-20

-20

-15

-10

-5

0

20

0

5

20

15

10

40

60

80

100

120

∆14C

0 2000 4000 6000 8000 

YEARS BEFORE PRESENT

0 2000 4000 6000 8000 

YEARS BEFORE PRESENT

RESIDUAL (DETRENDED) ∆14C 

A

B

200-year

cycles

irradiance (solar constant) and weather and climate has been suggested for 

more than 100 years but generally rejected by most scientists, who assume 

that the effect of solar variations would be small. However, recent satellite 

radiometer measurements and modeling studies indicate that small changes in 

total solar irradiance could produce global temperature changes of the 

magnitude suggested for climatic events such as the Little Ice Age (A.D. 

1550–1700). 

 The best known and documented solar cycle is the 11-year sunspot cycle 

(Schwabe Cycle; fig. 1), during which the number of dark blotches (sunspots) 

on the sun’s surface increases from a minimum to a maximum then back to a 

minimum over a period of about 11 years. The origin of sunspots is not fully 

understood, but thanks to satellites such as the Solar and Heliospheric 

Observatory (SOHO), scientists are learning rapidly. What is known is that 

during the period from A.D. 1645 to 1715 very few sunspots were observed. 

This period is called the Maunder Minimum (fig. 1) after E.W. Maunder, 

superintendent of the Solar Department of the Greenwich Observatory in 

England, who continued the observations of the German astronomer, Gustav 

Spörer (Eddy, 1976). Spörer noted a dearth of sunspots during a 70-year 

period beginning about A.D. 1645, and during an earlier 90-year period 

beginning about A.D. 1460 (now called the Spörer Minimum). An early 

association between sunspots and terrestrial phenomena was the observation 

that the number and intensity of aurora borealis sightings were greatest during 

sunspot maxima when the sun was most active (active sun), and lowest during 

sunspot minima (quiet sun). Another terrestrial observation was that the 

Maunder Minimum coincided with the coldest part of the Little Ice Age.

Recent studies have demonstrated that the production of radiocarbon (14C) 

also is related to solar activity. Radiocarbon is produced in the upper atmosphere by bombardment of atmospheric nitrogen (14N) with neutrons from 

outer space (cosmic rays). An increase in solar activity (more sunspots) is 

accompanied by an increase in the “solar wind,” which is an outflow of 

ionized particles, mostly protons and electrons, from the sun. The solar wind 

reflects cosmic rays and reduces 14C production. Therefore, the 14C concentration of the atmosphere is lower during sunspot maxima and higher during 

sunspot minima. The 14C concentration of the atmosphere is recorded in the 

shows that 14C production was much greater during the mid-Holoc

(approximately 7,000 years ago), and steadily decreased until abou

years ago. When this long-term secular variation trend in ∆14C is r

the residual values show distinct cycles with periods of about 2,00

(Hallstadtzeit Cycle; fig. 2B). 

A closer look at the tree-ring ∆14C reconstruction for the last 1,

shows that 14C was higher during the Maunder and Spörer Minima

shows that there were peaks in 14C production centered on about A

and 1050 (fig. 3). These two periods of increased 14C production a

inferred to have also been periods of minimum solar activity, and a

the Wolf and Oort Minima, respectively. These cycles in 14C produ

adiance 

ch 

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arch 

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Research 

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A, p. 

the 

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he 

 

h electric 

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ng of 

search, 

0

50

100

150

200

SUNSPOT NUMBER

Dalton MinimumMaunder Minimum

2000 1900 1800 1700

YEARS A.D.

Figure 1. Record of sunspot numbers from A.D. 1700 to 1990 showing the 11-year Schwabe 

Cycle.

Figure 2. Records of radiocarbon production, A, as measured in bi-decada

wood (∆14C) in which the age of the sample is known from counting tree rin

Sonnett, 1991) for the period from 10,000 calendar years ago to the present. 

