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Appendix C
Key Gaps in the Science of Climate Change
From the White House document at:
"Advancing the Science of Climate Change" Key Gaps in Science of Climate Change
[PDF]
Despite major investments in climate change science by the United States and
other nations over the past decade, and despite major accomplishments by
scientists throughout the world, numerous gaps remain in our understanding of
climate change. The National Academy of Sciences identified in its report,
Climate Change Science: An
Analysis of Some Key Questions,
(June 2001),
critical uncertainties about the science of climate change. Fundamentally,
the report indicated the need to better understand the causes of warming. The
National Academy of Sciences stated, "Greenhouse gases are accumulating in
Earth's atmosphere as a result of human activities, causing surface air
temperatures and subsurface ocean temperatures to rise. Temperatures are, in
fact, rising. The changes observed over the last several decades are likely
mostly due to human activities, but we cannot rule out that some significant
part of these changes is also a reflection of natural variability."
The National Academy of Sciences report goes on to identify a range of
specific areas that require additional study and research. The following
considerations provide important information that will be used in setting
priorities:
How much carbon is sequestered by oceans and terrestrial sinks and how much
remains in the atmosphere is uncertain:
"How land contributes, by location and processes, to exchanges of
carbon with the atmosphere is still highly uncertain. . . ." (p. 11)
"These estimates [of future carbon dioxide climate forcings] . . . are
only approximate because of uncertainty about how efficiently the ocean
and terrestrial biosphere will sequester atmospheric CO2." (p.
13)
"How much of the carbon from future use of fossil fuels will be seen as
increases in carbon dioxide in the atmosphere will depend on what
fractions are taken up by land and by the oceans. The exchanges with land
occur on various time scales, out to centuries for soil decomposition in
high latitudes, and they are sensitive to climate change. Their projection
into the future is highly problematic." (p. 18)
The feedbacks in the climate system that determine the magnitude and rate of
temperature increases are uncertain:
"Because there is considerable uncertainty in current understanding of
how the climate system varies naturally and reacts to emissions of
greenhouse gases and aerosols, current estimates of the magnitude of
future warming should be regarded as tentative and subject to future
adjustments (either upward or downward)." (p. 1)
"Much of the difference in predictions of global warming by various
climate models is attributable to the fact that each model represents
these [feedback] processes in its own particular way. These uncertainties
will remain until a more fundamental understanding of the processes that
control atmospheric relative humidity and clouds is achieved." (p. 4)
The direct and indirect effects of aerosols are uncertain:
"The greatest uncertainty about the aerosol climate forcing -- indeed, the
largest of all the uncertainties about global climate forcings -- is probably
the indirect effect of aerosols on clouds." (p. 14)
"The great uncertainty about this indirect aerosol climate forcing
presents a severe handicap both for the interpretation of past climate
change and for future assessments of climate changes." (p. 14)
"Climate forcing by anthropogenic aerosols is a large source of
uncertainty about future climate change." (p. 13)
"Because of the scientific uncertainties associated with the sources
and composition of carbonaceous aerosols, projections of future impacts on
climate are difficult." (p. 12)
The details and impacts of regional climate change resulting from global
climate change are uncertain:
"On the regional scale and in the longer term, there is much more
uncertainty" with respect to effects on agriculture and forestry. (p. 19)
"The Northern Hemisphere as a whole experienced a slight cooling from
1946-75, and the cooling during that period was quite marked over the
eastern United States. The cause of this hiatus in the warming is still
under debate." (p. 16)
"Health outcomes in response to climate change are the subject of
intense debate. . . .The understanding of the relationships between
weather/climate and human health is in its infancy and therefore the
health consequences of climate change are poorly understood. The costs,
benefits, and availability of resources for adaptation are also
uncertain." (p. 20)
"Changes in storm frequency and intensity are one of the more uncertain
elements of future climate change prediction." (p. 20)
The nature and causes of the natural variability of climate and its
interactions with forced changes are uncertain:
"Because of the large and still uncertain level of natural variability
inherent in the climate record and the uncertainties in the time histories
of the various forcing agents (and particularly aerosols), a causal
linkage between the buildup of greenhouse gases in the atmosphere and the
observed climate changes during the 20th
century cannot be unequivocally established." (p. 17)
"The value of indirect effect of ozone changes induced by solar
ultraviolet irradiance variations "remains highly uncertain." (p. 14)
The future usage of fossil fuels and the future emissions of methane are
uncertain:
"With a better understanding of the sources and sinks of methane, it
may be possible to encourage practices...that lead to a decrease in
atmospheric methane and significantly reduce future climate change." (p.
13)
"There is no definitive scientific basis for choosing among several
possible explanations for these variations in the rates of change of
global methane contributions, making it very difficult to predict its
future atmospheric concentrations." (p. 11)
In response to these gaps in our knowledge, the National Academy of Sciences
study also recommends, "research that couples physical, chemical, biological,
and human systems; an improved capability of integrating scientific knowledge,
including its uncertainty, into effective decision support systems, and an
ability to conduct research at the regional or sectoral level that promotes
analysis of the response of human and natural systems to multiple stresses."
The NAS report also indicates that to advance the understanding of climate
change, it will be necessary to have "a global observing system in support of
long-term climate monitoring and prediction [and] concentration on large-scale
modeling through increased, dedicated supercomputing and human resources." High
priority areas for further research also are identified in numerous recent
reports and documents, such as:
Global Environmental
Change: Research Pathways for the Next Decade (1999),
Capacity of U.S.
Climate Modeling to Support Climate Change Assessment Activities (1998),
Adequacy of
Climate Observing Systems (1999), and others.
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