WRITTEN STATEMENT BY
DR. THOMAS R. KARL
DIRECTOR, NATIONAL CLIMATIC
NATIONAL OCEANIC AND ATMOSPHERIC ADMINISTRATION
(NOAA)
FOR AN OVERSIGHT HEARING:
INTRODUCTION TO CLIMATE CHANGE
BEFORE THE
COMMITTEE ON GOVERNMENT REFORM
JULY 20, 2006
Mr. Chairman and Members of the Committee: As Director of the National
Climatic Data Center, which is part of the National Environmental Satellite,
Data, and Information Service (NESDIS) within the National Oceanic and
Atmospheric Administration (NOAA), and as Program Manger for one of five
different NOAA Climate Goal Programs (Climate Observations and Analysis), I am
pleased to have the opportunity to testify before you today. The
The U.S.
Climate Change Science Program (CCSP) integrates federal research on global
climate change, as sponsored by thirteen federal agencies.[1] CCSP is a multi-agency program charged with:
investigating natural and human-induced changes in the Earth's global environmental
system; monitoring, understanding, and predicting global change; and providing
a sound scientific basis for national and international decision-making. The CCSP combines the near-term focus of the
Administration’s Climate Change Research Initiative — including a focus on
advancing the understanding of aerosols and carbon sources and sinks and
improvements in climate modeling — with the breadth of the long-term research
elements of the US Global Change Research Program.
Since CCSP was created in 2002, the program has successfully integrated a wide range of research, climate science priorities and budgets of the thirteen CCSP agencies. CCSP integrates research and observational approaches across disciplinary boundaries and is also working to create more seamless approaches between theory, modeling, observations, and applications required to address the multiple scientific challenges posed by changes in climate. With an approximately $2 billion annual expenditure in 2006, CCSP is taking on the most challenging questions in climate science and is developing products to convey the most advanced state of knowledge to be used by federal, state and local decision makers, resource managers, the science community, the media, and the general public. Over the next two years CCSP will be completing a series of 21 Synthesis and Assessment Reports, the first of which was just released a few months ago. The collection of these Synthesis and Assessment Report will address many of the issues pertinent to this testimony.
I will provide an overview of the current understanding of the
atmosphere in terms of: the role that greenhouse gases play in the atmosphere;
evidence for how greenhouse gases are already influencing the climate in both
general and in specific ways; an introduction to the use of global climate
models, and some of the evidence that has led a number of assessments,
including the IPCC (2001), the National Research Council (2002), and the
Climate Change Science Program Synthesis and Assessment Report 1.1 (Karl et
al., 2006) to link the rise in temperature over the past several decades to
increases in greenhouse gases and related changes climate.
Atmospheric
Composition and Greenhouse Gases
The natural "greenhouse" effect is real, and is an essential
component of the planet's climate process. A small percentage (roughly 2 percent) of the
atmosphere is, and long has been, composed of greenhouse gases (water vapor,
carbon dioxide, ozone and methane). These
gases effectively prevent part of the heat radiated by the Earth's surface from
otherwise escaping to space. The response
of the global system to this trapped heat is a climate that is warmer than it
would be otherwise without the presence of these gases. In the absence of these greenhouse gases the
temperature on Earth 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 whereas water
vapor is semi-transparent to heat escaping from the Earth’s surface.
In addition to the natural greenhouse effect outlined above, there is a change
underway in the greenhouse radiation balance.
Some greenhouse gases are increasing in the atmosphere because of human
activities and increasingly trapping more heat. Direct atmospheric measurements made over the
past 50 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 nearly 35 percent over the
Industrial Era (since 1750), compared to the relatively constant abundance of
carbon dioxide over at least the preceding 750 years of the past millennium
(Figure 1). 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 the increase of methane 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 uncertainties
regarding 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 residence
time, on the order of decades to centuries (IPCC, 2001). This means they become well mixed throughout
the globe.