(detrended; dashed lines) record of ∆14C in which the long-term secular var

has been removed. The heavy red curve is a weighted smoothing function 

Hallstadtzeit Cycles with periodicities of about 2,000 years.

Printed on recycled paper U.S. Department of the Interior

U.S. Geological Survey

USGS Fact Sh

FOLD
YEAR A.D.

-30

-20

-10

0

10

20

0 100 200 300 400 500 600 700 800 900 1000 1100

CALENDAR YEARS BP

∆14C

Modern

Max.

Medieval

Max.

Maunder

Minimum Spörer

Minimum

Wolf

Minimum

Oort

Minimum

B

Figure 3. Details of the ∆14C record for the last 1,100 years, in A, years A.D., and B, 

calendar years BP. The sunspot record from A.D. 1700 to 1900 (in red) is repeated from 

figure 1. Note that the scale of ∆14C (Y-axis) is inverted relative to the same scale in figure 

2 so that minima in ∆14C correspond to maxima in sunspot number. Note also that maxima 

in ∆14C lead minima in sunspot number; this represents the length of time (20–60 years) 

that it takes the radiocarbon signal in the Earth's atmosphere to respond to changes in 

solar activity. The cycles in sunspot minima (∆14C maxima) with periodicities of about 200 

years are referred to as Suess Cycles.

1.0

0.001 0.01

100

2241

805

504

385

232

131

105

208 (Suess)

88 (Gleissberg)

1000 

0.5

0

RELATIVE SPECTRAL POWER

FREQUENCY (YEARS-1)

PERIOD (YEARS)

Figure 4. Maximum entropy method (MEM) power spectrum of the 

∆14C record. Periodicities in red are discussed in text. Adapted fro

Terrestrial Components of the Atmospheric 14C variation Spectrum” 

and Charles P. Sonnett in The Sun in Time, edited by C.P. Sonnett, M

M.S. Matthews. Copyright  1991 The Arizona Board of Regents. R

permission of the University of Arizona Press.

increase has occurred since 1970. Solar forcing can only a

the 0.36° increase; the rest could be due to greenhouse war

other cause. Thus, there appears to be a real relation betwe

and global temperature related to solar irradiance, with col

during sunspot minima. The period of low 14C production 

A.D. 1100 and 1250 is called the Medieval Maximum of s

3), and coincides in time with a warm interval referred to a

Warm Period. This is when the Vikings colonized southern 

eastern Canada (Vinland; http://www.anthro.mankato.msus

vikings/vikhome.html). The reasons for abandonment of t

colonies are still being debated, but a likely contributing fa

the cold climate associated with the Wolf Minimum (fig. 3

was generally low (14C production high) during the period 

with two distinct lows, the Spörer and Maunder Minima (

been in the 20th century that solar activity has increased (1

decreased) to levels that occurred during the Medieval Max

Far more variable than changes in total solar irradiance 

amount of energy emitted as ultraviolet, extreme ultraviole

in the continuous outflow of ionized solar particles (solar w

controls the properties of the Earth’s magnetosphere. The 

between solar activity, as measured by what solar physicist

“coronal mass ejections” or CME’s, and geomagnetic storm

documented in recent years. (See reviews by Cliver, 1994a

1995.) A connection between geomagnetic storms and clim

another link (in addition to solar irradiance) between the su

with geomagnetic storms as the intermediate step, driving 

troposphere, the lower 10 km of the Earth’s atmosphere wh

occurs. Roberts and Olson (1973) observed that strong sol

disturbances in the troposphere during winter were often fo

few days by a deepening of subpolar low-pressure systems 

TRIM

TRIM

and, by inference, in solar activity, have periodicities of about 200 years (socalled Suess Cycles; Damon and Sonnett, 1991), and cycles with similar 

periodicities can be seen in the longer ∆14C record (fig. 2). Note that the 

sunspot minima lead the ∆14C maxima by 20–60 years, which is the time it 

takes for the effect of solar activity on 14C production to cycle through the 

atmosphere-ocean system. When a spectral analysis is performed on the ∆14C 

record, a number of peaks appear in addition to spectral peaks at about 2,000 

and 200 years (fig. 4). Two other prominent spectral peaks occur with periodicities of about 400 and 88 years. The 88-year cycle is called the Gleissberg 

