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Climate Change 2013: The Physical Science Basis.

Changes in the sun’s energy affect how much energy reaches Earth’s system

Graph comparing solar irradiance and difference in global surface temperature. Solar irradiance has been regularly cycling, while global surface temperatures have been steadily increasing.

The sun’s energy received at the top of Earth’s atmosphere has been measured by satellites since 1978. It has followed its natural 11-year cycle of small ups and downs, but with no net increase (bottom). Over the same period, global temperature has risen markedly (top).

Click the image to view a larger version.
Source: USGCRP (2009).Climate is influenced by natural changes that affect how much solar energy reaches Earth. These changes include changes within the sun and changes in Earth’s orbit.

Changes occurring in the sun itself can affect the intensity of the sunlight that reaches Earth’s surface. The intensity of the sunlight can cause either warming (during periods of stronger solar intensity) or cooling (during periods of weaker solar intensity). The sun follows a natural 11-year cycle of small ups and downs in intensity, but the effect on Earth’s climate is small.[1]

Changes in the shape of Earth’s orbit as well as the tilt and position of Earth’s axis can also affect the amount of sunlight reaching Earth’s surface.[1][2]

The role of the sun’s energy in the past

Changes in the sun’s intensity have influenced Earth’s climate in the past. For example, the so-called “Little Ice Age” between the 17th and 19th centuries may have been partially caused by a low solar activity phase from 1645 to 1715, which coincided with cooler temperatures. The “Little Ice Age” refers to a slight cooling of North America, Europe, and probably other areas around the globe.[2]

Changes in Earth’s orbit have had a big impact on climate over tens to hundreds of thousands of years. In fact, the amount of summer sunshine on the Northern Hemisphere, which is affected by changes in the planet’s orbit, appears to drive the advance and retreat of ice sheets. These changes appear to be the primary cause of past cycles of ice ages, in which Earth has experienced long periods of cold temperatures (ice ages), as well as shorter interglacial periods (periods between ice ages) of relatively warmer temperatures.[1][2] 

Rates of Climate Change Have Varied Over Time

Image of a glacier calving. Click to learn about how rates of climate change have varied over time.

Click to learn about how rates of climate change have varied over time.

The recent role of the sun’s energy

Changes in solar energy continue to affect climate. However, over the last 11-year solar cycle, solar output has been lower than it has been since the mid-20th century, and therefore does not explain the recent warming of the earth.[2] Similarly, changes in the shape of Earth’s orbit as well as the tilt and position of Earth’s axis affect temperature on very long timescales (tens to hundreds of thousands of years), and therefore cannot explain the recent warming.