The Earth had existed in a certain state for tens of thousands of years. During that time, plant and animal life evolved, the oceans established certain levels of depth and salinity, and weather patterns were largely stable. While minute changes occurred over time, the planet remained largely stable. The last 150 years, however, have witnessed the constant expansion of manufacturing processes and mechanized transportation that have changed the environment. The release into the Earth's atmosphere of chemicals, for example, the chlorofluocarbons that were linked to depletion of the ozone layer of the stratosphere, have affected the environment in unanticipated (to most) ways.
The effects of climate change have been substantial and profound. The state of the Earth as it existed for many years has changed -- and many scientists attribute these changes to natural phenomena as well as to human interaction with the environment -- with resulting transformations in age-old weather patterns. This has a direct effect on the agricultural industry, upon which the entire world depends for sustenance. Regions of the world that were previously immune to severe weather patterns, or only experienced them very rarely, now experience hurricane-force winds and rainfall levels to which they are not accustomed. The increased prevalence of tornadoes is causing widespread destruction. And crop yields are being adversely affected by greater extremes in levels of rainfall interspersed with protracted periods of drought.
Agriculture has developed over thousands of years consistent with predictable weather patterns. As those patterns change, the ability of the earth to support such activities also changes. If farmers can no longer count on predictable weather patterns, they can no longer raise crops with the level of assurance needed to make the endeavor economically feasible.
Increased or changed rainfall and drought patterns also affect human migration. As regions long inhabited by subsistence communities are increasingly affected by climate change, they have no choice but to move, en masse, to a different region. These migrations can affect millions of people over time, creating humanitarian crises.