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The Effects of Galactic Cosmic Rays on Weather and Climate on Multiple Time Scales
This article has been accepted for publication in the Encyclopedia of Human Ecology, Kenneth E. F. Watt, editor. A shorter version is published in the Proceedings of the 17th Annual Pacific Climate Workshop, May 2000 and an updated shorter version is soon to be published in the Proceedings of the 19th Annual Pacific Climate Workshop, March 2002.
This article is available on my website: http://www.hartnell.cc.ca.us/faculty/mercurio/
September 14, 2002
In this article, evidence is presented that galactic cosmic rays (GCRs) are a major forcing agent on weather and climate on multiple time scales ranging from weekly through glacial-interglacial. Known effects of GCRs are used to explain phenomena and observations in the fields of meteorology, climatology, paleoclimatology and paleoecology. Evidence is presented that primary effects of increases in levels of GCRs are increases in the amounts of low clouds- especially over the tropics, increases in the albedo of low clouds and decreases of the temperature of and increases of the strength of the stratospheric polar vortex. This has widespread effects on atmospheric circulation including the El Nino Southern Oscillation (ENSO). Other effects of increases in levels of GCRs include increases in relative humidities and surface condensation, possible decreases in average amounts of precipitation and increases in storm intensities (vorticity area index). Secondary effects arising from these include decreases in surface temperatures, increases in equabilities and over the long term, a colder, more oscillating (more frequent Enso-Warm Events) tropical Pacific and increases in levels of glaciation. Levels of GCRs in the earth’s atmosphere are inversely related to the strengths of the solar magnetic and geomagnetic fields that modulate them. Variations in solar magnetic field structure are used to explain the origin of approximately weekly, monthly, quasibiennial, decadal, bidecadal, multidecadal and millennial scale climatic cycles. Changes in geomagnetism are used to explain glacial-interglacial and ~13,000 year cycles. The sum of the earth’s obliquity, inclination of the orbital plane with relation to the invariable plane of the solar system and inclination of the orbital plane with relation to the plane of the solar equator is used to calculate a hypothetical curve of geomagnetism that would result over the last three million years. Higher geomagnetism and lower levels of GCRs are attributed to a greater sum of these factors. The effects of a ~412,000 year geomagnetic cycle modulated by the earth’s orbital eccentricity are also considered. The curve obtained is compared to glacial-interglacial chronologies derived from ice core and deep sea core records. Effects of extended periods of very high levels of GCRs are used to explain glacial climates. The cycle of the changing time of the year of the earth’s maximum B angle (the maximum angle of inclination of the earth’s orbital plane with relation to the solar equatorial plane) is used to calculate the hypothetical 13,000 year cycle of geomagnetism which is used to explain the origin of climate cycles of around this length. Effects of GCR modulated climate are used to explain characteristics of prehistoric biological communities and their variations with glacial-interglacial chronology, the uniqueness of the Holocene, causes of Quaternary megafaunal extinctions and effects of resulting climates, environments and their changes on human prehistory. Predictable levels of GCRs in the future are used to predict future changes in climate.
There is a growing interest in the effects of galactic cosmic rays (GCRs) on the atmosphere and a growing awareness that GCRs could be a major factor in the determination of weather and climate. Although research on GCRs is in its early stages at this time, GCRs appear to be the best candidate for an extraterrestrial agent of low total energy input that is capable of having major effects on weather and climate. In this article, I take what is known about how GCRs are modulated and about how GCRs, in turn, modulate weather and climate and extend this information beyond what has been done previously to as many possible effects and as many known timescales of periodic change as possible. GCRs appear to fit very well as a primary forcing agent on virtually all of the timescales considered.
GCRs are inversely related in levels and effects to the small changes in solar radiation that, through their heat input, have long been considered the primary agent in climatic forcing. Because of this, GCRs help explain solar related climatic periodicities already established. Svensmark and Friis-Christensen (1997) describe GCRs as “a missing link in solar-climate relationships.” When solar radiation is low, GCR levels are high, and both of these result in increased cooling. The magnitude of the effects of GCRs on cooling through increasing low cloud cover and increasing cloud albedo, however, is much greater and increases in levels of GCRs could be the primary cause of global cooling (Svensmark 1998; Landscheidt 1998).
Svensmark (1998) states that the temperature change due to GCR modulation of cloud cover in the years 1975 to 1989 was 3 to 5 times the magnitude of temperature changes due directly to solar radiation. Data from Hartmann (1993) indicates that the change in the earth’s radiation budget over a solar cycle due to changes in cloudiness are equal to 80% of the total estimated radiative forcing from the increase in CO2 concentration during the last century. Fletcher (Pers. Comm.) states that a 1% change in cloud cover is equal in temperature effects to a 10% change in CO2 levels.
