Climate Change Indicators in the United States
Weather is the state of the atmosphere at any given time and place. Most of the weather that affects people, agriculture, and ecosystems takes place in the lower layer of the atmosphere, the troposphere. Familiar aspects of weather include temperature, precipitation, clouds, and wind. Severe weather conditions include hurricanes, tornadoes, blizzards, and droughts.
Climate is the long-term average of the weather in a given place. While the weather can change in minutes or hours, a change in climate is something that develops over longer periods of decades to centuries. Climate is defined not only by average temperature and precipitation, but also by the type, frequency, duration, and intensity of weather events such as heat waves, cold spells, storms, floods, and droughts. Weather can vary widely, and extreme events occur naturally, but average conditions tend to remain stable unless the Earth experiences a force that can shift the climate. At various times in the Earth’s history, the climate has changed in response to forces such as large volcanic eruptions, changes in greenhouse gas concentrations, and shifts in the Earth’s orbit around the sun.
What is happening?
The average temperature at the surface of the Earth has been increasing over the past century, primarily because human activities are adding large quantities of heat-trapping greenhouse gases to the atmosphere. Unusually warm days and nights have also become more common in some places. Generally, warmer surface temperatures lead to an increase in evaporation from the oceans and land, leading to an increase in globally averaged precipitation. However, while some regions can get more precipitation, shifting storm patterns and increased evaporation can cause some areas to experience more severe droughts than they have in the past. Scientific studies also indicate that extreme weather events such as storms, floods, and hurricanes are likely to become more intense. However, because these extremes already vary naturally, it may be difficult over short time periods to distinguish whether changes in their intensity and frequency can be attributed to larger climate trends caused by human influences.
Why does it matter?
Climate variations can directly or indirectly affect many aspects of society—in both positive and disruptive ways. For example, warmer average temperatures reduce heating costs and improve conditions for growing some crops; yet extreme heat can increase illnesses and deaths among vulnerable populations and damage some crops. Precipitation can replenish water supplies and support agriculture, but intense storms can damage property, cause loss of life and population displacement, and temporarily disrupt essential services such as transportation, telecommunications, energy, and water supplies.