Atmospheric Research 76 (2005) 272 – 287
www.elsevier.com/locate/atmos
Lightning and climate: A review
E.R. Williams*
Massachusetts Institute of Technology, Cambridge, MA, USA
Received 13 November 2003; accepted 23 September 2004
Abstract
Research on regional and global lightning activity and the global electrical circuit is summarized.
This area of activity has greatly expanded through observations of lightning by satellite and through
increased use of the natural (Schumann) resonances of the Earth–ionosphere cavity. The global
electrical circuit provides a natural framework for monitoring global change on many time scales.
Lightning is responsive to temperature on many time scales, but the sensitivity to temperature
appears to diminish at the longer time scales.
D 2005 Elsevier B.V. All rights reserved.
Keywords: Lightning; Climate; Global change; Global circuit; Schumann resonances
1. Introduction
The importance of lightning for climate studies is increasingly recognized. Thunderstorms are major players in the global redistribution of water substance, a key mediator of
both short and long wavelength radiation. The cloud buoyancy that drives vertical motion
in electrified convection results from a temperature differential of the order of 1 8C.
Temperature perturbations of this order have appreciable local effects in the highly
nonlinear process of cloud electrification. Temperature perturbations of this order are also
clearly important in the context of global warming. The global circuit framework has been
in place for decades but the ability to measure it reliably had developed only recently. New
4 Tel.: +1 78 1981 3744; fax: +1 78 1981 0632.
E-mail address: [email protected].
0169-8095/$ - see front matter D 2005 Elsevier B.V. All rights reserved.
doi:10.1016/j.atmosres.2004.11.014
E.R. Williams / Atmospheric Research 76 (2005) 272–287
273
measurements of global lightning from the ground and from space have greatly stimulated
the application and integration of lightning in climate studies.
Areas treated in this review include the relationship between lightning and rainfall in
the general circulation of the atmosphere, the manifestation of tropical bchimneysQ in the
lightning activity, the dependence of lightning activity on temperature, the candidacy of
thunderstorms and lightning as climate bextremesQ, the connection between lightning and
upper tropospheric water vapor, the increasingly investigated role of aerosol in
precipitation and electrification processes, the regional climatology of lightning, and
finally the issue of long-term variations.
2. Lightning and the general circulation of the atmosphere
Lightning is to the global electrical circuit as rainfall is to the general circulation. The
global distributions of these two quantities are substantially different. Lightning and
rainfall are products of the convective elements that energize the respective global
systems. TRMM satellite observations (Christian et al., 2003; Goodman, pers. comm.,
2003) now provide global maps of these two quantities and they show distinct differences,
as shown in Fig. 1. Lightning is dramatically dominated by land areas in the tropics
(Orville and Henderson, 1986; Zipser, 1994; Latham and Christian, 1998; Williams and
Fig. 1. Global lightning activity (top) based on observations with the Lightning Imaging Sensor and global rainfall
(bottom) based on observations with the Special Sensor Microwave Imager (SSM/I) (courtesy of S. Goodman,
NASA MSFC).
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E.R. Williams / Atmospheric Research 76 (2005) 272–287
Stanfill, 2002), with the three dchimneyT regions dominating the Walker circulation. In
contrast, rainfall is zonally uniform in the upwelling portion of the Hadley circulation.
These global contrasts emphasize key differences between the traditional dhot towersT of
the tropical ITCZ (Riehl and Malkus, 1958) and the continental thunderstorms. The most
important difference is the updraft speed (Lemone and Zipser, 1980; Jorgenson and
Lemone, 1994; Lucas et al., 1994; Williams and Stanfill, 2002). Rainfall production and
lightning activity have quite different dependencies on updraft speed. Only modest lifting
is necessary for rainfall and the tropical upwelling is associated with moderate updrafts
over large areas. Stronger, deeper lifting is needed for lightning, with a strong dependence
of lightning flash rate on updraft speed (Baker et al., 1995, 1999; Boccippio, 2001).
