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Warming Frequently Asked Questions Introduction What is the greenhouse effect,
and is it affecting our climate? Are greenhouse gases increasing? Is the climate
warming? Are El Niños related to Global Warming? Is the hydrological cycle
(evaporation and precipitation) changing? Is the atmospheric/oceanic circulation
changing? Is the climate becoming more variable or extreme? How important are
these changes in a longer-term context? Is sea level rising? Can the observed
changes be explained by natural variability? What about the future? Additional
Information All figures linked from this page with the exception of global
surface temperatures are from the IPCC 2001 report 'Climate Change 2001: The
Scientific Basis'.
Introduction This page is based on a brief synopsis of the 2001 report by the
Intergovernmental Panel on Climate Change, and the National Research Council's
2001 report Climate Change Science: An Analysis of Some Key Questions, as well
as NCDC's own data resources.
It was prepared by David Easterling and Tom Karl, National Climatic Data Center,
Asheville, N.C.
28801. One of the most hotly debated topics on Earth is the issue of climate
change, and the National Environmental Satellite, Data, and Information Service
(NESDIS) data centers are central to answering some of the most pressing global
change questions that remain unresolved.
The National Climatic Data Center contains the instrumental records that can
precisely define the nature of climatic fluctuations at time scales of a up to a
century.
Among the diverse kinds of data platforms whose data contribute to NCDC's
armamentarium are: Ships, buoys, weather stations, balloons, satellites, and
aircraft.
The National Oceanographic Data Center contains the subsurface data which reveal
the ways that heat is distributed and redistributed over the planet.
Knowing how these systems are changing and how they have changed in the past is
crucial to understanding how they will change in the future.
And, for climate information that extends from hundreds to thousands of years,
the paleoclimatology program, also at the National Climatic Data Center, helps
to provide longer term perspectives. Internationally, the Intergovernmental
Panel on Climate Change (IPCC), under the auspices of the United Nations (UN),
World Meteorological Organization (WMO), and the United Nations Environment
Program (UNEP), is the most senior and authoritative body providing scientific
advice to global policy makers.
The IPCC met in full session in 1990, 1995 and in 2001.
They address issues such as the buildup of greenhouse gases, evidence,
attribution, and prediction of climate change, impacts of climate change, and
policy options.
Listed below are a number of questions commonly addressed to climate scientists,
and brief replies (based on IPCC reports and other research) in common,
understandable language.
This list will be periodically updated, as new scientific evidence comes to
light. What is the greenhouse effect, and is it affecting our climate? The
greenhouse effect is unquestionably real and helps to regulate the temperature
of our planet.
It is essential for life on Earth and is one of Earth's natural processes.
It is the result of heat absorption by certain gases in the atmosphere (called
greenhouse gases because they effectively 'trap' heat in the lower atmosphere)
and re-radiation downward of some of that heat.
Water vapor is the most abundant greenhouse gas, followed by carbon dioxide and
other trace gases.
Without a natural greenhouse effect, the temperature of the Earth would be about
zero degrees F (-18°C) instead of its present 57°F (14°C).
So, the concern is not with the fact that we have a greenhouse effect, but
whether human activities are leading to an enhancement of the greenhouse effect.
Are greenhouse gases increasing? Human activity has been increasing the
concentration of greenhouse gases in the atmosphere (mostly carbon dioxide from
combustion of coal, oil, and gas; plus a few other trace gases).
There is no scientific debate on this point.
Pre-industrial levels of carbon dioxide (prior to the start of the Industrial
Revolution) were about 280 parts per million by volume (ppmv), and current
levels are about 370 ppmv.
The concentration of CO2 in our atmosphere today, has not been exceeded in the
last 420,000 years, and likely not in the last 20 million years.
According to the IPCC Special Report on Emission Scenarios (SRES), by the end of
the 21st century, we could expect to see carbon dioxide concentrations of
anywhere from 490 to 1260 ppm (75-350% above the pre-industrial concentration).
Is the climate warming? Yes.
Global surface temperatures have increased about 0.6°C (plus or minus 0.2°C)
since the late-19th century, and about 0.4°F (0.2 to 0.3°C) over the past 25
years (the period with the most credible data).
The warming has not been globally uniform.
Some areas (including parts of the southeastern U.S.) have, in fact, cooled over
the last century.
The recent warmth has been greatest over North America and Eurasia between 40
and 70°N.
Warming, assisted by the record El Niño of 1997-1998, has continued right up to
the present, with 2001 being the second warmest year on record after 1998.
Linear trends can vary greatly depending on the period over which they are
computed.
