air, which has increased the atmospheric uptake of radiated heat from the Earth’s surface. This has induced higher temperatures throughout the Earth, and that’s the mechanism of global warming. Annual fossil carbon dioxide emissions per year in 2001-2005 are estimated to be about 7 billion tons (carbon equivalent; hereinafter referred to as tC. Seven billion tC is equivalent to about 26 billion tons in terms of the amount of carbon dioxide), and this value is expected to further increase in the future. In the meanwhile, the maximum amount of carbon dioxide that can be absorbed yearly by nature is estimated to be about 3 billion tC (or about 11 billion tons in terms of the amount of carbon dioxide). In this light, it is necessary to maintain the GHG balance between emissions and

Figure 1-9: Mechanism of global warming

uptake in order to stabilize climate and prevent the expansion of adverse impacts.

A few scientists strongly doubt this theory that the increase in human-caused $ CO_{2} $ emissions is the major cause of global warming. However, when we study the trend after the Industrial Revolution and during the latter half of the 20th century in particular, we cannot explain the hike in $ CO_{2} $ level and temperature without consideration on the human-caused $ CO_{2} $ emission increase.

Those scientists who doubt the fatal contribution of human-caused carbon dioxide to global warming point out that the current amount of human-caused $ CO_{2} $ emissions of about 7 billion tC is far below the amount of carbon exchanged among the atmosphere, ocean, and terrestrial biosphere, which is estimated to be 90 billion tC. However, the value “7 billion tC” represents the annual emissions, and the accumulated amount of $ CO_{2} $ emissions caused by human activities is estimated to be 350 billion tC. This is equivalent to 60% to 70% of the amount of $ CO_{2} $ remaining in the atmosphere before the Industrial Revolution, and apparently exceeds the $ CO_{2} $ uptake potential in nature.

There is another theory supported by a few scientists, arguing that the increased $ CO_{2} $ concentration is not responsible for global warming and that, instead, the rise of temperature is responsible for the increase in $ CO_{2} $ concentration. This theory alleges that increased sea-surface temperature induces $ CO_{2} $ release into the air from the ocean, causing the increase in atmospheric $ CO_{2} $ concentration. However, suppose this alleged mechanism exists, it cannot theoretically explain the fact that $ CO_{2} $ concentration has rapidly risen for the past few decades, and the cause of the recent temperature hike either.

Recently, science academics of the G8 nations jointly announced the statement (in 2005) that claims urgent actions to be taken by G8 governments over the issue of global warming, on the grounds that most of the warming in recent decades can be attributed to the human-caused increases in $ CO_{2} $ emissions. The IPCC AR4 WG1 report also supports most of the observed increase in global average temperatures since the mid-20th century is very likely (probability of occurrence is more than 90%) due to the observed increase in anthropogenic greenhouse gas concentrations. Thus, the mechanism that global warming is attributed to human-caused GHG increase in the atmosphere, including carbon dioxide, is a general consensus among majority of scientists in the world.

According to the IPCC AR4 WG3 report (the report by Working Group III for the Fourth Assessment Report), there is substantial economic potential for the mitigation of global GHG emissions over the coming decades, that could offset the projected growth of global emissions or reduce emissions below current levels. Bottom-up studies indicate that the economic mitigation potential in 2030 is 9-17 GtCO $ _{2} $ -eq/yr in the carbon price of 20 US/tCO $ _{2} $ -eq, and 16-31 GtCO $ _{2} $ -eq/yr for 100 US/tCO $ _{2} $ -eq.

Table 1-1: Stabilization scenario

Category	CO2 concentration	CO2-eq Concentration	Global mean temperature increase above pre-industrial at equilibrium, using “best estimate” climate sensitivity	Peak year for CO2 emissions	Change in global CO2 emissions in 2050
ppm	ppm	°C	Year	%
I	350~400	445~490	2.0~2.4	2000~2015	-85~ -50
II	400~440	490~535	2.4~2.8	2000~2020	-60~ -30
III	440~485	535~590	2.8~3.2	2010~2030	-30~ +5
IV	485~570	590~710	3.2~4.0	2020~2060	+10~ +60
V	570~660	710~855	4.0~4.9	2050~2080	+25~ +85
VI	660~790	855~1130	4.9~6.1	2060~2090	+90~+140

Source: Compiled by the Ministry of the Environment based on the IPCC AR4 WG3 report