Increases in heat-trapping greenhouse gases 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 cover and sea ice
decrease, more of the Sun’s energy is absorbed by the planet, instead of being
reflected back to space by the snow cover and sea ice. Present evidence also suggests that as
greenhouse gases lead to temperature increases, evaporation increases leading
to more atmospheric water vapor (Trenberth, et al., 2005; Soden, B.J., et al.,
2005). Additional water vapor (which, as
mentioned above, is the dominant greenhouse gas) acts as a very important
feedback to further increase temperature. The most uncertain feedback is related to clouds. Specifically, changes in cloud frequency,
location, and height. The range of
uncertainty spans from a significant positive feedback to no feedback, or even
a slight negative feedback. Our present
understanding suggests that these feedback effects account for at least half of
the warming (IPCC, 2001; Karl and Trenberth, 2003). The exact magnitude of these feedback effects
remains a significant source of uncertainty related to our understanding of the
impact of increasing greenhouse gases. For
example, increases in evaporation and water vapor affect global climate in
other ways besides increasing temperature such as increasing rainfall and
snowfall rates, and accelerating drying during droughts. 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 by reflecting more solar
radiation back to space than the heat they absorb, and cause a negative forcing
or cooling of the climate system (e.g., sulfate aerosol). Other aerosols act in the same way as
greenhouse gases, 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 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 the clouds reflectivity
to sunshine. The magnitude of the
negative forcing of the indirect effect of aerosols is highly uncertain, but
may be larger than negative forcing of the direct effect of aerosols (IPCC,
2001).
Emissions of greenhouse gases and aerosols continue to alter the atmosphere in
ways that are expected to affect the climate, e.g., temperature and
precipitation extremes, reduction in snow cover and sea ice, changes in storm
track and intensity (IPCC, 2001). By altering the planet’s natural energy flows
changes in temperature, evaporation, precipitation, storms are affected. There are also natural factors which exert a
forcing on climate, e.g., changes in the Sun's energy output and short-lived (a
few years) aerosols in the stratosphere following episodic and explosive
volcanic eruptions. If we sum up all the
possible influences of natural and human climate forcings over the past several
decades then the increase of greenhouse gases are larger than all the other
forcings and continue to grow disproportionately larger (Karl and Trenberth,
2003; IPCC, 2001).
Human activities also have a large-scale
impact on the land surface. Changes in
land use through urbanization and agricultural practices, although not global,
are often most pronounced where people live, work, and grow food, and are part
of the human impact on climate. Large-scale
deforestation and desertification in Amazonia and the
There is no doubt that the composition
of the atmosphere is affected by human activities. Today greenhouse gases are the largest human
influence on atmospheric composition.
What exactly
is a climate model and why is it useful?
Many of the
scientific laws governing climate change and the processes involved can be
quantified and linked by mathematical equations. Figure 2 shows schematically the kinds of
processes that can be included in climate models. Among these are many earth system components
such as atmospheric chemistry, ocean circulation, sea-ice, land-surface
hydrology, biogeochemistry[3],
atmospheric circulation, etc. The
physics of many, though not all, of the processes governing climate change are
well understood, and may be described by mathematical equations. Linking these equations creates mathematical
models of climate that may be run on computers or super-computers. Coupled climate models can include mathematical
equations describing physical, chemical, and biogeochemical processes, and are
used because the climate system is composed of different interacting components.
In fact,
coupled climate models are the preferred way to approach climate modeling. This is because if we put all our
understanding into a single model, it would be too complex to run on any
existing computer systems. The decisions
for how to build any given climate model includes trade-offs between the
complexity of the model and number of Earth system components included, the
horizontal and spatial resolution within the model, and the number of years of
simulations the model can produce per day of computer time. Consequently, there is a hierarchy of model
complexity, often based on the degree to which approximations are required for each
model or component processes omitted.
Approximations
in climate models represent aspects of the models that require parameter
choices and “tuning.” As a simple
example, imagine a single cumulus cloud and how it has to be represented in a
global climate model. The cloud may
encompass only a few hundred meters in the vertical and horizontal extent, which
is much finer resolution than can be run on today’s coupled atmosphere and
ocean climate models. This then means that
in order to incorporate such clouds into the climate model, some approximations
have to be made regarding the statistical properties of such clouds within say
an area 100 or 1000 times larger than the cloud itself. This is referred to as model
parameterization, and the process of selecting the most appropriate parameters
to best simulate observed conditions is called model tuning. Similar methods are also required in today’s
state-of-the-science weather forecasting models.
An important
difference between weather forecasting models and climate models is that
weather models are initialized with a specific set of observations representing
today’s weather to precisely predict the weather “x” days or hours into the
future. The initial starting conditions
of the climate models, however, are not nearly as important. Climate models are used to simulate many
years of “weather” into the future with the intent of understanding the
difference in the collection of weather events at some point in the future,
compared to some other time in the past (often the climate of the last 30 years
or so). This comparison enables
scientists to study the output of climate model simulations to understand the
effect of various modifications of those aspects of the climate system that might
cause the climate to change. A key
challenge in climate modeling is to isolate and identify cause and effect –
which requires knowledge about the changes and variations of the external
forcings controlling climate, and a comprehensive understanding of climate feedbacks
(such as a change in the earth’s reflectivity because of a change sea ice or
cloud amount) and natural climate variability.