Temperature trends in the lower troposphere (between about 2,500 and 26,000 ft.)
from 1979 to the present, the period for which Satellite Microwave Sounding Unit
data exist, are small and may be unrepresentative of longer term trends and
trends closer to the surface.
Furthermore, there are small unresolved differences between radiosonde and
satellite observations of tropospheric temperatures, though both data sources
show slight warming trends.
If one calculates trends beginning with the commencement of radiosonde data in
the 1950s, there is a slight greater warming in the record due to increases in
the 1970s.
There are statistical and physical reasons (e.g., short record lengths, the
transient differential effects of volcanic activity and El Niño, and boundary
layer effects) for expecting differences between recent trends in surface and
lower tropospheric temperatures, but the exact causes for the differences are
still under investigation (see National Research Council report "Reconciling
Observations of Global Temperature Change"). An enhanced greenhouse effect is
expected to cause cooling in higher parts of the atmosphere because the
increased "blanketing" effect in the lower atmosphere holds in more heat,
allowing less to reach the upper atmosphere.
Cooling of the lower stratosphere (about 49,000-79,500ft.) since 1979 is shown
by both satellite Microwave Sounding Unit and radiosonde data, but is larger in
the radiosonde data. Relatively cool surface and tropospheric temperatures, and
a relatively warmer lower stratosphere, were observed in 1992 and 1993,
following the 1991 eruption of Mt.
Pinatubo.
The warming reappeared in 1994.
A dramatic global warming, at least partly associated with the record El Niño,
took place in 1998.
This warming episode is reflected from the surface to the top of the
troposphere. There has been a general, but not global, tendency toward reduced
diurnal temperature range (DTR), (the difference between high and low daily
temperatures) over about 50% of the global land mass since the middle of the
20th century.
Cloud cover has increased in many of the areas with reduced diurnal temperature
range.
The overall positive trend for maximum daily temperature over the period of
study (1950-93) is 0.1°C/decade, whereas the trend for daily minimum
temperatures is 0.2°C/decade.
This results in a negative trend in the DTR of -0.1°C/decade. Indirect
indicators of warming such as borehole temperatures, snow cover, and glacier
recession data, are in substantial agreement with the more direct indicators of
recent warmth.
Evidence such as changes in glacier length is useful since it not only provides
qualitative support for existing meteorological data, but glaciers often exist
in places too remote to support meteorological stations, the records of glacial
advance and retreat often extend back further than weather station records, and
glaciers are usually at much higher alititudes that weather stations allowing us
more insight into temperature changes higher in the atmosphere. Large-scale
measurements of sea-ice have only been possible since the satellite era, but
through looking at a number of different satellite estimates, it has been
determined that Arctic sea ice has decreased between 1973 and 1996 at a rate of
-2.8 +/- 0.3%/decade.
Although this seems to correspond to a general increase in temperature over the
same period, there are lots of quasi-cyclic atmospheric dynamics (for example
the Arctic Oscillation) which may also influence the extent and thickness of
sea-ice in the Arctic.
Sea-ice in the Antarctic has shown very little trend over the same period, or
even a slight increase since 1979.
Though extending the Antarctic sea-ice record back in time is more difficult due
to the lack of direct observations in this part of the world. Are El Niños
related to Global Warming? El Niños are not caused by global warming.
Clear evidence exists from a variety of sources (including archaeological
studies) that El Niños have been present for hundreds, and some indicators
suggest maybe millions, of years.
However, it has been hypothesized that warmer global sea surface temperatures
can enhance the El Niño phenomenon, and it is also true that El Niños have been
more frequent and intense in recent decades.
Recent climate model results that simulate the 21st century with increased
greenhouse gases suggest that El Niño-like sea surface temperature patterns in
the tropical Pacific are likely to be more persistent. Is the hydrological cycle
(evaporation and precipitation) changing? Overall, land precipitation for the
globe has increased by ~2% since 1900, however, precipitation changes have been
spatially variable over the last century.
Instrumental records show that there has been a general increase in
precipitation of about 0.5-1.0%/decade over land in northern mid-high latitudes,
except in parts of eastern Russia.
However, a decrease of about -0.3%/decade in precipitation has occurred during
the 20th century over land in sub-tropical latitudes, though this trend has
weakened in recent decades.
Due to the difficulty in measuring precipitation, it has been important to
constrain these observations by analyzing other related variables.
The measured changes in precipitation are consistent with observed changes in
streamflow, lake levels, and soil moisture (where data are available and have
been analyzed). Northern Hemisphere annual snow cover extent has consistently
remained below average since 1987, and has decreased by about 10% since 1966.
