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Small things can have a big impact. Take phytoplankton, the plants that populate Earth’s oceans. When zooplankton eat them, phytoplankton release a chemical called dimethyl sulfide (DMS) and this is what people are referring to when they talk about the “smell of the ocean.” Chemical reactions in the atmosphere convert DMS into sulfur-containing particles. Provide a surface for water vapor to condense. Do this often enough and the resulting clouds affect both local weather and, by reflecting sunlight into space, the world’s climate.
Other small things have similar big impacts. Sulfur from ships’ funnels also creates particles that form clouds, creating the puffy white “shiptrack” strings that can be seen in satellite images. Meanwhile, soot from burning fossil fuels has the opposite effect. It’s made up of dark particles that absorb solar energy, warming the air around them and discouraging cloud formation if the sulfur particles make it into the atmosphere in sufficient quantities (thanks to volcanic eruptions, perhaps). They can create a haze that blocks some sunlight from reaching the Earth’s surface.
Although scientists know in general terms how these processes work, they are very difficult to quantify. Uncertainties about the behavior of “aerosols”, as various small particles in the air are collectively known, are one of the main sources of scientific uncertainty in climate models. So they are a big reason for the error bars that surround estimates of how much warming the Earth will cause given a certain increase in the amount of carbon dioxide in its atmosphere.
Climate scientists hope that NASA’s new satellite, PACE (for “Plankton, Aerosol, Cloud, Ocean Ecosystem”), which was launched into Earth orbit on February 8, will reduce the uncertainties surrounding aerosols. PACE’s cameras will monitor the planet every one to two days to create a constantly updated census of very small things suspended in the oceans (plankton) and air (aerosols).
PACE’s main camera is sensitive to the spectrum of light between ultraviolet and near-infrared. For the oceans, this means that PACE will be able to distinguish different types of phytoplankton. “This is powerful because diatoms promote fish farming [and] Cyanobacteria can be harmful,” says Jeremy Vardell, a NASA oceanographer and PACE chief scientist. Two other instruments aboard PACE will provide information on the size and shape of the aerosols, making it possible for the first time to distinguish soot from sea spray. and particles produced by burning fossil fuels.
This could be “transformative” for climate models, says Gavin Schmidt, a climate scientist who also works at NASA. Modelers have had to compensate for the limited nature of existing aerosol data with informed estimates. As a result, different climate models vary considerably in their projections of how powerfully aerosols influence climate.
Such uncertainties affect the question of how air pollution affects climate change. Laws in Europe and North America have cut the amount of air pollution from fossil fuels since the 1980s. It is a boon for human health. But it has also lifted the smog curtain that was masking some of the warming caused by greenhouse-gas emissions. Cleaning up air pollution could have one of the most significant impacts on climate in the coming decades. Better data will allow better modelling.
Similarly, climatologists are divided on the impact of rules adopted by the International Maritime Organization, a UN arm, which has limited the amount of sulfur in ship fuel since January 2020. Some believe the lack of sulfur in ship exhaust may have contributed to the exceptionally hot temperatures recorded around the world in 2023. Others feel the impact was minimal.
There are many other questions that climatologists would like to know the answers to. A list of 18 projects is written on a whiteboard in Kirk Knobelspiese’s office at NASA’s Goddard Space Flight Center in Greenbelt, Maryland. This includes everything from gathering live data on volcanoes and wildfires to answering what happens when soot from the agricultural clearing fires that burn each year in West Africa reaches the top of ocean clouds, blocking the sun’s rays. His face turns black in front of him. The answers to all those questions depend on the behavior of small things. After decades of uncertainty, answers may be on the way.
© 2024, The Economist Newspaper Limited. All rights reserved.
From The Economist, published under license. Original content can be found at www.economist.com
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Published: 07 May 2024, 08:00 PM IST
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