Continental updrafts are stronger than oceanic ones but over a smaller total area.
The paucity of lightning found in the heavy rainfall over tropical oceans is indirect
evidence that the global circuit will not be strongly coupled with the Hadley circulation.
The abundance of lightning over land where high altitude cirrus production is most
prevalent (Kent et al., 1995) is indirect evidence that the Walker circulation will be
coupled with the global circuit.
3. The tropical chimneys
Satellite observations of lightning and rainfall have lent new scrutiny to the three
tropical dchimneyT regions that are well-recognized contributors to the global circuit
(Whipple, 1929). The ranking of these regions in lightning (Boccippio et al., 2000;
Christian et al., 2003; Williams and Satori, 2004) and rainfall now clearly indicate that
Africa is the most continental chimney (most lightning, least rainfall), the Maritime
Continent (Petersen et al., 1996) is the most maritime (most rainfall, least lightning), with
the South American continent as intermediate. South America is also strongly maritime
during the westerly wind regime of the wet season there, as documented by numerous
results in the NASA TRMM LBA program in 1999 (Cifeli et al., 2002; Petersen et al.,
2002; Halverson et al., 2002; Williams et al., 2002), and has been dubbed the dgreen
oceanT for that reason.
Evidence has long shown that South America dominates the ionospheric potential and
the DC global circuit (Whipple, 1929; Markson, 2003), whereas Africa is predominant in
lightning alone (Brooks, 1925; Christian et al., 2003) and therefore the AC global circuit
(Schumann resonances). Africa also appears to dominate in vigorous mesoscale
convective systems (MCS) (Toracinta and Zipser, 2001), in the energetic positive ground
flashes, which continental MCSs are known to produce (Armstrong, 2000; Patel, 2001;
Lyons et al., 2003), and the sprites caused by the positive ground flashes (Füllekrug and
Price, 2002).
The reasons for Africa’s strongly continental character and lightning dominance have
been attributed to surface characteristics (Williams and Stanfill, 2002; Williams and Satori,
2004) and to the effects of aerosol (McCollum et al., 2000; Rosenfeld, pers. comm., 2001;
Christian et al., 2003). A drier surface allows for the development of a deeper reservoir of
unstable air over the course of the diurnal cycle. A drier surface also allows for a higher
cloud base height and a suppression of warm rain coalescence beneath the 0 8C isotherm
E.R. Williams / Atmospheric Research 76 (2005) 272–287
275
(Williams et al., 2004, 2005). A more polluted boundary layer may keep the cloud droplets
small, suppress the warm-rain coalescence process, and allow for more liquid water to
enter the mixed phase region where it can invigorate the ice-based electrification process.
The relative importance of these distinct factors has not been resolved.
4. Lightning response to temperature
The global warming issue has dominated climate research for many years. Global
lightning is naturally integrated by Schumann resonances and electrified weather is
naturally integrated by the DC global circuit. The existence of these natural global
frameworks led logically to considerations of how lightning and electrified clouds respond
to temperature and temperature change. Work in this area is summarized in Table 1. A
large number of papers have appeared with many developments aided by new data sources
for global lightning, both optical (Boccippio et al., 2000; Christian et al., 2003) and rf
(Mackerras et al., 1998; Watkins et al., 1998; Huang et al., 1999; Moore and Idone, 1999;
Füllekrug and Constable, 2000; Price et al., 2002; Wood and Inan, 2002). Many time
scales have been explored for lightning variations: the diurnal (Price, 1993; Markson and
Price, 1999; Markson, 2003), the 5-day wave (Patel, 2001), the intraseasonal (Madden–
Julian oscillation) (Anyamba et al., 2000), the semiannual (Williams, 1994, 1999; Satori
and Zieger, 1996; Füllekrug and Fraser-Smith, 1998; Nickolaenko et al., 1998; Manohar et
al., 1999; Nickolaenko et al., 1999; Christian et al., 2003), the annual (Williams, 1994;
Nickolaenko and Rabinowicz, 1995; Heckman et al., 1998; Williams et al., 2000;
Christian et al., 2003) and the interannual, dominated by the ENSO (Williams, 1992;
Lopez and Holle, 1998; Reeve and Toumi, 1999; Goodman et al., 2000; Hamid et al.,
2001; Morales and Vergara, 2000, Satori and Zieger, 1999). Price and Rind (1992) have
predicted more lightning in a warmer world based on results with a general circulation
model with enhanced CO2 content. Smith et al. (2000), Gilmore and Wicker (2002), Carey
Table 1
Summary of studies on global variations in lightning and temperature
Time scale
Period,
day
Diurnal
1
Global 5-day wave 5
Intraseasonal (MJO) 30–60
Semiannual
182
Temperature Lightning
References
variation, 8C variation, %
1–2
?
