Earth's
Fidgeting Climate
From: NASA
Science News home
Is human activity warming the
Earth or do recent signs of climate change signal
natural variations? In this feature article,
scientists discuss the vexing ambiguities of our
planet's complex and unwieldy climate.
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to this story (requires RealPlayer)
October
20, 2000 -- Newspaper headlines trumpet
record-breaking temperatures, dwindling sea ice, and
retreating glaciers around the world. Concentrations
of atmospheric carbon dioxide, one of the greenhouse
gases responsible for scalding temperatures on Venus
and at least 33 degrees C of normal warming here on
Earth, are on the rise. Our planet seems destined for
a hot future!
But is it really? Or are we simply
experiencing a natural variation in Earth's climate
cycles that will return to "normal" in time?
Correlations between rising CO2
levels and global surface temperatures suggest that
our planet is on a one-way warming trend triggered by
human activity. Indeed, studies by paleoclimatologists
reveal that natural variability caused by changes in
the Sun and volcanic eruptions can largely explain
deviations in global temperature from 1000 AD until
1850 AD, near the beginning of the Industrial Era.
After that, the best models require a human-induced greenhouse
effect.
In spite of what may seem persuasive
evidence, many scientists are nonetheless skeptical.
They argue that natural variations
in climate are considerable and not well understood.
The Earth has gone through warming periods before
without human influence, they note. And not all of the
evidence supports global warming. Air
temperatures in the lower atmosphere have not
increased appreciably, according to satellite
data, and the sea ice around Antarctica has actually
been growing for the last 20 years.
It may surprise many people that
science -- the de facto source of dependable knowledge
about the natural world -- cannot deliver an
unqualified, unanimous answer about something as
important as climate change.
Why is the question so thorny? The
reason, say experts, is that Earth's climate is
complex and chaotic. It's so unwieldy that researchers
simply can't conduct experiments to check their ideas
in the usual way of science. They often rely, instead,
on computer models. But such models are only as good
as their inputs and programming, and today's computer
models are known to be imperfect.
Most scientists agree that no single
piece of data will likely resolve the global warming
debate. In the end, the best we can expect is a
scientific consensus based on a preponderance of
evidence.
The canary in the coal mine?
The recent discovery that Greenland's
ice sheet is thinning is a good example of our
climate's sometimes vexing ambiguity.
About
85 percent of Greenland is covered by a massive ice
sheet with an area of about 1,736,000 square
kilometers and an average thickness of about 1,500
meters. The volume of ice in the Greenland sheet
is estimated to be about 2,600,000 cubic kilometers --
enough ice to raise sea levels by 6.4 meters if it all
were to melt.
While it is only about one-seventh
the size of the Antarctic ice sheet, some scientists
think that watching the ice on Greenland provides
better clues about global warming.
"Even though Antarctica is
seven times the size of Greenland, because (Antarctica
is) kind of symmetrically positioned around the South
Pole, it doesn't really interact with climate up in
the more temperate regions the way Greenland
does," said Dr. William Krabill at NASA's Wallops
Flight Facility. Krabill is the project scientist for
the team that discovered the
thinning. "Greenland ... is likely to be a
better indicator of global climate change than
Antarctica," he noted.
Above: A map of
Greenland showing the changes in surface elevation
measured by Krabill's team. Gray indicates no change,
white indicates an increase in thickness and shades of
blue indicate a decrease in thickness, with darker
blue denoting greater thinning. [more
images]
Krabill's team used an airborne
laser to survey the altitude of the ice sheet's
surface during 1993 and 1994. They repeated their
survey in 1998 and 1999, making certain to retrace
their flight paths from the first survey as closely as
possible.
After incorporating some assumptions
that let them extend their measurements to the sheet's
edges, the scientists compared the second survey to
the first. They found that the ice sheet's surface was
slightly higher at the center but considerably lower
at the edges -- particularly the southeastern
edge.
The overall result: The ice sheet
lost at least 51 cubic kilometers of volume during
that five year period. Greenland appeared to be
melting!
Many newspaper headlines cried the
discovery as a sign of global warming -- which most
readers presumably took to mean
"anthropogenic," or human-caused, global
warming.
