Earth's climate varies naturally on a wide range of timescales, from seasonal variations (such as a particularly wet spring, hot summer, or snowy winter) to geological timescales of millions or even billions of years. Careful statistical analyses have demonstrated that it is very unlikely6 that natural variations in the climate system could have given rise to the observed global warming, especially over the last several decades. However, natural processes produce substantial seasonal, year-to-year, and even decade-to-decade variations that are superimposed on the long-term warming trend, as well as substantial regional differences. Improving understanding of natural variability patterns, and determining how they might change with increasing GHG emissions and global temperatures, is an important area of active research (see the end of this section and Chapter 6).
Natural climate variations can also be influenced by volcanic eruptions, changes in the output from the Sun, and changes in Earth's orbit around the Sun. Large volcanic eruptions, such as the eruption of Mount Pinatubo in 1991, can spew copious amounts of aerosols into the upper atmosphere. If the eruption is large enough, these aerosols can reflect enough sunlight back to space to cool the surface of the planet by a few tenths of a degree for several years.
The Sun's output has been measured precisely by satellites since 1979, and these measurements do not show any overall trend in solar output over this period. Prior to the satellite era, solar output was estimated by several methods, including methods based on long-term records of the number of sunspots observed each year, which is an indirect indicator of solar activity. These indirect methods suggest that there was a slight increase in solar energy received by the Earth during the first few decades of the 20th century, which may have contributed to the global temperature increase during that period (see Figure 2.2).
Perhaps the most dramatic example of natural climate variability is the ice age cycle. Detailed analyses of ocean sediments, ice cores, geologic landforms, and other data show that for at least the past 800,000 years, and probably the past several million years, the Earth has gone through long periods when temperatures were much colder than today and thick blankets of ice covered much of the Northern Hemisphere (including the areas currently occupied by the cities of Chicago, New York, and Seattle). These very long cold spells were punctuated by shorter, warm "interglacial" periods, including the last 10,000 years. Through a convergence of theory, observations, and
6 As discussed in Appendix D, very unlikely indicates a less than 1 in 10 chance of a statement being incorrect.
modeling, scientists have deduced that the ice ages were initiated by small recurring variations in the Earth's orbit around the Sun.
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