IntroductionOn Earth, a complex web of variables interacts to produce the planet's climates. Factors such as latitude, proximity to oceans and water, and topography all play a role in determining the climate of a specific region. On a global scale, climate is influenced by more dynamic processes, such as ocean circulation, atmospheric greenhouse gases, cloud cover, and rainfall distribution. Even natural hazards, such as the 1991 volcanic eruption of Mount Pinatubo in the Philippines, can alter Earth's climate for several years.
After months of predictive earthquake activity and lava dome growth, Mount Pinatubo in the Philippines erupted in June, 1991. (Image credit: U.S. Geological Survey)
Climate Change on EarthWith the exception of natural hazard events, climate patterns remain relatively stable over the course of a human lifetime. But on a geologic time scale, Earth's climate undergoes significant fluctuations. To understand the concept of climate change, it is important to distinguish between weather and climate. Weather refers to the day-to-day changes in atmospheric conditions in a specific location; whereas climate refers to the average, prevailing weather conditions at a given location over a longer period of time - such as a season or year - based on long-term records and data. For example, the weather on an August afternoon in the desert region of southern Arizona may be dry and sunny, but the long-term climate pattern of that region shows that summer monsoons typically occur at that time.
Researchers remove an ice core from the drill core barrel near the South Pole Remote Earth Science Observatory in Antarctica. (Image credit: U.S. Geological Survey)
Analysis of sediments, rock strata, and air bubbles trapped in glacial ice indicates that temperatures on Earth have fluctuated throughout geologic time. Climatologists are now discovering that climate change can happen more rapidly than was previously believed. Recent studies of ice core samples, which provide a record of past climate, show that Earth has experienced dramatic shifts in average global temperature in a short time span - changing by as much as 8 degrees C (15 degrees F) in a 10- to 30-year period.
This chart illustrates the cycle through which glacial water and ice eventually ends up in the oceans. Glacier and ice cap melt is responsible for about a third of sea level rise. (Image credit: Goddard Space Flight Center/NASA)
During the past 800,000 years, eight ice ages have occurred, during which glacial periods alternated with interglacial periods. Each glacial period lasts about 100,000 years and is followed by an interglacial period that lasts an average of 6,000 years, but sometimes as long as 28,000 years. We are currently living in an interglacial period, which began about 12,000 years ago and represents the second longest interglacial period.
The year 2005 was the most intense Atlantic hurricane season on record. These before-and-after images are of the Northern Chandeleur Islands, which lie about 100 km (60 miles) east of New Orleans, Louisiana. The first image was taken in July 2001. The second image shows the same site on August 31, 2005, two days after Hurricane Katrina made landfall on the Louisiana and Mississippi coastline. Storm surge and large waves from Katrina submerged the islands, stripped sand from the beaches, and eroded large sections of the marsh. Today, few recognizable landforms are left on the islands. (Image credit: U.S. Geological Survey)
The Mojave Desert in the western United States is a prime example of a microclimate. Because of its location in the "rain shadow" of the Sierra Nevada Mountains, it experiences less precipitation and more arid conditions than surrounding regions. (Image credit: U.S. Geological Survey)
Topographic features on the Earth's surface can create localized climate conditions, called microclimates, that differ from the general climate pattern in that region. For example, the Gobi Desert in Asia lies in the "rain shadow" of the Himalaya Mountains. Rain shadows occur on the leeward side of high mountains and experience lower precipitation rates and arid conditions.
Cities located near lakes or oceans enjoy cooler summers and warmer winters, since large bodies of water increase humidity and stabilize temperatures. Because of its close proximity to both San Francisco Bay and the Pacific Ocean, the city of San Francisco, California, often has several different microclimates going on simultaneously. Depending on where you are in relation to the bay, the ocean, and the coastal mountains, it can be warm and sunny, or cold, wet, and foggy.
Even cities and urban areas can spawn microclimates. Large amounts of asphalt, concrete, and brick absorb and hold heat, creating an effect known as an "urban heat island." In Phoenix, Arizona, average temperatures have risen more than 4 degrees C (7 degrees F) over the past 50 years, and experts attribute this to the city's tremendous growth spurt and accompanying increase in streets and building structures.