But is that the right conclusion?
"What you can say is, yes,
carbon dioxide (in the atmosphere) is at levels higher
than ever before, and carbon dioxide is a greenhouse
gas, so it's reasonable to say that there's warming
associated with the increase of carbon dioxide,"
said Dr. Waleed Abdalati, co-author of the paper that
announced the Greenland discovery.
"But you can't make the leap
yet that all the cars in the world have led to what
we're observing in the thinning of the Greenland ice
sheet," Abdalati said.
Left:
A graph showing the Earth's average global surface
temperature from 1880 to the late 1990s. The black
dotted line follows the averages for each year and the
red line traces the five-year average. There is a
clear upward trend over that time.
If there's one lesson to be learned
from science, it's that things are usually much more
complex than they at first appear. The warming trend
of the last century may seem to be the obvious
explanation for the thinning seen on Greenland, but
scientists are considering other possibilities.
"That's what science is
about," said Dr. Ellen Mosley-Thompson, a
research scientist at the Byrd Polar Research Center
at The Ohio State University.
"Just because you have an
hypothesis and immediately your experiment produces
support for it, you can't simply accept those results
(without a degree of skepticism),"
Mosley-Thompson said. "The whole idea is to play
devil's advocate on your own research before your
colleagues do."
Last century's warming trend is not
the only possible explanation for the thinning that
Krabill's team saw on Greenland.
In fact, ice cores taken as part of
another NASA-funded study suggest that natural
variation in snowfall may be partly to blame,
Mosley-Thompson said.
"The ice core data provide
evidence -- not necessarily conclusive -- that Bill's
results may in part reflect variability in snow
accumulation over his five-year observational
window," said Mosley-Thompson, who co-authored
the paper reporting these results with Dr.
Joseph McConnell, an associate research professor
at The Desert Research
Institute in Reno, Nevada. The results of the
study were published in the August 24 edition of the
journal Nature.
Other natural processes could
account for the thinning as well. Ocean currents might
have caused part of the change. Or the flux of warm
water into the North Atlantic caused by the 1990-1996
positive phase of the slow-moving North
Atlantic Oscillation could have had an influence.
The ice sheet could also be thinning in response to
the long-term warming of the planet since the
transition from the last glacial period about 10,000
years ago. Krabill, Dr. Ron Kwok of NASA's Jet
Propulsion Laboratory, and Abdalati mentioned these
scenerios during interviews with Science@NASA.
Scientists often refer to these
alternate explanations under the umbrella term of
"natural variability."
The ant on the hour hand
"For the ordinary person, it's
a common misperception that weather is not changing
... that last winter is about as cold as this winter
and last summer is about as warm ... and the world is
pretty much constant," Krabill said. "That's
not true. The Earth has gone through and continues to
go through cycles of warming and cooling. It's just
natural."
This natural variability often shows
an astounding degree of complexity, much of which
remains poorly understood.
"We've only begun making (large
scale) measurements in the last 100 to 150
years," Abdalati said. "And climatic
processes happen on very different time scales. There
are some, like ice ages, that are in the tens of
thousands or hundreds of thousands of years long. An
then there are atmospheric processes like weather,
which happen on the scales of hours and
days."
Other
climate cycles fall in between, such as the North
Atlantic Oscillation mentioned above, which is thought
to complete one cycle roughly every 20 to 30
years.
"And so you have all these
processes mixed together that have been going on for
thousands of years, and you're in the difficult
position of trying to separate something very recent
from the natural cycle without fully understanding
what that natural cycle is," Abdalati said.
Above: Knowing where a
relatively short interval of observation fits into the
long-term pattern is a difficult challenge for
scientists. A steady increase that appears to be a
trend may be a trend, but it may also be a small part
of a larger cycle.
Observing a system like climate that
varies on several time scales -- some of which
approach geological slowness -- could be likened to an
ant watching the hands of a clock, "perhaps with
the ant sitting on the hour hand," Abdalati
added.
Seen in this context, scientists
don't give much weight to the five-year snapshot of
the ice on Greenland.
