Understanding the natural variability of Earth’s climate is complicated enough, but more than ever, human activities are introducing powerful stresses on Earth’s delicately balanced climate system. Changes in Earth’s climate— whether abrupt or gradual, global or regional—have been going on throughout the planet’s long history and will continue. These changes are governed by complex interactions involving the atmosphere, the oceans, planetary volcanism, the cryosphere (ice), the biosphere (living things) and external forces such as variability of solar radiation and even the occasional asteroid impact.
Change in precipitation minus surface evaporation for the 2021-2040 period minus the average over 1950-2000 show the imminent transition to a more arid climate in southwestern North America
Over recent decades, it is becoming increasingly apparent that our climate is rapidly changing. The air is warming, more so at high latitudes; ice and permafrost are melting; sea level is rising as warmer sea water expands and ice and glacial melt water reach the ocean; changing patterns of precipitation and evaporation are stressing agricultural and water resources; and droughts and floods seem to be more intense. Increasingly, these recent climate changes are attributed to human activities.
If we can understand our climate system, we can prepare reliable projections of future global and regional climate trends. This will enable informed management of a sustainable society and help safeguard humankind’s future as well as the planet’s well-being.