0.5–1
1
~30–100
~50
~50
~50
Annual
365
4
50–100
ENSO
1000–2000
1
10–100
Decadal
3000–5000 0.3–0.6
~10
Whipple (1929), Price (1993),
Markson and Price (1999),
Christian et al. (2003), Markson (2003)
Patel (2001), Williams et al. (2001)
Anyamba et al. (2000)
Williams (1994), Satori and Zieger (1996),
Füllekrug and Fraser-Smith (1998),
Manohar et al. (1999), Christian et al. (2003),
Manohar and Kesarkar (2003)
Heckman et al. (1998), Christian et al. (2003),
Toumi and Qie (2004)
Williams (1992), Reeve and Toumi (1999),
Satori and Zieger (1999)
Kazadi and Kaoru (1996)
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E.R. Williams / Atmospheric Research 76 (2005) 272–287
et al. (2003), and Williams et al. (2004, 2005) have all called attention to enhancements in
clustered positive ground flash activity in the presence of elevated equivalent potential
temperature.
Physical mechanisms and hypotheses linking temperature and thermodynamics with
lightning and the global circuit are discussed in Williams et al. (2005) and will not be
repeated here. The evidence that lightning responds positively to temperature on all of the
foregoing time scales does not guarantee a pronounced global circuit response to
temperature. The key issue here is convective adjustment. On the diurnal time scale, for
example, the surface warms more readily than the upper atmosphere, as there is
insufficient time for the convection to bcommunicateQ with the upper troposphere. A
simple but radical form of forced convective adjustment, used largely for convenience in
global climate modeling, is the moist neutral state (Xu and Emanuel, 1989) in which
vertical temperature profiles are simply moist adiabats. This condition is not supported by
observations (Williams and Renno, 1993; Brown and Bretherton, 1997) but more
importantly, in the present context, would not sustain a global electrical circuit. An
atmosphere with finite but invariant CAPE (Emanuel et al., 1994), a less radical
convective adjustment, would support a global circuit (with possibly weak temperature
dependence) but is supported by neither observations (Gettelman et al., 2002) nor by
climate models (Ye et al., 1998). Within any framework involving finite CAPE, the
manner with which CAPE is evaluated is crucial to the water budget of the troposphere,
The pseudoadiabatic process dries the upper troposphere (Lindzen, 1990) by virtue of the
removal of cloud condensate the instant it forms. The reversible process (Xu and Emanuel,
1989) is more likely to flood the upper troposphere by virtue of the retention of all
condensate to the upper reaches of the convection. The reality is intermediate and
transcends the simplicities of parcel theory (Williams and Stanfill, 2002). The behavior of
the global electrical circuit on long time scales is a promising approach to the still
unresolved issues associated with convective adjustment in a turbulent atmosphere.
