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Climate is weather, averaged over time—usually a minimum of 30 years. Regional climate means the average weather trends in an area. For instance, summer along Colorado’s Front Range tends to mean warm days, a high likelihood of late-afternoon thunderstorms, and cool nights. Summer in southwestern India is the monsoon season, and massive thunderstorms tend to dominate. Global climate, an average of regional climate trends, describes the Earth’s climate as a whole.
Our climate has been constantly changing since Earth began, with periods of global warming and global cooling long before human beings and their activities began.
Modern weather measurements go back only 100 to 150 years, but we can reconstruct what Earth's climate was like in prehistoric times using clues from the Earth itself. Tree rings can show scientists what climate factors shaped each year of a tree's life, for example, and bubbles trapped in ice fields hold air from many thousands of years ago. Other clues come from the shells and other material deposited in layers of sediment on the bottom of lakes and oceans by creatures that thrived in different climate ranges.
We also know that the orbits of the Sun and Earth undergo a variety of cyclic shifts over thousands and millions of years. Tiny changes in the tilt of Earth and the asymmetry of its path around the Sun can make a big difference to regional climate—sometimes enough to trigger an ice age.
Natural changes in the oceans, which can unfold over several years, also impact Earth's climate. One of the best known of these phenomena is El Niño, a warming of the surface waters in the eastern tropical Pacific. The weather impacts related to El Niño and its counterpart, La Niña (a cooling of the eastern tropical Pacific), extend throughout the Pacific Rim to eastern Africa and beyond.
El Niño is normally accompanied by a change in atmospheric circulation called the Southern Oscillation. The ENSO phenomenon (standing for El Niño–Southern Oscillation) is one of the main sources of interannual, or year-to-year, variation in weather and climate around the world.
For more than 100 years, Earth's surface temperature has been monitored by a global network of land-based weather stations. These reports are supplemented by sea-surface and air temperature readings taken at points across the oceans, which cover 70 percent of the globe.
Together, these data show that Earth's surface air temperature has risen about 1.5°F (0.85°C) between 1880 and 2012, according to the 2013 report of the Intergovernmental Panel on Climate Change. This warming of the average temperature around the globe has been especially sharp since the 1970s.
Since the start of the industrial revolution in the late 1700s, humans have increasingly burned fossil fuels to create heat and electricity and to power vehicles and factories. Fossil fuels, such as coal, oil, and natural gas, emit greenhouse gases, especially carbon dioxide, as they combust. Greenhouse gases, which also include methane and water vapor, warm Earth's atmosphere by absorbing and trapping outgoing radiation from the Earth and reradiating some of this energy back to the surface.
The amount, or concentration, of CO2 gas in the atmosphere has risen more than 40 percent since the industrial revolution. CO2 concentration in the atmosphere is now at its highest point in the last 800,000 years. Each year, the CO2 concentration increases by about 0.5 percent.
Despite the normal variability of Earth's climate, which includes periods of natural warming and cooling, scientists now expect that the greenhouse gases added to the atmosphere from human activity will cause the overall temperature trend to continue increasing.
Some signs of climate change can already be seen. Glaciers are retreating, especially atop mountains at lower latitudes, close to the equator. In the Arctic the thickness and extent of summer sea ice have decreased dramatically over the last 50 years, and computer modeling by NCAR scientists shows that the Arctic’s summer ice may virtually disappear by the middle of this century. Meanwhile, snowfall over much of Antarctica is increasing, a paradoxical sign of warming temperatures in this frozen, arid land.
The timing of the annual cycles of plant growth and animal migration has also shifted in response to lengthening of the warm season over much of the Northern Hemisphere.
And while the vast majority of climate scientists agree with the consensus of the Intergovernmental Panel on Climate Change that Earth will continue to warm along with increasing greenhouse gases, the effects will be far more varied than a simple and uniform warming over the entire planet, because heating also alters the water cycle, among other changes. As a result, some regions will become considerably hotter or cooler, or wetter or drier, than others.
Scientists have understood the physics of the greenhouse effect—that some gases in the atmosphere trap heat—for more than a century. Because carbon dioxide is one such gas, and because it is produced when fuels are burned, scientists in the late 1800s were able to surmise that humans could one day burn enough fuels to cause the atmosphere to warm.
It is this basic understanding of climate physics that forms the core of today's climate models. These computer models use mathematical equations to describe the greenhouse effect, as well as how different parts of the Earth system interact. For example, some equations may describe how the surface temperatures of the oceans are linked to atmospheric circulation patterns.
Scientists are continually working to improve their models by refining the existing equations and adding new ones. To see how well their models are working, researchers run them over a historical time period and compare the model's results to observations of what really happened.
Scientists at NCAR conduct broad-ranging research on all aspects of climate using various tools and techniques, including climate models, radar and weather-balloon observations and satellite data.
NCAR is also home to the Community Earth System Model, which is freely available for the wider climate research community to use. Since the first version of the model was released in 1983, the model has become increasingly more sophisticated as the supercomputers needed to run them have become more powerful.
Scientists from virtually every corner of NCAR do research that at least touches on the climate. (For the most recent examples, check out our "Climate News" section on the right.) But for several labs, climate change is a primary focus. Those include: