English homework

The subject which I chose is the following one:
Is global warming a threat? What is the future of our planet? And even of mankind?
I would explain it, in an only one paragraph:
An increasing number of observations indicate that the world has warmed:
• The average surface temperature has increased over the 20th century by about 0.6°C. This increase occurred mainly from 1910 to 1945 and 1976 to 2000.
• Temperatures have risen during the past four decades in the lowest 8 kilometers of the atmosphere.
• Snow and ice extent have decreased.
• The sea level has risen by 10 to 20 cm during the 20th century.
• Precipitations is likely to have increased in the 20th century in some land areas and decreased in others
• There were relatively small global increases in severe drought or severe wetness over the century but an increase in droughts in some regions in recent decades.
Climate has and will always vary for natural reasons. However, human activities are increasing significantly the concentrations of some gases in the atmosphere, such as greenhouse gases (mainly CO2), which tend to warm the earth surface, and anthropogenic aerosols, which mostly tend to cool it.
Projected changes in climate are expected to have both beneficial and adverse effects on water resources, agriculture, natural ecosystems and human health. But the larger the changes in climate the more the adverse effects should dominated. For instance, some crop and forest productivities could benefit from a small climate change. But for many other natural systems, the adverse effects should be dominant, especially if warming exceeds a few degrees. Human populations are expected to face increasing flooding and heat waves but reduced cold spells. The geographic range for infectious diseases should increase.
They project for the next century that, without specific changes:
• Global mean temperature should increase by between 1.4 and 5.8°C.
• The Northern Hemisphere cover