２１００年に２０億人超が「危険な暑さ」に直面　英研究

【パリＡＦＰ＝時事】地球温暖化の抑制に向けた現行政策に変更がなければ、２１００年までに地表温度は産業革命前に比べ２．７度上昇すると予測する論文が２２日、科学誌「ネイチャー・サステナビリティー」に掲載された。その場合、世界の予想人口９５億人の２割強に当たる２０億人以上が、生命に危険が及ぶほどの酷暑環境に置かれる恐れがあるとしている。（写真は資料写真）
　英エクセター大学のグローバルシステム研究所の所長で、論文の筆頭著者ティム・レントン氏は「地表の居住環境が一変し、居住可能地域に大きな変動が起きる可能性がある」と予想した。
　このシナリオでは、生命を脅かす極暑にさらされる人口が最も多い国は、インド（６億人）、ナイジェリア（３億人）、インドネシア（１億人）、フィリピンとパキスタン（ともに８０００万人）など。
　ただし、世界の気温上昇を産業革命前と比べて１．５度に抑える「パリ協定」の目標を達成した場合は、酷暑環境下の人口は世界全体の約５％に当たる５億人以下にとどまる見通し。
　レントン氏は「地球温暖化による損失は経済面から語られることが多いが、私たちの研究では、気候変動危機に取り組まなかった場合の人的損失が驚くほど大きいことが浮き彫りになった」と強調。「気温が現在に比べ０．１度上昇するごとに、約１億４０００万人が危険な暑さにさらされることになる」と説明した。
　研究では、年平均気温が２９度を上回る場合を「危険な暑さ」と定義。人類史上、居住地域は年平均気温が温帯で１３度、熱帯では規模は小さいが２７度の地域周辺に分布している。
　すでに２９度に近づいている地域では、酷暑環境下に移行するリスクが高くなる。２９度またはそれ以上の高温が続けば、死亡率の上昇、労働生産性や農作物収量の低下、紛争や感染症の増加につながることが研究で明らかになっている。
　こうしたリスクが高まっているのは、人口が急増している赤道周辺地域だ。熱帯では、気温がそれほど高くなくても湿度が高いため、発汗による気化熱で体温を下げることができず、死に至る恐れがある。
　結局、酷暑の影響を最も受けるのは、１人当たりの温室効果ガス排出量が最も少ない貧困国だと、研究チームは指摘している。 【翻訳編集ＡＦＰＢＢＮｅｗｓ】
〔ＡＦＰ＝時事〕（2023/05/23-17:05）

Current policies to limit global warming will expose more than a fifth of humanity to extreme and potentially life-threatening heat by century's end, researchers warned Monday.
Earth's surface temperature is on track to rise 2.7 degrees Celsius above preindustrial levels by 2100, pushing more than two billion people -- 22 percent of projected global population -- well outside the climate comfort zone that has allowed our species to thrive for millennia, the scientists reported in Nature Sustainability.
The countries with the highest number of people facing deadly heat in this scenario are India (600 million), Nigeria (300 million), Indonesia (100 million), as well as the Philippines and Pakistan (80 million each).
That's a profound reshaping of the habitability of the surface of the planet, and could lead potentially to the large-scale reorganisation of where people live, said lead author Tim Lenton, director of the Global Systems Institute at the University of Exeter.
Capping global warming at the 2015 Paris climate treaty target of 1.5C would sharply reduce the number of those at risk to less than half-a-billion, some five percent of the 9.5 billion people likely to inhabit the planet six or seven decades from now, according to the findings.
Just under 1.2C of warming to date has already amplified the intensity or duration of heatwaves, droughts and wildfires beyond what could have occurred absent the carbon pollution generated by burning fossil fuels and forests. The last eight years were the hottest on record.
The costs of global warming are often expressed in financial terms, but our study highlights the phenomenal human cost of failing to tackle the climate emergency, said Lenton.
For every 0.1C of warming above present levels, about 140 million more people will be exposed to dangerous heat.
- Profoundly unjust -
The threshold for dangerous heat used in the new findings is a mean annual temperature (MAT) of 29C.
Across history, human communities have been densest around two distinct MATs -- 13C (in temperate zones) and to a lesser extent 27C (in more tropical climes).
Global warming is pushing up the thermostat everywhere, but the risk of tipping into lethal heat is clearly higher in regions already close to the 29C red line.
Sustained high temperatures at or beyond that threshold, studies have shown, are strongly linked to greater mortality, reduced labour productivity and crop yields, along with more conflict and infectious disease.
As recently as 40 years ago, only 12 million people worldwide were exposed to such extremes.
That number has today increased five-fold, and will climb ever more steeply in coming decades, the study found.
The risk is accentuated in regions straddling the equator, where human populations are expanding most rapidly: tropical climes can become deadly even at lower temperatures when high humidity prevents the body from cooling itself through sweating.
Episodes of extreme humid heat have doubled since 1979.
Those most exposed to extreme heat live mostly in poorer countries with the smallest per capita carbon footprints, the authors say.
According to the World Bank, India emits on average about two tonnes of CO2 per person every year and Nigerians about half-a-tonne annually, compared to less than seven tonnes per person in the European Union and 15 in the United States.
Carbon-cutting pledges by governments and companies not yet translated into action would stop the rise in global temperatures at -- or even below -- 2C, allowing hundreds of millions to avoid catastrophic heat.
But scenarios even worse than the 2.7C world that would result from current policies cannot be excluded either, the authors warn.
If past and continuing emissions trigger the release of natural carbon stores, such as in permafrost, or warm the atmosphere more than anticipated, temperatures could climb nearly four degrees above mid-19th century levels, they said.