While many scoff at the idea that global warming is even occurring, federal agencies have been collecting data on the recent rise of global average temperatures. According to the National Oceanic and Atmospheric Administration, average surface temperatures on Earth have risen about 0.74 degree Celsius (1.3 degrees Fahrenheit) since the late 19th century. For the past 50 years, average temperatures have risen of 0.13 degree Celsius (0.23 degree Fahrenheit) per decade -- almost twice that of the previous century.

A planet's temperature depends on the stability between energy entering and leaving the planet and its atmosphere. When energy from the sun is taken in, Earth heats up. When the sun’s energy is sent back into space, the Earth does not receive heat from that energy. Scientists have identified three primary factors that could drive the planet into a state of global warming: the greenhouse effect, radiation from the sun reaching the Earth and the reflectivity of the atmosphere.

Gases like water vapor, carbon dioxide and methane draw in energy from direct sunlight as it passes through the atmosphere. They also slow or stop Earth's radiation of warmth into space. In this way, greenhouse gases behave like a layer of insulation, making the planet warmer than it would be -- an phenomenon typically referred to as the “greenhouse effect." Since the Industrial Revolution in the middle of the 18th century, human activities have added significantly to climate change by releasing carbon dioxide and other greenhouse gases into the environment. These gases have ramped up the greenhouse effect and caused surface temperature to increase, according to the Environmental Protection Agency. The main human activity influencing the amount and pace of climate change is greenhouse gas emissions from the incineration of fossil fuels.

Global warming can also be a result of shifts in how much solar energy reaches Earth. These shifts include transformations in solar activity and alterations in Earth’s orbit around the sun. Changes occurring in the sun itself can affect the intensity of the sunlight that reaches Earth’s surface. The intensity of the sunlight can result in either warming, during intervals of more robust solar intensity, or cooling during periods of weakened solar intensity. The well-documented period of chillier temperatures between the 17th and 19th centuries, dubbed the Little Ice Age, may have been spurred by a low solar phase from 1645 to 1715. Also, shifts in the Earth's orbit around the sun have been linked to past cycles of ice ages and glacial growth.

When sunlight gets to Earth, it is either reflected or absorbed depending on factors in the atmosphere and on Earth’s surface. Light-colored features and areas, like snowfall and clouds, tend to reflect most the sun's rays, while darker objects and surfaces, like the ocean or dirt, tend to take in more sunlight. Reflectivity of the Earth is also affected by small particles or fluid droplets from the atmosphere called aerosols. Light-colored aerosols that reflect sunlight, like debris from volcanic eruptions or sulfur emissions from the incineration coal, have a cooling effect. Those that soak up sunlight, such as soot, have a warming effect. Volcanoes have also affected reflectivity by releasing particles into upper atmosphere that typically reflect sunlight back to space. Deforestation, reforestation, desertification and urbanization also contribute to the Earth's reflectivity.