One naturally appeals to long time scales to understand convective adjustment and global
warming in the observations. The time scale of convenience (more than plan) is the
interannual, dominated by the tropical El Nino Southern Oscillation (ENSO). There remains
much debate about the relevance of the ENSO variations (Lindzen, 1995; Bauer et al., 2001)
to the problem of global warming by CO2 but, nevertheless, it remains a primary btest-bedQ
for observations. Foremost among these is the controversial issue of the water vapor
feedback in response to global warming (Sun and Held, 1996; Held and Soden, 2000).
The response of global lightning to temperature on the ENSO time scale is important
both from the standpoint of the water vapor feedback (Price, 2000; Soden, 2000) and from
the standpoint of convective adjustment. In considering all the results, a consistent feature
is for greater lightning in the warm (dEl NinoT) phase of the ENSO cycle. But here the
internal consistency of the observations ceases. Williams (1992) found a doubling in
Schumann resonance amplitude during the 1972 ENSO event that correlated with the
tropical temperature anomaly (Hansen and Lebedeff, 1987). Reeve and Toumi (1999)
found a positive interannual correlation but the lightning changes were extratropical.
Regional lightning studies on the ENSO time scale show a tropical enhancement in the
warm phase in one case (Hamid et al., 2001) and extratropical regional enhancements in
the other two cases (Goodman et al., 2000; Morales and Vergara, 2000).
E.R. Williams / Atmospheric Research 76 (2005) 272–287
277
Ongoing Schumann resonance observations show little change (b 10%) over the recent
(1997–1998) ENSO event, in seeming contradiction to results reported in Williams (1992).
The reasons of this difference are currently unknown. This modest ENSO response stands
in marked contrast with the systematic annual variation of global lightning (Heckman et
al., 1998; Goodman and Cecil, 2001; Christian et al., 2003; Williams and Satori, 2004)
that is close to a factor-of-two in amplitude, with a maximum during NH summer when the
Earth is warmest (on account of the greater continentality of the NH). Satori and Zieger
(1999) have inferred systematic changes in the meridional location of tropical thunderstorm regions on the basis of observations of Schumann resonance frequency variations on
the ENSO time scale. An investigation of ENSO anomalies in NASA OTD and LIS
observations is currently underway (D. Boccippio, pers. comm., 2003).
5. Upper tropospheric water vapor
The upper troposphere water vapor, despite its relatively small magnitude, is more
important than the water vapor in the boundary layer as a greenhouse substance for two
reasons. First, the energy transport by convective air motions in the lower troposphere
effectively short-circuits the radiative transfer relatively more than at upper levels (Brunt,
1952). Second, from a purely radiative standpoint, the boundary layer is substantially more
infrared-opaque than the upper troposphere (where the absolute humidity is comparatively
quite small) and so changes in water vapor at lower levels have relatively little effect (Held
and Soden, 2000). An important new finding in the area of lightning and climate is the
strong correlation between upper tropospheric water vapor and global lightning activity.
Price (2000) has used Schumann resonance measurements to compare with global
measurements of upper tropospheric water vapor and finds excellent agreement. This
finding is consistent with the thunderstorm’s role as an ice factory and a supplier of small
ice particles in the detraining anvils that spread out near tropopause level over areas
substantially greater than the updraft area. The predominance of the three tropical chimney
regions as water vapor sources is consistent with model computations of the thunderstorm
ice factory (Baker et al., 1995, 1999) and with global satellite observations of upper
tropospheric cirrus cloud (Kent et al., 1995).
On the basis of both model results and local observations of thunderstorms, Baker et al.
(1995), (1999) have suggested that lightning be used as a surrogate for the small ice
particle transport to the upper troposphere, which then contributes to water vapor there by
sublimation. This suggestion deserves attention because lightning is more easily measured
than water vapor. A remaining puzzle in this context is why the Maritime Continent, with
the least lightning activity (Christian et al., 2003), shows the most extensive area of highlevel cirrus (Kent et al., 1995; Wang et al., 2003), whereas Africa, with the greatest
lightning activity (Williams and Satori, 2004), shows the least area of high-level cirrus. A
likely resolution of this puzzle is that the Maritime Continent has very frequent, high-top
convection with modest updraft strengths (because the wet surface quells the instability).
