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dence (but low) that global cloud amounts have tended to increase. On a regional
basis this is supported by reductions in evaporation as measured by pan evapori-
meters over the past several decades in Russia and the United States, and a world-
wide reduction in the land surface diurnal temperature range. Moreover, an increase
in water vapor has been documented over much of North America and in the tropics
(IPCC, 2001).
Changes in water vapor are very important for understanding climate change
since water vapor is the most important greenhouse gas in the atmosphere. The
measurement of changes in atmospheric water vapor is hampered by both data
processing and instrumental difficulties for both weather balloon and satellite retrie-
vals. The latter also suffers from discontinuities among successive satellites and
errors introduced by changes in orbits and calibrations. Upper tropospheric water
vapor is particularly important for climate feedbacks, but, as yet, little can be said
about how it has varied over the course of the past few decades.
4 IS THE ATMOSPHERIC=OCEANIC CIRCULATION CHANGING?
Sur prisingly, there is a considerable lack of reliable information about changes in
atmospheric circulation, even though it is of daily concern to much of the world
since it relates to day-to-day weather changes. Analyses of circulation are performed
every day on a routine basis, but the analysis schemes have changed over time,
making them of limited use for monitoring climate change. Moreover, even the
recent reanalysis efforts by the world’s major numerical weather prediction centers,
whereby the analysis scheme is fixed over the historical record, contains time-varying
biases because of the introduction of data with time-dependent biases and a changing
mix of data (e.g., introducing satellite data) over the course of the reanalysis
(Trenberth and Guillemot, 1997). Even less information is available on
measured changes and variations in ocean circulation.
A few major atmospheric circulation features have been reasonably well
measured because they can be represented by rather simple indices. This includes
the El Nin
˜
o–Southern Oscillation (ENSO) index, the North Atlantic Oscillation
(NAO) index, and the Pacific–North American (PNA) circulation pattern index.
There are interesting decadal and multidecadal variation, but it is too early to
detect any long-term trends. Evidence exists that ENSO has varied in period, recur-
rence interval, and strength of impact. A rather abrupt change in ENSO and other
aspects of atmospheric circulation seems to have occurred around 1976–1977. More
frequent ENSOs with rare excursions into its other extreme (La Nin
˜
a) became much
more prevalent. Anomalous circulation regimes associated with ENSO and large-
amplitude PNA patterns persisted in the North Pacific from the late 1970s into the
late 1980s, affecting temperature anomalies. Moreover, the NAO has been persistent
in its association with strong westerlies into the European continent from the late
1980s until very recently when it abruptly shifted. As a result, temperature anomalies
and storminess in Europe have abruptly changed over the past 2 years compared to
the past 7 or 8 years.
146 OBSERVATIONS OF CLIMATE AND GLOBAL CHANGE FROM REAL-TIME MEASUREMENTS
Increases in the strength of the Southern Hemisphere circumpol ar vortex during
the 1980s have been documented (van Loon et al., 1993; Hurrell and van Loon,
1994) using station sea level pressure data. This increase was associated with a
delayed breakdown in the stratospheric polar vortex and ozone deficit in the Antarctic
spring. A near-global sea level pressure data set has been used to identify changes in
circulation patterns in the Indian Ocean. Allan et al. (1995) and Salinger et al. (1995)
find that circulation patterns in the periods 1870–1900 and 1950–1990 were more
meridional than those in the 1900–1950 period, indicating intensified circulation
around anticyclones. These changes may be related to changes in the amplitude of
longwave troug hs to the south and west of Australia and the Tasman Sea=
New Zealand area and a subsequent decrease in preci pitation in Southwest Australia
(Nicholls and Lavery, 1992; Allan and Haylock, 1993).
5 IS THE WEATHER AND CLIMATE BECOMING MORE
EXTREME OR VARIABLE?
Climate and weather extremes are of great interest. Due to inadequate monitoring as
well as prohibitively expensive access to weather and climate data held by the
world’s national weather and environmental agencies, only limited reliable informa-
tion is available about large-scale changes in extreme weather or climate variability.
The time-dependent biases that affect climate means are even more difficult to
effectively eliminate from the extremes of the distributions of various weather and
climate elements. There are a few areas, however, where regional and global changes
in weather and climate extremes have been reasonably well documented (Easterling
et al., 2000a).
