The ocean is warmer, more acidic and less productive. Melting glaciers and ice sheets are causing sea level rise, and coastal extreme events are becoming more severe_and more to come if no or delayed action, said the report.
While sea level has risen globally by around 15 cm during the 20th century, it is currently rising more than twice as fast – 3.6 mm per year – and accelerating, the new report of the UN body for assessing science related climate change, IPCC has revealed.
Glaciers and ice sheets in polar and mountain regions, on the other hand, are losing mass, contributing to an increasing rate of sea level rise, together with expansion of the warmer ocean, the same report noted.
The IPCC (Intergovernmental Panel on Climate Change) Special Report on the Ocean and Cryosphere in a Changing Climate, approved this week, provides new evidence for the benefits of limiting global warming to the lowest possible level – in line with the goal that governments set themselves in the 2015 Paris Agreement.
It highlights the urgency of prioritizing timely, ambitious and coordinated action to address unprecedented and enduring changes in the ocean and cryosphere.
The ocean and the cryosphere – the frozen parts of the planet – play a critical role for life on Earth. A total of 670 million people in high mountain regions and 680 million people in low-lying coastal zones depend directly on these systems. Four million people live permanently in the Arctic region, and small island developing states are home to 65 million people, the report said.
Global warming has already reached 1°C above the pre-industrial level, due to past and current greenhouse gas emissions. And there is overwhelming evidence that this is resulting in profound consequences for ecosystems and people. The ocean is warmer, more acidic and less productive. Melting glaciers and ice sheets are causing sea level rise, and coastal extreme events are becoming more severe, it futher pointed.
“The open sea, the Arctic, the Antarctic and the high mountains may seem far away to many people,” said Hoesung Lee, Chair of the IPCC. “But we depend on them and are influenced by them directly and indirectly in many ways – for weather and climate, for food and water, for energy, trade, transport, recreation and tourism, for health and wellbeing, for culture and identity.”
“If we reduce emissions sharply, consequences for people and their livelihoods will still be challenging, but potentially more manageable for those who are most vulnerable,” Lee said.
Continuing sea level rise
Sea level will continue to rise for centuries, noted the report. It could reach around 30-60 cm by 2100 even if greenhouse gas emissions are sharply reduced and global warming is limited to well below 2°C, but around 60-110 cm if greenhouse gas emissions continue to increase strongly.
“In recent decades the rate of sea level rise has accelerated, due to growing water inputs from ice sheets in Greenland and Antarctica, in addition to the contribution of meltwater from glaciers and the expansion of warmer sea waters,” said Valérie Masson-Delmotte, Co-Chair of IPCC Working Group I. “The wide range of sea level projections for 2100 and beyond is related to how ice sheets will react to warming, especially in Antarctica, with major uncertainties still remaining.”
Changing ocean ecosystems
Warming and changes in ocean chemistry are already disrupting species throughout the ocean food web, with impacts on marine ecosystems and people that depend on them, the report said.
To date, the ocean has taken up more than 90% of the excess heat in the climate system. By 2100, the ocean will take up 2 to 4 times more heat than between 1970 and the present if global warming is limited to 2°C, and up to 5 to 7 times more at higher emissions.
Ocean warming reduces mixing between water layers and, as a consequence, the supply of oxygen and nutrients for marine life, the report said.
Meanwhile, the ocean has taken up between 20 to 30% of human-induced carbon dioxide emissions since the 1980s, causing ocean acidification. Continued carbon uptake by the ocean by 2100 will exacerbate ocean acidification.
Ocean warming and acidification, loss of oxygen and changes in nutrient supplies, are already affecting the distribution and abundance of marine life in coastal areas, in the open ocean and at the sea floor, the report said.
“Cutting greenhouse gas emissions will limit impacts on ocean ecosystems that provide us with food, support our health and shape our cultures,” said Hans-Otto Pörtner, Co-Chair of IPCC Working Group II. “Reducing other pressures such as pollution will further help marine life deal with changes in their environment, while enabling a more resilient ocean.”
The report futher pointed that sea level rise will increase the frequency of extreme sea level events, which occur for example during high tides and intense storms.
Indications are that with any degree of additional warming, events that occurred once per century in the past will occur every year by mid-century in many regions, increasing risks for many low-lying coastal cities and small islands.
Without major investments in adaptation, they would be exposed to escalating flood risks, the report shows. Some island nations are likely to become uninhabitable due to climate-related ocean and cryosphere change, the report said, but habitability thresholds remain extremely difficult to assess.
Increases in tropical cyclone winds and rainfall are exacerbating extreme sea level events and coastal hazards. Hazards will be further be intensified by an increase in the average intensity, magnitude of storm surge and precipitation rates of tropical cyclones, especially if greenhouse gas emissions remain high.
Marine heatwaves, meanwhile, have doubled in frequency since 1982 and are increasing in intensity. They are projected to further increase in frequency, duration, extent and intensity.
Their frequency will be 20 times higher at 2°C warming, compared to pre-industrial levels. They would occur 50 times more often if emissions continue to increase strongly.
Declining Arctic sea ice, thawing permafrost
The extent of Arctic sea ice is declining in every month of the year, and it is getting thinner.
If global warming is stabilized at 1.5°C above pre-industrial levels, the Arctic ocean would only be ice-free in September – the month with the least ice – once in every hundred years. For global warming of 2°C, this would occur up to one year in three.
Permafrost ground that has been frozen for many years is warming and thawing and widespread permafrost thaw is projected to occur in the 21st century.
Even if global warming is limited to well below 2°C, around 25% of the near-surface (3-4 meter depth) permafrost will thaw by 2100. If greenhouse gas emissions continue to increase strongly, there is a potential that around 70% near-surface permafrost could be lost.
Arctic and boreal permafrost hold large amounts of organic carbon, almost twice the carbon in the atmosphere, and have the potential to significantly increase the concentration of greenhouse gases in the atmosphere if they thaw.
It is unclear whether there is already a net release of carbon dioxide or methane due to the ongoing thaw of the Arctic permafrost. In the future, increased plant growth can increase the storage of carbon in soils and offset carbon release from permafrost thaw, but not at the scale of large changes on the long term.
Changes in high mountains
Glaciers, snow, ice and permafrost are declining and will continue to do so. This is projected to increase hazards for people, for example through landslides, avalanches, rockfalls and floods.
Smaller glaciers found for example in Europe, eastern Africa, the tropical Andes and Indonesia are projected to lose more than 80% of their current ice mass by 2100 under high emission scenarios. The retreat of the high mountain cryosphere will continue to adversely affect recreational activities, tourism, and cultural assets.
As mountain glaciers retreat, they are also altering water availability and quality downstream, with implications for many sectors such as agriculture and hydropower.
“We will only be able to keep global warming to well below 2°C above pre-industrial levels if we effect unprecedented transitions in all aspects of society, including energy, land and ecosystems, urban and infrastructure as well as industry.
“The ambitious climate policies and emissions reductions required to deliver the Paris Agreement will also protect the ocean and cryosphere – and ultimately sustain all life on Earth,” said Debra Roberts, Co-Chair of IPCC Working Group II.
Read more at https://www.ipcc.ch/srocc/home/