The Earth's path of orbit around the Sun varies from a highly elliptical pattern to an almost circular one. This change in orbit occurs roughly every 100,000 years and affects the amount of solar energy that reaches the top of the Earth's atmosphere. The Earth's orbit also affects a change in the timing of the equinoxes and solstices, or changes of seasons. Currently, the Earth is closer to the Sun in January than it is in July, but when this precession of equinoxes takes place in 13,000 years, it will be closer in July. When this happens, the Northern Hemisphere will experience less variation in average temperatures between seasons.
Changes inside the Sun can affect temperatures on Earth. The Sun is an active star, and like all active stars, its activity varies. Sometimes there is less solar activity, which results in cooler periods on Earth. NASA scientists believe one such change in solar activity is responsible for what is known as "The Little Ice Age," or a period during Medieval times where temperatures in Europe and North America were slightly cooler than they are today.
When volcanoes on Earth's surface erupt, they emit natural aerosols and carbon dioxide, the build-up of which can contribute to climate change. Aerosol emissions temporarily block out the Sun's light and can contribute to brief, short-term cooling before they leave the atmosphere. Severe volcanic eruptions are though to be the cause of temperature variations of up to 5 degrees Fahrenheit. Carbon dioxide has the opposite effect: It builds up in the atmosphere and amplifies the natural greenhouse effect by trapping infrared energy in the atmosphere and directing it back toward the Earth.
In the last century, Earth has experienced a warming period not attributed wholly to natural factors. Human activities beginning in the industrial area have led to warmer average temperatures worldwide. Industrial activity has increased carbon dioxide and aerosol emissions and further amplified the greenhouse effect. Land development by humans, such as replacing forests with farmland or concrete, has also changed the way the Earth absorbs and reflects the Sun's energy and led to changes in temperature, climate and even weather.