IT WAS one of the largest volcanic eruptions in the past 10 000 years. Mount
Changbai in China blasted 50 cubic kilometres of rock into the air, darkening
the skies and plunging the world into a wintry gloom. Radiocarbon dating of
trees buried by the eruption put it at early in the 11th century. But it took
Keith Briffa at the University of East Anglia (UEA) in Norwich and painstaking
analysis of tree ring growth in the Urals, to pinpoint the precise year:
1032.
Every ring in every tree around the world provides a record of the climate in
the year it was formed. If temperatures fall, tree rings become narrower and
denser—as happens if volcanic dust high in the atmosphere deflects
sunlight for a year or more. Briffa and his colleagues from the Climatic
Research Unit (CRU) at UEA have successfully demonstrated that tree rings can
reliably record sudden and dramatic shocks to the climate system, from Changbai
to the eruption of Mount Pinatubo in June 1991.
But it is not just climatic shocks that interest Briffa and other scientists
around the world involved in dendrochronology, the science of tree-ring
analysis. Studying tree ring sequences across longer timescales can also reveal
more general trends, including the natural pattern, range and variability of
global temperature fluctuations. And that, Briffa believes, has the potential to
answer one of the most vital questions of our time: has human activity really
started to warm the planet?
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A major project funded by the European Union and involving Briffa and
colleagues in 15 institutions and nine countries from Ireland to eastern
Siberia, hopes to determine whether the warming seen across the planet in the
past century is within the limits of normal natural variability, or the start of
man-made global warming. They are using tree rings in attempts to construct a
year-by-year history of temperatures across northern Europe and Asia over the
past 10 000 years, since the waning of the last ice age. The project is called
ADVANCE-10K (Analysis of Dendrochronological Variability and Associated Natural
Climates in Eurasia—the last 10 000 years).
The search for an irrefutable “sign” of anthropogenic warming is hotting up.
This summer, the scientific working group of the UN’s Intergovernmental Panel on
Climate Change (IPCC) concluded that “the balance of evidence suggests a
discernible human influence on global climate”. Few investigators doubt that the
world has warmed recently. Nor that the enhanced “greenhouse effect” of
pollution from gases, such as carbon dioxide will warm the planet. But many are
increasingly aware of how little is known about the natural variability in
climate within which they are searching for a definitive “signal” of human
influence.
Simon Tett is one prominent IPCC researcher concerned about this gap in our
knowledge. He works at the Hadley Centre for climate modelling at Britain’s
Meteorological Office in Bracknell which houses one of the world’s five leading
global circulation models, a mathematical version of how the atmosphere works
that can simulate climate changes over decades or even centuries. “In the past,”
he says, “our estimates of natural variability have been based on climate
models.” But this autumn, those estimates were thrown into turmoil by a paper
published in the journal The Holocene. In it, Tim Barnett of the
Scripps Institution of Oceanography, part of the University of California, San
Diego, compared model estimates of natural temperature fluctuations over the
past 400 years with the best evidence from the real world—from
thermometers in the past century and “proxy data”, such as Briffa’s tree rings,
corals, sediment cores and ice cores.
It was bad news for the modellers. The two models examined—one German,
the other American—could account for only 20 per cent to 60 per cent of
the actual variability revealed by the proxy data. “Of course we don’t have to
believe the proxy data. They certainly have problems attached to them. But my
belief is that both the models, and proxy data too, underestimate real
variability,” says Barnett. And the greater the natural variability, the more
difficult it is to determine if there are signs of anthropogenic warming.
The models’ errors are not so surprising. As Barnett points out, they do not
yet include the effects of “forcing” mechanisms that alter temperature, such as
solar cycles and volcanic eruptions. Nor can they fully mimic the largest
year-on-year variability in the natural climate system, the El Niño
oscillation in the Pacific Ocean. Nonetheless, the findings should serve as a
warning. A greater natural variability in climates might make it harder to
discover a genuine human signal. But Barnett says another worry is that today’s
models miss so much natural climatic variability that they “might lead us to
believe that an anthropogenic signal had been found when, in fact, that may not
be the case”.
Barnett knows how easily such misinterpretations can happen. In 1995 he was a
main author of a key chapter in the IPCC scientific assessment which
investigated “the detection of climate change and attribution of causes”. It
formulated the IPCC case that the evidence points towards a human influence on
climate, but warned repeatedly that great uncertainties remained. “We wrote a
long list of caveats in that chapter,” says Barnett, adding that they resisted
pressure from within the IPCC to dilute the list. Even so, when the findings
were first leaked to The New York Times last year, it was under the
headline “Scientists finally confirm human role in global warming”.
