The 2001 Assessment of Climate Change
Testimony of
Dr. Kevin E. Trenberth*
National
Center for Atmospheric Research**
before
The
U. S. Senate Committee on Environment and Public Works
The
United States Senate
Room
628 of the Dirksen Senate Building
9:30
a.m., May 2, 2001
Introduction
My name is Kevin
Trenberth. I am the Head of the Climate
Analysis Section at NCAR, the National Center for Atmospheric Research. I am especially interested in global-scale
climate dynamics; the observations, processes and modeling of climate changes
from interannual to centennial time scales.
I have served on many national and international committees including
National Research Council/National Academy of Science committees, panels and/or
boards. I served on the National
Research Council Panel on Reconciling
observations of global temperature change, whose report was published in
January 2000. I co-chaired the
international CLIVAR Scientific Steering Group of the World Climate Research
Programme (WCRP) from 1996 to 1999 and I remain a member of that group as well
as the Joint Scientific Committee that oversees the WCRP as a whole. CLIVAR is short for Climate Variability and
Predictability and it deals with variability from El Niño to global warming. I have been involved in the global warming
debate and I have been extensively involved in the Intergovernmental Panel on
Climate Change (IPCC) scientific assessment activity as a lead author of
individual chapters, the Technical Summary, and Summary for Policy Makers (SPM)
of Working Group (WG) I.
The IPCC is a body of
scientists from around the world convened by the United Nations jointly under
the United Nations Environment Programme (UNEP) and the World Meteorological
Organization (WMO) and initiated in 1988.
Its mandate is to provide policy makers with an objective assessment of
the scientific and technical information available about climate change, its
environmental and socio-economic impacts, and possible response options. The
IPCC reports on the science of global climate change and the effects of human
activities on climate in particular. Major assessments were made in 1990, 1995
and now 2001. Each new IPCC report
reviews all the published literature over the previous 5 years or so, and
assesses the state of knowledge, while trying to reconcile disparate claims and
resolve discrepancies, and document uncertainties.
WG I deals with how the
climate has changed and the possible causes.
It considers how the climate system responds to various agents of change
and our ability to model the processes involved as well as the performance of
the whole system. It further seeks to
attribute recent changes to the possible various causes, including the human
influences, and thus it goes on to make projections for the future. WG II deals with impacts of climate change
and options for adaptation to such changes, and WG III deals with options for
mitigating and slowing the climate change, including possible policy options. Each WG is made up of participants from the
United Nations countries, and for the 2001 assessment, WG I consisted of 123
lead authors, 516 contributors, 21 review editors, and over 700 reviewers. The IPCC process is very open. Two major reviews were carried out in
producing the report, and skeptics can and do participate, some as
authors. The strength is that the
result is a consensus report. The SPM
was approved line by line by governments in a major meeting. The rationale is that the scientists
determine what can be said, but the governments determine how it can best be
said. Negotiations occur over wording
to ensure accuracy, balance, clarity of message, and relevance to understanding
and policy. The latest report (IPCC
2001) reaffirms in much stronger language that the climate is changing in ways
that cannot be accounted for by natural variability and that “global warming”
is happening. A summary and commentary
is given in Trenberth (2001).
Observed Climate Change
Analyses of observations of surface
temperature show that there has been a global mean temperature increase of
about 1.2°F over the past one hundred years. The calendar year 1998 is the warmest on
record, exceeding the previous record held by 1997. Preliminary annual global mean temperatures in the latest year,
2000, were about the same as for 1999.
The 1990s are the warmest decade on record. Synthesis of information from tree rings, corals, ice cores and
historical data further indicates that these years are the warmest in at least
the past 1000 years for the Northern Hemisphere, which is as far back as
annual-resolution hemispheric estimates of temperatures can be made. The melting of glaciers over most of the
world and rising sea levels confirm the reality of the global temperature
increases. The warming is observed over
land and ocean, and over both hemispheres.
