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How to monitor climate change from space

Cyril Crevoisier
Cyril Crevoisier
CNRS Research Director at the Dynamic Meteorology Laboratory (LMD*) at the École Polytechnique (IP Paris)

Fol­low­ing the COP26 in Glas­gow, there has been much talk about how to mon­i­tor cli­mate change around the world and its effects. In the Dynam­ic Mete­o­rol­o­gy Lab­o­ra­tory (LMD), we focus on Earth obser­va­tions using satel­lites to improve our under­stand­ing of our plan­et’s cli­mate and changes that are hap­pen­ing. This is pos­si­ble main­ly thanks to progress in satel­lite tech­nol­o­gy, and our abil­i­ty to analyse the data gathered.

Watch­ing Earth from the skies

We have two objec­tives when we observe the Earth from out­er orbit. First, we aim to under­stand the plan­et as a whole, ask­ing fun­da­men­tal ques­tions like: how did Earth become to be the way it is now? How is it evolv­ing? But we are also look­ing at key cri­te­ria that hold answers to some of today’s biggest soci­etal issues, such as the UN Sus­tain­able Devel­op­ment Goals (many of which are cli­mate relat­ed)1 or under­stand­ing risks of hur­ri­canes, earth­quakes and oth­er nat­ur­al disasters.

Efforts are glob­al, with dif­fer­ent States around the world turn­ing their focus to var­i­ous tech­ni­cal areas. In France, at the Nation­al Cen­tre for Space Stud­ies (CNES), NASA was our top part­ner for a long time along with the Euro­pean Space Agency (ESA).  But in recent years we have seen some big changes in the Earth Obser­va­tion sec­tor. Col­lab­o­ra­tions with India, Chi­na and oth­er small­er Euro­pean agen­cies in Ger­many and the UK have swelled. And, with the advent of nanosatel­lites [tiny, light­weight satel­lites designed for high­ly spe­cif­ic mis­sions] dozens of new, small­er space agen­cies have been cre­at­ed around the world work­ing on diverse projects – so many that we strug­gle to keep up with all of them!

More­over, the field has received both pub­lic and polit­i­cal recog­ni­tion – espe­cial­ly thanks to the COP21 in 2015 that saw the selec­tion of 2 French space pro­grams ded­i­cat­ed to the mon­i­tor­ing of CO2 and CH4: Micro­Carb2 and Mer­lin3, respec­tive­ly. Add to that the oth­er game-chang­er for us: the Euro­pean Space pro­gramme, Coper­ni­cus4. Oper­a­tional since 2014, it com­pris­es now 8 satel­lites (known as Sen­tinels) in orbit around the globe, each observ­ing dif­fer­ent com­part­ments of the Earth sys­tem. Anoth­er 10 are in prepa­ra­tion for con­tin­u­ous launch­es up to 2030 and we are already plan­ning the next pro­gramme with at least 6 more! 

The pro­gramme is ded­i­cat­ed to pro­vid­ing autonomous and inde­pen­dent access to infor­ma­tion in the domains of envi­ron­ment and secu­ri­ty on a glob­al lev­el in order to help ser­vice providers, pub­lic author­i­ties and oth­er inter­na­tion­al organ­i­sa­tions. All Coper­ni­cus data is also open source. This means it is free to access for agen­cies, lab­o­ra­to­ries or oth­er enti­ties around the world who may wish to use it (includ­ing com­mer­cial enter­pris­es). And one of its big­ger users is the sci­en­tif­ic community.

Air pol­lu­tion in Europe return­ing to pre-pan­dem­ic lev­els in March 2021 © Euro­pean Union, Coper­ni­cus Sentinel‑2 imagery

Tech­no­log­i­cal inno­va­tion for new capabilities

In France, we have three fields of excel­lence, which allow us to study pre­cise changes in so-called essen­tial cli­mate vari­ables, a set of 54 geo­phys­i­cal vari­ables that crit­i­cal­ly con­tribute to the char­ac­ter­i­sa­tion of Earth’s cli­mate, of which about 60% can be addressed only by satel­lite data: altime­try, opti­cal imagery and atmos­pher­ic sounding. 

Using altime­try, from the pio­neer­ing mis­sions TOPEX/Poseidon, Jason and now Sentinel‑6, we can mon­i­tor sea water lev­els over time, a huge­ly impor­tant fac­tor in glob­al cli­mate change. Our sys­tem can keep track of ocean depth changes and is able to see the 3.3 mm annu­al increase, which has dri­ven the 10 cm rise in sea lev­els over the last 30 years!