Interannual temperature variability has not changed significantly over the past
century. On shorter time scales and higher frequencies, e.g., days to a week, tempera-
ture variability may have decreased across much of the Nor thern Hemisphere (Karl
and Knight, 1995). Related to the decrease in high-frequency temperature variability
there has been a tendency for fewer low-temperature extremes, but widespread
changes in extreme high temperatures have not been noted.
Trends in intense rainfall have been examined for a variety of countries. Some
evidence suggests an increase in intense rainfalls (United States, tropical Australia,
Japan, and Mexico), but analyses are far from complete and subject to many discon-
tinuities in the record. The strongest increases in extreme precipitation are docu-
mented in the United States and Australia (Easterling et al., 2000b)
Intense tropical cyclone activity may have decreased in the North Atlantic, the
one basin with reasonably consistent tropical cyclone data over the twentieth century,
but even here data prior to World War II is difficult to assess regarding tropical
cyclone strength. Elsewhere, tropical cyclone data do not reveal any long-term
trends, or if they do they are most likely a result of inconsistent analyses. Changes
in meteorological assimilation schemes have complicated the interpretations of
changes in extratropical cyclone frequency. In some regions, such as the North
Atlantic, a clear trend in activity has been noted, as also in significant wave height s
5 IS THE WEATHER AND CLIMATE BECOMING MORE EXTREME OR VARIABLE? 147
in the northern half of the North Atlantic. In contrast, decreases in storm frequency
and wave heights have been noted in the south half of the North Atlantic over the
past few decades. These changes are also reflected in the prolonged positive excur-
sions of the NAO since the 1970s.
6 IS THE RADIATIVE FORCING OF THE PLANET CHANGING?
Understanding requi res a time history of forcing global change. The atmospheric
concentration of CO
2
, an important greenhouse gas because of its long atmospher ic
residence time and relatively high atmospheric concentration, has increased substan-
tively over the past few decades. This is quite certain as revealed by precise measure-
ments made at the South Pole and at Mauna Loa Observatory since the late 1950s,
and from a number of stations distributed globally that began operating in subse-
quent decades. Since atmospheric carbon dioxide is a long-lived atmospheric consti-
tuent and it is well mixed in the atmosphere, a moderate number of well-placed
stations operating for the primary purpose of monitoring seasonal to decadal
changes provides a very robust estimate of global changes in carbon dioxide.
To understand the causes of the increase of atmospheric carbon dioxide, the
carbon cycle and the anthropogenic carbon budget must be balanced. Balancing
the carbon budget requires estimates of the sources of carbon from anthropogenic
emissions from fossil fuel and cement production, as well as the net emission from
changes in land use (e.g., deforestation). These estimates are derived from a combi-
nation of modeling, sample measurem ents, and high-resolution satellite imagery. It
also requires measurements for the storage in the atmospher e, the ocean uptake, and
uptake by forest regrowth, the CO
2
and nitrogen fer tilization effect on vegetation, as
well as any operating climate feedback effects (e.g., the incre ase in vegetation due to
increased temperatures). Many of these factors are still uncertain because of a
paucity of ecosystem measurements over a sustained period of time. Anthropogenic
emissions from the burning of fossil fuel and cement production are the primary
cause of the atmospheric increase.
Several other radiatively important anthropogenic atmospheric trace constituents
have been measured for the past few decades. These measurements have confirmed
significant increases in atmospheric concentrations of CH
4
,N
2
O, and the halocar-
bons including the stratospheric ozone destructive agent of the chloroflourocarbons
and the bromocarbons. Because of their long lifetimes, a few well-placed high-
quality in situ stations have provided a good estimate of global change. Stratospheric
ozone depletion has been monitored both by satellite and ozonesondes. Both obser-
ving systems have been crucial in ascertaining changes of stratospheric ozone that
was originally of interest, not because of its role as a radiative forcing agent, but its
ability to absorb ultraviolet (UV) radiation prior to reaching Earth’s surface. The
combination of the surface- and space-based observing systems has enabled much
more precise measurements than either system could provide by itself. Over the past
few years much of the ozonesonde data and satellite data has been improved using
148 OBSERVATIONS OF CLIMATE AND GLOBAL CHANGE FROM REAL-TIME MEASUREMENTS
information about past calibration methods, in part because of differences in trends
between the two observing systems.
