| Abstract from DBPedia | Globally, sea levels are rising due to human-caused climate change. Between 1901 and 2018, the globally averaged sea level rose by 15–25 cm (6–10 in), or 1–2 mm per year on average. This rate is accelerating, with sea levels now rising by 3.7 mm per year. Climate scientists expect further acceleration during the 21st century. Climate change heats (and therefore expands) the ocean and melts land-based ice sheets and glaciers. Between 1993 and 2018, the thermal expansion of water contributed 42% to sea level rise; melting of temperate glaciers, 21%; Greenland, 15%; and Antarctica, 8%. Over the next 2000 years, the sea level is predicted to rise by 2–3 m (7–10 ft) if global warming is limited to 1.5 °C, by 2–6 m (7–20 ft) if it peaks at 2 °C and by 19–22 metres (62–72 ft) if it peaks at 5 °C. Sea level rise has a substantial lag in its response to Earth temperature changes. This means that it is virtually certain to continue for a long time, and that its extent in the short term (i.e. around 2050) is insensitive to temperature changes between now and then. Thus, there's confidence that 2050 levels of sea level rise combined with the 2010 population distribution (i.e. absent the effects of population growth and human migration) would result in ~150 million people under the water line during high tide and ~300 million in places which are flooded every year – an increase of 40 and 50 million people relative to 2010 values for the same. At the same time, the impact on temperature from changes in greenhouse gas emissions over the longer term would greatly influence longer-term sea level rise: by 2100, the spread between the lowest and the highest plausible emission trajectories would result in the sea level rise of 0.38–0.77 m (1 ft 3 in – 2 ft 6 in) when using the best-understood median estimates. When compared to 2050 levels, the difference between the low and high end of that range is equivalent to the difference between ~40 million more people under the water line during high tide and ~50 million more in places which are flooded every year (190 and 350 million people) and ~80 and ~90 million more for the same metrics (230 and 390 million people), respectively. Under the highest emission scenario, less-understood processes may also lead to sea level rise of well over one metre (3+1⁄2 ft) by 2100, and the levels of two metres (6+1⁄2 ft) cannot be excluded. Applying this extent of sea level rise to the 2010 population distribution could mean as many as 520 million more people under the water line during high tide and 640 million in places which are flooded every year. While the rise in sea levels ultimately impacts every coastal and island population on Earth and leads to higher storm surges, more dangerous tsunamis, damage in cities, loss and degradation of agricultural land and ultimately permanent loss of land and the displacement of populations, it does not occur uniformly due to local factors like tides, currents, storms, tectonic effects and land subsidence. For instance, sea level rise along US coasts (and along the US East Coast in particular) is already higher than the global average, and it is expected to be 2 to 3 times greater than the global average by the end of the century. At the same time, Asia will be the region where sea level rise would impact the most people: eight Asian countries – Bangladesh, China, India, Indonesia, Japan, the Philippines, Thailand and Vietnam – account for 70% of the global population exposed to sea level rise and land subsidence. Altogether, out of the 20 countries with the greatest exposure to sea level rise, 12 are in Asia. Finally, the greatest near-term impact on human populations will occur in the low-lying Caribbean and Pacific islands – many of those would be rendered uninhabitable by sea level rise later this century. Societies can adapt to sea level rise in three different ways: implement managed retreat, accommodate coastal change, or protect against sea level rise through hard-construction practices like seawalls or soft approaches such as dune rehabilitation and beach nourishment. Sometimes these adaptation strategies go hand in hand, but at other times choices have to be made among different strategies. For instance, a managed retreat strategy is difficult if the population in the area is quickly increasing: this is a particularly acute problem for Africa, where the population of low-lying coastal areas is projected to increase by around 100 million people within the next 40 years. Poorer nations may also struggle to implement the same approaches to adapt to sea level rise as richer states, and sea level rise at some locations may be compounded by other environmental issues, such as subsidence in so-called sinking cities. Coastal ecosystems typically adapt to rising sea levels by moving inland; however, they might not always be able to do so, due to natural or artificial barriers.海面上昇（かいめんじょうしょう）とは海洋の平均水位の上昇のこと。要因として地球温暖化に端を発する海水の熱膨張や、大陸氷床の融解などがある。平均海水面、つまり波浪やうねり、津波・高潮などの短周期変動をならして平均化した水面の上昇を指す。 地球の長い歴史をみると、顕著な海面上昇と海面低下は何度も発生している（海水準変動を参照）。これは260万年前以降の第四紀にもみられ、特に氷期が終わって間氷期に向かい温暖化していく時期に、数十mもの海面上昇が起こったと推定されている。6000年前までの約1万年間にも、間氷期開始に伴う100m近い海面上昇が発生している。しかし、ここ数千年では大きくは変化せず、過去3千年間は平均0.1 － 0.2mm/年程度の上昇量であった。しかし近年は地球温暖化の影響により、その数十倍のペースの海面上昇が観測され、さらに加速するものと懸念されている。 現在では、一般的に「海面上昇」といえば19世紀以降の地球温暖化の影響と推定されるものを指す。地球史上の特定の時期に関して「海面上昇」と呼ぶこともあるが、この記事では特に断りがない限り地球温暖化によるものを取り上げる。 (Source: http://dbpedia.org/resource/Sea_level_rise) |
