African Dust The New CO2?
Chad, located along the southern edge of the Sahara Desert, is the dustiest place on Earth. Recognizing that aerosols plays a vital role in climate and biophysical feedback in Earth's environmental system, climate researchers are turning to dust as a major driver of climate change. A new article, to be published in PNAS, identifies the Bodélé Depression in Chad as the producer of about half the mineral aerosols emitted from the Sahara. According to Richard Washington et al. dust could be a “tipping element” where “small features of the atmospheric circulation, such as the Bodélé Low-Level Jet, could profoundly alter the behavior of this feature.” With the impact of CO2 diminished due to a cooling climate climate, researchers are searching for new hazards: Is African dust the new carbon dioxide?
The role of aerosols, both mineral and biological, has been under intense scrutiny in recent years. The IPCC AR4 reported aerosols as one of the most potent climate forcings about which science understood very little. The paper “Dust as a tipping element: The Bodélé Depression, Chad” is but the latest paper to call attention to the importance of African dust. I have previously reported on the impact of Saharan dust storms on the tropical Atlantic (see “African Dust Heats Up Atlantic Tropics”) and how aerosol levels from dust storms and volcanoes alone would account for much of observed global temperature rise (see “Arctic Aerosols Indicate Melting Ice Not Caused By CO2”). Now, in the PNAS paper, researchers claim that the Bodélé area may be a “tipping element” in the context of climate change, due to its location and prodigious dust output.
The Bodélé Depression is Chad’s lowest point and it is one of the world’s most active sources of dust storms. On November 9, 2006, NASA’s Terra satellite took this picture of a dust storm in the Bodélé Depression showing two visible plumes. The dust plumes appear as pale beige brush strokes against a tan background. To the southwest of the dust storm is Lake Chad, which appears mostly dark green. Irrigation and declines in rainfall have reduced the size of the lake, once as big as Lake Erie in North America, to only 5 percent of its size in 1966.
Dust storm captured by the Moderate Resolution Imaging Spectroradiometer (MODIS). NASA.
So much dust is produced by the Bodélé Depression that it provides most of the mineral dust to the Amazon forest. In a paper in Environmental Research Letters, Ilan Koren et al. reported that, based on satellite observations, dust is continuously transport 3,000 miles (5000 km) from Saharan sources to the Caribbean Sea and North America in the Northern summer and to the Amazon basin during the Northern winter. According to the PNAS paper:
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.
There is evidence that during the Last Glacial Maximum atmospheric dust concentration was as much as an order of magnitude greater than present-day values, but dust levels are highly variable. Modern observations suggest that between 1960 and 2000 annual mean African dust generation may have varied by a factor of 4. It is thought that minor modifications to global circulation patterns could cause dust production from the Bodélé to increase significantly or reduce it to near zero. “We argue that the Bodélé is indeed capable of profound changes in future emissions,” state Washington et al. “Although the full consequences of these change have not yet been quantified, the case exists for doing so.”
TOMS satellite annual average aerosol index (AI) 1979 –1992. Washington et al.
Indeed, recent analysis of aerosols' complex heating and cooling influences have led to a greatly reduced impact for atmospheric CO2 levels. As previously reported, a large aerosol cooling implies a correspondingly large climate sensitivity. Conversely, reduced aerosol cooling implies lower GHG warming, which in turn implies lower model sensitivity. The upshot of this is that CO2 sensitivity values used in models for the past quarter of a century have been set too high. With global temperatures flattening and even declining since 2002 it looks like all those climate models that had been tweaked to match last century's temperature curve—thereby pumping up the importance of human carbon dioxide emissions—have quietly left the world stage.
The new found emphasis on dust has been bearing scientific fruit in a rash of recent papers including one published in Quaternary Research in November of 2007. This paper by Elsa Jullien et al. reports a link between low-latitude “dusty events” and “icy Heinrich events,” which occur at higher latitudes. As you may recall from previous posts, Heinrich events happened when the American ice sheets became unstable. That gave rise to enormous ice surges which formed vast flotillas of drifting ice floes in the Atlantic Ocean and noticeable increases in ice rafted debris deposits. The last event (H1) some 14,000 years ago set the stage for the onset of the Holocene warming. According to the abstract of the paper by Jullien et al.:
We explore low-latitude Heinrich events equivalents at high resolution, in a piston core recovered from the tropical north-western African margin. They are characterized by an increase of total dust, lacustrine diatoms and fibrous lacustrine clay minerals. Thus, low-latitude events clearly reflect severe aridity events that occurred over Africa at the Saharan latitudes, probably induced by southward shifts of the Inter Tropical Convergence Zone. At a first approximation, it seems that there is more likely synchronicity between the high-latitude Heinrich Events (HEs) and low-latitude events (LLE), rather than asynchronous behaviours.
Heinrich events, numbered 1 through 6, occurred during a period of extensive sea-ice in the North Atlantic similar to today’s Arctic Sea. Heinrich events are relatively brief and tend to occur at the boundaries of major climatic transitions as indicated by δ18O proxy measurements. Event number six, the oldest accepted event, occurred around 60,000 years ago. Heinrich event 1 marked the onset of the termination which signaled the end of the last interglacial. It is thought that fresh water released during the Heinrich events disrupted deep-water formation, thereby prompting switches between glacial and interglacial modes of thermohaline circulation. If these recent papers linking dust to HEs prove accurate then it is possible that large amounts of dust from the Sahara is a sign of cooling, not global warming.
Dirty icebergs are characteristic of Heinrich events. NOAA.
Again science has uncovered significant factors not considered by the IPCC's models, with significant impact on the doomsday predictions made by climate change alarmists. The important link between dust and climate was only suspected when the IPCC was busy cranking out dire global warming scenarios with their computer models. None of the models accurately accounted for aerosols and their affect on other forcings like albedo and the hydrological cycle. Now some scientists even suspect that dust is an important trigger for larger climate change events, a “tipping element.”
“Several factors distinguish the Bodélé as a potential tipping element,” argue Washington et al. “it is the largest single source of mineral dust on the planet, producing about half of the Sahara’s mineral aerosol loadings.” They list the following reasons for thinking dust could be a tipping element: “Mineral dust plays a key role in modifying climate through interaction with cloud physics and radiative heating. It is also involved in numerous biophysical feedbacks both in the oceans and on land.” Even so, the Bodélé is a very small region and the effects of dust on the world's environmental system are still mostly unknown. The authors almost sadly conclude “The Bodele, at this stage, qualifies as a potential tipping element until more work can be done to quantify the radiative impacts and biogeochemical consequences of mineral aerosols.”
Is dust then the new carbon dioxide, a villain to base predictions of disruptive climate change on? Probably not, aerosols' complex environmental interactions can't be dumbed down into a simple formulation as rising CO2 levels were. When trying to frighten the public simple sells, and dust's interaction with the environment is far from simple. But one simple point is becoming clearer and clearer: Climate modeling centered on CO2 levels, instead of providing a solid foundation for global warming theory, has proven to be less substantial than the dust blowing out of Africa.
Be safe, enjoy the interglacial and stay skeptical.
Dust storms in Africa can affect climate world wide.