Figure 3 depicts the IPCC (1995) best estimate of the radiative forcing associ ated
with various atmospheric constituents. Unfortunately, measurements of most of the
forcings other than those already discussed have low or very low confidence, not
only because of our uncertainty about their role in the physical climate system, but
because we have not adequately monitored their change. For example, estimates of
changes in sulfate aerosol concentrations are derived from model estimates of source
emissions, not actual atmospheric concentrations. The problem is complicated
because of the spatially varying concentrations of sulfate due to its short atmospheric
lifetime. Another example is measurements of solar irradiance, which have been
taken by balloons and rockets for several decades, but continuous measurements of
top-of-the-atmosphere solar irradiance did not begin until the late 1970s with the
Nimbus 7 and the Solar Maximum Mission satellites. There are significant absolute
differences in total irradiance between satellites, emphasizing the critical need for
overlaps between satellites and absolute calibration of the irradiance measurements
to determine decadal changes. Spectrally resolved measurements will be a key
element in our ability to model the effects of solar variability, but at the present
time no long-term commitment has been made to take these measurements. Another
important forcing that is estimated through measured, modeled, and estimated
changes in opti cal depth relates to the aerosols injected high into the atmosphere
by major volcanic eruptions. The aerosols from these volcanoes are sporadic and
usually persist in the atmosphere for at most a few years. Improved measurements of
Figure 3 (see color insert) Estimates of globally and annually averaged radiative forcing (in
W=m
2
) for a number of agents due to changes in concentrations of greenhouse gases and
aerosols and natural changes in solar output from 1750 to the present day. Error bars are
depicted for all forcings (from IPCC, 2001). See ftp site for color image.
6 IS THE RADIATIVE FORCING OF THE PLANET CHANGING? 149
aerosol size distribution and compo sition will help better understand this agent of
climate change.
7 WHAT CAN WE DO TO IMPROVE OUR ABILITY TO DETECT
CLIMATE AND GLOBAL CHANGE?
Even after extensive reworking of past data, in many instances we are incapable of
resolving important aspects concerning climate and global change. Virtually every
monitoring system and data set requires better data quality, continuity, and fewer
time-varying biases if we expect to conclusively answer questions about how the
planet has changed, because of the need to rely on observations that were never
intended to be used to monitor the physical characteristics of the planet of the course
of decades. Long-term monitoring, capable of resolving decade-to-century-scale
changes, requires different strategies of operation.
In situ measurements are currently in a state of decay, decline, or rapid poorly
documented change due to the introduction of automated measurements without
adequate precaution to understand the difference between old and new observing
Figure 4 Global, annual-mean radiative forcings (Wm
2
) due to a number of agents for the
period from pre-industrial (1750) to the present. The height of the vertical bar denotes the
central or ‘‘best’’ estimate, no bar indicates that it is not possible to provide a ‘‘best’’ estimate.
The vertical line indicates an estimate of the uncertainty range and the level of scientific
understanding is a subjective judgement about the reliability of the forcing estimate based on
such factors as assumptions, degree of knowledge of the physical=chemical mechanisms, etc.
(From IPCC 2001). See ftp site for color image.
150
OBSERVATIONS OF CLIMATE AND GLOBAL CHANGE FROM REAL-TIME MEASUREMENTS
systems. Satellite-based systems alone will not and cannot provide all the necessary
measurements. Much wiser implementation and monitoring practices must be
adopted for both space-based and surface-based observing systems in order to
adequately understand global change. The establishment of the Global Climate
Observing System (GCOS) is a high priority (Spence and Townsend, 1995), and
continued encouragement by the World Meteorological Organization (WMO) of a
full implementation of this system in all countr ies is critical. Fur thermore, in the
context of the GCOS, a number of steps can be taken to improve our ability to
monitor climate and global change.
These include:
1. Prior to implementing changes to existing environmental monitoring systems
or introducing new observing systems, standard practice should include an
assessment of the impact of these changes on our ability to monitor
environmental variations and changes.
2. Overlapping measurements in time and space of both the old and new
observing systems should be standard practice for critical environmental
variables.
3. Calibration, validation, and knowledge of instrument, station, and=or plat-
form history are essential for data interpretation and use. Changes in
instrument sampling time, local environmental conditions, and any other
factors pertinent to the interpretation of the observations and measurements
should be recorded as a mandatory part of the observing routine and be
archived with the original data. The algorithms used to process observations
must be well documented and available to the scientific community. Docu-
mentation of changes and improvements in the algorithms should be carried
along with the data throughout the data archiving process.