Model
simulations of climate over specified periods can be verified and validated
against the observational record. Models
that prove to describe climate variability and change well can be used as a
tool to increase our understanding of the climate system. Once evaluated and validated, climate models can
then be used for predictive purposes. Given
specific forcing scenarios, climate models can provide viable projections of
future climate. In fact, climate models
have become the primary means to predict climate, although prediction is ultimately
likely to be achieved through a variety of means, including the observed rate
of global climate change.
How do we know
the global air temperature is increasing?
There has now been a comprehensive analysis of the changes of
temperatures near the surface and throughout much of the atmosphere in the April
2006 Climate Change Science Program Synthesis and Assessment Report 1.1. This report addressed the nagging issue of
differences in the rate of warming between measurements derived near the
surface and those taken from the atmosphere.
The surface air temperatures are derived from several different analyses
teams using various combinations of ocean ships and buoys, land observations
from weather reporting stations, and satellite data. Atmospheric data sets have been derived using
satellites, weather balloons, and a combination of the two.
Considering all the latest satellite, balloon and surface records, the
CCSP report concluded there is no significant discrepancy between the rates of global
temperature change over the past several decades at the surface compared to
changes higher in the atmosphere. The
report does, however, acknowledge there are still uncertainties in the tropics,
and this is primarily related to data from weather balloons. There is uncertainty as to whether scientists
have been able to adequately adjust for known biases and errors in the data,
especially in the tropics where many developing nations struggle to routinely
launch weather balloons and process these measurements.
Globally, data indicate that rates of temperature change have been
similar throughout the atmosphere since 1979 when satellite data were first
available, and the rates of temperature change have been slightly greater in
the atmosphere compared to the surface air temperature since 1958 (the time at
which weather balloons had adequate spatial coverage for global calculations). The global surface temperature time series,
shown in Figure 3, indicates warming on even longer time scales, with
acceleration since 1976.
Instrumental temperature measurements are not our only evidence for
increasing global temperatures. The
observed increased melting of glaciers can be used to estimate the rate of
temperature increase since the late 19th Century. Estimates of the near-surface temperature
based on glacial melting are very similar to estimates based on instrumental temperature
data. There has been a 15-20 percent
reduction in Arctic sea ice since the 1970s, a 10 percent decrease in snow
cover since the 1970s, and shortened periods of lake and river ice cover (about
2 weeks shorter since the 19th century). Also, ocean heat content has significantly
increased over the past several decades (IPCC, 2001).
Why do we
think humans are influencing the Earth’s climate?
The scientific community has been actively working on detection and
attribution of climate change as related to human activities since the
1980s. As described above, one set of
tools often used to examine these issues are mathematical computer models of
the climate. Outstanding issues in
modeling include specifying forcing mechanisms (e.g., the causes of climate
variability and change) within the climate system; properly dealing with
complex feedback processes that affect carbon, energy, and water sources, sinks
and transports; and improving simulations of regional weather, especially
extreme events. Today’s inadequate or
incomplete measurements of the various forcing mechanisms, with the exception
of well-mixed greenhouse gases, add uncertainty when trying to simulate past
and present climate. Confidence in our
ability to predict future climate depends on our ability to use climate models
to attribute past and present climate change to specific causes.
Recent carbon dioxide emission trends are upward with increases between
0.5 and 1 percent per year over the past few decades. Concentrations of both reflective and
nonreflective aerosols are also estimated to be increasing. Radiative forcings[4]
from greenhouse gases dominate over the net cooling forcings from aerosols and the
global temperature has exceeded the bounds of natural variability. This has been the case since about 1980. As an example of how models are used to detect
human influence on the climate system Figure 4 shows that without including all
the observed forcing mechanisms the models cannot replicate the observed global
temperature changes. There are many
other aspects of the climate system besides global surface temperatures that
have been tested for human influences.
Today, there is convincing evidence from a variety of climate change detection
and attribution studies pointing to human influences on climate. These include regional analyses of changes in
temperature, the paleoclimatic[5]
temperature record, three dimensional analysis of atmospheric temperature
change, changes of free atmospheric temperature, changes in sea ice extent and
other components of the cryosphere, changes in ocean heat content, and new
studies on extreme weather and climate events.
