La Tierra

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:

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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: USGRCP (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 reflectivity 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. [1]

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 1700s can be explained by natural causes, such as changes in solar energy, volcanic eruptions, and natural changes in greenhouse gas (GHG) concentrations.

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.

physical mechanism it is important to examine in detail the basis for the suggested link between solar activity variations and climate. In this search it must be realized that it is not possible to define climate in a simple and unique way. Even a simple number like the yearly global average temperature is subject to variations without any immediate cause in terms of a change in the radiative forcing because the existence of internal oscillations in the atmosphere and the complex coupling to the oceans. Similarly, solar activity can not be characterized by a simple number, at least not until we have identified the exact physical mechanism that is involved. Different solar activity parameters show different solar cycle variations and different long term trends. Therefore it is not surprising that studies dealing with different climate and solar activity parameters do not always agree. These studies may not all be particularly credible in their methodology and use of data. Beyond all these caveats, however,

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