Africa, in contrast, has a larger Bowen ratio (Williams and Satori, 2004), a stronger diurnal
cycle in temperature, more powerful but less frequent deep convection, and frequent
lightning but less cirrus.
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E.R. Williams / Atmospheric Research 76 (2005) 272–287
6. Climate extremes
The dtailsT of geophysical distribution functions are often the most volatile and variable.
A good example is the distribution of photons from the sun. An important prediction of
climate models is for larger numbers of extreme events (rainfall, floods, severe storms,
hurricanes) in a warmer climate (IPCC Report, 1995; Kunkel, 2003). Atmospheric
electrical studies naturally focus on extremes of convection and extremes in lightning.
Thunderstorms are extreme clouds at the convective scale. Mesoscale convective
complexes (MCCs) (Laing and Fritsch, 1997) are extreme cloud systems at the mesoscale.
The positive lightning flashes in MCCs (Huang et al., 1999; Lyons et al., 2003) are likely
the most extreme energetically and are causal to sprites in the mesosphere (Williams,
2001a,b).
Given these extremes, natural questions arise relevant to lightning and climate:
(1) Are mean thunderstorm flash rates larger in a warmer world? Using the annual time
scale as a surrogate for climate (Lindzen, 1995), Williams et al. (2000) found dnoT.
Using flash rates from the LIS between warm and cold phases of ENSO, Mushtak
and Williams (unpublished results, 2002) found dnoT.
(2) Are there more thunderstorms in a warmer world? Using warm land areas as a
surrogate for warmer climate, the answer is dyesT. Using the annual time scale as a
surrogate, the answer is also dyesT. Using general circulation models, the answer is
dyesT (Price and Rind, 1992).
(3) Are there more MCCs in a warmer world? With comparisons between warm and
cold phases of ENSO in extratropical South America (Velasco and Fritch, 1987), the
answer is dyesT.
(4) Are there more Schumann resonance Q-bursts (and associated sprites) with extreme
positive ground flashes in a warmer world? Work in progress using the ENSO as a
climate surrogate does not show strong effects.
In general, the evidence in Table 1 for diminished lightning sensitivity to temperature
on longer time scales casts some doubt on the existence of a reliable climate surrogate
(Lindzen, 1995) on shorter time scales in the lightning context. This is the problem of
convective adjustment, whose understanding is still incomplete.
7. The role for aerosol
The influence of the atmosphere aerosol on fair weather electricity has been known for
a century. An important role in climate has been realized only within the last couple of
decades. The development of innovative satellite methods for diagnosing cloud microphysics (Rosenfeld and Lensky, 1998; Rosenfeld and Woodley, 2003) in recent years has
greatly expanded the interest in aerosol as mediator of cloud microphysics, precipitation,
cloud electrification and lightning. The presumed role of increased aerosol concentration is
a reduced mean droplet size, a suppression of warm rain coalescence, and an enhancement
of the cloud water reaching the mixed phase region (Williams et al., 2002). Distinguishing
E.R. Williams / Atmospheric Research 76 (2005) 272–287
279
the aerosol influence from purely thermodynamic/dynamic effects has proven challenging,
as the published observations attest.
A three-fold enhancement of cloud-to-ground lightning flash density over Houston,
Texas in a 12-year record (Orville et al., 2001; Steiger et al., 2002) raises the issue of
pollution or heat island effect as a cause (Huff and Changnon, 1972). Though reductions in
rainfall (Rosenfeld, 1999) have been documented in association with pollution elsewhere,
the surplus of precipitation downwind from Houston by Shepherd and Burian (2003) casts
doubt on a dominant influence by pollution in this case. Both the precipitation
enhancement and the strong afternoon peak in the Houston lightning anomaly point to
a heat island effect as the primary causal agent. Further study is warranted and anticipated.
