Good morning, Mr. Chairman and members of the
Committee. I am Tom Karl, Director of the National Oceanic and
Atmospheric Administration’s (NOAA’s) National Climatic Data
Center (NCDC). The NCDC is the largest archive of weather and
climate data in the world and it is one of three data centers
operated by NOAA’s National Environmental Satellite Data and
Information Services line office, within the Department of
Commerce. I have been invited to discuss the science of climate
change.
The information I present to you today is based
on the findings from two assessments, one carried out
internationally and one carried out nationally. Specifically, I
refer to the 2001 report of the Intergovernmental Panel on
Climate Change (IPCC) and the National Academy of Sciences (NAS)
June 6, 2001, report, "Climate Change Science: Analysis of
Some Key Questions." Over the past three years, I have had
the privilege of working with my scientific peers as a
Coordinating Lead Author and panel member, respectively, on each
of these two recent assessments.
The IPCC assessment took almost three years to
prepare and represents the work of hundreds of scientific
authors worldwide. It is based on the scientific literature, and
was carefully scrutinized by hundreds of scientific peers
through an extensive peer review process. The independent NAS
report was requested by the current administration, and was a
consensus report compiled by an 11-member panel of leading U.S.
climate scientists, including a mix of scientists who have been
skeptical about some findings of the IPCC and other assessments
on climate change. The NAS panel addressed a series of questions
posed by the present administration.
First, I want to emphasize two fundamental
issues of importance. These have been long-known, are very well
understood, and have been deeply underscored in all previous
reports and other such scientific summaries.
* The natural "greenhouse" effect
is real, and is an essential component of the planet's climate
process. A small percentage (roughly 2%) of the atmosphere
is, and long has been, composed of greenhouse gases (water
vapor, carbon dioxide, ozone and methane). These effectively
prevent part of the heat radiated by the Earth's surface from
otherwise escaping to space. The global system responds to this
trapped heat with a climate that is warmer, on the average, than
it would be otherwise without the presence of these gases. In
the absence of these greenhouse gases the temperature would be
too cold to support life as we know it today. Of all the
greenhouse gases, water vapor is by far the most dominant, but
other gases are more effective at trapping heat energy from
certain portions of the electromagnetic spectrum where water
vapor is semi-transparent to heat escaping from the Earth’s
surface.
In addition to the natural greenhouse effect
above, there is a change underway in the greenhouse radiation
balance, namely:
* Some greenhouse gases are increasing in the
atmosphere because of human activities and increasingly trapping
more heat. Direct atmospheric measurements made over the
past 40-plus years have documented the steady growth in the
atmospheric abundance of carbon dioxide. In addition to these
direct real-time measurements, ice cores have revealed the
atmospheric carbon dioxide concentrations of the distant past.
Measurements using air bubbles trapped within layers of
accumulating snow show that atmospheric carbon dioxide has
increased by more than 30% over the Industrial Era (since 1750),
compared to the relatively constant abundance that it had over
the preceding 750 years of the past millennium. The predominant
cause of this increase in carbon dioxide is the combustion of
fossil fuels and the burning of forests. Further, methane
abundance has doubled over the Industrial Era, but its increase
has slowed over the recent decade for reasons not clearly
understood. Other heat-trapping gases are also increasing as a
result of human activities. We are unable to state with
certainty the exact rate at which these gases will continue to
increase because of uncertainties in future emissions as well as
how these emissions will be taken up by the atmosphere, land,
and oceans. We are certain, however, that once in the atmosphere
these greenhouse gases have a relatively long life-time, in the
order of decades to centuries. This means they become well mixed
throughout the globe.
*The increase in heat-trapping greenhouse
gases due to human activities are projected to be amplified by
feedback effects, such as changes in water vapor, snow cover,
and sea ice. As atmospheric concentrations of carbon dioxide
and other greenhouse gases increase, the resulting increase in
surface temperature leads to less sea ice and snow cover helping
to raise temperatures even further. As snow and sea ice
decrease, more of the Sun’s energy is absorbed by the planet
instead of being reflected back to space by the underlying snow
and sea ice cover. Present evidence also suggests that as
greenhouse gases increase, evaporation increases leading to more
atmospheric water vapor. Additional water vapor acts as a very
important feedback to further increase temperature. Our present
understanding suggests that these feedback effects account for
about 60% of the warming. The magnitude of these feedback
effects and others, such as changes in clouds, remain a
significant source of uncertainty related to our understanding
of the impact of increasing greenhouse gases. Increases in
evaporation and water vapor affect global climate in other ways
besides increasing temperature such as increasing rainfall and
snowfall rates.
The increase in greenhouse gas concentrations in
the atmosphere implies a positive radiative forcing, i.e., a
tendency to warm the climate system.
*Particles (or aerosols) in the atmosphere
resulting from human activities can also affect climate. Aerosols
vary considerably by region. Some aerosol types act in a sense
opposite to the greenhouse gases and cause a negative forcing or
cooling of the climate system (e.g., sulfate aerosol), while
others act in the same sense and warm the climate (e.g., soot).