It is not an urban heat island effect.
Further supporting evidence comes from the substantial retreat and
thinning of Arctic sea ice, increased temperatures throughout the upper layers
of the global oceans, decreases in Northern Hemisphere snow cover and in the
freezing season of lakes and rivers.
There is good evidence for
decadal changes in the atmospheric circulation and for ocean changes. These mean that increases in temperature are
not uniform or monotonic; some places warm more than the average and some
places cool. A good example is the past
winter, where it was cold and temperatures were well below average in most of
the lower 48 states, but Alaska had its warmest winter on record, averaging 9°F above normal. Similarly it was very warm throughout Europe.
Changes in precipitation and
other components of the hydrological cycle vary considerably
geographically. It is likely that
precipitation has increased by perhaps 1%/decade in the 20th Century over most
mid and high latitude continents of the Northern Hemisphere. Over the United States, surface temperatures
have not risen as much as over Eurasia and instead it has become wetter, with
more very heavy events, and this pattern has been shown to be a response to the
general warming of the tropical oceans (Hoerling et al. 2001). Changes in climate variability and extremes
are beginning to emerge.
One persistent issue has been
the discrepancy in trends from the so-called satellite temperature record and
that at the surface. The satellite
record begins in 1979 and measures microwave radiation from the Earth coming
from about the lowest 5 miles of the atmosphere. Consequently it does not measure the same thing as the surface
temperature. Climate models run with
increasing greenhouse gases suggest that warming in the lower atmosphere should
be larger than at the surface whereas the observed record shows much less
warming from 1979–1999 and this has been highlighted by skeptics. However, when observed stratospheric ozone
depletion is included, the models suggest that the surface and tropospheric
temperatures should increase at about the same rate. In fact this is what has happened from about 1960 to the present
based on balloon observations that replicate the satellite record after
1979. The shorter satellite record is
influenced by El Niño and effects of volcanic eruptions, and thus the Mt.
Pinatubo eruption in 1991 leads to a relative downward trend in the lower atmosphere. Other effects, such as from cloudiness
changes, have not been quantified but also influence the two records
differently. Accordingly, the small
trend in the satellite record is not inconsistent with the observed larger
trend in surface temperatures (NRC 2000).
Human Influences
The amount of carbon dioxide
in the atmosphere has increased by about 31% since the beginning of the
industrial revolution, from 280 parts per million by volume (ppm) to 367 ppm,
owing mainly to combustion of fossil fuels and the removal of forests. In the absence of controls, future
projections are that the rate of increase in carbon dioxide amount may
accelerate and concentrations could double from pre-industrial values within
the next 50 to 100 years. Several other
greenhouse gases are also increasing in concentration in the atmosphere from
human activities (especially biomass burning, agriculture, animal husbandry,
fossil fuel use and industry, and through creation of landfills and rice
paddies). These include methane,
nitrous oxide, the chlorofluorocarbons (CFCs) and tropospheric ozone, and they
tend to reinforce the changes from increased carbon dioxide. However, the observed decreases in lower
stratospheric ozone since the 1970s, caused principally by human-introduced
CFCs, contribute to a small cooling.
Human activities also affect
the tiny airborne particulates in the atmosphere, called aerosols, which
influence climate in other ways.
Aerosols occur in the atmosphere from natural causes; for instance, they
are blown off the surface of deserts or dry regions. The eruption of Mt. Pinatubo in the Philippines in June 1991
added considerable amounts of aerosol to the stratosphere which, for about two
years, led to a loss of radiation at the surface and a cooling. Human
activities contribute to aerosol particle formation mainly through injection of
sulfur dioxide into the atmosphere (which contributes to acid rain)
particularly from power stations, and through biomass burning. A direct effect of resulting sulfate aerosols,
which are seen as the milky whitish haze from airplane windows, is the
reflection of a fraction of solar radiation back to space, which tends to cool
the Earth’s surface. Other aerosols
(like soot) directly absorb solar radiation leading to local heating of the
atmosphere, and some absorb and emit infrared radiation. A further influence of aerosols is that many
act as nuclei on which cloud droplets condense, affecting the number and size
of droplets in a cloud and hence altering the reflection and the absorption of
solar radiation by the cloud. Because
man-made aerosols are mostly introduced near the Earth’s surface where they can
be washed out of the atmosphere by rain, they typically remain in the
atmosphere for only a few days and they tend to be concentrated near their
sources such as industrial regions.