GCRs are the only particles hitting the earth with enough energy to penetrate the stratosphere and troposphere. They are modulated by the sun’s and earth’s magnetic fields. GCRs are a major determinant of levels of ionization in the troposphere. The ionization of the lower atmosphere by GCRs is the meteorological variable subject to the largest solar cycle modulation (Svensmark 1998).
Levels of ionization affect levels of aerosols suitable as cloud condensation nuclei necessary for cloud formation. Because of this, levels of ionization are a major determinant of relative humidities, levels of condensation, levels of low cloudiness and cloud albedos that, in turn, are major determinants of temperatures, levels of surface moisture and levels of equability. Clouds formed from greater amounts of condensation nuclei, such as sulfate aerosols, are brighter and longer lived and may be more effective at cooling the earth than other clouds because of their greater albedo (reflectivity) (Rodhe 1999; Rosenfeld 2000). These clouds would also be likely to produce less precipitation.
GCRs could affect broader aspects of clouds and atmospheric circulation as well. There is evidence that they may affect storm intensities (vorticity area index) (Herman and Goldberg 1978; Tinsley and Dean 1991). GCRs also appear to be a major determinant of the temperature and strength of the stratospheric polar vortex that has a strong effect on global atmospheric circulation including the El Nino-Southern Oscillation (ENSO).
Levels of GCRs have changed over past millennia in response to changes in solar magnetic and geomagnetic fields as indicated by levels of Carbon 14 (C14) present in fossils and levels of Beryllium 10 (Be10) present in ice cores and sediments. Higher past levels of GCRs are indicative of cooler conditions and increased glaciation and lower past levels of GCRs are indicative of warmer conditions and decreased glaciation.
In addition to improving understanding of the origins of major aspects of weather and climate and the course of climate history, understanding of levels and effects of GCRs can improve the understanding of past environments and changes in ecosystems, including extinctions. This is because plant community structure is strongly affected by long term changes in levels of condensation and relative humidity, equability and precipitation distribution.
2. GCR Modulation by Solar Magnetism and the ~11 Year and ~22 Year Solar
Changes in cloudiness and temperatures have been related to the ~11 and ~22 year solar (sunspot) cycles (Ely 1995; Svensmark and Friis-Christensen 1997; Svensmark 1998; Soon et al. 2000). Change in the solar constant over ~11 year sunspot cycles is rather small at ~0.1 % to be able to account for observed changes but changes in levels of GCRs may be able to. Levels of global cloudiness were observed to increase between 3 and 4 % from solar maximum to solar minimum over an ~11 year cycle period studied (Svensmark 1998). A strong correlation is present only in low cloudiness (two miles or lower), which is the type that would increase cooling (Bailunas and Soon 2000). Changes in the brightness of the planet Neptune over the solar cycle appear to be due to similar changes in cloudiness (Bailunas and Soon 2000) and also vary by 3 to 4 % over the solar cycle with maximum brightness at solar minimum. These changes occur because the solar magnetic field is lower at solar minimum, allowing more GCRs to reach the earth and other planets at this time.
The brightness of the earth as measured by changes in earthlight reflecting off of the moon has also been observed to change similarly over the ~11 year solar cycle (Schneider 2001). Interestingly, this measurement of the earth’s brightness primarily measures the brightness of the lower latitudes that are the areas where the modulation of low cloudiness by GCRs is the greatest. Bago and Butler (2000) found that there was little correlation of levels of low cloudiness to GCR levels in polar regions, but a significant degree of correlation in the tropics. Kristjansson (2001) also found a correlation to middle latitude oceanic low cloud cover.
~11 year solar cycles alternate between two phases referred to as parallel (-) and antiparallel (+) with the transitions occurring around solar maximum. Consistently greater levels of GCRs reach the earth over most of the antiparallel cycles (Figure1). This results in ~22 year cycles which correlate well to meteorological cycles including such phenomena as major droughts on the western Great Plains and variations in precipitation from cyclonic storms in the westerlies in Southern California. A curve of Los Angeles yearly precipitation totals shows an approximately 22 year cycle with generally progressively increasing totals during antiparallel cycles and generally progressively decreasing totals during parallel cycles (Figure 2). This results in a pattern in which wettest years often occur around antiparallel to parallel solar maxima and the driest years often occur around parallel to antiparallel solar maxima. Generally higher precipitation totals are also seen during ENSO-Warm Event (El Nino) and Pacific Decadal Oscillation (PDO) warm phase years and generally lower totals during ENSO-Cold Event (La Nina) and PDO cold phase years. A PDO warm phase and an ENSO-Warm Event often occur following the antiparallel to parallel transition adding to higher rainfall totals at these times.
