Cambridge, MA — A review of more than 200 climate studies led by
researchers at the Harvard-Smithsonian Center for Astrophysics has
determined that the 20th century is neither the warmest century nor
the century with the most extreme weather of the past 1000 years. The
review also confirmed that the Medieval Warm Period of 800 to 1300
A.D. and the Little Ice Age of 1300 to 1900 A.D. were worldwide
phenomena not limited to the European and North American continents.
While 20th century temperatures are much higher than in the Little
Ice Age period, many parts of the world show the medieval warmth to
be greater than that of the 20th century.

Smithsonian astronomers Willie Soon and Sallie Baliunas, with
co-authors Craig Idso and Sherwood Idso (Center for the Study of
Carbon Dioxide and Global Change) and David Legates (Center for
Climatic Research, University of Delaware), compiled and examined
results from more than 240 research papers published by thousands of
researchers over the past four decades. Their report, covering a
multitude of geophysical and biological climate indicators, provides
a detailed look at climate changes that occurred in different regions
around the world over the last 1000 years.

“Many true research advances in reconstructing ancient climates have
occurred over the past two decades,” Soon says, “so we felt it was
time to pull together a large sample of recent studies from the last
5-10 years and look for patterns of variability and change. In fact,
clear patterns did emerge showing that regions worldwide experienced
the highs of the Medieval Warm Period and lows of the Little Ice Age,
and that 20th century temperatures are generally cooler than during
the medieval warmth.”

Soon and his colleagues concluded that the 20th century is neither
the warmest century over the last 1000 years, nor is it the most
extreme. Their findings about the pattern of historical climate
variations will help make computer climate models simulate both
natural and man-made changes more accurately, and lead to better
climate forecasts especially on local and regional levels. This is
especially true in simulations on timescales ranging from several
decades to a century.

–Historical Cold, Warm Periods Verified–

Studying climate change is challenging for a number of reasons, not
the least of which is the bewildering variety of climate indicators –
all sensitive to different climatic variables, and each operating on
slightly overlapping yet distinct scales of space and time. For
example, tree ring studies can yield yearly records of temperature
and precipitation trends, while glacier ice cores record those
variables over longer time scales of several decades to a century.

Soon, Baliunas and colleagues analyzed numerous climate indicators
including: borehole data; cultural data; glacier advances or
retreats; geomorphology; isotopic analysis from lake sediments or ice
cores, tree or peat celluloses (carbohydrates), corals, stalagmite or
biological fossils; net ice accumulation rate, including dust or
chemical counts; lake fossils and sediments; river sediments; melt
layers in ice cores; phenological (recurring natural phenomena in
relation to climate) and paleontological fossils; pollen; seafloor
sediments; luminescent analysis; tree ring growth, including either
ring width or maximum late-wood density; and shifting tree line
positions plus tree stumps in lakes, marshes and streams.

“Like forensic detectives, we assembled these series of clues in
order to answer a specific question about local and regional climate
change: Is there evidence for notable climatic anomalies during
particular time periods over the past 1000 years?” Soon says. “The
cumulative evidence showed that such anomalies did exist.”

The worldwide range of climate records confirmed two significant
climate periods in the last thousand years, the Little Ice Age and
the Medieval Warm Period. The climatic notion of a Little Ice Age
interval from 1300 to1900 A.D. and a Medieval Warm Period from 800 to
1300 A.D. appears to be rather well-confirmed and wide-spread,
despite some differences from one region to another as measured by
other climatic variables like precipitation, drought cycles, or
glacier advances and retreats.

“For a long time, researchers have possessed anecdotal evidence
supporting the existence of these climate extremes,” Baliunas says.
“For example, the Vikings established colonies in Greenland at the
beginning of the second millennium that died out several hundred
years later when the climate turned colder. And in England, vineyards
had flourished during the medieval warmth. Now, we have an
accumulation of objective data to back up these cultural indicators.”

The different indicators provided clear evidence for a warm period in
the Middle Ages. Tree ring summer temperatures showed a warm interval
from 950 A.D. to 1100 A.D. in the northern high latitude zones, which
corresponds to the “Medieval Warm Period.” Another database of tree
growth from 14 different locations over 30-70 degrees north latitude
showed a similar early warm period. Many parts of the world show the
medieval warmth to be greater than that of the 20th century.

The study — funded by NASA, the Air Force Office of Scientific
Research, the National Oceanic and Atmospheric Administration, and
the American Petroleum Institute — will be published in the Energy
and Environment journal. A shorter paper by Soon and Baliunas
appeared in the January 31, 2003 issue of the Climate Research
journal.

Headquartered in Cambridge, Massachusetts, the Harvard-Smithsonian
Center for Astrophysics (CfA) is a joint collaboration between the
Smithsonian Astrophysical Observatory and the Harvard College
Observatory. CfA scientists organized into six research divisions
study the origin, evolution, and ultimate fate of the universe.