They therefore affect climate with a very strong regional pattern and
usually produce cooling. In contrast,
the greenhouse gases are not washed out.
Their long lifetimes ensure a build up in amounts over time, as is
observed to be happening.
The determination of the
climatic response to the changes in heating and cooling is complicated by
feedbacks. Some of these can amplify
the original warming (positive feedback) while others serve to reduce it (negative
feedback). If, for instance, the amount
of carbon dioxide in the atmosphere were suddenly doubled, but with other
things remaining the same, the outgoing long-wave radiation would be reduced
and instead trapped in the atmosphere.
To restore the radiative balance, the atmosphere must warm up and, in
the absence of other changes, the warming at the surface and throughout the
troposphere would be about 1.2°C. In reality, many
other factors will change, and various feedbacks come into play, so that the
best IPCC estimate of the average global warming for doubled carbon dioxide is
2.5°C.
In other words, the net effect of the feedbacks is positive and roughly
doubles the response otherwise expected.
The main positive feedback comes from increases in water vapor with
warming.
In 2001, the IPCC gave
special attention to this topic. The
many issues with water vapor and clouds were addressed at some length in
Chapter 7 (of which I was a lead author, along with Professor Richard Lindzen
(M.I.T.), and others). Recent
possibilities that might nullify global warming (Lindzen 2001) were considered
but not accepted because they run counter to the prevailing evidence, and the
IPCC (Stocker et al. 2001) concluded that “the balance of evidence favours a
positive clear sky water vapour feedback of the magnitude comparable to that
found in the simulations.”
Increases in greenhouse gases
in the atmosphere produce global heating (“global warming”) which leads to
expectations for increases in global mean temperatures (often mistakenly
thought of as global warming), but other changes in weather are also
important. In particular, surface
heating enhances the evaporation of moisture and thus enhances the hydrological
cycle (see Trenberth 1999). Global
temperature increases signify that the water-holding capacity of the atmosphere
increases and, together with enhanced evaporation, this means that the actual
atmospheric moisture should increase, as is observed to be happening in many
places. Because water vapor is a powerful
greenhouse gas, this provides a positive feedback. It also follows that naturally-occurring droughts are likely to
be exacerbated by enhanced drying. Thus
droughts, such as those set up by El Niño, are likely to set in quicker, plants
wilt sooner, and the droughts may become more extensive and last longer with
global warming. Once the land is dry
then all the solar radiation goes into raising temperature, bringing on
sweltering heat waves. Further,
globally there must be an increase in precipitation to balance the enhanced
evaporation. The presence of increased
moisture in the atmosphere implies stronger moisture flow converging into
precipitating weather systems. This
leads to the expectation of enhanced rainfall and snowfall events, which are
also being observed in many areas. In general,
it is observed that where an increase in precipitation occurs, more falls as
heavy events, increasing risk of flooding.
Modeling and Attribution of Climate
Change
The best climate models
encapsulate the current understanding of the physical processes involved in the
climate system, the interactions, and the performance of the system as a
whole. They have been extensively
tested and evaluated using observations.