Thus, there is considerable confidence that the observed warming,
especially the period since 1970s is mostly attributable to increases in
greenhouse gases (IPCC, 2001; Karl et al., 2006; Stott et al., 2001; Stott et
al., 2006; Tett et al., 2002; Hegerl et al., 2001; Gillet et al., 2002; Zhang
et al., 2006; Allen, 2005; Zweirs and Zhang, 2003; Stone and Allen, 2005; Karoly
and Wu, 2005; and many others).
Changes in
Extremes in the
The U.S. Climate Change Science Program Synthesis and Assessment Report
3.3 will specifically address the issue of changes in extreme events, focusing
on
Increasing air temperature leads to increased water vapor in the
atmosphere. By raising the air
temperature, the capacity of the atmosphere to hold more water vapor is
increased, which defines the upper bounds of the amount of precipitation that
can occur during short term (~daily or less) extreme precipitation events. Surface
moisture, if available (as it always is over the oceans), effectively acts as
the “air conditioner” of the surface – as heat used for evaporation moistens
the air rather than warming it. Therefore,
another consequence of global heating of the lower troposphere is accelerated
land-surface drying and more atmospheric water vapor (the dominant greenhouse
gas). Satellite measurements now confirm
a significant increase in atmospheric water vapor (Trenberth et al., 2005; Dai
et al.,2005), consistent with theoretical expectations given the rate of
observed atmospheric warming during the past several decades. Accelerated drying, without an increase in
precipitation, increases the incidence and severity of droughts (Dai et al., 2004),
whereas additional atmospheric water vapor increases the risk of heavy
precipitation events (Trenberth et al., 2003).
Increases in global temperature also increase sea surface temperatures,
one of several important factors affecting the hurricane intensity.
NOAA’s
Changes
in Heavy and Extreme Precipitation
Basic theory, climate model simulations, and empirical evidence (Figure
6) confirm that warmer climates, owing to increased water vapor, lead to more
intense precipitation events even when the total precipitation remains
constant, and with prospects for even stronger events when precipitation
amounts increase. Figure 7 depicts the
aggregate land-surface world-wide changes in heavy precipitation events over
the last half of the 20th century with an associated geographic
depiction of where changes in heavy precipitation have occurred, with most
areas showing increases. World-wide, an
increase of a few percent in heavy precipitation events is evident since the
middle of the 20th century, particularly in the middle and high
latitudes. By the end of the 21st
Century a conservative estimate of the projected increase in the amount of
precipitation that would occur in one day (a one-in-twenty-year heaviest daily
precipitation event) is between 10-20 percent (Zweirs and Zhang, 2005). This assumes carbon dioxide does not exceed
550 ppmv[6].
The practical implications of addressing these changes are seen in
NOAA’s recent update of the
In many geographic regions increases in extreme precipitation is
occurring, even when changes in total precipitation are relatively constant
(Groisman et al., 2003 and 2005; Alpert, 2005).
Changes
in Drought Severity and Frequency
Drought is a recurring feature of the climate system. In other words, we have had major droughts in
the past, and expect to have major droughts in the future. At any given time, at least part of the
For the continental
record occurred from 1933 to 1938. In July 1934, 80 percent of the
moderate or greater drought (Figure 8), and 63 percent was experiencing
severe to
extreme drought. During
1953-1957, severe drought covered up to 50 percent of the
country. Paleoclimatic data
(e.g. tree ring measurements) have been used to reconstruct
drought patterns for the period prior to the modern instrumental record
(Cook et al.,
1999 and 2004). These
reconstructions show that during most of the past two millennia
the climate of the western
Western drought from 1999 to 2004 that strongly affected the
exceeded in severity as recently as the 19th century. Within the past millennium there
have been severe droughts in both the western
multiple decades.
Long-term warming trends have also led to changes in the
timing of snow melt and stream flows, especially in the West. This is resulting in earlier peak stream flows
and diminished summer-time flows.
Drought is a recurring feature of the climate in the
Changes
in Hurricane Intensity and Frequency
Tropical storms and particularly hurricanes, are an important issue of
concern for the
An important consideration in hurricane intensity is a trend toward
warmer sea surface temperatures, particularly in the tropical Atlantic and Gulf
of Mexico, indicating climate change may play some role in the increased hurricane
intensity (Emanuel, 2005; Webster et
al., 2005). Another factor is a slow
cycle of natural fluctuations in atmospheric conditions and ocean temperatures
in the
What does the future hold for hurricane activity? In the near term, it is expected that
favorable conditions for Atlantic hurricanes will persist for the next decade
or so based on previous active periods.
