Now or never, that is how the IPCC message could also be summed up. The IPCC (Intergovernmental Panel on Climate Change) has presented its sixth report since it was founded in 1988. In it, the intergovernmental body under the umbrella of the United Nation warns that it will soon be impossible to keep the warming of the earth below two degrees, let alone 1.5 degrees Celsius, unless we immediately start to drastically reduce greenhouse gas emissions. The earth has warmed by 1.07 degrees Celsius since the end of the 19th century due to human activities. Average warming is likely to reach or exceed 1.5 degrees over the next two decades. Heat waves are becoming more frequent and precipitation patterns are changing. Nevertheless, the researchers emphasize that mankind still has scope for action.
What is it about
It is the first comprehensive report on the state of the global climate since the three parts of the fifth assessment report were published in 2013 and 2014. The current sixth report also has three parts, the first has now been published. He deals with the physical principles of climate change. The other parts, which are due to appear in 2022, will deal with consequences, adaptation and climate protection. For the current report, 234 volunteer authors evaluated around 14,000 studies; the result can be viewed as a consensus of research. the “Summary for Decision Makers” was agreed sentence by sentence with the delegates of the 195 IPCC member states. The aim of this process is to establish agreement on the facts on a political level as well.
Scenarios and temperature
The scientists looked at five scenarios as examples. The horror scenario called SSP5-8.5 is considered unrealistic from today’s perspective. The one with the lowest emissions, SSP1-1.9, but also. So far, there are many indications that the world is about on a middle path. For the corresponding scenario, the researchers expect two degrees of warming by the middle of the century and 2.7 degrees by 2100. The neighboring scenarios end up at 1.8 degrees and 3.6 degrees by 2100. No scenario remains below 1.5 degrees, only the most optimistic reaches 1.6 degrees by the middle of the century and drops back to 1.4 degrees by 2100.
There has been significant progress in so-called climate sensitivity. It indicates the warming that occurs when the CO₂ concentration in the atmosphere doubles compared to the pre-industrial value. It is a measure of how sensitive the earth is to more greenhouse gases. In the fifth assessment report, it was given as 1.5 to 4.5 degrees Celsius. The researchers now calculate it more precisely to probably 2.5 to four degrees Celsius; the best estimate is around three degrees.
Ice and oceans
The IPCC states that man-made climate change is very likely behind the decline in Arctic sea ice, glaciers, snow cover in spring and Greenland’s ice sheet, but the situation with the Antarctic ice sheet is less clear. Since 1901 the sea level has risen by around 20 centimeters, most recently by 3.7 millimeters annually. This will continue: for centuries to millennia, according to the researchers, the ice will continue to melt, no matter what. By 2100, the sea level is likely to rise by another 28 to 101 centimeters, depending on emissions. However, due to uncertainties about melting processes, with high emissions even two meters by 2100, five meters by 2150 and 15 meters by 2300 cannot be ruled out. Over the next 2,000 years, sea level rise is likely to reach at least two, in extreme cases up to 22 meters.
Weather extremes
One of the greatest advances compared to the fifth assessment report is the assignment of individual weather events to climate change. For heat waves in particular, a clear increase can be demonstrated for all parts of the world and man-made global warming can be identified as the cause with a high degree of certainty. In addition, hot spells are increasingly accompanied by droughts and fires. Tropical cyclones have become more violent, if not necessarily more frequent. Marine heat waves would have roughly doubled in frequency. “The data clearly show that climate extremes increase as global warming progresses,” says climate researcher Sonia Seneviratne from the Swiss Federal Institute of Technology in Zurich (ETHZ) and lead author of the relevant chapter. “Even very small changes in global warming can have huge effects.”
Water cycle
There is a high probability that climate change will lead to more frequent and more intense heavy rain. From a global perspective, extreme rain could intensify by around seven percent for every degree of warming. Such a trend has been observed in most regions since 1950, including Western Europe. In Asia and West Africa, the greenhouse gases have presumably led to an intensification of the monsoons. While precipitation is likely to increase in high latitudes and in the Pacific region, parts of the subtropics and tropics are likely to receive less rain. In snowy areas, the snowmelt will start earlier. Overall, a warmer climate will create and intensify “very wet and very dry” seasons, with effects such as droughts and floods.
Tipping points
For the first time, the new report goes into more detail on so-called tipping points, the exceeding of which causes an irreversible change in the climate system, such as the melting of the West Antarctic ice sheet. Or the weakening of the so-called Atlantic meridional overturning current (AMOC), which also includes the Gulf Stream. It is likely to continue throughout this century. The scientists consider a complete collapse to be relatively unlikely if global warming is limited to 1.5 or two degrees Celsius. But you can’t rule it out either.
Regional consequences
“There have been some breakthroughs in regional climate modeling,” says Douglas Maraun from the University of Graz, an author of the relevant chapter. In Western and Central Europe, including Germany, for example, more floods as a result of rain and overflowing rivers are expected, as well as more droughts. In the Mediterranean region, warming, drought, less rainfall and higher forest fire risks combine to create a particularly dangerous mix. In the Alps, the “sharp declines in glaciers, permafrost, snow cover” that are already being observed will continue.
At the same time, effects can also occur regionally that run counter to global trends in the short term. “Climate fluctuations play a major role at the regional level,” says Maraun. “If we look into the future over the next ten to 30 years, the exact opposite of what we expect in the long term can happen regionally.” In particular, the jet stream and the influence of the Arctic on the weather in mid-latitudes are not yet sufficiently understood.
What can still be saved
“There are now many things that we can no longer prevent,” says Dirk Notz from the University of Hamburg, one of the lead authors for the chapter on ice and sea levels. “But the report also shows that there is still a lot of room for maneuver.” If mankind were to switch to ambitious climate protection now, this would have an effect on the measured CO₂ concentrations “within years”, according to the report. Within 20 years, the temperature difference compared to scenarios with high emissions would become apparent. Other effects would take longer, the reaction time of the ice sheets is particularly long. But by the end of the century at the latest, the differences between the scenarios are dramatic.