. Observations with very stable and precise detectors on several Earth-orbiting satellites show that the Sun’s total radiative input to the Earth, termed the solar irradiance, is not a constant, but instead varies over time scales of days and years. Measurements from five independent space-based radiometers since 1978 (top) have been combined to produce the composite solar irradiance (bottom) over two decades. They show that the Sun’s output fluctuates during each 11-year sunspot cycle, changing by about 0.1 percent between maximums (1980 and 1990) and minimums (1987 and 1997) in magnetic activity. Temporary dips of up to 0.3 percent and a few days’ duration are due to the presence of large sunspots on the visible hemisphere. The larger number of sunspots near the peak in the 11-year cycle is accompanied by a rise in magnetic activity that creates an increase in luminous output that exceeds the cooling effects of sunspots. Here the total irradiance just outside our atmosphere, called the solar constant, is given in units of watts per square meter, where one watt is equivalent to one joule per second. The capital letters are acronyms for the different radiometers, and offsets among the various data sets are the direct result of uncertainties in their scales. Despite these offsets, each data set clearly shows varying radiation levels that track the overall 11-year solar activity cycle. (Courtesy of Claus Fröhlich.)
. The pressure of our atmosphere (right scale) decreases with altitude (left scale). This is because fewer particles are able to overcome the Earth’s gravitational pull and reach higher altitudes. The temperature (bottom scale) also decreases steadily with height in the ground-hugging troposphere, but the temperature increases in two higher regions that are heated by the Sun. They are the stratosphere, with its critical ozone layer, and the ionosphere. The stratosphere is mainly heated by ultraviolet radiation from the Sun, and the ionosphere is created and modulated by the Sun’s X-ray and extreme ultraviolet radiation.
. A satellite map showing an exceptionally low concentration of ozone, called the ozone hole, that forms above the South Pole in the local spring. In October 1990 it had an area larger than the Antarctic continent, shown in outline below the hole. Eventually spring warming breaks up the polar vortex and disperses the ozone-poor air over the rest of the planet. (Courtesy of NASA.)
. During the Sun’s 11-year activity cycle, the upper-atmosphere temperatures fluctuate by factors of two, and neutral and electron densities by factors of ten. Enhanced magnetic activity on the Sun produces increased ultraviolet and X-ray radiation that heats the Earth’s upper atmosphere and causes it to expand, resulting in higher temperatures and greater densities at a given altitude in our atmosphere. (Courtesy of Judith Lean.)
. Variations in the air temperature over land in the Northern Hemisphere (solid line) closely fit changes in the length of the sunspot cycle (dashed line). Shorter sunspot cycles are associated with increased temperatures and more intense solar activity. This suggests that solar activity is at least partly responsible for the rise in global temperatures over the last century, and that the Sun can substantially moderate or enhance global warming brought about by human increases of carbon dioxide and other greenhouse gases in the atmosphere.
. The annual global-mean temperature near the Earth’s surface is plotted as a temperature difference from the long-term mean value. At more than 0.5 degrees Celsius above the mean, recent global temperatures maintain a warming trend for the past two decades. Natural temperature fluctuations prohibit a clear detection of human-induced warming in the early part of the twentieth century, but human increases of greenhouse gases in the atmosphere may be noticeably contributing to the recent rise in temperatures. The global mean temperature for the period 1880 to 1997 is 13.8 degrees Celsius (56.9 degrees Fahrenheit). [Courtesy of Michael Changery, the National Climatic Data Center (NCDC) of the National Oceanic and Atmospheric Administration (NOAO).]
. A reconstruction of the Sun’s brightness over the past 400 years indicates that the global temperature fluctuations from 1600 to 1800 were mainly due to variations in solar activity. Cooling by haze from volcanoes played a role during the next 100 years. The global temperature increases after the mid-1970’s, are mainly caused by heat-trapping greenhouse gases emitted by industrial economies. Here the decadally averaged values of Northern Hemisphere summer surface temperatures (scale at left) are compared with the reconstructed solar total irradiance (symbols, scale at right). The dark solid line is paleoclimate temperature data, primarily tree rings, and the gray dashed line is instrumental temperature data. (Courtesy of Judith Lean.)
. This reconstruction of the hemisphere temperature record suggests that the Northern Hemisphere has been warmer in the 20th century than in any other century of the last thousand years. The sharp upward jump in the temperature during the last 100 years was recorded by thermometers at and near the Earth’s surface. Earlier fluctuations were reconstructed from “proxy” evidence of climatic change contained in tree rings, lake and ocean sediments, and ancient ice and coral reefs. The farther back in time the reconstruction is carried, the larger the range of possible error, denoted by the light shaded regions. (Courtesy of Michael E. Mann.)
. Three independent indices demonstrate the existence of long decreases in the level of solar activity, such as the Maunder and Spörer Minimum. The observed annual mean sunspot numbers (scale at right) also follows the 11-year solar activity cycle after 1700. The curve extending from 1000 to 1900 is a proxy sunspot number index derived from measurements of carbon-14 in tree rings. Increased carbon-14 is plotted downward (scale at left-inside), so increased solar activity and larger proxy sunspot numbers correspond to reduced amounts of radiocarbon in the Earth’s atmosphere. Open circles are an index of the occurrence of auroras in the Northern Hemisphere (scale at left-outside). (Courtesy of John A. Eddy.)
. This section of a painting by Pieter Bruegel the Elder depicts a time when the average temperatures in Northern Europe were much colder than they are today. Severe cold occurred during the Maunder minimum, from 1645 to 1715, when there was a conspicuous absence of sunspots and other signs of solar activity. This picture was painted in 1565, near the end of another dearth of sunspots, called the Spörer minimum. (Courtesy of the Kunsthistorisches Museum, Vienna.)
. The advance and retreat of glaciers are controlled by changes in the Earth’s orbital shape or eccentricity, and variations in its axial tilt and wobble. They alter the angles and distances from which solar radiation reaches Earth, and therefore change the amount and distribution of sunlight on our planet. The global ebb and flow of ice is inferred from the presence of lighter and heavier forms of oxygen, called isotopes, in the fossilized shells of tiny marine animals found in deep-sea sediments. During glaciations, the shells are enriched with oxygen-18 because oxygen-16, a lighter form, is trapped in glacial ice. The relative abundance of oxygen 18 and oxygen 16 (top) is compared with periodic 41,000-year variations in the tilt of the Earth’s axis (middle) and in the shape, or eccentricity – longer 100,000-year variation, and wobble, or precession, of the Earth’s orbit – shorter 23,000-year variation (bottom).
. Ice core data indicate that changes in the atmospheric temperature over Antarctica closely parallel variations in the atmospheric concentrations of two greenhouse gases, carbon dioxide and methane, for the past 160,000 years. When the temperature rises, so does the amount of these two greenhouse gases, and vice versa. This strong correlation has been extended by a deeper Vostok ice core, to 3,623 meters in depth and the past 420,000 years. The carbon dioxide and methane increases may have contributed to the glacial-interglacial changes by amplifying orbital forcing of climate change. The ice core data does not include the past 200 years, shown as a dashed lines at the right. The present-day levels of carbon dioxide and methane are unprecedented during the past four 100,000-year glacial-interglacial cycles.