I need to write the opening of a story which has the topic Life in the futuristic world?

600-900 words really isn't that many and the question already tells you what to concentrate those words on. Just talk about how the Earth has been ruined by global warming (teachers eat that stuff up!), the Earth will be hotter, the equatorial region will be inhospitable and everyone will be moving to higher latitudes where rain still falls and plants will grow. Half of the world's population has been wiped out by disease and hunger and civilized societies as we know them have collapsed, leading to war over usable farming land, water and living space.
Use a lot of descriptive words and the 600-900 words will fly past.