Natural signs
Suggestive though the evidence may be, Barnett and his coauthors insist that
the uncertainties, especially concerning natural variability, have still to be
answered. Researchers are now paying even more attention to natural records of
temperature, including corals (which also have an annual temperature imprint),
sediments on oceanic shelves, ice sheets—and of course, Briffa’s tree
rings. He believes the information from tree rings and ice cores will
“complement each other, focusing at best on different timescales”. Annual and
decade-to-decade variations, which can tell you a lot about variability, show up
clearly in tree rings but are much harder to detect in ice cores, in which
longer term trends stand out. Moreover, ice cores, by their very nature, have a
more restricted geographical distribution than trees. And plotting spatial, as
well as chronological, variability is a vital part of improving the
understanding of climate change. So the modellers are now queuing at Briffa’s
door to find out what his tree-ring data shows about the real world beyond the
computer simulations. “Five years ago, they wanted nothing to do with the palaeo
community,” says Briffa. “But now they realise that they need our data. We can
help them to define natural variability.” He has already collaborated with
Barnett, and Tett paid his first visit to the dendrochronology laboratory in
Norwich in November.
Over the next two years, Briffa and his colleagues will assemble data from
the forests of nine countries in Eurasia to increase the body of available proxy
data on past climate change. Building the chronologies is painstaking work. The
approximate age of the trees can be obtained by radiocarbon dating. But to
construct long, continuous series in which each growth ring is dated to a
precise year, requires meticulous comparison of year-to-year ring patterns,
measured in many overlapping samples from one area. As researchers find more and
more unique matches between groups of trees, they can assemble longer and longer
series to complete the whole jigsaw over thousands of years.
The best data, says Briffa, come from analysing both ring width and the
maximum density of wood in each ring. By firing X-rays through the wood,
researchers can now analyse the density of rings as narrow as 30 micrometres
across—the equivalent of a tree’s girth growing by 2 centimetres a
century. Ring growth is a product of many factors, including genetic variation
within tree populations, past climate, the age of the tree and soil moisture.
Briffa says that it is even possible that “over very long timescales, trees may
adapt to changes”, obscuring the variations in climate that he is looking for,
hence the need for meticulous comparisons.
Such factors mean that the relationships between ring growth and summer
temperature are not precise. But, comparisons between the recent rings and known
climatic data show that the rings can capture at least 50 per cent of the summer
temperature variability. This means that if you draw a graph of summer
temperature fluctuations based on tree ring analysis, it will show around half
the year-on-year variability measured by thermometers. In particular, the growth
of cell walls late in the growing season, says Briffa, “appears to depend
directly on the average mean temperature”.
The project’s most recent work has analysed temperature variations as far
back as 6000 BC, using data from living trees, up to 400 years old and well
preserved dead logs, from deep inside the Arctic Circle in Sweden and Finland.
It also provides a fairly complete series. At such high latitudes, near today’s
northern limit for the Scots pine, small perturbations in average summer
temperatures can greatly affect the number of days when temperatures rise above
5 °C, the threshold for growth. The result is dramatic differences in the
thickness and density of tree rings.
In charge of the work in Sweden is Wibjörn Karlén, a geographer
at the University of Stockholm who has pioneered the collection of dead wood
from around Lake Torneträsk, pulling logs and stumps from lakes where some
have lain for thousands of years. The published year-on-year ring record extends
back 2000 years from the present day, but will reach back 8000 years. The
temperature graphs produced at Torneträsk show “pronounced variability on
all timescales, from year-on-year variations right up to century-on-century,”
says Briffa.
Climate of change
During the past two millennia, they show long cool spells between 500 and
850, between 1100 and 1350 and especially between 1580 and 1750. Summer
temperatures in these cold spells were between 0.25 °C and 0.5 °C cooler
than the long-term average for the period between AD 500 and the present day.
There were also long warm spells, when temperatures were about 0.3 °C above
the long-term mean—between 900 and 1100 and between 1360 to 1560.
Further back, results from Torneträsk suggest a strong warm era from
4000 to 3300 BC, and cool periods ending around 4100, 3000 and 1600 BC. “We have
a continuous growth record for every year between 5500 BC and the present
day—except for an intriguing gap around 300 BC, where we cannot find any
trees spanning the period,” says Briffa. The same gap, lasting a few decades,
turns up in data from northern Finnish tree rings. It suggests some major
calamity which the trees did not survive, or which distorted their ring patterns
so badly that they can’t be matched with each other to form a chronology.
“What all this means,” says Briffa, “is that the old image of the 10 000
years since the end of the last ice age—the Holocene era—as
climatically tranquil looks increasingly inaccurate. It is vital to chart these
rapid climate changes if we are to judge how unusual recent climate is or why it
has occurred.”