Cycle and is thought to be the result of amplitude modulation of the 11-year 

Schwabe Cycle (Sonnett and Finney, 1990). A 22-year periodicity (not shown 

in fig. 4), the so-called “double sunspot” or Hale Cycle, also may be the result 

of amplitude modulation of the 11-year Schwabe Cycle. The prominent 200-

year cycles of Maunder-like minima (so called “Suess wiggles”) may be the 

result of modulation of the ≈ 2,000-year cycle (Sonnett and Finney, 1990). 

This makes it difficult to assign a cause to all but the basic 11-year sunspot 

cycle. The Maunder Minimum occurred during the coldest part of the Little 

Ice Age, but what can be said about global temperature during other sunspot 

minima and, for that matter, during sunspot maxima?

Satellite radiometer measurements made over the last 20 years have shown 

that total solar irradiance varies 0.1 percent over one 11-year sunspot cycle, 

but that irradiance varies considerably with the wavelength of the electromagnetic radiation (Lean and others, 1995a). The variation of 0.1 percent in 

total solar irradiance over one sunspot cycle translates to a global tropospheric 

temperature difference of 0.5° to 1.0°C (Labitzke and van Loon, 1993). 

Irradiance is greatest during sunspot maxima and lowest during sunspot 

minima—which appears to be counterintuitive because the dark sunspots 

might be expected to dim the sun. However, bright regions surrounding the 

sunspots, called faculae, cause the sun to brighten at peak activity (Lean and 

Foukal, 1988). Lean and others (1995a) estimated that during the Maunder 

Minimum, total solar irradiance was reduced by 0.2 percent relative to a 

present quiet sun (minimum of the mid-1990’s), but total ultraviolet (UV) 
30°

Winter

40

80

120

ZONAL FLOW

Departure From Climatology

(700 mb; feet)

Polar Jet

0.2

0.4

100 300 500 700 900 1

VARVE (CALENDAR) YEARS 

MM SM WM OM

100 300 500 700 900 1

-0.8

-0.6

-0.4

-0.2

0

0.6

0.8

1.0

0.4

0.2

-1.0

 400-yr comp

21.5% of total v

0.6

100 300 500 700 900 1

-0.8

-0.6

-0.4

-0.2

0

0.2

0.4

RELATIVE CONTRIBUTION OF THE

400-YR PERIOD TO VARIABILITY

IN PERCENT ALUMINUM

RELATIVE CONTRIBUTION OF THE

84-YR PERIOD TO VARIABILITY

IN PERCENT ALUMINUM

 84-y

14% of 

g)

50

0.02

the tree-ring calibrated 

from “Solar and 

um” by Paul E. Damon 

t, M.S. Giampapa, and 

s. Reprinted by 

Figure 5. Location of North America with respect to the region of greatest increase in 

zonal winds following occasional solar-geomagnetic disturbances at the 700 millibar (mb; 

approximately 4 km elevation) level during the winter (Stolov and Shapiro, 1974), and the 

position of the core of the polar jet in winter and summer at 6,000 cal. yr. BP as computed 

by Kutzbach (1987). Shading represents lowering (in feet) of the 700 mb level relative to 

the average height in winter for the period from 1947 to 1970 (”climatology”). The darkest 

areas with the largest departure in central North America correspond to a 7 percent 

increase in mean geostrophic westerly flow. Arrow shows the surface-wind trajectory 

during the mid-Holocene in north-central Minnesota as determined from sand dune 

alignment (Grigal and others, 1976).

