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| Method for predicting the Earth's
future from climates of the distant past |
According to the IPCC Third Assessment Report, if global warming
continues, climate changes in the 21st century may be far more drastic
than we have ever seen; not just over the last century during which
meteorological observation began, but over the last one thousand
years. In order to understand the Earth's mechanisms well enough
to predict future climate change, it is necessary to understand
not only the climate variation of the 20th century but also the
biogeochemical mechanism of climate change over the last several
tens of thousands of years, when drastic climate changes similar
to the current global warming took place. We believe that by studying
the reproducibility of past climates, we will be able to validate
the general circulation model (GCM), which is the model used for
global warming prediction. Dr. Abe-Ouchi will introduce the efforts
of the Paleoclimate Research Group, which will contribute to the
IPCC Fourth Assessment Report.
Dr. Ayako Abe-Ouchi Group Leader
Paleoclimate Research Group, Global Warming Research Program |
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| Understanding the
climate of the distant past |
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| In order to infer and verify climate changes of the distant past,
it is necessary to have data that can be used as proxies to indicate
the Earth's conditions at that time. For example, gas bubbles and
ice stored in glaciers and ice sheets*1, as well as pollen,
are some of the important proxies. By analyzing the air contained
in the bubbles, paleoclimate scientists found that during the Ice
Age of 20,000 years ago, the quantity of carbon dioxide (CO2)
in the atmosphere was approximately half the current amount, and it
increased rapidly after the Industrial Revolution. Moreover, by chemically
analyzing bottom sediments and the types of plankton living in the
ocean using the collected sediment, we are able to see the climatic
conditions and ocean currents of ancient times. The deep-ocean drilling
vessel "Chikyu"*2, scheduled to sail in 2006,
is expected to play an important role in collecting data on past global
environment changes. |
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| Capturing the past,
present and future using the Earth system model |
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How can evidence of past climate change be used to predict future
global warming? First, it can help us understand the characteristics
of the Earth's natural climate change that occurred without human
intervention. For example, data over the past 500,000 years shows
that the atmospheric temperature, water temperature, ice-sheet volume
(sea level) and amount of CO2 in
the atmosphere changed at the same rate on a large scale, and that
warm climate ages, like that of the present, and ice ages occurred
alternately. In order to clarify the mechanism of the ice age cycle,
our group conducted a detailed wavelet analysis of several time series
of proxy data (figure on right). However, this analysis alone does
not clarify the direct causes of climate change or mechanisms which
may cause amplification of other effects. Thus we are developing an
Earth system model with various elements such as atmosphere, ocean,
ice sheet, and carbon cycle that can be used to test both past and
present conditions in order to actively conduct numerical experiments
under various assumptions. Development of the Earth system model,
which is also expected to play an important role in global warming
prediction, is part of MEXT's "Project for Sustainable Coexistence
of Humans, Nature, and the Earth" conducted at FRSGC. In our
group, as we clarify the processes of ice sheet disappearance and
ice age termination from the ice age up to the present day, we also
predict the rise of sea levels, which is expected to occur in the
future due to global warming. |
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| The importance of
paleoclimate research will be recognized in the IPCC Fourth Assessment
Report |
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Data sets of past climate change are useful for directly validating
the model for global warming prediction. Our group has participated
in joint international research that simulates the climates of 6,000
to 9,000 years ago using multiple climate models. A variety of evidence
suggests that the Sahara Desert experienced significant precipitation
and was mostly covered with vegetation over the indicated period of
time. However, when we conducted numerical experiments, we were unable
to create sufficient precipitation over the desert by using the atmospheric
general circulation model. We published our result in 1998, which
was included in the IPCC Third Assessment Report. Later, we found
that it is possible to create sufficient precipitation to support
vegetation in the desert when considering the changes in ocean currents
and plant distribution on land. We are working to complete a paper
to be presented in the IPCC Fourth Assessment Report.
Recently, the importance of paleoclimate research has been recognized
by policy makers, and research methods have advanced. As a result,
the inclusion of an independent chapter on paleoclimate is under consideration
for the IPCC Fourth Assessment Report. Considering that global warming
has become a real problem, we believe that validating the mechanism
for the climate change that actually occurred in the past will become
increasingly important.
- Example of the results of data analysis for ice
age and inter-ice age cycles by the Paleoclimate Research Group,
Global Warming Research Program
Details of wavelet transformation are shown by deep sea temperature,
atmospheric temperature, CO2 concentration, and sea level at the
ice age terminations over the past four periods. Dark blue indicates
the timing of the maximum change of each factor at the ice age termination.
This analysis confirms that the interval between changes in the
atmospheric CO2 concentration and sea level (the ice sheet volume)
is 7,000 years. On the other hand, the result obtained by Fourier
analysis to presume the phase of all signals over 100,000 years
was 15,000 years. Moreover, the figure shows that sea-level change
occurred after changes in three other factors-Antarctic air temperatures,
seawater temperatures in the southern hemisphere, and CO2 concentration-but
at a much faster rate than other factors.
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Continental-scale glacier. Currently, ice sheets only exist in Greenland
and the Antarctic. |
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Scientific exploration vessel with the mission to clarify
global environment changes, the processes of change in the Earth's
crust, the material cycle inside the Earth, and the underground
life zone and fluids in the Earth's crust, with the ability
to dig to depths of 7,000m beneath the sea floor and to operate
at deep-sea depths of 2,500m (target depth is 4,000m). |
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