Opti­cal imagery allows us to fol­low what’s hap­pen­ing on Earth with extreme spa­tial res­o­lu­tion (up to 10 metres). Using this tech­nique, mis­sions like TRISHNA allow us to analyse ground humid­i­ty and fol­low crop har­vests from the skies as a way of keep­ing track of human-dri­ven changes to the planet’s sur­face5

Final­ly, atmos­pher­ic sounders let us mea­sure radi­a­tion com­ing from var­i­ous lay­ers of the atmos­phere across the whole light spec­trum – even that which we can­not see – to offer indi­ca­tions for the pres­ence of green­house gas­es and oth­er major pol­lu­tants. These mea­sures are key to under­stand­ing the com­po­si­tion of the Earth’s atmos­phere and, more impor­tant­ly, how it is chang­ing over time due to human activ­i­ty. The IASI instru­ment devel­oped by CNES in coop­er­a­tion with the Euro­pean Organ­i­sa­tion for the Exploita­tion of Mete­o­ro­log­i­cal Satel­lites (EUMETSAT)67 has recent­ly pro­vid­ed a unique view on the trans­port all around the world of car­bon monox­ide emit­ted by dra­mat­ic Cal­i­forn­ian fires and allowed track­ing sev­er­al desert storms respon­si­ble for “yel­low sky” in Europe.

Data treat­ment back on land 

Satel­lites take var­i­ous mea­sures or images from the upper atmos­phere, yet most of the data is analysed on the ground. How­ev­er, it is very rare that we can direct­ly mea­sure exact­ly what we need. Hence, to make sense of satel­lite mea­sure­ments we need to inter­pret them in terms of geo­phys­i­cal infor­ma­tion by design­ing inno­v­a­tive algo­rithms, such as machine learn­ing, capa­ble of trans­form­ing data into use­ful infor­ma­tion. More­over, to be use­ful and add infor­ma­tion to the one already pro­vid­ed by ground-based obser­va­tion net­works, we need to reach a high lev­el of accu­ra­cy. For instance, to iso­late the very small sig­na­tures of cli­mate change, we need to be able to detect a small trend of 0.1 K annu­al increase… from mea­sure­ments made at 800 km from the surface! 

Anoth­er chal­lenge is to cre­ate an inte­grat­ed obser­va­tion sys­tem that can com­bine space data with mea­sures from the sur­face or in the air [such as weath­er bal­loons or research aicrafts] in an inte­grat­ed obser­va­tion sys­tem. Our goal: to link these obser­va­tions togeth­er in order to gen­er­ate mean­ing – and for that we need accu­rate mea­sure­ments and numer­i­cal mod­els of the Earth sys­tem to assim­i­late them.

Plan­ning the next step 

What we need for the com­ing years is inno­va­tion… and more con­ti­nu­ity in space mis­sions. Inno­va­tion to observe new geo­phys­i­cal vari­ables, such as cloud con­vec­tion, and to improve the obser­va­tions: to allow study­ing low-scale process­es, it is need­ed to increase the spa­tial res­o­lu­tion from 10 to 2 m in car­tog­ra­phy or from 75 to 15 km in altime­try. Con­ti­nu­ity to mon­i­tor glob­al change: if we con­sid­er cli­mate stud­ies, it takes at least 20 years to see trends in the data. Back in the 1950s mis­sions last­ed only 5 years. Where­as, these days, we are now clos­er to 12 years. Even so, we still need more per­ma­nen­cy, cou­pled with the capac­i­ty to link data from one plat­form to anoth­er: this is key if we wish to make long-stand­ing com­par­isons in cli­mate data year on year. As such, we need long-term pro­grams with long-term budgets. 

And this is chal­leng­ing also from a man­age­ment point of view! For instance, the IASI mis­sion was designed from the start to last 20 years, by build­ing three satel­lites before 2006, of which one was launched straight away and the oth­er two were kept in stor­age for 4–8 years. In that time tech­nol­o­gy can change and engi­neers move on or retire, tak­ing the skills and com­pe­ten­cies away with them. We are already plan­ning anoth­er three mis­sions for launch in 2023, 2030 and 2037. So, we need to be able to hold onto our engi­neers and assure that the satel­lites age well whilst they wait in the hangar for over a decade before they are launched!



Cyril Crevoisier

Cyril Crevoisier

CNRS Research Director at the Dynamic Meteorology Laboratory (LMD*) at the École Polytechnique (IP Paris)

Cyril Crevoisier and his team study atmospheric climate variables, and greenhouse gases in particular, using spatial and airborne observations. He is a Research director at the CNRS, and head of the "Atmosphere, biosphere and climate by remote sensing" team at the Dynamic Meteorology Laboratory (*LMD: a joint research unit of CNRS, École Polytechnique - Institut Polytechnique de Paris, ENS, Sorbonne University). He also chairs the Scientific Committee for Earth Sciences at the National Center for Space Studies (CNES).

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