Climate change could directly threaten the water supply for 670 million people

The United Nations-backed Intergovernmental Panel on Climate Change published a report on Wednesday investigating what is likely to happen to oceans, glaciers and ice caps if the world fails to bring carbon emissions under control. Here are some of the key findings from the latest science:

Shrinking glaciers

A total of 670 million people in high mountain regions depend on glaciers for their water supplies. But climate change is causing these glaciers to retreat rapidly.

The melting has already started to cause an initial surge in water runoff, temporarily increasing water supply - a phenomenon witnessed in Asia, the European Alps and Alaska. But as glacial water storages shrink, this peaking effect will swing into reverse, and rivers could eventually run dry.

Smaller glaciers in places such as eastern Africa, the tropical Andes and Indonesia are projected to lose more than 80% of their current ice mass by 2100 if emissions remain high.

Melting ice sheets, rising seas

As climate change caused by burning fossil fuels drives temperatures higher, the ocean warms, causing it to expand. This expansion in turn causes sea levels to rise.

Scientists have found that water draining from melting glaciers and ice sheets in polar and mountain regions is now also contributing to this rise in sea levels.

While global sea levels rose by about 15 centimeters during the 20th century, the report said sea levels are now rising twice as fast, and accelerating.

There is now so much warming baked in to the global climate system that sea levels will continue to rise for centuries. Assuming greenhouse gas emissions fall sharply, and warming is limited to well below 2 degrees Celsius as stipulated in the 2015 Paris Agreement, then sea-level rise could be limited to about 30-60 cm by the end of the century.

But if greenhouse gas emissions continue to increase strongly, the sea-level rise could reach 60-110 cm by 2100.

High tides, storm surges

If warming continues, then storm surge 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.

An increase in the intensity and magnitude of storm surges and the precipitation rates of tropical cyclones will increase the dangers, especially if greenhouse gas emissions remain high.

Collapse of ocean ecosystems

To date, the ocean has absorbed more than 90% of the excess heat in the climate system. By 2100, the ocean will absorb up to two to four times more heat than between 1970 and now if warming is limited to 2°C, and up to five to seven times more at higher emissions.

Ocean warming and acidification, the loss of oxygen and changes in nutrient supplies are already hitting marine ecosystems hard.

Marine heatwaves have doubled in frequency since 1982 and are expected to get worse. They are projected to occur 20 times more frequently at 2°C warming, compared to pre-industrial levels. If emissions continue to increase strongly, marine heatwaves will occur 50 times more often.

Disappearing arctic sea ice, thawing permafrost

Arctic sea ice is declining in every month of the year, and 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. If global warming reached 2°C, this phenomenon would occur up to one year in three.

Permafrost that has been frozen for many years is thawing. Even if global warming is limited to well below 2°C, around 25% of the upper layer of the world's permafrost will thaw by 2100. If greenhouse gas emissions continue to increase strongly, it is possible that around 70% of this upper permafrost could be lost.

Arctic and boreal permafrost hold vast amounts of ancient carbon and could significantly increase the concentration of greenhouse gases in the atmosphere if they thaw. It is unclear whether thawing Arctic permafrost has already started to produce a net release of carbon dioxide or methane, a much more powerful short-term warming agent.