Satellites are detecting plastic pollution in the ocean worldwide

The harm­ful effects of plastic pol­lu­tion on human health and biod­iversity are well doc­u­mented. How­ever, last sum­mer, the nego­ti­at­ing com­mit­tee meet­ing in Geneva once again failed to reach an inter­na­tion­al treaty against plastic pol­lu­tion. This pol­lu­tion is accu­mu­lat­ing in the oceans: 75 to 199 mil­lion tonnes of plastic are thought to have ended up in the oceans in 2021¹, and annu­al dis­charges now amount to tens of mil­lions of tonnes. Plastic accounts for more than 80% of the debris present in aquat­ic sys­tems². “There is cur­rently no glob­al assess­ment of pol­lu­tion in the sea,” laments Audrey Has­son, ocean­o­graph­er and coordin­at­or of the GEO Blue Plan­et Ini­ti­at­ive. Yet this would enable reduc­tion tar­gets to be set and the effect­ive­ness of pre­ven­tion policies to be mon­itored.” For sev­er­al years now, sci­ent­ists have been try­ing to use satel­lites for this purpose.

Between 1950 and 2017, approx­im­ately 9.2 bil­lion tonnes of plastic were pro­duced. Of this volume, 7 bil­lion tonnes became waste. This waste is sent to land­fill, enters uncon­trolled waste streams or is dumped in the envir­on­ment. Most of the plastic waste in the oceans comes from land-based sources. Once at sea, it is car­ried by the cur­rents³. “Without a glob­al view, it is very dif­fi­cult to find solu­tions to the prob­lem of plastic pol­lu­tion,” com­ments François Gal­gani. “How­ever, the tools avail­able to provide this glob­al view are very lim­ited, and satel­lite meas­ure­ments are one of the few that can do so.”

Sci­ent­ists use three main tech­niques to quanti­fy plastic debris and its move­ments⁴: numer­ic­al mod­els, in situ meas­ure­ments (at sea on board a ship, for example) and satel­lite mon­it­or­ing. The use of satel­lite data to study plastic pol­lu­tion is a recent devel­op­ment, dat­ing back to the 2010s, and two major stud­ies were pub­lished in 2020 ⁵⁶. In the same year, the European Space Agency sup­por­ted a cam­paign⁷ ded­ic­ated to meas­ur­ing mar­ine debris from space for the first time.

In prac­tice, the data used con­sists of high-res­ol­u­tion images in the vis­ible spec­trum, as well as infrared and radar images. “There is no ded­ic­ated satel­lite mis­sion, so we use data from the Sen­tinel 1, 2 and 3 Earth obser­va­tion satel­lites,” explains Audrey Has­son. Stud­ies show that it is pos­sible to detect plastic clusters with a min­im­um sur­face area of 5 m² using Sentinel‑2. “We can only detect large pieces of plastic; the res­ol­u­tion of the detec­tion depends on the res­ol­u­tion of the instru­ment used,” adds Audrey Has­son. It is there­fore impossible to detect indi­vidu­al pieces of plastic, only clusters of debris.

How­ever, the chal­lenge of detect­ing plastic by satel­lite is that it is neces­sary to ascer­tain the nature of the object observed. How can plastic be dis­tin­guished from algae, drift­wood or foam? A pub­lic­a­tion in Nature has suc­ceeded for the first time in devel­op­ing an indic­at­or: the plastics index. Depend­ing on its nature, each object reflects light dif­fer­ently, a prop­erty that can be observed in its spec­tral sig­na­ture. Sci­ent­ists have thus char­ac­ter­ised the spec­tral sig­na­ture of plastic to dis­tin­guish it from oth­er mater­i­als in Sen­tinel images. “Plastics mix with foam or veget­a­tion, so dif­fer­en­ti­ation remains very com­plic­ated,” cau­tions Audrey Has­son. François Gal­gani, an ocean­o­graphy research­er at Ifre­mer, adds: “Today, some detec­tion meth­ods are quite reli­able, such as in the case of con­tain­er losses. We also expect to be able to loc­ate aban­doned fibre­glass boats, which rep­res­ent a reser­voir of 500,000 to one mil­lion tonnes of plastic.”

To date, space-based detec­tion of plastic pol­lu­tion remains at the R&D stage and no oper­a­tion­al applic­a­tions exist. In a recent study ⁸[3], sci­ent­ists show that space-based detec­tion is not widely favoured by the sci­entif­ic com­munity: among the 46 sci­ent­ists sur­veyed, field obser­va­tions and drone meas­ure­ment cam­paigns are the most com­monly used tech­niques, ahead of satel­lite imagery and GPS tracking.

To over­come cur­rent tech­no­lo­gic­al lim­it­a­tions, an inter­na­tion­al team has developed a new ana­lys­is meth­od. Their goal: to bypass the dif­fi­cult step of identi­fy­ing the nature of the debris. Sci­ent­ists show that cer­tain types of float­ing waste clusters (regard­less of their nature) that are recog­nis­able by their shape are indic­at­ors of recent pol­lu­tion incid­ents ⁹[4]. The team states: “Des­pite the lim­it­a­tions of space tech­no­logy, satel­lite detec­tions are suf­fi­cient to map hot­spots and cap­ture trends, provid­ing an unpre­ced­en­ted view of mar­ine debris from its source to its final des­tin­a­tion.” This data is invalu­able for bet­ter anti­cip­at­ing pol­lu­tion peaks and thus lim­it­ing them.

Anoth­er aven­ue: launch­ing ded­ic­ated satel­lite mis­sions. “We hope to launch the first nanosatel­lite from the Des­de­mone uni­ver­sity pro­gramme with­in three years,” says Jérôme Ben­ven­iste, Pres­id­ent of the Com­mit­tee on Space Research (COSPAR) and former seni­or expert in ocean­o­graphy and hydro­logy at the European Space Agency. Thanks to this nanosatel­lite mon­it­or­ing sys­tem, the pro­gramme aims to meas­ure the con­cen­tra­tions and flows of macro-plastic waste in the Medi­ter­ranean in order to mon­it­or, anti­cip­ate and act in near real time, while provid­ing train­ing in space-related pro­fes­sions¹⁰. Anoth­er aven­ue is being explored: detect­ing micro­plastics by satellite.

“Once the debris degrades, it becomes micro­plastic and is undetect­able by satel­lite,” explains Jérôme Ben­ven­iste. “One of the European Space Agency’s pro­grammes aims to test wheth­er the spec­tral sig­na­ture of sea­wa­ter changes in regions with high levels of microplastics.”

Anaïs Maréchal