Under the existing, fossil-fuel-intensive and high-population-growth trajectory, we will come to experience within one life time extreme and prolonged heat waves that, coupled with existing humidity levels, will cause incidences of human mortality in southern Asia and North America. We are likely to see wildfires and water restriction in Europe; crop failures in South America; vector and water-borne diseases in Africa and Central America. The planet will have harsher and more frequent tropical cyclones, landslides, droughts, storm surges and flooding. Due to sea level rise we will see coastal erosions as well as and salt water contaminations. Ocean water will become more acidic and its temperature rise.
These phenomena and patterns will continue in subsequent centuries—but in more acute forms.
Under the existing, fossil-fuel-intensive and high-population-growth trajectory, in a few human lifetimes global temperatures may correspond to those earth had last around 40 million years ago. However, it is one thing to have a climate shift over millions of years or even over many thousands of years, allowing most species to migrate, evolve, or work their way to new, suitable habitats. It is entirely different to turn the terrestrial-temperatures dial 6°C (≈11°F) to 10°C (18°F) over a period of a few life times—along with extreme drought and precipitation events—for a planetary ecosystem that is largely bankrupt with only isolated, hemmed-in pockets of intact nature.