4. The capability must be established to routinely assess the quality and
homogeneity of the historical database for monitoring environmental varia-
tions and change, including long-term high-resolution data capable of
resolving important extreme environmental events.
5. Environmental assessments that require knowledge of environmental varia-
tions and change should be well integrated into a global observing syst em
strategy.
6. Observations with a long uninterrupted record should be maintained. Every
effort should be made to protect the data sets that document long-term
homogeneous observations. Long term may be a century or more. A list of
prioritized sites or observations based on their contribution to long-term
environmental monitoring should be developed for each element.
7. Data-poor regions, variables, regions sensitive to change, and key
measurements with inadequate temporal resolution should be given the
highest priority in the design and implementation of new environmental
observing systems.
7 WHAT CAN WE DO TO IMPROVE OUR ABILITY TO DETECT CHANGE? 151
8. Network designers, operators, and instrument engineers must be provided
environmental monitoring requirements at the outset of network design. This
is particularly impor tant because most observing systems have been designed
for purposes other than long-term monitoring. Instruments must have
adequate accuracy with biases small enough to resolve environmental
variations and changes of primary interest.
9. Much of the development of new observation capabilities and much of the
evidence supporting the value of these observations stem from research-
oriented needs or programs. Stable, long-term commitments to these obser-
vations, and a clear transition plan from research to operations, are two
requirements in the development of adequate environmental monitoring
capabilities.
10. Data management systems that facilitate access, use, and interpretation are
essential. Freedom of access, low cost, mechanisms that facilit ate use
(directories, catalogs, browse capabilities, availability of metadata on station
histories, algorithm accessibility and documentation, on-line accessibility to
data, etc.), and quality control should guide data management. International
cooperation is critical for successful management of data used to monitor
long-term environmental change and variability.
REFERENCES
Allan, R. J., and M. R. Haylock (1993). Circulation features associated with the winter rainfall
decrease in southwestern Australia, J. Climate 6, 1356–1367.
Allan, R. J., J. A. Lindesay, and C. J. C. Reason (1995). Multidecadal variability in the climate
system over the Indian Ocean region during the austral summer, J. Climate 8, 1853–1873.
Easterling, D. R., G. Meehl, C. Parmesan, S. Changnon, T. Karl, and L. Mearns (2000a).
Climate extremes: Observations, modeling, and impacts, Science 289, 2068–2074.
Easterling, D. R., J. L. Evans, P. Ya. Groisman, T. R. Karl, K. E. Kunkel, and P. Ambenje
(2000b). Observed variability and trends in extreme climate events: A brief review, Bull.
Am. Meteor. Soc. 81, 417–426.
Elliott, W. P. (1995). On detecting long-term changes in atmospheric moisture, Climatic
Change 31, 219–237.
Groisman, P. Y., and D. R. Legates (1995). Documenting and detecting long-term precipitation
trends: Where we are and what should be done, Climatic Change 31, 471–492.
Huffman, G. J., R. F. Adler, P. Arkin, A. Chang, R. Ferraro, A. Gruber, J. Janowiak, A. McNab,
B. Rudolf, and U. Schneider (1997). The Global Precipitation Climatology Project (GPCP)
Combined Precipitation Dataset, Bull. Am. Meteor. Soc. 78, 5–20.
Hurrell, J. W., and K. E. Trenberth (1996). Satellite versus surface estimates of air temperature
since 1979, J. Climate 9, 2222–2232.
Hurrell, J. W., and H. van Loon (1994). A modulation of the atmospheric annual cycle in the
Southern Hemisphere, Tellus 46A, 325–338.
152
OBSERVATIONS OF CLIMATE AND GLOBAL CHANGE FROM REAL-TIME MEASUREMENTS
IPCC (1992). Climate Change, 1992, Supplementary Report, WMO=UNEP, J. T. Houghton,
B. A. Callander, and S. K. Varney (Eds.), New York, Cambridge University Press,
pp. 62–64.
IPCC (2001). Climate Change, 2001: The Scientific Basis. Contribution of Working Group 1 to
the Third Assessment Report of the Intergovernmental Panel on Climate Change,
J. T. Houghton, Y. Ding, D. J. Griggs, M. Noguer, P. J. van der Linden, X. Dai, K. Maskell,
and C. A. Johnson (Eds.), New York, Cambridge University Press.