FIGURE 1. The top curve is the annual mean variation in cosmic ray flux as measured by ionization chambers from 1937 to 1994 (adapted from Svensmark 1998). The bottom curves are neutron flux, which is a proxy for galactic cosmic ray flux, from the neutron monitor in Climax, Colorado from 1951 to 2000 and sunspot number (adapted from University of Chicago/LASR GIF image). Note the differences in the shapes of the curves of GCRs in antiparallel (+) and parallel (-) solar cycles and the differences in GCR levels at solar maxima.
It has be observed that longer ~11 year cycles are associated with cooler global temperatures and shorter ones with warmer global temperatures and this may also be due to corresponding variations in the GCR flux (Svensmark and Friis-Christensen 1997). A curve of annual GCR intensities over the last four solar cycles shows higher GCR levels at the sunspot maxima of the longer ~11 year cycles (Figure1). Solar maximum may be the time when the greatest variations in GCR levels occur and this could be an important factor in periods of cooling, especially on century and millennial time scales.
FIGURE 2. The top curve is the Arctic Oscillation Index with average values (adapted from Kerr 1999d). More positive conditions indicative of a stronger, colder polar vortex are up in direction. The upper middle curve is July to June yearly U. S. Weather Bureau precipitation totals for Los Angeles civic center. The bottom curves are the Pacific Decadal Oscillation (PDO) and Southern Oscillation Index (SOI) (adapted from Mantua et. al 1997). In the SOI curve, values indicative of ENSO-Warm Event conditions are above the line and values indicative of ENSO-Cold Event conditions are below the line and in the PDO curve, warmer sea surface temperatures are above the line and colder sea surface temperatures below the line. Note the general relationship between antiparallel (+) solar cycles and a more positive average Arctic Oscillation, progressively increasing Los Angeles precipitation totals, more positive (ENSO-Cold Event) SOI conditions and the opposite for parallel (-) solar cycles. Also note the general relationship between ENSO-Warm Event conditions and higher Los Angeles precipitation totals.
3. GCRs and the Polar Vortex, Arctic Oscillation and El Nino Southern
Levels of GCRs appear to have a relationship to the state of the stratospheric polar vortex and, more indirectly, to the state of the El Nino Southern Oscillation (ENSO). The polar vortex circles the globe at around 450N, and has widespread climatic effects over the Northern Hemisphere. Higher levels of GCRs are one of several factors that appear to be associated with a stronger, colder polar vortex. Assuming some lag time, some indications of a ~22 year periodicity in winter stratospheric temperatures, with increasingly colder average conditions in antiparallel cycles, can be seen in Figure 3. A stronger, colder polar vortex is in a general way associated with ENSO-Cold Event (La Nina) conditions and, on a short-term basis, a warmer, weaker polar vortex is often associated with ENSO-Warm Event (El Nino) conditions. Some indications of this can also be seen in Figure 3.
An exception to this is ENSO-Warm Events associated with large, sulfurous volcanic eruptions, which are associated with a stronger, colder polar vortex. This is because, in addition to levels of GCRs, volcanic effects on the stratospheric equator to pole temperature gradient are also a determinant of the strength and temperature of the polar vortex. Sulfurous gasses and aerosols heat the stratosphere and can increase the stratospheric equator to pole temperature gradient, which results in a stronger, colder polar vortex (Kerr 1993a). These sulfurous gasses and aerosols also cause high latitude ozone depletion, which can further cool and strengthen the polar vortex. Other effects of volcanic sulfurous gasses and aerosols are changes in atmospheric circulation that can produce an ENSO-Warm Event (Chanin 1993) and lower global tropospheric and surface temperatures after around a year’s time. A major factor in volcanically related ENSO-Warm Events is weakened Trade Winds probably resulting from large positive tropospheric temperature anomalies over North America caused by the sulfurous gasses and aerosols.
The Arctic Oscillation Index is a measure of the variations in the strength of the polar vortex. Its positive phase is the result of a colder polar stratosphere, the effects of which propagate into the troposphere (Schindell et al. 1999). This results in a stronger, colder polar vortex, a northerly path for the polar front jet stream, mild and wet northerly latitudes, limited penetration of cold into continental interiors, dryness in Mediterranean latitudes, stronger trade winds and a weaker subtropical jet stream. The negative phase has the opposite effects and results in a more southerly polar front jet stream, wetter Mediterranean latitudes, a colder midwestern US, Western Europe and Russia, weaker trade winds, and a stronger subtropical jet stream (Stricherz 1999; Chanin 1993). The effects on worldwide weather patterns of the positive phase of the Arctic Oscillation is similar in some ways to those caused by ENSO-Cold Events and the effects of the negative phase are similar in some ways to those caused by ENSO-Warm Events.
The effects of the strength of the polar vortex on the strength of the trade winds actually provides a direct link between GCRs, the polar vortex and ENSO. Effects on ENSO may at least partly occur through effects on the strength of the Trade Winds that are directly related to the strength of the polar vortex (Stricherz 1999; Chanin 1993). Weaker Trade Winds are associated with ENSO-Warm Events.
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