日本経済新聞 2007/11/18
温暖化対策 2050年まで300兆円必要 IPCC報告「今後20ー30年がカギ」
国連の気候変動に関する政府間パネル(IPCC)は１７日、スペイン・バレンシアで開いた総会で第四次報告を採択した。
地球温暖化の進行を抑えるには、2050年までに全世界の国内総生産(GDP)の最大5.5%(約300兆円に相当)が必要になると試算。「今後20−30年間の削減努力と投資が大きな影響を与える」と、国際社会の取り組みを訴えた。
今年前半に開いた３つのIPCC作業部会の評価報告を統合したもので、６年ぶりの研究成果となる。国連の播基文事務総長は同日の記者会見で「国際社会の協力による持続的な行動で破滅的なシナリオは回避できる」と強調した。
報告は早急な対策がなければ、地球の平均気温が今世紀末に最大で6.4度上昇するなど事態が深刻化すると警告。気温の上昇幅を2-3度に抑えなければ世界的に損失が拡大すると指摘したうえで、影響の抑制には５０年までに二酸化炭素(C02)など温暖化ガスの排出を半減させる必要があるとした。
最終的には世界GDPの5.5%(年平均で0.12%)の投資コストが必要だとする一方、温暖化ガスの排出削減を００年レベルまでにとどめるならぱ、投資コストも世界GDPの４%前後で済むと予想。だが、その場合の気温上昇幅は3度を超え、被害が飛躍的に増える恐れがあると分析している。
| IPCC報告の骨子 ・温暖化は疑う余地がない ・気温上昇のほとんどは人間活動によってもたらされた ・現在の政策を続ければ温暖化ガスは今後20-30年増加する ・早急な対策がなければ地球の平均気温は今世紀末に最大で6.4度上昇する ・温暖化の進行を抑えるには2050年までに約300兆円が必要 ・今後20-30年の努力と投資が温暖化ガスの安定化のカギとなる |
2050年までに排出半減
IPCC第四次報告の主な内容は次の通り。
・気温上昇のほとんどは人間活動によってもたらされた可能性がかなり高い。
・今後20−30年間の気候変動緩和の努力と投資が(気温上昇を)低く安定させられるかどうかに大きな影響を与える。
・温暖化ガス濃度を抑えようするほど、早期に排出量の削減に転じなければいけない。
・温暖化の緩和策と影響への適応策をとれば、気候変動リスクをかなり減らすことができる。
・気候変動の影響はコスト負担を生む可能性がかなり高く、コストは気温上昇に伴い増える。
・温暖化進行を抑えるには2050年までに全世界のGDPの最大5.5%(約300兆円)のコストが必要。
・対策の厳しさに応じ、今世紀末の気温は２０世紀末比で1.1−6.4度上昇、海面は18−59センチの範囲で上がると予測される。
・気温の上昇幅を2−3度に抑えなければ損失が拡大する。影響抑制には50年までに温暖化ガスの排出を半減させる必要がある。
・温暖化ガス濃度を安定させても気温と海面の上昇は何世紀も続く。
・寒い日や夜、霜が降りる日が減少し、熱波や大雨の頻度が増している可能性が高い。
・温暖化は二酸化炭素の陸や海洋への取り込み量を減らす恐れがある。
・社会的弱者が温暖化の影響を受けやすい。
・２０世紀末からの気温上昇が1.5−2.5度を超えると、20-30%の生物種で絶滅リスクが高まる可能性がある。
Summary for Policymakers
of the Synthesis Report of the IPCC Fourth Assessment Report
http://www.ipcc.ch/pdf/assessment-report/ar4/syr/ar4_syr_spm.pdf
1. Observed changes in climate and their effects
Warming of the climate system is unequivocal, as is now evident from observations of increases in global average air and ocean temperatures, widespread melting of snow and ice, and rising global average sea level
Observational evidence from all continents and most oceans shows that many natural systems are being affected by regional climate changes, particularly temperature increases.
There is medium confidence that other effects of regional climate change on natural and human environments are emerging, although many are difficult to discern due to adaptation and non-climatic drivers.