"You know, five years is a
pretty short amount of time in glaciological
terms," Krabill said. "To try to make
inferences about 'Global Climate Change' in capital
letters from a five-year period of time is a pretty
risky business."
Other modern data sets are not much
longer. The era of satellite observation is only about
30 to 40 years old -- a mere blink in climatological
terms. And the widespread network of
weather-measurement stations in the developed world is
about 150 years old.
The Ghost of Climates Past
Greater insight about the role of
natural variability may come from the field of
paleoclimatology -- a specialized branch of
climatology that uses scientific sleuthing to summon
the ghost of climates past.
The
"fingerprints" of Earth's climate hundreds
or even thousands of years ago remain imprinted in the
rings of temperature-sensitive trees, the chemicals
trapped in ancient ice, and the layers of sediment on
the ocean floor.
Several studies by
paleoclimatologists have suggested that natural
variability can't fully explain the warming of the
last century.
For example, Dr.
Thomas J. Crowley, a geologist at Texas A&M
University, used similar techniques to reconstruct
basic climate data -- such as average global
temperature -- back to 1000 A.D.
Crowley examined natural climate
variations in a simple computer climate model caused
by two external influences: fluctuations in the sun's
intensity and aerosols injected into the atmosphere by
volcanoes.
He deduced the history of solar flux
from concentrations of carbon-14 in tree rings and of
beryllium-10 in ice cores. Then, he deciphered past
volcanic activity from sulfate aerosol deposits in ice
cores.
Crowley ran the computer climate
model with the solar and volcanic forcing terms, then
compared the average temperatures it produced with a
temperature record constructed from tree-ring
data.
Despite the relative simplicity of
his model, Crowley found good agreement between the
temperature fluctuations it calculated for the years
1000 AD to 1850 AD and the fluctuations actually
measured from tree rings during that interval. Over
that 850-year period, fluctuations in solar intensity
along with volcanic eruptions could account for
roughly 50 percent of the variation seen in the
tree-ring record -- give or take 10 percent.
Something happened, however, after
1850. Crowley's model could only account for about 25
percent of the observed temperature changes. Something
else was needed -- volcanic eruptions and solar
variability were not enough.
Crowley then introduced a
human-triggered greenhouse effect to the model and it
produced a much better match.
"It all comes out as indicating
that you can't resort to (natural variability) to
explain the recent warming," Crowley said.
"The (recent) warming is consistent with a
greenhouse effect but inconsistent with any
explanation from natural variability."
So with the weight of a 1,000-year
climate record on human shoulders, can scientists
finally say that they've proven humanity is causing an
unnatural warming of the globe?
Not necessarily.
"The time series we developed
is statistically significant -- highly
significant," Crowley said. "That doesn't
prove something is right, but it still makes a good
case that we're on the right track."
The problem with the "P"
word --Proof!
From a statistical point of view, no
single scientific result based on real-life data ever
deserves absolute confidence. There always lingers the
possibility -- however small -- that the apparent
results are due to chance patterns in the data, i.e.,
"noise."
In the case of Crowley's study,
statistical tests show that the probability of his
results being due to chance is less than 1 percent.
Usually, anything less than 5 percent is considered
credible.
"This is not mathematics where
you can prove something and write Q.E.D. at the bottom
of the page," Crowley said. "This is
geoscience. It's a dirtier field, and usually you make
statistical arguments."
The abbreviation stands for the
Latin phrase quod erat demonstrandum, which means,
"which was to be demonstrated."
Mathematicians use "Q.E.D." to indicate the
end of the written proof of a theorem, which, if
correct, is considered absolute.
Above: The general trend
of average global temperatures from Precambrian time
-- when multicellular organisms first evolved -- to
the present. Notice that average global temperatures
vary by roughly 10 degrees Celsius between warm and
cool periods. Image courtesy of Dr.
Chris Scotese.
In addition to the caveats inherent
to statistics, conclusions from studies like Crowley's
that are based on computer simulations of the world's
climate are plagued by questions of how well computer
models portray the real thing.
To prove causation, scientists must
perform experiments under controlled conditions on the
system being studied, manipulating the system to
understand what causes what. Other scientists repeat
the experiments to show that the explanation is
reliable.