For the longer term, climate models project an increase in the intensity
of strong hurricanes late in the 21st Century Knutson, (2006). Specifically, this translates to increases in
wind speed and about a ½ category increase in intensity on the commonly used
Saffir-Simpson Hurricane Intensity Scale as tropical sea surface temperatures increase
by nearly 2°C. Given those conditions (stronger hurricanes and warmer
tropical sea surface temperatures) climate models also predict an increase in storm
rainfall rates of about 20 percent (Knutson 2006). However, it is unclear if the total number of
hurricanes will change in future years (IPCC, 2001).
New analyses of precipitation rates for different strengths of
landfalling Atlantic tropical cyclones (both hurricanes and tropical storms)
over the southeastern
Overall, the issue of hurricanes and climate change is an ongoing
debate. The scientific community has
varying viewpoints on the magnitude of influence of global climate change on
hurricanes and how long the current active period will last. NOAA recognizes the debate and continues to
study hurricane development, intensity, activity, and modeling.
Conclusion
The state of the science continues to indicate that modern climate
change is affected by human influences, primarily human-induced changes in
atmospheric composition. These
perturbations result mainly from emissions associated with energy use, but on
local and regional scales, urbanization and land use changes are also important
contributors to climate change. While
there is still considerable uncertainty about the rates of change that can be
expected, it is clear these changes will be increasingly manifested in
important and tangible ways, such as changes in extremes of temperature and
precipitation, decreases in seasonal and perennial snow and ice extent, sea
level rise, and now there is accumulating evidence to suggest that there will
be increases in hurricane intensity and related heavy and extreme
precipitation. Furthermore, while there
has been progress in monitoring and understanding the causes of climate change,
there remain many scientific, technical, and institutional challenges to
precisely planning for, adapting to, and mitigating the effects of climate
change. The U.S. Climate Change Science
Program is addressing the scientific dimensions of these challenges by
facilitating the creation and application of knowledge of the Earth’s global
environment through research, observations, decision support, and
communication. The program’s vision is
to improve the nation’s ability to manage the risks and opportunities of change
in the climate and related environmental systems. Within the next two years the program will
produce a series of synthesis and assessment reports that describe the
scientific state-of-the-art on a range of key issues, thereby providing further
important contributions to the nation’s discussions of climate change. As mentioned above, the first in the series
(Climate Change Science Synthesis and Assessment Product 1.1) was released
earlier this year on the topic of temperature trends, and has already made a
valuable contribution to the national dialogue.
Thank you again, Mr. Chairman for allowing me the opportunity to help
inform the Committee about climate change.
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Figure 1: Changes in atmospheric concentration of carbon dioxide, methane,
and nitrous oxide since 1000 A.D. (from IPCC, 2001).
Figure 2. Components of the climate system and the
interactions among them, including the human component. All these components
have to be modeled as a coupled system that includes the oceans, atmosphere,
land, cryosphere, and biosphere.
Figure 3: Globally averaged surface air temperature and carbon dioxide
concentration (parts per million by volume) since 1880 (Updated from Karl and
Trenberth, 2003).
Figure 4: Climate model simulations of the global air temperature for
the period 1860-2000. Figure 4a includes
only natural forcing mechanisms such as volcanic eruptions and solar
variability; 4b includes only anthropogenic greenhouse gas increases; and 4c
includes both natural and anthropogenic forcing mechanisms (from IPCC 2001).
Figure 5: The U.S. Climate Extremes Index (CEI) is the average of the
percent of
Figure 6: The diagram shows that warmer climates (red) have a higher
percentage of total rainfall coming from heavy and very heavy events. The data are based on a worldwide distribution
of observing stations, but each have the same seasonal mean precipitation
amount of 230 (±5) mm. For cool climates
(blue), there are more daily precipitation events than in warmer climates
(Adapted from Karl and Trenberth, 2002).
The various cloud and rain symbols reflect the various daily
precipitation rates and have been categorized in the top panel of this figure
to reflect the approximate proportion of the various precipitation rates for
cool, moderate, and warm climates across the globe.
Figure 7: Changes in the contribution of heavy precipitation events to
the annual total amount. The annual
values are smoothed by the orange line to better represent decadal variability
and change. Globally there has been a change of nearly two percent since the
mid-20th century (from Alexander et al., 2006: Global observed
changes in daily climate extremes of temperature and precipitation. J. Geophys. Res., D05109, doi:10.1029/2005JD006290).
Figure 8: The percentage of the contiguous
Figure 9: The number of hurricanes striking the