Cây xanh luôn nhả khí oxy sau khi hoàn tất quá trình quang hợp dưới sự trợ giúp của ánh sáng măt trời chất diệp lục, con người và các loài động vật cấp cao khác luôn cần khí oxy để duy trì sự sống. Theo tính toán của các nhà khoa học Nhật Bản, 1 hecta rừng hoăc vườn cây rậm rạm có diện tích tương đương hấp thụ 1000Kg CO2 mỗi ngày. Trang 23 a) Theo thống kê ước tính năm 2020 Việt Nam có 14,6 triệu ha rừng. Tính lượng oxy của rừng cây thải ra biết cứ 44 kg phân tử CO2 thì sẽ tạo ra 32 kg phân tử O2 trong năm 2020 là bao nhiêu tấn? b) Trung bình một người sẽ cần 9,5 tấn không khí để thở trong vòng 1 năm, với khí oxy chiếm 23% số lượng này. Tính đến ngày 31 tháng 12 năm 2020, dân số Việt Nam ước tính là 97.757.118 người (Nguồn: https://danso.org/viet-nam/). Một cây cao 30m; trưởng thành có thể hấp thụ khoảng 22,7 Kg khí CO2 trong một năm. Với dân số như trên thì cần bao nhiêu cây xanh?

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