Steiger and Orville (2003) have more recently identified a lightning enhancement over oil
refineries near Lake Charles, Louisiana that they interpret as an aerosol effect. The sharp
discontinuity of flash density at the Texas and Louisiana coastlines suggests a primary role
for surface properties in controlling the lightning activity, however. Heat island and aerosol
effects are also under investigation in Brazil (Naccarato et al., 2003).
Experiments to verify enhanced lightning activity predicted by the aerosol hypothesis
were carried out by Williams et al. (2002) in Brazil. In the lightning-active pre-monsoon
period, the comparisons in measures of electrification between polluted and clean
conditions cast doubt on a primary role for aerosol in influencing the lightning. Recent
observations by Andreae et al. (2004), even earlier in the polluted transition season in
Brazil, suggest that aerosol can invigorate the convection. These results appear contrary to
the earlier findings of Williams et al. (2002). Quantitative measures of macroscopic
quantities (lightning, rainfall, and CAPE) were however lacking in the more recent study.
During the less active wet season, the distinction between aerosol and thermodynamic
contributions is even less clear-cut. Williams and Stanfill (2002) proposed tests to
distinguish the aerosol hypothesis from the traditional thermal hypothesis on the basis of
how lightning activity varies with island area. These results also support the thermal
hypothesis with islands acting in oceans as bheat islandsQ in the continent. Further studies
with islands in the same context have also been carried out (Williams et al., 2004, 2005),
with a similar result.
Studies in North America showing enhancements in cloud-to-ground flashes with
positive polarity (Lyons et al., 1998a; Murray et al., 2000) have been attributed to
incursions of smoke from fires in Mexico and subsequent ingestion by these thunderstorms. Substantial enhancements in peak current have also been identified. Later studies
in Florida with LISDAD (Lightning Imaging Sensor Data Acquisition and Display)
(Williams et al., 2004, 2005) identified thunderstorms that produced almost exclusively
positive ground flashes but were also found in conditions of extraordinary instability.
Subsequent exploration of thunderstorms in Brazil forming over smoke from biomass
burning (Williams et al., 2002) does not corroborate the results found earlier in North
America (Lyons et al., 1998a; Murray et al., 2000). This difference remains a puzzle.
Different aerosol types in different regions may be a factor. In light of earlier results
showing clustered positive ground flashes beneath storms developing in strong instability
(MacGorman and Burgess, 1994), it seems prudent that both aerosol and instability be
examined in future work. An important role of cloud base height has been identified
recently by Williams et al. (2004, 2005) and Williams and Satori (2004). More recent work
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E.R. Williams / Atmospheric Research 76 (2005) 272–287
(Jungwirth et al., 2003) suggests that aerosol chemistry also be explored in the context of
positive cloud-to-ground lightning.
Kuntz and Thuma (1999) have raised skepticism for the traditional storm-based
mechanism for the global electrical circuit and postulated that the gravity-driven descent of
negatively charged aerosol particles is the primary current source. They have not
substantiated their idea with observations, however.
8. Regional lightning climatology
The rapid expansion of lightning detection systems, both on the ground and in space,
has enabled a parallel expansion in the climatology of lightning. The behavior of lightning
has been investigated in Brazil (Petersen and Rutledge, 2001; Petersen et al., 2002;
Williams et al., 2002; Pinto et al., 1999, 2003; Pinto and Pinto, 2003), in Africa (Boccippio
et al., 2000; Patel, 2001; Füllekrug and Price, 2002), in India (Manohar et al., 1999;
Manohar and Kesarkar, 2003), in Spain (Rivas Soriano et al., 2001; De Pablo and Rivas
Soriano, 2002), in the Tibetan Plateau (Toumi and Qie, 2004), in Indonesia (Hamid et al.,
2001; Hidyat and Ishii, 1998), in Israel (Altaratz et al., 2003), the Mediterranean Sea
(Rivas Soriano and de Pablo, 2002; Altaratz et al., 2003), in the United States (Lyons et al.,
1998b; Huffines and Orville, 1999; Williams et al., 1999; Zajac and Rutledge, 2000;
Boccippio et al., 2001; Orville et al., 2002), over oceans (Füllekrug et al., 2002a), in the
tropics (Toracinta et al., 2002; Mushtak et al., 2005), in hurricanes (Cecil and Zipser, 1999,
2002; Cecil et al., 2002), and in mesoscale convective systems (Toracinta and Zipser,
2001).
