Severe environmental threats such as rise in the sea level and sea surface temperature, and land erosion could render about 30,000 people in the Sunderbans homeless in the islands of the world's largest delta by 2020. Disclosing this, the director of the School of Oceanography, Saugata Hazra, said: ''About four lakh people are already affected in the past few years and more than 600 families have migrated to other islands due to erosion and coastal flooding, while crops and property worth Rs. 950 million have been damaged.''