November 5, 2004
Japan Agency for Marine-Earth Science and Technology


Reproduction of the Climate Change in the 20th Century by a Numerical Climate Model
-Attribution of Causes of the Global Mean Surface Air Temperature Change: Temperature Rise in Late 20th Century Attributed to Human Activities-

Center for Climate System Research, University of Tokyo,
Professor, Akimasa Sumi
Professor, Masahide Kimoto
National Institute for Environmental Studies
Senior researcher, Toru Nozawa
Senior researcher, Seita Emori
Frontier Research Center for Global Change, Japan Agency for Marine-Earth Science and Technology
Group leader (as an additional post), Seita Emori

(Released by the Ministry of the Environment press club, the press association of the Ministry of Education, Culture, Sports, Science and Technology, and the press association of Tsukuba University Town)

1. Outline

The joint research team of Center for Climate System Research (CCSR) of the University of Tokyo, National Institute for Environmental Studies (NIES), and Frontier Research Center for Global Change (FRCGC) of the Japan Agency for Marine-Earth Science and Technology has succeeded in the reproduction of the variation of global mean surface air temperature observed in the 20th century, using a numerical model which calculates the atmosphere and the ocean of the Earth. In this calculation, the external factors of climate variations that were ignored in previous attempts have been included. As a result of numerical experiments by separately including the factors, it has been strongly suggested that the temperature rise in this recent years (about the last 30 years of the 20th century) is attributable to human activities. On the other hand, it has been suggested that the temperature rise in the early 20th century (between 1910 and 1945) is attributable to natural causes. It is expected that further analysis of this output will be useful to obtain more findings besides on temperature increase.

This research was carried out by the Global Environment Research Fund of the Ministry of the Environment, and the research fund of “Project for Sustainable Coexistence of Human, Nature, and the Earth” by the Ministry of Education, Culture, Sports, Science and Technology.

2. Background

Observational data shows that the mean surface air temperature of the Earth had raised approximately 0.6ºC in 100 years of the 20th century. Various research institutions in the world have conducted numerical experiments to reproduce the climate of the 20th century using supercomputers. In these experiments, various observed factors of climate variations, such as increased greenhouse gases, are given to the model as external conditions. In the previous experiments, due to the lack of information, only a part of the factors that are considered to be important were included. Now, the lacked information was collected and arranged. It has become then possible to conduct climate reproduction experiment of the 20th century including all the possible factors so far considered, such as the land use change and the change in carbon aerosols including soot emitted by human activities.

3. Outline of Calculations

The resolution of the climate model used is 300 km for the atmosphere and 100 km for the ocean, a standard one in the world. The experiments conducted are for the past 150 years including 20th century. The following eight factors of climate variations have been included.

(1) Variations of solar insolation
(2) Changes in aerosols that reached stratosphere due to large-scale volcanic eruption
(3) Increase in greenhouse-gas concentrations (carbon dioxide, methane, nitrous oxide, and Halocarbon)
(4) Decrease in stratospheric ozone concentration since the middle of 1970's
(5) Increase in tropospheric ozone concentration due to human activities
(6) Increase in sulfur dioxide emissions (precursor of sulfate aerosol) due to industrial activities
(7) Increase in carbonaceous aerosol emissions such as soot, due to human activities
(8) Change in land use

The factors (1) and (2) above are natural causes of climate variations, while those from (3) to (8) are due to human activities. Factors of climate variations were considered separately, such as including only natural factors or only human-activity factors, in order to understand the contribution of factors to the changes in mean surface air temperature observed.

4. The Results of Experiments

When considering all the factors of climate variations (Figure1 top panel), the model has reproduced the warming trend very realistically both in the early 20th century (1910 to 1945) and in the late 20th century (the last 30 years of the 20th century). On the other hand, in the case of ignoring all the factors (Figure 1 bottom panel), the observed temperature variations have not been reproduced at all.

5. Warming Trend in Recent Years

The observed remarkable warming trend since 1970 has been reproduced very realistically when only the factors due to human activities are considered (Figure 1 second top panel). However, when only the natural factors are considered, (Figure1 third top panel), the observed temperature variation has not been reproduced at all. From this analysis, it has been strongly suggested that the global warming trend in this recent years is caused by human activities. The factors due to human activities are classified roughly into the two: global warming caused by increase in greenhouse gases and global cooling caused by the increase in tropospheric aerosols. It is considered that the warming was actually observed because the former effect was greater than the latter.

6. The Warming Trend in the Early 20th Century

A trend of temperature rise was also observed from 1910 to 1945 in the early 20th century. Here, in the numerical experiments, when only the factors due to human activities (Figure 1 second top panel) are considered, it has hardly been reproduced. On the other hand, in the case of considering the natural causes (Figure 1 third top panel), a warming trend similar to that observed has successfully been reproduced. Therefore, it has been suggested that the warming trend in the early 20th century is attributable to natural causes. There are only two natural factors, namely, large-scale volcanic eruptions and changes in solar insolation. It appears that the temperature rise in the early 20th century was caused by an increase of solar insolation (Figure 2 bottom panel, around years 1910-1950) according to the time series of the two factors (Figure 2 bottom panel). However, when considering solely the change in solar insolation (Figure 2 middle panel), the warming trend seen in the observational data has not been reproduced. The mean surface air temperature of the Earth had been a little low from the late 19th century to the early 20th century because the volcanic eruptions were active (Figure 2 bottom panel, around 1885 to1915). However, since the Earth was recovered from the cooling due to volcanic activities at the same time when the solar insolation began to increase (1910 to 1950 in Figure 2 bottom panel), it appears that the warming trend seen in the observation was obtained through combined effect of the two factors.

(Additional comments)

Although, already in the IPCC third assessment report (2001), it was pointed out using numerical climate models that temperature rise in the early 20th century might be attributable to natural causes, and the temperature rise in the end of the 20th century was attributable to human activities, some important processes and factors of climate variations were not yet considered in the previous calculations. The reliability of the previous findings has been enhanced because the aforementioned problems have been solved, and thus all the factors of climate variations so far considered have been included in the present calculation.

Contact:

Mr. Miura
Research Cooperation Section, Komaba branch office of Kashiwa-region
Center for Climate System Research, University of Tokyo
Tel: 03-5453-3953 Fax: 03-5453-3964 URL: http://www.ccsr.u-tokyo.ac.jp/

Mr. Tanabe
Planning/Public Relations Office
National Institute for Environmental Studies
Tel: 029-850-2303 Fax: 029-851-2854 URL: http://www.nies.go.jp/

Japan Agency for Marine-Earth Science and Technology

Ms. Ota
Frontier Research Center for Global Change
Tel:045-778-5687 Fax:045-778-5497 URL: http://www.jamstec.go.jp/frcgc/jp/
Mr. Gochou
Public Relations Division, Administration Department
Tel:046-867-9066 Fax: 046-867-9055 URL: http://www.jamstec.go.jp/