The new IPCC climate report

Sea-level rise

Sea levels are rising faster now than in the previous two millennia, and the rise will continue to accelerate – regardless of the emissions scenario, even with strong climate mitigation. (This is due to the inertia in the system.) The new IPCC scenarios to 2100 are shown the following graph.

Figure 3 Rise of the global sea level until the year 2100, depending on the emissions scenario.

This is perhaps the biggest change over the 4th IPCC report: a much more rapid sea-level rise is now projected (28-98 cm by 2100). This is more than 50% higher than the old projections (18-59 cm) when comparing the same emission scenarios and time periods.

With unabated emissions (and not only for the highest scenario), the IPCC estimates that by the year 2300 global sea levels will rise by 1-3 meters. [Correction: the document actually says: “1 m to more than 3 m”]

Already, there are likely more frequent storm surges as a result of sea level rise, and for the future this becomes very likely.

Land and sea ice

Over the last two decades, the Greenland and Antarctic ice sheets have been losing mass, glaciers have continued to shrink almost worldwide, and Arctic sea ice and Northern Hemisphere spring snow cover have continued to decrease in extent.

The Greenland ice sheet is less stable than expected in the last report. In the Eemian (the last interglacial period 120,000 years ago, when the global temperature was higher by 1-2 °C) global sea level was 5-10 meters higher than today (in the 4th IPCC report this was thought to be just 4-6 meters). Due to better data very ​​high confidence is assigned to this. Since a total loss of the Greenland ice sheet corresponds to a 7 meters rise in sea level, this may indicate ice loss from Antarctica in the Eemian.

In the new IPCC report the critical temperature limit at which a total loss of the Greenland ice sheet will occur is estimated as 1 to 4°C of warming above preindustrial temperature. In the previous report that was still 1.9 to 4.6 °C – and that was one of the reasons why international climate policy has agreed to limit global warming to below 2 degrees.

With unabated emissions (RCP8.5) the Arctic Ocean will likely become virtually ice-free in summer before the middle of the century (see figure). In the last report, this was not expected until near the end of the century.

Figure 4 The ice cover on the Arctic Ocean in the 2-degree world (left) and the 4-degree world (right).

Rainfall

The IPCC expects that dry areas become drier due to global warming, and moist areas even wetter. Extreme rainfall has likely already been increasing in North America and Europe (elsewhere the data are not so good). Future extreme precipitation events are very likely to become more intense and more frequent over most land areas of the humid tropics and mid-latitudes.

Oceans

At high emissions (red scenario above), the IPCC expects a weakening of the Atlantic Ocean circulation (commonly known as the Gulf Stream system) by 12% to 54% by the end of the century.

Last but not least, our CO2 emissions not only cause climate change, but also an increase in the CO2 concentration in sea water, and the oceans acidify due to the carbonic acid that forms. This is shown by the measured data in the graph below.

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