This is mostly due to a decrease in spring and summer snowfall over both the
Eurasian and North American continents since the mid-1980s.
However, winter and autumn snow cover extent has shown no significant trend for
the northern hemisphere over the same period. Improved satellite data shows that
a general trend of increasing cloud amount over both land and ocean since the
early 1980s, seems to have reversed in the early 1990s, and total cloud amount
of land and ocean now appears to be decreasing.
However, there are several studies that suggest regional cloudiness, perhaps
especially in the thick precipitating clouds has increased over the 20th
century. Is the atmospheric/oceanic circulation changing? A rather abrupt change
in the El Niño - Southern Oscillation behavior occurred around 1976/77 and the
new regime has persisted.
There have been relatively more frequent and presistent El Niño episodes rather
than the cool La Niñas.
This behavior is highly unusual in the last 120 years (the period of
instrumental record).
Changes in precipitation over the tropical Pacific are related to this change in
the El Niño - Southern Oscillation, which has also affected the pattern and
magnitude of surface temperatures.
However, it is unclear as to whether this apparent change in the ENSO cycle is
caused by global warming. Is the climate becoming more variable or extreme? On a
global scale there is little evidence of sustained trends in climate variability
or extremes.
This perhaps reflects inadequate data and a dearth of analyses.
However, on regional scales, there is clear evidence of changes in variability
or extremes. In areas where a drought or excessive wetness usually accompanies
an El Niño, these dry or wet spells have been more intense in recent years.
Other than these areas, little evidence is available of changes in drought
frequency or intensity. In some areas where overall precipitation has increased
(ie.
the mid-high northern latitudes), there is evidence of increases in the heavy
and extreme precipitation events.
Even in areas such as eastern Asia, it has been found that extreme precipitation
events have increased despite total precipitation remaining constant or even
decreasing somewhat.
This is related to a decrease in the frequency of precipitation in this region.
Many individual studies of various regions show that extra-tropical cyclone
activity seems to have generally increased over the last half of the 20th
century in the northern hemisphere, but decreased in the southern hemisphere.
It is not clear whether these trends are multi-decadal fluctuations or part of a
longer-term trend. Where reliable data are available, tropical storm frequency
and intensity show no significant long-term trend in any basin.
There are apparent decadal-interdecadal fluctuations, but nothing which is
conlusive in suggesting a longer-term component. Global temperature extremes
have been found to exhibit no significant trend in interannual variability, but
several studies suggest a significant decrease in intra-annual variability.
There has been a clear trend to fewer extremely low minimum temperatures in
several widely-separated areas in recent decades.
Widespread significant changes in extreme high temperature events have not been
observed. There is some indication of a decrease in day-to-day temperature
variability in recent decades. How important are these changes in a longer-term
context? Paleoclimatic data are critical for enabling us to extend our knowledge
of climatic variability beyond what is measured by modern instruments.
Many natural phenomena are climate dependent (such as the growth rate of a tree
for example), and as such, provide natural 'archives' of climate information.
Some useful paleoclimate data can be found in sources as diverse as tree rings,
ice cores, corals, lake sediments (including fossil insects and pollen data),
speleothems (stalactites etc), and ocean sediments.
Some of these, including ice cores and tree rings provide us also with a
chronology due the nature of how they are formed, and so high resolution climate
reconstruction is possible in these cases.
However, there is not a comprehensive 'network' of paleoclimate data as there is
with instrumental coverage, so global climate reconstructions are often
difficult to obtain.
Nevertheless, combining different types of paleoclimate records enables us to
gain a near-global picture of climate changes in the past.
For the Northern Hemisphere summer temperature, recent decades appear to be the
warmest since at least about 1000AD, and the warming since the late 19th century
is unprecedented over the last 1000 years.
Older data are insufficient to provide reliable hemispheric temperature
estimates.
Ice core data suggest that the 20th century has been warm in many parts of the
globe, but also that the significance of the warming varies geographically, when
viewed in the context of climate variations of the last millennium. Large and
rapid climatic changes affecting the atmospheric and oceanic circulation and
temperature, and the hydrological cycle, occurred during the last ice age and
during the transition towards the present Holocene period (which began about
10,000 years ago).
Based on the incomplete evidence available, the projected change of 3 to 7°F
(1.5 - 4°C) over the next century would be unprecedented in comparison with the
best available records from the last several thousand years. Is sea level
rising? Global mean sea level has been rising at an average rate of 1 to 2
mm/year over the past 100 years, which is significantly larger than the rate
averaged over the last several thousand years.