In contrast to the long-lived nature of carbon dioxide
(centuries), aerosols are short-lived and removed from the lower
atmosphere relatively quickly (within a few days). Therefore,
human generated aerosols exert a long-term forcing on climate
only because their emissions continue each day of the year.
Aerosol effects on climate can be manifested directly by their
ability to reflect and trap heat, but they can also have an
indirect effect by changing the lifetime of clouds and changing
their reflectivity to sunshine. The magnitude of the negative
forcing of the indirect effect of aerosols is highly uncertain,
but may be larger than the direct effect of aerosols.
Emissions of greenhouse gases and aerosols due
to human activities continue to alter the atmosphere in ways
that are expected to affect the climate. There are also natural
factors which exert a forcing on climate, e.g., changes in the
Sun's energy output and short-lived (about 1 to 2 years)
aerosols in the stratosphere following episodic and explosive
volcanic eruptions. The forcing estimates in the case of the
greenhouse gases are greater than for these two other forcing
agents.
What do these changes in the forcing agents
mean for changes in the climate system? What climate changes
have been observed? How well are the causes of those changes
understood? Namely, what are changes due to natural factors, and
what are changes due to the greenhouse-gas increases? Is there a
safe level of greenhouse gas concentrations? And, what does this
potentially imply about the climate of the future? These
questions bear directly on our understanding of the science of
climate change.
* There is a growing set of observations that
yields a collective picture of a warming world over the past
century. The global-average surface temperature has
increased over the 20th Century by 0.4 to 0.8° C (0.7 to
1.4°F). This occurred both over land and the oceans. The
average temperature increase in the Northern Hemisphere over the
20th Century is likely to have been the largest of any century
during the past 1,000 years, based on "proxy" data
(and their uncertainties) from tree rings, corals, ice cores,
and historical records. The 1990s are likely to have been the
warmest decade and 1998 the warmest year of the past 1000 years.
Other observed changes are consistent with this warming. There
has been a widespread retreat of mountain glaciers in non-polar
regions. Snow cover, sea ice extent and sea ice thickness, and
the duration of ice on lakes and rivers have all decreased.
Ocean heat content has increased significantly since the late
1940s, the earliest time when we have adequate computer
compatible records. The global-average sea level has risen
between 10 to 20 centimeters (4 to 8 inches), which is
consistent with a warmer ocean occupying more space because of
the thermal expansion of sea water and loss of land ice.
*It is likely that the frequency of heavy and
extreme precipitation events has increased as global
temperatures have risen. This is particularly evident in
areas where precipitation has increased, primarily in the mid
and high latitudes of the Northern Hemisphere. Other extremes
have decreased such as the frequency of extremely cold weather
and the frequency of frost during the period of the instrumental
record , e.g., 50 to 200 years depending on location.
* There is new and stronger evidence that most
of the warming observed over the last 50 years is attributable
to human activities. The 1995 IPCC climate-science
assessment report concluded: "The balance of evidence
suggests a discernible human influence on global climate."
There is now a longer and more closely scrutinized observed
temperature record. Climate models have evolved and improved
significantly since the previous assessment. Although many of
the sources of uncertainty identified in 1995 still remain to
some degree, new evidence, longer and more precise data sets,
and improved understanding support the updated conclusion.
Namely, recent analyses have compared the surface temperatures
measured over the last 1000, 140, and 50 years to those
simulated by mathematical models of the climate system, thereby
evaluating the degree to which human influences can be detected.
Both natural climate-change agents (solar variation and
episodic, explosive volcanic eruptions) and human-related agents
(greenhouse gases and aerosols) were included. The natural
climate-change agents alone do not explain the warming.
* Scenarios of future human activities indicate
continued changes in atmospheric composition throughout the 21st
century. The atmospheric abundances of greenhouse gases and
aerosols over the next 100 years cannot be predicted with high
confidence, since the future emissions of these species will
depend on many diverse factors, e.g., world population,
economies, technologies, and human choices, which are not
uniquely specifiable. Rather, the IPCC assessment aimed at
establishing a set of scenarios of greenhouse gas and aerosol
abundances, with each based on a picture of what the world
plausibly could be over the 21st Century. Based on these
scenarios and the estimated uncertainties in climate models,
e.g., feedback effects, the resulting projection for the global
average temperature increase by the year 2100 ranges from 1.3 to
5.6° C (2.3°to10.1°F). Approximately half of the uncertainty
in this range is due to model uncertainties related to feedback
effects and half is due to different scenarios of future
emissions. Regardless of these uncertainties, such a projected
rate of warming would be much larger than the observed 20th
Century changes and would very likely be without precedent
during at least the last 10,000 years. The corresponding
projected increase in global sea level by the end of this
century ranges from 9 to 88 centimeters (4 to 35 inches).
Uncertainties in the understanding of some climate processes
make it more difficult to project meaningfully the corresponding
changes in regional climate. Future climate change will, of
course, depend on the technological developments that enable
reductions of greenhouse gas emissions.