Since the Earth's climate is beyond
the reach of such experimentation, scientists instead
run computer simulations of global climate. These
models are always much simpler than the Earth's
climate itself. In fact, it's theoretically impossible
to create a "perfect" model of climate that
includes all the detail of the real system.
"The climate system is too
complex," Mosley-Thompson said. "Even the
most complex climate model doesn't get it right. And
why is that? Because who writes the climate models?
Humans. What is a climate model? It's a set of
equations that describes what we think we know. If
you're not cognizant of a particular phenomenon, then
how can you incorporate it into a climate model?"
The fact that different computer
models often produce different forecasts doesn't offer
much reassurance. For example, one model predicted
that the Southeastern U.S. would become more
jungle-like in the next century, while another model
predicted the same region would become a dried-out
savanna, according to Dr.
John Christy, a professor of atmospheric science
at the University of Alabama in Huntsville.
However, scientists can establish
some degree of confidence in their computer models by
seeing if the model can accurately "predict"
past climate patterns that are known to science.
"Models in isolation may not be
believable, but when ... a model can simulate a number
of different observed climate responses, the results
have more weight than mere calculation," Crowley
said. "That still doesn't prove the point, but it
minimizes the value of the argument, 'It's only a
model.'"
Putting the pieces together
Ultimately, the verdict from science
about the extent and cause of global climate change
may not come from one particular study or
observation.
"I think from the viewpoint of
thoughtful scientists, there's not going to be any
single indicator, but rather there's going to be a
concurrency of lots of indicators that's going to be
convincing," said Dr.
Tony Rosenbaum, a professor at the University of
Florida who specializes in the politics of
environmental issues.
Left: Much
of the energy coming from the Sun is in the form of
short-wavelength radiation, which passes through the
atmosphere. The Earth's surface re-radiates that
energy as long-wavelength radiation -- such as
infrared -- which is trapped by greenhouse gases,
primarily water vapor, carbon dioxide and methane.
While the "big picture"
view of all the evidence from research around the
world may offer scientists their best chance to
understand global warming, no mathematical tools exist
for combining all the data into a definitive,
objective conclusion. Scientists' only option is
to weigh the evidence and make a professional
judgment.
"There is a dichotomy between
what is realistic and achievable, and what some people
would like to hold as the ideal proof,"
Mosley-Thompson said.
"I don't think this discussion
lends itself to standard statistical testing,"
she continued. "Certainly you can test,
statistically, the output of one climate model against
another, a climate model against observation, an ice
core data against observation -- but those are
snippets .... What does it mean for reality? There's
where the translation becomes difficult."
When drawing a conclusion from the
mosaic of evidence, different scientists will use
different criteria, Crowley said.
"I think that there are many
scientists that are still locked into the idea that we
have to prove something (about causes of climate
change) beyond a reasonable doubt," Crowley said.
"I don't think we necessarily have to do that.
... In a court of law you can convict based on a
weight of evidence. ... That's the way you approach
this type of problem."
Because such conclusions are based
on a scientist's professional judgment, disagreement
is inevitable.
"There is enormous room for
differences of opinion among equally competent
scientists of good will," Rosenbaum concluded.
"There are always people -- and
reasonable people -- who fall on both sides of the
argument," agreed Abdalati. "And there are
reasons for that. So the best we can hope for is a
consensus."
Related Links:
U.S.
Environmental Protection Agency global warming
homepage -- Includes facts and figures about
global warming.
Greenland's
Receding Ice -- Information, images and
animations about the discovery of thinning ice on
Greenland by NASA scientists. Produced by NASA's
Goddard Space Flight Center.
IPCC
(Intergovernmental Panel on Climate Change)
-- The homepage of the international delegation of
scientists charged with the task of investigating
the evidence on climate change and producing reports
for world leaders and the public.
Contrary
Thermometers -- A Science@NASA article
discussing atmospheric temperature trends measured
by satellites that conflict with the predicts of
current computer climate models.
The
North Atlantic Oscillation -- An
informational Web site from Columbia University.
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