CO2 a livelli record, superate stabilmente le 400 ppm

di Michele Costanzo 48

CO2
Negli ultimi mesi si sono moltiplicati i segnali preoccupanti sullo stato del clima sulla Terra. Un quadro per molti versi critico e distribuito su più fronti, dal riscaldamento globale ai cambiamenti climatici fino ai disastri naturali. Fenomeni molto complessi e tra loro correlati che sempre più spesso hanno un pesante impatto negativo sull’ambiente e su milioni di persone. Gli ultimi dati presentati poche ore fa dall’Organizzazione Meteorologica Mondiale (World Meteorological Organization – WMO) aggiungono un ulteriore tassello a questo quadro puntando l’obiettivo su uno dei parametri più significativi a livello globale, vale a dire la concentrazione di CO2 (anidride carbonica) in atmosfera.

Un anno di dati preoccupanti

L’ultimo biennio sembra ormai destinato a passare agli archivi per una lunga sequenza di dati preoccupanti su clima ed ambiente. Che la situazione avesse raggiunto un elevato grado di criticità si era intuito giù durante i lavori della COP21 alla fine del 2015. Ma da li in avanti le cifre allarmanti si sono costantemente ripresentate. I dati GISS/NASA, ad esempio, hanno più volte evidenziato come nel corso del 2016 si siano registrati ripetuti picchi storici nelle temperature medie. Nella scorsa estate un rapporto di Greenpeace ha anche evidenziato come gli effetti del riscaldamento globale siano stati particolarmente severi sull’Artico. Il recentissimo accordo di Kigali ha ribadito i rischi dell’effetto serra ed ha messo in campo nuove azioni di contenimento.

In questo scenario i dati pubblicati nel WMO Greenhouse Gas Bulletin aggiungono una ulteriore chiave di lettura ai fenomeni in corso. L’Organizzazione Meteorologica Mondiale ha infatti stabilito che nel 2015 la concentrazione di CO2 in atmosfera ha raggiunto nuovi livelli record superando la soglia delle 400 parti per milione (ppm). In passato sforamenti di questo valore simbolico si erano avuti in alcuni periodi dell’anno o in alcune zone limitate. Ora però la barriera delle 400 ppm è stata stabilmente superata a livello globale ed i dati acquisiti per il 2016 suggeriscono che anche nell’anno in corso non si registrerà alcuna inversione di tendenza.

El Niño e la CO2 in atmosfera

Tra le case che negli ultimi due anni hanno portato ad un incremento della CO2 in atmosfera, il WMO individua anche El Niño. Questo fenomeno climatico si manifesta con particolare intensità da ormai molti mesi ed ha come conseguenza diretta estesi fenomeni di siccità nella zone tropicali. In una sorta di effetto domino la riduzione delle foreste nella fascia tra i due tropici ha ridotto le capacità di riassorbimento della CO2 aumentando la parte che resta in atmosfera. La stessa siccità innescata da El Niño ha anche aumentato diffusione e dimensione degli incendi a loro volta fonte di CO2.

Se El Niño è tra i responsabili dei picchi di concentrazione di anidride carbonica, è comunque evidente che la tendenza di fondo resta quella di un costante incremento. Negli anni ’80 ad esempio la concentrazione di CO2 in atmosfera era ancora al di sotto delle 350 ppm.

Lo stato della CO2

Secondo i dati del bollettino WMO l’anidride carbonica (CO2) rappresenta il 65 del forzante radioattivo dei gas serra di lungo periodo. Si tratta quindi della componente più rilevante nel meccanismo dell’effetto serra che intrappola nell’atmosfera una maggiore quantità di energia (e quindi di calore). In questo senso la dinamica della concentrazione di CO2 è particolarmente significativa e direttamente collegata con l’attività dell’uomo. Nell’era pre-industriale si registravano infatti 278 ppm in una condizione di sostanziale equilibrio degli scambi tra atmosfera, oceani e biosfera.

Nel 2015 la concentrazione media annua ha come detto raggiunto le 400 ppm con un incremento del 144% rispetto ai livelli pre-industriali. Questa condizione evidenzia un sistema non più in equilibrio in cui le nuove emissioni di CO2 non riescono più ad essere bilanciate.

Metano e protossido di azoto

Il metano è il secondo gas serra in atmosfera responsabile del 17% del forzante radioattivo. Mentre il 40% del metano in atmosfera è di origine naturale, il restante 60% dipende dall’azione umana ed in particolare dall’allevamento di bestiame, dall’agricoltura e dall’uso di combustibili fossili. Nel 2015 la concentrazione di metano in atmosfera ha raggiunto le 1845 parti per miliardo (ppb) con un incremento del 256% rispetto all’era pre-industriale.

Una dinamica simile è seguita anche dal protossido di azoto (N2O), gas che ha un ruolo importante nel fenomeno della riduzione dello strato di ozono. La sua presenza in atmosfera è dovuta al 40% all’azione antropica. La concentrazione di protossido di azoto ha raggiunto le 328 ppb con un incremento del 121% rispetto all’era pre-industriale.

Tra gli altri gas serra si registra la progressiva riduzione dei clorofluorocarburi. Queste sostanze sono state messe progressivamente al bando con il Protocollo di Montréal in quanto considerate responsabili del così detto buco dell’ozono. Al contrario è in crescita la concentrazione di idrofluorocarburi, potenti gas serra al centro del recente accordo di Kigali.

Dati per la COP22

Nel presentare il nuovo bollettino sui gas serra, il WMO sottolinea l’importanza dell’accordo di Kigali che in estensione del Protocollo di Montréal prevede la progressiva eliminazione degli idrofluorocarburi con l’obiettivo di ridurre l’effetto serra. Il WMO però evidenzia anche come il problema fondamentale resti legato alla CO2 che come detto è il gas maggiore responsabile dell’effetto serra.

Le cifre fornite dal WMO saranno al centro dei dibattiti e delle trattative anche alla prossima COP22 in programma a Marrakech dal 7 al 18 novembre 2016.

Photo | Thinkstock

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