They are exceedingly useful tools for carrying out numerical climate
experiments, but they are not perfect, and so have to be used carefully
(Trenberth 1997). Key issues in global
climate change remain those of firstly detecting whether the recent climate is
different than should be expected from natural variability, and secondly
attributing the climate changes to various causes, including the human
influences. The latest models have
increasingly been able to reproduce the climate of the past century or so. Also their estimates of natural variability
are compatible with those from the paleoclimate reconstructions. As a result, they can break down the
contributions to the warming into components.
Increases in solar luminosity probably were responsible for some of the
warming from about 1910 to 1950 (perhaps as much as 0.3°F), but the warming of about 0.7°F in the past 30 years can only be
accounted for by the increases in greenhouse gases in the atmosphere. Consequently, after much debate in the final
plenary, the IPCC (2001) carefully crafted the following: “In the light of new
evidence, and taking into account the remaining uncertainties, most of the
observed warming over the last 50 years is likely to have been due to the
increase in greenhouse gas concentrations.”
In 1995 the IPCC assessment
concluded that “the balance of evidence suggests a discernible human influence
on global climate” (IPCC 1996). Since
then the evidence has become much stronger — from the recent record warmth, the
improved paleo-record that provides context, better understanding of the role
of stratospheric ozone depletion, improved modeling and simulation of the past
climate, and improved statistical analysis.
Thus the headline in IPCC (2001) is “There
is new and stronger evidence that most of the warming observed over the last 50
years is attributable to human activities.” The best assessment of global warming is that the human climate
signal emerged from the noise of background variability in the late 1970s.
Biggest impact is likely to
be felt by making the extremes more extreme.
For any change in mean climate, there is likely to be an amplified
change in extremes. The wide range of
natural variability associated with day-to-day weather means that we are
unlikely to notice most small climate changes except for the extremes. Extremes are exceedingly important to both
natural systems and human systems and infrastructure, as we are adapted to a
range of natural weather variations, and it is these extremes that exceed
tolerances and cause nonlinear effects: the so-called “straw that breaks the
camel’s back.” For instance, floods
that used to have an expected return period of 100 years may now recur in 50 or
30 years. In practice, this effect may
be experienced in floods through dams or levees that break, inundating the
surrounding countryside and urban areas, resulting in loss of life, water
damage, and more subtle effects such as polluted drinking waters.
The attribution of the recent
climate change to the increases in greenhouse gases in spite of uncertainties
related to aerosols has direct implications for the future. Because of the long lifetime of carbon
dioxide and the slow penetration and equilibration of the oceans, there is a
substantial future commitment to further global climate change even in the
absence of further emissions of greenhouse gases into the atmosphere. Future projections of climate change depend
on future emissions. They are given by
the IPCC and not detailed here. In spite
of differences among models and the many uncertainties that exist, the models
produce some consistent results. All
show considerable warming. All show
larger changes over high northern latitudes and the northern continents,
including North America, because land warms up faster than the oceans. Further research is needed to understand why
the models respond as they do, and to reduce the uncertainties. While some changes arising from global
warming are benign or even beneficial, the rate of changes as projected exceed
anything seen in nature in the past 10,000 years and are apt to be disruptive
in many ways. The economic effects of
the weather extremes are substantial and clearly warrant attention in policy
debates.
References
Hoerling, M. P., J. W. Hurrell, and T.
Xu, 2001: Tropical origins for recent North Atlantic climate change. Science, 292, 90–92.
IPCC, 1996: Climate Change 1995: The Science of Climate Change. Eds. J. T. Houghton et al., Cambridge University
Press, Cambridge, U.K. 572 pp.
IPCC, 2001: Climate Change 2001: The Scientific Basis. Eds. J. T. Houghton, et
al., Cambridge University Press, Cambridge, U.K. (in press).
Lindzen, R. S., M-D. Chou, and A. Y. Hou,
2001: Does the Earth have an adaptive infrared iris? Bulletin of the American Meteorological Society, 82, 417–432.
National Research Council (NRC) 2000:
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85 pp.
Stocker, T., G. K. C. Clarke, H. Le
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