Earth’s temperature is a balancing act

Earth’s temperature depends on the balance between energy entering and leaving the planet’s system . When incoming energy from the sun is absorbed by the Earth system, Earth warms. When the sun’s energy is reflected back into space, Earth avoids warming. When energy is released back into space, Earth cools. Many factors, both natural and human, can cause changes in Earth’s energy balance, including:

Models that account only for the effects of natural processes are not able to explain the warming over the past century. Models that also account for the greenhouse gases emitted by humans are able to explain this warming.
Source:  (2009)

• Changes in the greenhouse effect, which affects the amount of heat retained by Earth’s atmosphere
• Variations in the sun’s energy reaching Earth
• Changes in the of Earth’s atmosphere and surface

These factors have caused Earth’s climate to change many times.

Scientists have pieced together a picture of Earth’s climate, dating back hundreds of thousands of years, by analyzing a number of indirect measures of climate such as ice cores, tree rings, glacier lengths, pollen remains, and ocean sediments, and by studying changes in Earth’s orbit around the sun.

The historical record shows that the climate system varies naturally over a wide range of time scales. In general, climate changes prior to the Industrial Revolution in the can be explained by natural causes, such as changes in solar energy, volcanic eruptions, and natural changes in greenhouse.

Recent climate changes, however, cannot be explained by natural causes alone. Research indicates that natural causes are very unlikely to explain most observed warming, especially warming since the mid-20th century. Rather, human activities can very likely explain most of that warming.

The Greenhouse Effect causes the atmosphere to retain heat

When sunlight reaches Earth’s surface, it can either be reflected back into space or absorbed by Earth. Once absorbed, the planet releases some of the energy back into the atmosphere as heat (also called infrared radiation). Greenhouse gases ) like water vapor carbon dioxide), and methane () absorb energy, slowing or preventing the loss of heat to space. In this way, act like a blanket, making Earth warmer than it would otherwise be. This process is commonly known as the “greenhouse effect”.

The Role of the Greenhouse Effect in the Past

In the distant past (prior to about 10,000 years ago),levels tended to track the glacial cycles. During warm periods,  levels have been higher. During cool ‘glacial’ periods, levels have been lower.  This is because the heating or cooling of Earth’s surface can cause changes in greenhouse gas concentrations. ^[1] These changes often act as a positive feedback, amplifying existing temperature changes.

The Recent Role of the Greenhouse Effect

Since the Industrial Revolution began around 1750, human activities have contributed substantially to climate change by adding and other heat-trapping gases to the atmosphere. These greenhouse gas emissions have increased the greenhouse effect and caused Earth’s surface temperature to rise. The primary human activity affecting the amount and rate of climate change is greenhouse gas emissions from the burning of fossil fuels.

The Main Greenhouse Gases

The most important directly emitted by humans include  oxide ), and several others. The sources and recent trends of these gases are detailed below.

Carbon dioxide 
Carbon dioxide is the primary greenhouse gas that is contributing to recent climate change.  is absorbed and emitted naturally as part of the carbon cycle, through animal and plant respiration, volcanic eruptions, and ocean-atmosphere exchange. Human activities, such as the burning of fossil fuels and changes in land use, release large amounts of carbon to the atmosphere, causing  concentrations in the atmosphere to rise.