels increased by 62m per year and 40mm per year on average, respectively. Rainfall would seem to be the primary driver of [storage] variations, while management of the large lakes and dam building in the northern part of the region also contribute. The observed rainfall trend was negligible during the study period, but a 12% increase is predicted by 2100. The northern part of region encompasses the Grand Ethiopian Renaissance Dam on the Blue Nile River at Ethiopia’s north-west border with Sudan, which Egypt has strongly denounced because of the possibility of reduced flow through the Nile. Construction of the dam began in 2011 and is ongoing. Filling of the 74-km3 reservoir will probably produce a temporary increasing [storage] trend in its immediate vicinity.
3. Terrestrial water storage has been increasing in tropical western Africa. Precipitation was three percent below normal in 2000-2002 and three percent above normal during the rest of the Grace period. This appears to be the primary cause of storage accumulation, although the possible contribution of the many dams being built in this part of Africa is unknown. Because inter-annual variability of rainfall is substantial in the region, disregarding the dams it is likely that the change rate of storage will oscillate around zero over the coming decades. By 2100, rainfall is predicted to decrease by six percent; hence, the dam construction may be timely.4. Decreasing terrestrial water storage in the region between the coast of central Africa and the northern Congo River basin seems to be caused by natural inter-annual variability, although it has been suggested that the surface run-off rate has been enhanced by deforestation. Between 1999 and 2002, rainfall averaged four percent above normal, while it averaged one percent below normal during the rest of the Grace period, including two very dry years in 2014 and 2015.
5. The negative trend along the coast of south-eastern Africa reflects a recent severe drought, which has caused major food shortages. Rainfall was four percent below average during the Grace period, including annual accumulations that were below normal in five of the last eight years and barely above normal in the other three. Water levels in Lake Malawi, which is in the centre of the region, are well correlated with regional terrestrial water storage. The lake declined at a mean rate of 78mm per year during the period, accounting for 2.3 gigatons per year of the observed [storage] trend. Hence, it is likely that the apparent trend is primarily caused by natural variability, although a six percent decrease in rainfall is predicted during this century.
6. A weak negative trend extends across arid Africa north of 19°N, excluding Morocco. Precipitation during the Grace period was seven percent above normal, which suggests that the consumptive use of fossil groundwater to stimulate agriculture and economic development is the cause.