Karl, T. R., R. W. Knight, and N. Plummer (1995). Trends in high-frequency climate variability
in the twentieth century, Nature 377, 217–220.
Luers, J. K., and R. E. Eskridge (1995). Temperature corrections for the VIZ and Vaisala
radiosondes, Appl. Meteor. 34, 1241–1253.
National Research Council (NRC) (2000). Reconciling Observations of Global Temperature
Change, Report of the Panel on Reconciling Temperature Observations, Washington, DC,
National Academy Press.
Nicholls, N., and B. Lavery (1992). Australian rainfall trends during the twentieth century, Int.
J. Climatology 12, 153–163.
Reynolds, R. W. (1988). A real-time global sea surface temperature analysis, J. Climate 1,
75–86.
Rossow, W. B., and B. Cairns (1995). Monitoring changes in clouds, Climatic Change 31,
175–217.
Salinger, M. J., R. Allan, N. Bindoff, J. Hannah, B. Lavery, L. Leleu, Z. Lin, J. Lindesay,
J. P. MacVeigh, N. Nicholls, N. Plummer, and S. Torok (1995). Observed variability and
change in climate and sea level in Oceania, in Greenhouse: Coping with Climate Change,
W. J. Bouma, G. I. Pearman, and M. R. Manning (eds.), CSIRO, Melbourne, Australia 100–
126.
Sevruk, B. (1982). Methods of correcting for systematic error in point precipitation measure-
ments for operational use, Hydrology Rep. 21, World Meteorological Organization, Geneva
589.
Spence, T., and J. Townsend (1995). The Global Climate Observing System (GCOS), Climatic
Change 31, 131–134.
Spencer, R. W. (1993). Global oceanic precipitation from the MSU during 1979–1992 and
comparison to other climatologies, J. Climate 6, 1301–1326.
Spencer, R. W., and J. R. Christy (1992a). Precision and radiosonde validation of satellite
gridpoint temperature anomalies, Part I: MSU channel 2. J. Climate 5, 847–857.
Spencer, R. W., and J. R. Christy (1992b). Precision and radiosonde validation of satellite
gridpoint temperature anomalies, Part II: A tropospheric retrieval and trends during
1979–90, J. Climate 5, 858–866.
Trenberth, K. E., and C. J. Guillemot (1997). Evaluation of the atmospheric moisture and
hydrologic cycle in the NCEP reanalysis, Clim. Dyn. 14, 213–231.
van Loon, H., J. W. Kidson, and A. B. Mullan (1993). Decadal variation of the annual cycle in
the Australian dataset, J. Climate 6, 1227–1231.
REFERENCES 153
CHAPTER 13
WHY SHOULD WE BELIEVE
PREDICTIONS OF FUTURE CLIMATE?
JOHN MITCHELL
1 INTRODUCTION
Three-dimensional models of the atmosphere are based on laws of classical physics,
including the conservation of momentum (the Navier–Stokes equations), heat (the
first law of thermodynamics, the perfect gas law), mass and water vapor, allowing for
sources and sinks. The state variables are temperat ure. the northerly and easterly
wind components, and water vapor. For the ocean, salt is included rather than water
vapor, winds are replaced by currents, and the equation of state for seawater is
used (Fig. 1).
The state variables are held on a three-dimensional grid, which for current atmo-
spheric models is typically 200 to 300 km in the horizontal and on about 20 levels in
the vertical. This gives over 60,000 basic variables. The equations are solved to
produce the rates of change of these variables, and hence new values of the state
variables a small time interval ahead. This process is repeated over and over again to
produce the evolution of the syst em. In practice, a variety of numerical techniques
(explicit, implicit, semi-implicit, semi-Lagrangian) and spatial representations
(spectral and finite difference) are used. The time step varies typically from
10 min to an hour.
Other variables are diagnosed each time step from the state variables (e.g., cloudi-
ness, precipitation and latent heat release, radiative heating rates, surface evapora-
tion, g round wetness, and snow cover) and, where appropriate, are used for the
source and sink terms in the basic equations. Process es that occur on a scale too
small to be represented explicitly by the model grid have to be included approxi-
Handbook of Weather, Climate, and Water: Dynamics, Climate, Physical Meteorology, Weather Systems,
and Measurements, Edited by Thomas D. Potter and Bradley R. Colman.
ISBN 0-471-21490-6 # 2003 John Wiley & Sons, Inc.
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