Figure3-2-1 Linkages between Acidity of Surface Seawater and Formation of Calcium Carbonate

※Aragonite: Calcite and aragonite are two types of calcium carbonate with different crystal forms. Living organisms that form shells with aragonite more significantly affected by ocean acidification include sea butterflies (planktonic conches) and corals.

Source: Center for Global Environmental Research, National Institute for Environmental Studies, "What We Should Know about Global Warming"

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widespread mortality, unless there is thermal adaptation or acclimatization by corals.

Further, there have been changes in oceans and forests, which are the vital foundations for the living organisms. Before the Industrial Revolution, when the atmospheric concentration of carbon dioxide was around 280ppm, the sea surface had a pH of 8.17. At present, when the carbon dioxide concentration has reached around 380ppm, the sea surface pH has already declined to around 8.06 (Figure 3-2-1). In oceans, there are many living organisms that have calcareous shells and skeletons. For examples, shellfish form shells for self-protection, while fish use calcium carbonate for ear stones that maintain their physical balance. Corals leave calcareous skeletons to nurture the next generation. However, when the carbon dioxide concentration in seawater rises with atmospheric carbon dioxide dissolving into seawater, acid $ \left(\mathrm{H}^{+}\right) $ generated from carbon dioxide neutralizes carbonate ion $ \left(\mathrm{CO}_{3}^{2-}\right) $ , the material for calcium carbonate, reducing the carbonate ion concentration and making the production of calcium carbonate difficult. According to German Council of Science and Humanities, in order to avoid a critical impact on marine organisms that form calcareous shells, the decline in pH since the Industrial Revolution should not exceed 0.2. Meanwhile, in order to keep atmospheric temperature increases within two degrees C, the carbon dioxide concentration must be kept from exceeding 450ppm. If the carbon dioxide concentration is 450ppm, the decline in seawater pH would be around 0.17, barely below the 0.2 target to avoid a critical impact on marine organisms. Incidentally, both the atmospheric temperature rise target of 2 degrees C, above which climate change is likely to cause a major impact, and the pH decline target to avoid a critical impact on marine organisms have the same carbon dioxide concentration target of 450ppm.

A study on wildfires by the University of California, et al, and referenced by the IPCC Fourth Assessment Report found that since the 1970s, wildfire in the western United States increased in years when temperatures from spring to summer increased by about 2 degrees Celsius. Thus, large wildfires increased suddenly since the mid-1980s, and it has been reported that the frequency of wildfires is about four times and the forested area burned from 1987 to 2003 is 6.7 times the area from 1970 to 1986. In terms of the impact on ecosystems, the IPCC Fourth Assessment Report states that approximately 20-30% of plant and animal species assessed so far are likely to be at increased risk of extinction if increases in global average temperature exceed 1.5-2.5°C.

Turning to the impact on ecosystems in Japan, “Comprehensive Assessment of Climate Change Impacts to Determine the Dangerous Level of Global Warming and Appropriate Stabilization Target of Atmospheric GHG (greenhouse gas) Concentration” (hereafter referred to as “Project for Comprehensive Projection of Climate Change Impacts”), a strategic research and development area project financed by the Global Environment Research Fund of the Ministry of the Environment, projects a decrease in suitable habitats for beech forest an expansion of pine distribution areas at risk of pine wilt. It also projects that the impact and damage will likely decrease considerably if the greenhouse gas concentration is contained at the stringent stabilization target of 450ppm, but occurrence of a measure of damage cannot be avoided (Figure 3-2-2).

The degradation on biodiversity also affects global warming. The annual amount of carbon dioxide naturally absorbed by the earth is about 3.1 billion tons-C, of which terrestrial ecosystems (forests, grasslands and farmland, etc.) are estimated to absorb about 1.8 billion tons-C. As discussed in Section 1, the decline in forest area has not halted, with the capacity to absorb carbon dioxide decreasing gradually. Thus, the decline and degradation of forest ecosystems are likely to accelerate global warming. And if the atmospheric carbon dioxide concentration increases, the acidification of oceans that absorb 25% of global carbon dioxide emissions progresses further, threatening a serious impact on marine ecosystems.