Natural Climate Variability

Earth's climate varies naturally on a wide range of time scales. Many of these variations are caused by complex interactions between the fast-moving, less-dense atmosphere and the more massive, slower-to-respond oceans. For example, the El Niño-Southern Oscillation (ENSO), which is caused by ocean-atmosphere interactions in the tropical Pacific Ocean, is a source of significant year-to-year variability around the world. The "warm" or "El Niño" phase is characterized by warmer-than-normal sea surface temperatures in the eastern equatorial Pacific. El Niño years are often associated with significant, predictable regional variations in temperature and rainfall across many remote parts of the world; in the United States, for example, El Niño years typically exhibit wetter-than-normal conditions in Southern California and the southern Great Plains. Global temperatures also tend to be slightly warmer during years with strong El Niño events, such as 1998, and slightly cooler during "cool" or "La Niña" years, such as 2008.

A multitude of other patterns of natural climate variability have also been identified, and many of these are associated with strong regional climate variations. Higher-latitude oscillations, such as the Northern and Southern Annular Modes (Thompson and Wallace, 2000, 2001), the Pacific Decadal Oscillation (Guan and Nigam, 2008; Mantua et al., 1997), the North Atlantic Oscillation, and the Atlantic Multidecadal Oscillation (Guan and Nigam, 2009), have a large influence on regional climate at decadal time scales, with impacts on, for example, salmon fisheries in the Pacific Northwest (Hare et al., 1999; Mantua and Hare, 2002) and the number of hurricanes making landfall in North America (Dailey et al., 2009). The exceptionally cold and snowy winter experienced on the East Coast of the United States during 2009-2010, which was balanced by warmer-than-normal temperatures in much of northeastern Canada and the high Arctic, can be attributed in part to a strong North Atlantic Oscillation event. Natural climate oscillations on multidecadal and longer time scales could also exist (e.g., Enfield et al., 2001; Schlesinger and Ramankutty,1994), though the instrumental record is too short and too sparse to unambiguously attribute their causal mechanisms (e.g., Zhang et al., 2007a).

Climate variations can also be forced by natural processes including volcanic eruptions, changes in the output from the sun, and changes in Earth's orbit around the sun. Large, explosive volcanic eruptions, like Tambora in 1815, Krakatoa in 1883, El Chichon in 1983, and Pinatubo in 1991, spew copious amounts of sulfate aerosols into the stratosphere, cooling the Earth for several years (Briffa et al., 1998). The Pinatubo eruption is particularly noteworthy because it occurred in an era with widespread satellite and ground-based observations that allowed for the resulting aerosol distribution and climate response to be accurately quantified. These data indicate that aerosols induced a peak climate forcing of -2.5 W/m2 several months after the Pinatubo eruption (Harries and Futyan, 2006) and that global surface temperatures dipped approximately 0.9°F (0.5°C) 2 years later, then recovered over the next several years as aerosol levels gradually declined (Trenberth and Dai, 2007). Data from Pinatubo and other volcanic eruptions have been used to estimate the strength of climate feedbacks that operate on relatively short time scales, such as the feedback associated with the correlation between temperature and water vapor in the atmosphere, and for calibrating and validating climate model results (e.g., Soden et al., 2002).

While there has not been a net increase in the Sun's energy output over the past few decades (see Figure 6.9), the small variations in solar output associated with the 11 -year solar cycle do lead to temperature and circulation change in the upper atmosphere (Shindell et al., 1999), may affect weather patterns in the tropical Pacific (Meehl et al., 2009a), and could potentially be associated with small variations in Earth's average surface temperature (Camp and Tung, 2007; Lean and Woods, in press). There is also evidence that changes in solar activity influence Earth's climate on longer time scales. For example, the "Little Ice Age" (Matthes, 1939), a period with slightly cooler temperatures between the 17th and 19th centuries, may have been caused in part by a low solar activity phase from 1645 to 1715 called the Maunder Minimum (Eddy, 1976; Shindell et al., 2001) (see Figure 6.10). Estimates of variations in solar output on even longer time scales—going back thousands of years—have also been produced by analyzing cosmogenic isotopes in tree rings and ice cores (e.g., Weber et al., 2004). However, these estimates, and hence the extent of solar influence on global climate on these time scales, are even more uncertain (Lean and Woods, in press).

Perhaps the most dramatic example of natural climate variability is the Ice Age cycle (Figure 6.11). Detailed analyses of ocean sediments, ice cores, and other data (see, e.g.,

0 100,000 200,000 300,000 400,000 500,000 600,000 700,000 800,000 Years Before Present

FIGURE 6.11 Analysis of ice core data extending back 800,000 years documents (top) the Earth's changing CO2 concentration and (bottom) estimated temperatures in the Antarctic region. Until the past century, natural factors caused atmospheric CO2 concentrations to vary within a range of about 180 to 300 ppm. Note that time progresses from right to left in this figure, and that neither temperature changes nor the rapid CO2 rise (to 388 ppm) over the past century are shown. SOURCES: Based on data from (top) Luthi et al. (2008) and (bottom) Jouzel et al. (2007). Data available at http://www.ncdc.noaa.gov/paleo/icecore/ an tarctica/dom ec/dom ec_epica_data.html.

0 100,000 200,000 300,000 400,000 500,000 600,000 700,000 800,000 Years Before Present

FIGURE 6.11 Analysis of ice core data extending back 800,000 years documents (top) the Earth's changing CO2 concentration and (bottom) estimated temperatures in the Antarctic region. Until the past century, natural factors caused atmospheric CO2 concentrations to vary within a range of about 180 to 300 ppm. Note that time progresses from right to left in this figure, and that neither temperature changes nor the rapid CO2 rise (to 388 ppm) over the past century are shown. SOURCES: Based on data from (top) Luthi et al. (2008) and (bottom) Jouzel et al. (2007). Data available at http://www.ncdc.noaa.gov/paleo/icecore/ an tarctica/dom ec/dom ec_epica_data.html.

Luthi et al., 2008) 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 Chicago, New York, and Seattle). These very long cold spells were punctuated by shorter "interglacial" periods—including the last 10,000 years, during which time the climate appears to have been relatively stable.

Through a convergence of theory, observations, and modeling, scientists have deduced that the ice ages were initiated by small recurring variations in Earth's orbit around the sun, which modulated the magnitude and seasonality of sunlight received at the Earth's surface in a persistent way. Over many thousands of years, these relatively small changes in solar forcing resulted in gradual changes in and feedbacks between the cryosphere and biosphere that slowly but persistently changed the abundance of GHGs in the atmosphere, reinforcing the changes in solar forcing and ultimately driving a global temperature change on the order of 9°F ± 2°F (5°C ± 1°C) between glacial and interglacial periods (EPICA Community Members, 2004; Jansen et al., 2007). Because GHGs acted as a feedback rather than as a forcing during the Ice Age cycles, temporal variations in GHGs typically lag, rather than lead, the estimated temperature changes in Figure 6.11.

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