2. Causes of change
Global greenhouse gase (GHG) emissions due to human activities
have grown since pre-industrial times, with an increase of 70%
between 1970 and 2004
Global atmospheric concentrations of CO2, methane (CH4) and nitrous oxide (N2O) have increased markedly as a result of human activities since 1750 and now far exceed pre-industrial values determined from ice cores spanning many thousands of years.
Most of the observed increase in globally-averaged temperatures since the mid-20th century is very likely due to the observed increase in anthropogenic 人為的 GHG concentrations. It is likely there has been significant anthropogenic warming over the past 50 years averaged over each continent (except Antarctica)
Advances since the TAR(Third Assessment Report) show that discernible human influences extend beyond average temperature to other aspects of climate.
Human influences have:
・ very likely contributed to sea level rise during the latter half of the 20th century
・ likely contributed to changes in wind patterns, affecting extra-tropical storm tracks and temperature patterns
・ likely increased temperatures of extreme hot nights, cold nights and cold days
・ more likely than not increased risk of heat waves, area affected by drought since the 1970s and frequency of heavy precipitation events.
Anthropogenic warming over the last three decades has likely had a discernible influence at the global scale on observed changes in many physical and biological systems.
3. Projected climate
change and its impacts
There is high agreement and much evidence that with current
climate change mitigation policies and related sustainable
development practices, global GHG emissions will continue to grow
over the next few decades.
Continued GHG emissions at or above current rates would cause further warming and induce many changes in the global climate system during the 21st century that would very likely be larger than those observed during the 20th century
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There is now higher confidence than in the TAR in projected patterns of warming and other regional-scale features, including changes in wind patterns, precipitation, and some aspects of extremes and sea ice.
Studies since the TAR have enabled more systematic understanding of the timing and magnitude of impacts related to differing amounts and rates of climate change.
Altered frequencies and intensities of extreme weather, together with sea level rise, are expected to have mostly adverse effects on natural and human systems.
Anthropogenic warming and sea level rise would continue for centuries due to the timescales associated with climate processes and feedbacks, even if GHG concentrations were to be stabilised.
Anthropogenic warming could lead to some impacts that are abrupt or irreversible, depending upon the rate and magnitude of the climate change.
4. Adaptation and
mitigation options
A wide array of adaptation options is available, but more
extensive adaptation than is currently occurring is required to
reduce vulnerability to climate change. There are barriers,
limits and costs, which are not fully understood.
Adaptive capacity is intimately connected to social and economic development but is unevenly distributed across and within societies.
Both bottom-up and
top-down studies indicate that there is high agreement and much
evidence of 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.
While top-down and bottom-up studies are in line at the global
level there are considerable differences at the sectoral level.
A wide variety of policies and instruments are available to governments to create the incentives for mitigation action. Their applicability depends on national circumstances and sectoral context.
Many options for reducing global GHG emissions through international cooperation exist. There is high agreement and much evidence that notable achievements of the UNFCCC and its Kyoto Protocol are the establishment of a global response to climate change, stimulation of an array of national policies, and the creation of an international carbon market and new institutional mechanisms that may provide the foundation for future mitigation efforts. Progress has also been made in addressing adaptation within the UNFCCC and additional international initiatives have been suggested.
In several sectors, climate response options can be implemented to realise synergies and avoid conflicts with other dimensions of sustainable development. Decisions about macroeconomic and other non-climate policies can significantly affect emissions, adaptive capacity and vulnerability.
5. The long-term
perspective
Determining what constitutes “dangerous anthropogenic
interference with the climate system” in relation to Article 2 of the
UNFCCC involves value judgements. Science can support informed
decisions on this issue, including by providing criteria for
judging which vulnerabilities might be labelled “key”.
The five “reasons for concern”
identified in the
TAR remain a viable framework to consider key vulnerabilities.
These “reasons”
are assessed here
to be stronger than in the TAR. Many risks are identified with
higher confidence. Some risks are projected to be larger or to
occur at lower increases in temperature.