9. Long period variations
A major stimulus in the long-standing debate about sun–weather relationships has been
the finding of correlation between galactic cosmic rays (GCR) and global cloud cover
(Svensmark and Friss-Christensen, 1997; Carslaw et al., 2002) on the 11-year solar cycle.
Udelhofen and Cess (2001) more recently found, using a longer record of cloud cover
observations in the U.S., coherency with the solar cycle but, in this case, the cloud cover
was out of phase on the 11-year cycle in GCR. The overall picture is unclear on the basis
of these two sets of observations. Tinsley (2000) and Tinsley and Yu (2004) have
suggested cloud microphysical mechanisms linking solar wind variations to atmospheric
energetics. The predominance of dwarmT cloud involvement in the long period correlations
(Marsh and Svensmark, 2000) casts doubt on those mechanisms involving glaciation,
though warm cloud albedo variation remains a possibility.
Correlations between ionospheric potential and GCR were reported earlier by Markson
(1981) on time scales much shorter than the 11-year solar cycle. One possible qualitative
explanation for a positive correlation between GCR and cloudiness would be an increase
in the columnar resistance caused by the cloudiness and hence an increase in ionospheric
potential for fixed supply current by electrified weather. Unfortunately, the variations of
ionospheric potential over the solar cycle are not well known, though Tinsley (1996)
E.R. Williams / Atmospheric Research 76 (2005) 272–287
281
presents measurements not at variance with theoretical model results (Roble and Hays,
1979).
A systematic decline in the global electrical circuit over the 20th century has been
claimed by Harrison (2002) on the basis of surface records of potential gradient in the
United Kingdom. The physical interpretation is based on correlations of Markson (1981)
between CR and ionospheric potential on shorter time scales and his physical
interpretation for this correlation (Markson, 1978). Given the evidence for correlated
behavior between the DC global circuit and lightning (Williams, 1992), Harrison’s results
would imply a substantial reduction (factor-of-two) in global lightning over the course of
the previous century, when global temperature is on the increase (Hansen and Lebedeff,
1987). An alternative explanation for Harrison’s observations is based on the documented
local decline in pollution in the UK (Novakov et al., 2003), with little change in the global
circuit (Williams, 2003).
Evidence for global changes in the frequencies and quality factors of Schumann
resonances on the 11-year time scale has been found by Füllekrug et al. (2002b) and Satori
et al. (2004). These variations are attributed to photon ionization in the upper dissipation
layer of the Earth–ionosphere cavity.
10. Conclusions
Global lightning activity and the global electrical circuit form a natural framework for
investigating climate issues. Global signals are evident on many different time scales and
forcing mechanisms for many of these time scales are well known. In considering the
coupling between the global circuit and the general circulation of the atmosphere, one needs
to draw distinctions between latent heating/rainfall and electrification/lightning because the
former is prevalent with shallow, gentle lifting of air, whereas the latter is caused by deeper,
stronger lifting. Key questions remain about the long-term response of lightning and the
global circuit to temperature change. High quality measurements of the global circuit may
serve as key diagnostics for adjustments in cloud buoyancy of the order of 1 8C.
Acknowledgements
I thank Steve Goodman for the use of Fig. 1. The author’s work on global lightning has
been supported by the Physical Meteorology section of the National Science Foundation
on Grant ATM-0003346, by the TRMM program at NASA GSFC on Grant NAG5-9637
and by the U.S.–Israel Binational Science Foundation.
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