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世界氣候科學家紛紛發聲：如果我們想將人類造成的全球暖化限制在1.5以內？我們也許可以。但考慮到我們的起點，這將是艱難的。
這是一個結論 新報告 by the UN’s Intergovernmental Panel on Climate Change (IPCC). The focus on 1.5? is the result of years of international negotiation. Starting in 1994, a central aim of the UN’s climate change efforts (the Framework Convention on Climate Change, or UNFCCC) was to stabilise greenhouse gas concentrations at a level that would “prevent dangerous anthropogenic interference with the climate system”. Much was written on what this meant, particularly the word “dangerous”.
氣候變化的負面影響是連續的，並且確定氣候變化變得危險的點是困難和有爭議的。 另一方面，如果沒有一些目標努力，氣候變化談判就很困難。
Fifteen years later, the UNFCCC’s Copenhagen Accord introduced a 2? target, and its 2015 巴黎協定 was even more specific: it “aims to strengthen the global response to the threat of climate change … by holding the increase in … temperature to well below 2? above pre-industrial levels and pursuing efforts to limit the … increase to 1.5?”.
IPCC向制定政策的UNFCCC提供科學建議，而IPCC本身從未說過溫度目標。 然而，它確實列出了五個“關注原因”的氣候變化風險。 其中包括諸如“獨特和受威脅的生態系統和文化”（如珊瑚礁）和“極端天氣事件”等影響，每個影響的評級從“不可檢測”到“非常高”。 IPCC最近的（2014） 第五次評估 of the scientific evidence found that at around 1.5? warming there was a transition 從中度到高度的風險 受威脅的生態系統和文化以及極端天氣事件。 因此，巴黎和IPCC評估之間存在一致性。
氣候變化將使某些類型的極端天氣更加普遍。 德魯麥克阿瑟/ Shutterstock
The Paris Agreement asked the IPCC to report on the impacts of global warming of 1.5?, and this new publication is the result. Its tone is not “we must avoid 1.5? warming”, as you might think from many commentators, but more “if we want to avoid 1.5? warming, this is what must be done”. The report contrasts the impact of 1.5? and 2? warmings, giving information on what would be gained by the extra effort needed to limit warming to 1.5?.
As the IPCC’s reports are largely based on a critical assessment and synthesis of published scientific papers, many of its latest conclusions are unsurprising. There are many well recognised uncertainties in understanding climate change - for instance, even if we set a course aiming to hit 1.5? (which is mostly determined by future CO? emissions), we could end up hitting, say, 1? or 2? instead. The report provides uncertainty ranges in its estimates and confidence levels, based on expert judgement.
The new report tells us that human activity has already caused about 1? of global warming, while at the present rate of warming (0.2? per decade) we’ll hit 1.5? by about 2040. National pledges made as part of the Paris Agreement still mean we are on course for warming of about 3? by 2100, meaning four of the five “reasons for concern” would then be in the high to very-high risk category.
Achieving the 1.5? target will require anthropogenic CO? emissions to decline by 45% by 2030 (relative to 2010). By 2050, they will need to reach “net zero” - any further CO? emissions due to human activity would then have to be matched by 故意刪除 of CO? already in the atmosphere, including by planting trees. Net zero would have to occur by around 2075 to meet a 2? target.
泰國的再造林項目。 Somrerk Witthayanantw / Shutterstock
Many illustrations are given for the difference between 1.5? and 2? worlds. At 1.5?, summertime Arctic sea ice is projected to disappear once per century, compared to once per decade at 2?; 8% of plants that have been studied would lose half their climatically-suitable area, compared to 16%; sea level rise would be 10cm less (with 10m fewer people impacted at today’s population levels); and while coral reefs might decline by a further 80% at 1.5?, they could virtually disappear at 2?.
The report identifies various routes by which emissions cuts would limit warming to 1.5?; each makes assumptions about future changes in, for example, economic strategy, population growth and the rate at which low carbon energy is adopted. The IPCC recognises the challenges are “unprecedented in scale” but notes, for example, “the feasibility of solar energy, wind energy and electricity storage mechanisms have substantially improved over the past few years”.
The report is sensitive to the fact that changes required to meet 1.5? must be consistent with the UN’s wider 可持續發展目標。 限制氣候變化將有助於實現與健康，清潔能源，城市和海洋相關的目標。 但是，“如果不加以謹慎管理”，對其他人（貧困，飢餓，水，能源獲取）可能產生負面影響。
那麼接下來呢？ 當然，這些結論將在很多層面上進行廣泛辯論，但在12月初在波蘭卡托維茲舉行的下一次會議上，將關注聯合國氣候變化框架公約的回應。
關於作者
Keith Shine，Regius氣象學和氣候科學教授， 雷丁大學
相關書籍
at InnerSelf 市場和亞馬遜