Figure 6. A, Plot of percent aluminum in bulk sediment in E

years for the past 1,550 yr. MM, SM, WM, and OM indicate 

Spörer, Wolf, and Oort sunspot minima, respectively, and LIA 

intervals of Little Ice Age and Medieval Warm Period, respe

bution of 400-yr period reconstructed by singular spectrum 

sediments of Elk Lake for the past 1,550 yr. C, Relative contr

structed by singular spectrum analysis for percent Al data i

past 1,550 yr.

warming or some 

tween solar activity 

colder temperatures 

on between about 

f sunspot activity (fig. 

o as the Medieval 

hern Greenland and 

sus.edu/prehistory/ 

of the Greenland 

g factor may have been 

. 3). Solar activity 

od A.D. 1300 to 1900, 

a (fig. 3). It has only 

d (14C production 

Maximum (fig. 3).

nce are changes in 

olet, and X-rays, and 

ar wind), which 

he connection 

cists refer to as 

orms has been well 

94a, 1994b, and 

climate might form 

e sun and climate, 

ng zonal winds in the 

where weather 

solar-geomagnetic 

n followed within a 

ms over the North 

Pacific. Stolov and Shapiro (1974) studied the 700-millibar heights (about 4 

km) in the northern hemisphere between lat 20° and 70° N. in relation to solar 

geomagnetic disturbances over the period from 1947 to 1970. They noted 

that the most significant response to these disturbances was a decrease in the 

700-millibar height in winter relative to the average for the same period 

(“climatology”) 4 days following a disturbance. The decrease is greatest over 

the North American Great Lakes area between long 70° and 110° W. and lat 

40° and 70° N. It corresponds to an increase by as much as 7 percent in 

tropospheric westerly flow, which translates to an increase in mean zonal 

wind velocity of about 0.6 m s-1 (fig. 5). Statistical analyses by Tinsley and 

Deen (1991) of 33 years of meteorological data show a clear correlation 

between the intensity of cyclonic circulation in mid-latitude oceanic regions 

with changes in cosmic-ray flux following geomagnetic storms. The main 

effect of geomagnetic disturbances would be to increase westerly zonal winds 

from the North Pacific. These changes in zonal winds occur on a day-to-day 

time scale as well as on a decadal time scale related to the sunspot cycle.

These are several measured connections between solar geomagnetic storms 

and “weather” at the Earth’s surface. Because geomagnetic storms are related 

to solar activity and cosmic-ray flux, and because 14C is produced by cosmic 

rays, records of past 14C production, as recorded for example in tree rings, 

might also serve as a paleowind indicator. The tree-ring record of 14C 

production over the last 10,000 years shows that long-term production of 14C 

was low during the last 4,000 years relative to that during the mid-Holocene 

(8,000 to 4,000 years BP; fig. 2A). The Earth’s geomagnetic field strength 

FOLD
0

0 200

400 600 

VARVE THIC

VARVE YEARS

0 1500 1700

0

0 1500 1700

lags=25

0 1500 1700

lags=25

t

ce

A

B

C

Winkler

Ward

Culberson

Jeff Davis

Van Horn

Fort Stockton

Castile

core

us time in varve 

ls of the Maunder, 

indicate time 

Relative contri-

%Al data in 

4-yr period reconof Elk Lake for 

(dipole moment) during this 4,000-year mid-Holocene interval was the lowest 

that it has been during the Holocene (Damon and others, 1989). A weaker 

geomagnetic field allows more cosmic rays to enter the atmosphere producing 

more 14C. The cosmic-ray flux also affects cloud microphysical processes, 

and consequently the general atmospheric circulation (Tinsley and Deen, 

1991; Tinsley, 1998). Dust records from annually laminated (varved) 