Projected increase from 1990-2100 is anywhere from 0.09-0.88 meters, depending
on which greenhouse gas scenario is used and many physical uncertainties in
contributions to sea-level rise from a variety of frozen and unfrozen water
sources. Can the observed changes be explained by natural variability, including
changes in solar output? Since our entire climate system is fundamentally driven
by energy from the sun, it stands to reason that if the sun's energy output were
to change, then so would the climate.
Since the advent of space-borne measurements in the late 1970s, solar output has
indeed been shown to vary.
There appears to be confirmation of earlier suggestions of an 11 (and 22) year
cycle of irradiance.
With only 20 years of reliable measurements however, it is difficult to deduce a
trend.
But, from the short record we have so far, the trend in solar irradiance is
estimated at ~0.09 W/m2 compared to 0.4 W/m2 from well-mixed greenhouse gases.
There are many indications that the sun also has a longer-term variation which
has potentially contributed to the century-scale forcing to a greater degree.
There is though, a great deal of uncertainty in estimates of solar irradiance
beyond what can be measured by satellites, and still the contribution of direct
solar irradiance forcing is small compared to the greenhouse gas component.
However, our understanding of the indirect effects of changes in solar output
and feedbacks in the climate system is minimal.
There is much need to refine our understanding of key natural forcing mechanisms
of the climate, including solar irradiance changes, in order to reduce
uncertainty in our projections of future climate change. In addition to changes
in energy from the sun itself, the Earth's position and orientation relative to
the sun (our orbit) also varies slightly, thereby bringing us closer and further
away from the sun in predictable cycles (called Milankovitch cycles).
Variations in these cycles are believed to be the cause of Earth's ice-ages
(glacials).
Particularly important for the development of glacials is the radiation receipt
at high northern latitudes.
Diminishing radiation at these latitudes during the summer months would have
enabled winter snow and ice cover to persist throughout the year, eventually
leading to a permanent snow- or icepack.
While Milankovitch cycles have tremendous value as a theory to explain ice-ages
and long-term changes in the climate, they are unlikely to have very much impact
on the decade-century timescale.
Over several centuries, it may be possible to observe the effect of these
orbital parameters, however for the prediction of climate change in the 21st
century, these changes will be far less important than radiative forcing from
greenhouse gases. What about the future? Due to the enormous complexity of the
atmosphere, the most useful tools for gauging future changes are 'climate
models'.
These are computer-based mathematical models which simulate, in three
dimensions, the climate's behavior, its components and their interactions.
Climate models are constantly improving based on both our understanding and the
increase in computer power, though by definition, a computer model is a
simplification and simulation of reality, meaning that it is an approximation of
the climate system.
The first step in any modeled projection of climate change is to first simulate
the present climate and compare it to observations.
If the model is considered to do a good job at representing modern climate, then
certain parameters can be changed, such as the concentration of greenhouse
gases, which helps us understand how the climate would change in response.
Projections of future climate change therefore depend on how well the computer
climate model simulates the climate and on our understanding of how forcing
functions will change in the future. The IPCC Special Report on Emission
Scenarios determines the range of future possible greenhouse gas concentrations
(and other forcings) based on considerations such as population growth, economic
growth, energy efficiency and a host of other factors.
This leads a wide range of possible forcing scenarios, and consequently a wide
range of possible future climates. According to the range of possible forcing
scenarios, and taking into account uncertainty in climate model performance, the
IPCC projects a global temperature increase of anywhere from 1.4 - 5.8°C from
1990-2100.
However, this global average will integrate widely varying regional responses,
such as the likelihood that land areas will warm much faster than ocean
temperatures, particularly those land areas in northern high latitudes (and
mostly in the cold season). Precipitation is also expected to increase over the
21st century, particularly at northern mid-high latitudes, though the trends may
be more variable in the tropics. Snow extent and sea-ice are also projected to
decrease further in the northern hemisphere, and glaciers and ice-caps are
expected to continue to retreat. Additional Information/links Intergovernmental
Panel on Climate Change U.S.
Environmental Protection Agency World Data Center for Greenhouse Gases A
Paleoclimate perspective on global warming EL Niño/La Niña Climate of 2003
Climate of 2002 Climate of 2001, comprehensive (large pdf file)/ brief (html)
Climate of 2000, comprehensive (large pdf file)/ brief (html) Climate of 1999
Climate of 1998 U.S.
Climate Returns to Heat of the 1930s as Global Warmth Continues National Oceanic
and Atmospheric Administration National Climatic Data Center Federal Building
151 Patton Avenue Asheville NC 28801-5001 www.ncdc.noaa.gov Energy Guardian 2"
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