Hence the intense interest in ADVANCE-10K. In northern Finland, local diving
clubs picked some 3000 samples from lakes during the summer, while the Institute
of Krasnoyarsk in Siberia has been surveying tree remains on the frozen tundra
of the Taimyr Peninsula, far north of today’s tree line. Well-preserved logs
from trees that grew between 5000 and 8000 years ago have been found in river
sediments in this region.
On the Yamal Peninsula, north of the west Siberian plain, it is so cold that
temperatures during some summers possibly never reach the 5 °C threshold for
tree growth, and frequently only partial tree rings form. Stepan Shiyatov of the
Institute of Plant and Animal Ecology in Ekaterinburg in Russia has collected
2000 samples of ancient larch and spruce, some of which grew more than 10 000
years ago. Yamal is the only northern Eurasian site where trees spanning the 300
BC gap have been identified. “Early analysis suggests that this period may have
been the coldest in the 3200-year continuous ring series produced here to date,”
says Briffa.
Meanwhile, the first long data sets are starting to emerge from Siberia. In
July 1995, Briffa, with colleagues in Europe and America, published a paper on
“unusual 20th-century summer warmth in a 1000-year temperature record from
Siberia”. A complete tree-ring chronology from AD 914, pieced together from
larches near the Yamal Peninsula, suggested that average summer temperatures
since 1901 have been higher than for any similar length of time during the
chronology. It estimated that from 1600, the depth of the “little ice age”, to
the present day, there has been a 1.14 °C warming. The first eight decades
of the 20th century were 0.13 °C warmer than the next warmest period, nine
centuries before, in 1202-91.
Turbulent times
The Siberian chronology also shows that Europe’s “little ice age” extended
east of the Urals, but that the medieval warm period did not. These long trends,
however, disguise sharp short-term anomalies. The 11th century seems to have
been a particularly turbulent time in the Urals: 1045 was the warmest summer of
the millennium, whilst 1032, the probable year of the Changbai eruption, was the
coldest summer in a thousand years. Within a single year however, temperatures
had switched back, and 1033 produced the second warmest summer of the millennium
at more than 2 °C above the mean.
The project team is also assembling a network of shorter chronologies from
living trees across the whole of northern Eurasia. This winter, the timber is
being analysed at the laboratories of the Institute of Forest, Snow and
Landscape Research in Birmensdorf—the Swiss home of Fritz Schweingruber,
one of the world’s top tree-ring analysts. The project will also carry out a new
analysis on the large number of samples of ancient European oaks collected over
the past 30 years from bogs and river beds, archaeological sites and even the
beams of old houses. “There is a massive amount of existing data on European oak
rings,” says Briffa. “But much of this was collected for dating buildings and
archaeological sites. They require a lot of work to reinterpret them for climate
.”
Tree rings in Eurasia, corals in the tropics, high latitude ice cores, oak
rings from northern Europe and sediment cores from oceanic shelves—the
patterns of temperature change revealed by analyses of these different sources
may always remain too fragmented to reveal unambiguous trends in global average
temperatures. But this may not matter. “Frankly, global averages are not central
to the issue of attributing climate change,” says Barnett. “What will ultimately
prove whether or not we are altering the climate will be the patterns of
temperature change”—geographical patterns, seasonal patterns and vertical
patterns in the atmosphere. Barnett and Phil Jones of the CRU have formed a
small “detections group”, under the IPCC umbrella, to look for these tell-tale
patterns. If they find them, it will mean that even if the real variability in
temperature is greater than present models allow, it may not entirely mask the
effects of human activity. “We are systematically looking at the patterns, past
and present, of all the main forcings on climate,” Barnett says. They will
investigate how the world’s climate systems respond to volcanoes, to changes in
the ocean circulation, to solar cycles and so on—all of which can “force”
changes in climate patterns. “Then we will compare those patterns with what we
are seeing today. What we hope is that the current patterns of temperature
change prove distinctive and quite different from the patterns of natural
variability in the past.” And if that turns out to be the case, he says, “we
will be able to close down this issue of attribution, perhaps within three to
five years”.
Here, the climate models will again come into play. If current climate change
also accords with what the models predict from global warming, then it will
indeed look as if there is a human hand on the planet’s thermostat.
The models all suggest that anthropogenic global warming will show a very
distinctive pattern. For instance, they predict that it will be greatest in the
northern latitudes of the great continental land masses, such as Eurasia. The
prediction has given added significance to the project’s discovery that recent
summer temperatures in northern Siberia are higher than for a millennium.
Briffa grins at the prospect. “The trend seems to be accelerating. It has
certainly been very warm since we undertook that study. The spring of 1990 was
way above any measured in the last century. We are getting reports back from
Stepan that tree growth in the Urals is exploding. It does suggest a major
warming—and it is what the climate models predict.” Caution prevails, but
perhaps the elusive pattern of human-influenced global warming is now emerging
amid the larch groves on the sunny hills of northern Siberia.