sediments from Elk Lake, Minn., show centennial-scale variations of 

increased dustiness during periods of reduced solar activity such as during the 

Little Ice Age (Dean, 1997) and the mid-Holocene (Anderson, 1992; Dean 

and others, 1996). Cycles with periodicities of 200 years in the dust record in 

Elk Lake match 200-year cycles (Suess Cycles) in radiocarbon during the 

4,000-year mid-Holocene period of lower geomagnetic field (Anderson, 

1992). The mid-Holocene may have been a period of greatly increased eolian 

(windblown) activity in parts of North America, but particularly in the Great 

Plains (Dean and others, 1996). A varve-calibrated eolian dust record (as 

indicated by aluminum content) for the last 1,500 years in Elk Lake shows 

distinct cycles with dominant periodicities of 400 and 84 years (fig. 6), both 

of which are prominent features of the maximum entropy 14C power spectrum 

(fig. 4). The most prominent 400-year cycle coincides in time with the Little 

Ice Age, peaking during the Maunder Minimum (fig. 6A, B). The most 

prominent 84-year periodicities (Gleissberg period?) coincide in time with the 

Medieval Warm Period between the Oort and Wolf Minima (fig. 6A, C).

The Elk Lake record is but one of many examples in the literature of 

paleoproxy records purporting to exhibit cycles with periodicities matching 

those of solar cycles. Many are based on sparse dating so that time scales are 

questionable. But others, based on annual calibration (varves, tree rings, 

corals, and ice cores), present more convincing evidence. Commenting on the 

oxygen-isotope record in the recent Greenland GISP2 ice core, Stuiver and 

others (1997, p. 263) concluded, “The overall agreement between the timing, 

estimated order of magnitude of temperature change, and phase lag of the 18O 

and ∆14C signals *** is suggestive of solar forcing of Greenland climate for 

the current millennium.” But the record for the rest of the Holocene is less 

convincing. The dust record in the GISP2 ice core exhibits cycles with 

distinct periodicities of 11, 22, 90, and 200 years, believed 

with the Schwabe, Hale, Gleissberg, and Suess solar cycle

and others, 1997; Ram and Stolz, 1999). The physical con

solar activity and dust is postulated to be variations in prec

the Greenland dust-source areas caused by changes in clou

microphysics, which in turn are affected by the cosmic-ray 

others, 1997).

 Two unpublished examples of decadal to centennial sc

ancient and one modern, further illustrate the types of pale

that might have a solar connection. The Upper Permian (2

Formation in the Delaware Basin of West Texas and New 

contains more than 200,000 annual laminations (varves) o

and anhydrite (CaSO4), and anhydrite and halite (NaCl). T

in the Late Permian was located near the equator on the w

Pangea continent, but was partly isolated from the vast Pa

the west (Anderson and Dean, 1995). Precipitation of diff

minerals was the result of a delicate dynamic balance betw

reflux of marine waters (Kirkland and others, 2000). Give

simple chemical system in delicate balance, climatic fluctu

are recorded as cycles in these pure evaporite sediments. 

cycle in the Castile is a salinity cycle with an average peri

years in which the cycle begins, in the extreme, as a lamin

thin calcite and anhydrite laminae. The anhydrite laminae 

thicker, and the cycle ends, in the extreme, with anhydrite 

This cycle and longer period cycles have been discussed in 

example, Dean and Anderson, 1978, 1982; Anderson, 198

period cycles, such as the ones illustrated in figure 8 for an 

varve thickness, have not. This record shows cycles with 

about 20 years superimposed on longer cycles with an ave

200 years. Could these be related to “double sunspot” (H

“Suess wiggles”? In fact, could the dominant 2,500-year 

Formation be related to the dominant 2,000-year cycle in t

(Hallstadtzeit Cycle; fig. 2B)?

Figure 8 (above). Smoothed (moving average) 800-year record of calcite-anhydr

the Halite I Member of the Permian Castile Formation measured in a core from W

R.Y. Anderson and W.E. Dean, unpublished data.

Figure 7 (left). Locality of the Permian Castile Formation core, Winkler County, T

0 40 10 50 KILOMETERS 20 30

FOLD
0.3

0.32

0.34

0.36

0.38

0.4

PERCENT Ti

2000 1980 1960 1940 1920 1900 1880 1860 

YEAR A.D.

