Climate models that simulate the airborne African dust that influences Atlantic Ocean hurricanes are not up to the task of accurately representing the characteristics of that dust.
In a new study, researchers led by Amato Evan, a climate scientist at Scripps Institution of Oceanography, UC San Diego, examined the performance of 23 state-of-the-art global climate models used in the Fifth Assessment Report of the Intergovernmental Panel on Climate Change. The researchers found that none of them yielded accurate data on dust characteristics.
“The models systematically underestimate dust emission, transport and optical depth, and year-to-year changes in these properties bear little resemblance to observations,” the authors wrote. “These findings cast doubt on the ability of these models to simulate the regional climate and the response of African dust to future climate change.”
African dust influences everything from the transport of nutrients across continents to the planet’s energy budget. Improvement of the “skill” or accuracy of computer models, though, is hampered by a lack of comprehensive data from the regions where the dust originates. Evan said the inaccessibility of these regions often has nothing to do with nature; the Sahara Desert, for instance, is a large source of African dust but is also the home of political unrest and the lair of various terrorist organizations.
“I am in France trying to set up a new set of observations over the Sahara that are related to the dust question, but at present neither myself nor my French colleagues can make the trip out to southern Algeria to set up the equipment ourselves because of regional unrest,” Evan said shortly after the publication of the study.
Observations are needed to constrain models, helping to narrow down the range of possible dust behaviors that are an integral part of computer models of the global climate. Researchers at Scripps Oceanography have pioneered observations of aerosols in other world regions and their effects on climate. For example, Scripps Distinguished Professor of Climate and Atmospheric Sciences Veerabhadran Ramanathan has since the 1990s documented the influence of what has been termed the atmospheric brown cloud, a layer of particulate pollution several kilometers thick that forms over South Asia many times per year. This brown cloud has a major effect on agriculture and public health in that region and goes on to have global effects.
Kim Prather, an atmospheric chemistry professor who holds appointments in the UC San Diego Department of Chemistry and Biochemistry as well as at Scripps Oceanography, studies the long-range transport of pollutant aerosols to study how particulate matters traverses oceans to influence precipitation and other phenomena from one continent to another. In 2013, the Center for Aerosol Impacts on Climate and the Environment, directed by Prather, received a five-year, $20 million grant from the National Science Foundation to study how interactions between air and sea involving aerosols alter the chemistry of the atmosphere to influence climate.
In its Fifth Assessment Report released in stages starting in September 2013, the Intergovernmental Panel on Climate Change (IPCC) incorporated data about African dust generated from climate models. IPCC reports are not themselves original research but are syntheses of the most recent published climate research available when the reports are being compiled and presented to national policymakers around the world. The IPCC report acknowledged the large uncertainty in climate forcing by changes in all aerosols. Evan was a contributing author to Chapter 14 of the report, “Climate Phenomena and their Relevance for Future Regional Climate Change,” in which authors acknowledged that there is uncertainty in regional climate projections associated with aerosols.
“However, I think we showed (in this study) that the level of inaccuracy was just way beyond what anyone assumed it to be,” said Evan.
“All hurricanes that make landfall in the U.S. form and intensify in the tropical North Atlantic,” Evan added. “So how will global warming affect these hurricanes? Well, to really know this we need to get this dust question right, and at present we just can’t do that.”
The study, “An analysis of Aeolian dust in climate models,” appears in the American Geophysical Union journal Geophysical Research Letters. Co-authors include Owen Doherty of Scripps Oceanography, Cyrille Flamant of Laboratoire Atmosphère, Milieux, Observations Spatiales, CNRS and Université Pierre et Marie Curie in Paris, and Stephanie Fiedler of the School of Earth and Environment, University of Leeds, Leeds.
Funding for the research was provided by Agence Nationale de la Recherche in France, Laboratoire d’excellence Institute Pierre Simon Laplace in France, and NOAA.