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

Understanding the climate of the distant past

 
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.

 
Capturing the past, present and future using the Earth system model
 

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.

 
The importance of paleoclimate research will be recognized in the IPCC Fourth Assessment Report

 
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.
*1 Continental-scale glacier. Currently, ice sheets only exist in Greenland and the Antarctic.
*2 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).

Frontier Newsletter/No.23
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