Understanding about the relationship between impacts (the basis
for “reasons for concern”
in the TAR) and
vulnerability (that includes the ability to adapt to impacts) has
improved.
| ・ | Risks
to unique and threatened systems. There is new and stronger evidence of observed impacts of climate change on unique and vulnerable systems (such as polar and high mountain communities and ecosystems), with increasing levels of adverse impacts as temperatures increase further. An increasing risk of species extinction and coral reef damage is projected with higher confidence than in the TAR as warming proceeds. There is medium confidence 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 over 1980-1999 levels. Confidence has increased that a 1-2°C increase in global mean temperature above 1990 levels (about 1.5-2.5°C above pre-industrial) poses significant risks to many unique and threatened systems including many biodiversity hotspots. Corals are vulnerable to thermal stress and have low adaptive capacity. Increases in sea surface temperature of about 1-3°C are projected to result in more frequent coral bleaching events and widespread mortality, unless there is thermal adaptation or acclimatization by corals. Increasing vulnerability of indigenous communities in the Arctic and small island communities to warming is projected. |
| ・ | Risks
of extreme weather events. Responses to some recent extreme events reveal higher levels of vulnerability than the TAR. There is now higher confidence in the projected increases in droughts, heatwaves, and floods as well as their adverse impacts. |
| ・ | Distribution
of impacts and vulnerabilities. There are sharp differences across regions and those in the weakest economic position are often the most vulnerable to climate change. There is increasing evidence of greater vulnerability of specific groups such as the poor and elderly in not only developing but also developed countries. Moreover, there is increased evidence that low-latitude and less-developed areas generally face greater risk, for example in dry areas and mega-deltas. |
| ・ | Aggregate
impacts. Compared to the TAR, initial net market-based benefits from climate change are projected to peak at a lower magnitude of warming, while damages would be higher for larger magnitudes of warming. The net costs of impacts of increased warming are projected to increase over time. |
| ・ | Risks
of large-scale singularities. There is high confidence that global warming over many centuries would lead to a sea level rise contribution from thermal expansion alone which is projected to be much larger than observed over the 20th century, with loss of coastal area and associated impacts. There is better understanding than in the TAR that the risk of additional contributions to sea level rise from both the Greenland and possibly Antarctic ice sheets may be larger than projected by ice sheet models and could occur on century time scales. This is because ice dynamical processes seen in recent observations but not fully included in ice sheet models assessed in AR4 could increase the rate of ice loss. |
There is high confidence that neither adaptation nor mitigation alone can avoid all climate change impacts; however, they can complement each other and together can significantly reduce the risks of climate change.
Many impacts can be
reduced, delayed or avoided by mitigation. Mitigation efforts and
investments over the next two to three decades will have a large
impact on opportunities to achieve lower stabilisation levels.
Delayed emission reductions significantly constrain the
opportunities to achieve lower stabilisation levels and increase
the risk of more severe climate change impacts.
There is high agreement and much evidence that all stabilisation levels assessed can be achieved by deployment of a portfolio of technologies that are either currently available or expected to be commercialised in coming decades, assuming appropriate and effective incentives are in place for their development, acquisition, deployment and diffusion and addressing related barriers.
The macro-economic costs of mitigation generally rise with the stringency of the stabilisation target. For specific countries and sectors, costs vary considerably from the global average.
In 2050, global average macro-economic costs for mitigation towards stabilisation between 710 and 445ppm CO2-eq are between a 1% gain and 5.5% decrease of global GDP. This corresponds to slowing average annual global GDP growth by less than 0.12 percentage points.
Estimated global macro-economic costs in 2030 and 2050. Costs are relative to the baseline for least-cost trajectories towards different long-term stabilisation levels.
Stabilisation levels
(ppm CO2-eq)Median GDP
reduction (%)Range of GDP
reduction(%)Reduction of average annual
GDP growth rates (percentage
points)2030 2050 2030 2050 2030 2050 445 - 535 Not available < 3 < 5.5 < 0.12 < 0.12 535 - 590 0.6 1.3 0.2 to 2.5 slightly negative to 4 < 0.1 < 0.1 590 - 710 0.2 0.5 -0.6 to 1.2 -1 to 2 < 0.06 < 0.05
Responding to climate change involves an iterative risk management process that includes both adaptation and mitigation and takes into account climate change damages, co-benefits, sustainability, equity, and attitudes to risk.