Gulf of Calif

BC06

28°

26°

24°

22°

BC50

M

Baja California

Figure 10. Percent titanium (Ti) versus years A.D. (red) in 

sediments from a box core collected in the eastern Gulf of 

California between the mouths of Rios Yaqui and Mayo. 

The record of sunspot number from A.D. 1820 to 1990 is 

repeated from figure 1 (green).

(p) p

time interval recovered in core BC50 (fig. 9) taken farther offshore. The 

cycles are interpreted as due to greater and lesser riverine influx of volcanic 

rock debris from the Sierra Madre. The titanium cycles coincide almost 

exactly with 10- to 12-year cycles of precipitation as reconstructed from tree 

rings (Frittz, 1991). The cycles are more distinct in sediments deposited prior 

to A.D. 1940, which may reflect damming of rivers during the 20th century. 

Could variations in rainfall and river flow on the west slope of the Sierra 

Madre somehow be connected with the solar sunspot cycle as are changes in 

precipitation postulated for dust sources in Greenland ice cores (Ram and 

Stolz, 1999)?

 It might be argued that if you look hard enough, you can find a correlation 

between solar cycles and anything, such as cycles in the stockmarket. Nevertheless, enough evidence exists to support the hypothesis of Denton and 

Karlén (1973) that solar cycles, as recorded in the 14C variations, may be empirical indicators of paleoclimates. Cycles in sedimentary records with 

periodicities similar to those of solar cycles do not mean causation. The 

challenge to geologists and atmospheric scientists is to test these correlations 

with reasonable models of how solar cycles could affect the atmosphere and 

geologic processes at the surface of the Earth. 

Figure 9. Locality of sediment samples of cor

of California.

0 200 100 300 KILO

TRIM

TRIM
0

50

100

150

200

SUNSPOT NUMBER

1820

yo

Rio Fuerte

Sinaloa

economic resources: Berlin, Springer-Verlag, p. 61–78.

Cliver, E.W., 1994a, Solar activity and geomagnetic storms; The first 40 

years: Eos, v. 75, p. 569–576.

———1994b, Solar activity and geomagnetic storms; The corpuscular 

hypothesis: Eos, v. 75, p. 609–613.

———1995, Solar activity and geomagnetic storms; From M regions and 

flairs to coronal holes and CMEs: Eos, v. 76, p. 75–83.

Crowley, T.J., and Kim, K-Y, 1996, Comparison of proxy records of climate 

change and solar forcing: Geophysical Research Letters, v. 23, p. 359–362.

Damon, P.E., Cheng, S., and Linick, T.W., 1989, Fine and hyperfine structure 

in the spectrum of secular variations in atmospheric 14C: Radiocarbon, v. 

31, p. 697–703.

Damon, P.E., and Sonnett, C.P., 1991, Solar and terrestrial components of the 

atmospheric 14C variation spectrum, in Sonnett, C.P., Giampapa, M.S., and 

Matthews, M.S., eds., The sun in time: Tucson, Ariz., The University of 

Arizona Press, p. 360–388.

Dean, W.E., 1997, Rates, timing, and cyclicity of Holocene eolian activity in 

north-central United States—Evidence from varved lake sediments: 

Geology, v. 25, p. 331–334.

Dean, W.E., Ahlbrandt, T.S., Bradbury, J.P., and Anderson, R.Y., 1996, 

Regional aridity in North America during the middle Holocene: The 

Holocene, v. 6, p. 145–155.

Dean, W.E., and Anderson, R.Y., 1978, Salinity cycles—Evidence for deepwater deposition of the Castile and lower Salado Formations, Delaware 

Basin, Texas and New Mexico, in Symposium on the Ochoan rocks of 

southeastern New Mexico and west Texas: New Mexico Bureau of Mines 

and Mineral Resources Circular 159, p.15–20.