There is a basic scientific aspect that has been
underscored with very high confidence in all of the IPCC
climate-science assessment reports (1990, 1995, and 2001). It is
repeated here because it is a key (perhaps "the" key)
aspect of a greenhouse-gas-induced climate change:
* A greenhouse-gas warming could be reversed
only very slowly. This quasi-irreversibility arises because
of the slow rate of removal (centuries) from the atmosphere of
many of the greenhouse gases and because of the slow response of
the oceans to thermal changes. For example, several centuries
after carbon dioxide emissions occur, about a quarter of the
increase in the atmospheric concentrations caused by these
emissions is projected to still be in the atmosphere.
Additionally, global average temperature increases and rising
sea levels are projected to continue for hundreds of years after
a stabilization of greenhouse gas concentrations (including a
stabilization at today's abundances), owing to the long time
scales (decades to centuries) on which the deep ocean adjusts to
climate change. Because of its large specific heat capacity and
mass, the world ocean can store large amounts of heat and remove
this heat from direct contact with the atmosphere for long
periods of time.
*It is presently not possible to generally
define a safe level of greenhouse gases. This issue was
specifically addressed in the recent NAS study. There are
several difficulties related to answering this question. First,
as I have indicated, there are still large uncertainties related
to the projected rate and magnitude of climate change. The
determination of an acceptable concentration of greenhouse gases
depends on knowing this as well as knowledge of the risks and
vulnerabilities to climate change. A range of climate
sensitivities and emission scenarios could be used to explore
sensitivities to climate change. A first attempt was reported in
the National Climate Assessment and the recent IPCC report.
Analyses reveal that sectors and regions vary in their
sensitivity to climate change, but generally those societies and
systems least able to adapt and those regions with the largest
changes are at greatest risk. This includes the poorer nations
and sectors of our society, natural ecosystems, and those
regions likely to see the largest changes. For example, on
average, the largest increases of temperature and relative
changes in precipitation projected by all models are in the mid
to high latitudes of the Northern Hemisphere. Clearly, as the
rate and magnitude of climate change increases, the risk of
exceeding a safe level of greenhouse gases also increases. This
includes the possibility of surprises. As greenhouse gases
continue to increase there is an ever increasing, but still very
small chance, that the climate system could respond in an
unpredictable fashion. Examples include a shut-down of the
transport of heat in the North Atlantic Ocean thermohaline
circulation which could lead to large regional climate
anomalies, melting of the Greenland Ice Sheet or the Antarctic
Ice Shelf, substantial increases in hurricane intensity, and
other possibilities. None of these changes are foreseen at
present, but we cannot absolutely dismiss the possibility of a
surprisingly large and rapid change in climate.
*Because there is considerable uncertainty in
current understanding of how the natural variability of the
climate system reacts to emissions of greenhouse gases and
aerosols, current estimates of the magnitude and impacts of
future warming are subject to future adjustments (either upward
or downward). Nonetheless, it is noteworthy that our best
estimates of climate sensitivity to greenhouse gases have
essentially remained unchanged over the past three decades,
since the first National Academy of Sciences report on this
topic back in the 1960s. In addition to the uncertainty related
to the rate and magnitude of climate change, there is
considerable uncertainty related to quantifying the impact of
climate change on natural and human systems.
*To address these uncertainties, several
areas of study have been identified in the assessments.
Because understanding the climate system and its impacts is so
complex, progress will be hindered by the weakest link in the
chain. At the present time, there are several weak links that
need to be addressed. First and foremost a climate observing
system is needed to monitor long-term change for basic variables
needed to describe the climate system. Current observing systems
yield uncertainties in several key parameters, especially on
regional and local space scales. Although we have been able to
link observed changes to human activities, it is not possible to
quantitatively identify the specific contribution of each
forcing factor, which is required for the most effective
strategy to prevent large or rapid climate change.
To address these uncertainties, the President
has directed the Cabinet-level review of U.S. climate change
policy. Based on the Cabinet’s initial findings, the President
in his June 11 remarks committed his Administration to invest in
climate science. He announced the establishment of the U.S.
Climate Change Research Initiative to study areas of uncertainty
and to identify areas where investments are critical. He
directed the "Secretary of Commerce, working with other
agencies, to set priorities for additional investments in
climate change research, review such investments, and to provide
coordination amongst federal agencies. We will fully fund
high-priority areas for climate change science over the next
five years. We’ll also provide resources to build climate
observation systems in developing countries and encourage other
developed nations to match our American commitment."
I would like to underscore that we will use the
descriptions of the uncertainties identified in the NAS report
as the basis for development of U.S. research in climate. Cited
areas of uncertainty include:
-
Feedbacks in the climate system that
determine the magnitude and rate of temperature increases
and related precipitation changes
-
Future usage of fossil fuels
-
Carbon sequestration on land and in the
ocean
-
Details of regional climate change
-
Natural climate variability and the
interaction of these modes with other climate forcings
including greenhouse gases and the direct and indirect
effects of aerosols