Desertificazione
La desertificazione è un processo di degradazione del suolo nelle zone aride, semiaride e subumide, che può essere causato da diversi fattori, tra cui le attività umane e i cambiamenti climatici. Il terreno sfruttato intensamente per attività agricole, pastorizia o per necessità industriali, perde progressivamente la capacità di sostenere la vita animale e vegetale, non è dunque più in grado di alimentare gli ecosistemi. La desertificazione non riguarda solo la perdita della capacità di risorse di un terreno, ma anche la perdita di offrire servizi per l’uomo.
L’effetto della desertificazione, uno dei primi fenomeni di dissesto ambientale, è stato analizzato per la prima volta a Nairobi nel 1977, nella Conferenza delle Nazioni Unite sulla Desertificazione (UNCOD, United Nations Conference on Desertification). Si decise di intervenire con l’adozione del Piano d’Azione per Combattere la Desertificazione (PACD, Plan of Action to Combat Desertification). Dal 1984 si è attivato anche il Programma Ambientale delle Nazioni Unite (UNEP) affinché la desertificazione venisse prevenuta o almeno controllata. Da quella data ad oggi, tuttavia oltre 3590 milioni di ettari di territorio sono stati colpiti dalla desertificazione che ogni anno causa la perdita di oltre 24 miliardi di tonnellate di terra coltivabile.
Tra i principali fattori che determinano la desertificazione, si riconosce lo sfruttamento dei terreni da pascolo: quando il bestiame supera il livello di tollerabilità del terreno, il suolo comincia a degradare perché ben presto alle specie vegetali si sostituiscono piante annuali e arbusti, a cui si sostituiscono poi specie erbacee. Il continuo calpestio delle mandrie e delle greggi, l’impoverimento della vegetazione, lascia il terreno scoperto all’azione di acqua e vento e dunque all’erosione. Allo stesso modo l’attività agricola intensiva, il disboscamento, e la cattiva gestione dei sistemi di irrigazione, impoveriscono il terreno e lo rendono soggetto a desertificazione.
L’attività umana è responsabile della desertificazione delle Grandi Pianure degli Stati Uniti. Agli inizi degli anni ’30 vaste aree fertili degli USA vennero arati in profondità per essere coltivati con cereali, nel 1931 la siccità e l’erosione del vento crearono tempeste di sabbia di dimensioni mai viste. Una catasfrofe ecologica simile si verificò in Russia negli anni ’50, quando si avviarono all’agricoltura intere zone fertili dell’Unione Sovietica. In Africa una delle maggiori cause della desertificazione viene dall’uomo, dall’abbattimento intensivo di foreste per ottenere spazi da destinare all’agricoltura e al pascolo, ma anche per reperire legna da ardere. Nel Burkina Faso, nel Senegal, nel Sudan il tasso di desertificazione è tra i più alti del Pianeta. La cattiva gestione delle risorse idriche è responsabile invece della salinizzazione dei terreni, e dunque della loro desertificazione, fenomeno che ha colpito maggiormente Stati Uniti e Australia, ma anche Egitto, Pakistan, Siria e Iraq. Attualmente il 65% della superficie europea delle aree agricole aride, semi-aride, secco-subumide è stato colpito da desertificazione, mentre il 10% e il 31% dei terreni italiani sono soggetti rispettivamente a forte e medio rischio di erosione.
Per approfondire:
[Fonti: Wikipedia; Istituto Ponte Valtellina; Arpa]
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