———1982, Continuous subaqueous deposition of the Permian Castile 

Evaporites, Delaware Basin, Texas and New Mexico, in Handford, C.R., 

Loucks, R.G., and Davies, G.R., eds., Depositional and diagenetic spectra 

of evaporites—A core workshop: SEPM Core Workshop 3, p. 324–353.

Denton, G.H., and Karlén, W., 1973, Holocene climatic variations; Their 

pattern and possible cause: Quaternary Research, v. 3, p. 155–205.

Eddy, J.A., 1976, The Maunder Minimum: Science, v. 192, p. 1189–1202.

Frittz, H.C., 1991, Reconstructing large-scale climatic patterns from tree-ring 

data: Tucson, Ariz., The University of Arizona Press, 286 p.

Grigal, D.F., Severson, R.C., and Goltz, G.E., 1976, Evidence of eolian 

activity in north-central Minnesota 8,000 to 5,000 yr ago: Geological 

Society of America Bulletin, v. 87, p. 1251–1254.

Haigh, J.D., 1996, The impact of solar variability on climate: Science, v. 272, 

p. 981–984.

Kirkland, D.W., Denison, R.E., and Dean, W.E., 2000, Parent brine of the 

Castile evaporites (Upper Permian), Texas and New Mexico: Journal of 

Sedimentary Research, v. 70, p. 749–761.

Kutzbach, J.E., 1987, Model simulations of the climatic patterns during the 

deglaciation of North America, in Ruddiman, W.F., and Wright, H.E., Jr., 

eds., North America and adjacent oceans during the last deglaciation: 

Boulder, Colo., Geological Society of America, The Geology of North 

America, Volume K-3, p. 425–446.

Letters, v. 22, p. 31953198.

Ram, M., and Stolz, M.R., 1999, Possible solar influences on the 

of the GISP2 ice core from central Greenland: Geophysical R

Letters, v. 26, p. 1043–1046.

Ram, M., Stolz, M., and Koenig, G., 1997, Eleven year cycle of d

concentration variability observed in the dust profile of the GI

from Central Greenland; Possible solar connection: Geophysi

Letters, v. 24, p. 2359–2362.

Roberts, W.O., and Olson, R.H., 1973, Geomagnetic storms and w

300 mb trough development in the North Pacific–North Ameri

Journal of Atmospheric Science, v. 30, 135–140.

Sonnett, C.P., and Finney, S.A., 1990, The spectrum of radiocarb

Philosophical Transactions of the Royal Society of London, v. 

413–426.

Stolov, H.L., and Shapiro, R., 1974, Investigation of the response

general circulation at 700 mbar to solar-geomagnetic disturban

of Geophysical Research, v. 79, p. 2161–2170.

Stuiver, M., Braziunas, T.F., Grootes, P.M., and Zielinski, G.A., 1

evidence for solar forcing of climate in the GISP2 oxygen isot

Quaternary Research, v. 48, p. 259–266.

Stuiver, M., and Pearson, G.W., 1986, High-precision calibration 

radiocarbon time scale, A.D. 1950–500 B.C.: Radiocarbon, v. 

p. 805–838.

Tinsley, B.A., 1998, Solar wind inputs to cloud microphysics thro

charge accumulation by cloud droplets: Eos, v. 79, p. 4.

Tinsley, B.A., and Deen, G.W., 1991, Apparent tropospheric resp

MeV-GeV particle flux variations; A connection via electrofre

supercooled water in high-level clouds: Journal of Geophysica

v. 96, p. 2283–2296.

nd BC50, Gulf 

For more information please conta

Walter E. Dean

U.S. Geological Survey

P.O. Box 25046, MS 980

Denver Federal Center

Denver, CO 80225-0046

303-236-5760

303-236-5349 Fax

dean@usgs.gov

alifornia

FOLD

This Fact Sheet is also available online at

 http://greenwood.cr.usgs.gov/pub/fact-sheets/fs-0095-00