*Economist
Small things can have big effects.
Take the plant plankton that populate the Earth’s oceans. When zooplankton eat
them, the phytoplankton release a chemical called dimethyl sulphide (dms) and
it is this that people are referring to when they speak of the “smell of the
sea”. Chemical reactions in the atmosphere turn dms into sulphur-containing
particles that offer a surface for water vapour to condense on. Do that enough
times and the result is a cloud. Clouds, in turn, affect both the local weather
and, by reflecting sunlight into space, the world’s climate.
Other tiny things have similarly
extensive effects. Sulphur from ships’ funnels also makes particles that seed
clouds, producing strings of puffy white “shiptracks” that can be seen in
satellite pictures. Soot from burning fossil fuels, meanwhile, has the opposite
effect. It is made of dark particles that absorb solar energy, warming the air
around them and discouraging cloud formation. If sulphur particles make it high
enough in the atmosphere (thanks to a volcanic eruption, perhaps) they can form
a haze that blocks some sunlight from reaching Earth’s surface.
But although scientists know in
general terms how these processes work, quantifying them is much harder.
Uncertainties about the behaviour of “aerosols”, as various small particles in
the air are collectively known, are one of the main sources of scientific
uncertainty in climate models. They are therefore a big reason for the error
bars that surround projections of how hot Earth will become for a given
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 8th, will reduce those uncertainties
around aerosols. pace’s cameras will sweep the planet every one to two days to
create a continually updated census of the very small things that are suspended
in the oceans (plankton) and the air (aerosols).
pace’s main camera is sensitive to
the spectrum of light between the ultraviolet and the near infrared. For the
oceans, that means pace will be able to distinguish different types of
phytoplankton. “That is powerful because diatoms fuel fisheries [and]
cyanobacteria can be harmful,” says Jeremy Werdell, an oceanographer at nasa
who is pace’s chief scientist. Two other instruments mounted on pace will offer
information about the size and shape of aerosols, making it possible for the
first time to distinguish soot from sea spray and particles produced by burning
fossil fuels.
That could be “transformative” for
climate models, says Gavin Schmidt, a climate scientist who also works at nasa.
Modellers have had to compensate for the limited nature of the existing aerosol
data with informed guesswork. As a result, different climate models vary
considerably in their estimates of how powerfully aerosols affect the climate.
Such uncertainties affect questions
about how air pollution influences climate change. Laws in Europe and North
America have cut the amount of air pollution from fossil fuels since the 1980s.
This is a boon for human health. But it has also lifted a smoggy veil that was
masking some of the warming caused by greenhouse-gas emissions. Cleaning up air
pollution could be one of the most important influences on the climate in the
coming decades. Better data will allow better modelling.
Similarly, climatologists are divided
on the effect of rules adopted by the International Maritime Organisation, part
of the United Nations, which capped the amount of sulphur in ship fuel starting
in January 2020. Some believe the reduction in sulphur in ship exhausts may
have contributed to the exceptionally hot temperatures recorded around the
world in 2023. Others think the effect was minimal.
There are plenty more questions
climatologists would like answered. Scribbled on the whiteboard in Kirk
Knobelspiesse’s office at nasa’s Goddard Space Flight Centre in Greenbelt,
Maryland, is a list of 18 projects. It ranges from gathering live data on
volcanoes and forest fires to answering what happens when soot from
agricultural clearing fires that burn annually in west Africa ends up on the
tops of marine clouds, darkening the face they present to the sun. The answers
to all those questions depend on the behaviour of tiny things. After decades of
uncertainty, answers may be on the way.