The Intergovernmental Panel on Climate Change (IPCC) warns in its sixth comprehensive reportthat 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 reduce greenhouse gas emissions quickly and drastically. The earth warmed by 1.07 degrees Celsius due to human activities from the end of the 19th century to the last decade. On average over the next two decades, warming is likely to reach or exceed 1.5 degrees, regardless of how emissions develop. Heat waves are becoming more frequent and precipitation patterns are changing. While many of these changes are now inevitable, the researchers also emphasize that humanity still has the opportunity to act.
What kind of report is that?
It is the first comprehensive IPCC 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. Parts two and three, due to appear in 2022, will deal with consequences, adaptation and climate protection. 234 honorary authors were in charge of the current report; they evaluated around 14,000 studies. Numerous experts have participated in the revision of the text, so that the result can largely be viewed as a consensus of the global research community. Last week, the “Executive Summary for Decision-Makers” was finally agreed sentence by sentence with the delegates of the governments of the 195 IPCC member states until there was agreement on this text as well. The aim of this process is to establish a consensus on the facts at the political level as well.
Vulnerable planet: Earth, captured by the “Apollo 11” mission on the way to the moon, 1969.
(Photo: AP / AP)
Scenarios and temperature forecasts
As an example, the scientists looked at five scenarios: two with very low emissions called SSP1-1.9 and SSP2-2.6, a medium one called SSP2-4.5 and two with high emissions, SSP3-7.0 and SSP5-8.5. The abbreviation before the hyphen describes the social and economic development. Physically relevant is only the number after the hyphen, which stands for “radiative forcing”, ie how many greenhouse gases arrive in the atmosphere. From today’s perspective, the horror scenario SSP5-8.5 is unrealistic. The lowest, SSP1-1.9, but also. The IPCC does not evaluate current policy, it is not its job, but so far there is some evidence that the world is closest to the middle path. For this scenario, the researchers expect two degrees of warming by the middle of the century and 2.7 degrees by 2100. The neighboring, by no means excluded scenarios land at 1.8 degrees or 3.6 degrees by 2100. Not a single scenario remains below 1, 5 degrees, only the most optimistic will reach 1.6 degrees by the middle of the century and drop to 1.4 degrees by 2100.
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. As a result of the melting ice, the sea level has risen by around 20 centimeters since 1901, 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, in the extremely optimistic scenario, in which emissions will drop quickly from now on, sea level is likely to rise by a further 28 to 55 centimeters. Should emissions continue to rise, it would be 63 to 101 centimeters by the end of the century. However, due to uncertainties about certain melting processes in the case of high emissions, even two meters by 2100 and five meters by 2150 cannot be ruled out. The long-term prognoses are also drastic: Over the next 2,000 years the sea level rise is likely to be two to three meters if the 1.5 degree target is still achieved. In the other extreme case, with hardly any emissions and up to five degrees long-term warming, it could be up to 22 meters.
Weather extremes
According to the report, climate change has changed weather and climate extremes in all regions of the world. Compared to 1950, heat waves have become more frequent and stronger and they come more and more often accompanied by droughts and fires; The scientists also note an increase in heavy rain. Tropical cyclones in the highest categories have become more violent, although 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. The most recent heat wave in southern Europe and in the west of the USA and Canada as well as the heavy rainfalls in Germany are exactly what the climate models predict more and more often: more weather extremes that are so far unprecedented. “Even very small changes in global warming can have huge effects.”
Assignment to climate change
One of the greatest advances compared to the fifth assessment report is the assignment of individual weather events to climate change. So-called attribution research calculates how much climate change has made heat waves, droughts or heavy rain events more likely. 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. Some heat waves, such as those recently in the western United States and Canada, would have been almost impossible without man-made climate change. And the more the world warms up, the greater the likelihood of heat extremes, and to a much greater extent than other weather extremes.
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, which is likely to continue in the course of 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. In any case, such unlikely events would become more likely with every additional tonne of CO₂ released into the atmosphere.
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 degrees per degree of warming. Such a trend has been observed in most regions since 1950, including Western Europe. In Asia and West Africa, greenhouse gases are believed to have increased 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 as warming increases. 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. In the oceans, marine heat waves are likely to increase and the oxygen content to decrease, and according to the IPCC report it is “practically certain” that the oceans will become more acidic due to the CO₂ input.
Regional consequences
While man-made climate change aggravates extreme weather overall, the location and frequency of such events also depend on other factors, such as changes in regional air currents. “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 have already been observed will continue with increasing warming, according to the IPCC.
At the same time, effects can also occur at the regional level that run counter to global trends in the short term. A change in the jet stream, the air flow at high altitudes, could also lead to lower temperatures in some regions in the coming years. “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.
Risks to humans
All regions of the world have to do with several climate changes (e.g. rising temperatures with less precipitation), some of which are also mutually reinforcing. How high the risks are, however, depends largely on the degree of warming, and thus on how quickly greenhouse gas emissions fall. Extreme temperatures, which are threatening to agriculture and health, are reached significantly more often in a two-degree world than in a 1.5-degree world. The same applies to heavy rain and floods. There are special risks in cities, where heat waves can be particularly severe due to dense development. Seaside locations have to be prepared for more flood events as a result of rising sea levels.
Sensitivity of the earth
There has been significant scientific 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 of 280 parts per million; it is a measure of how sensitive the earth is to more greenhouse gases. In the previous report it was given as 1.5 to 4.5 degrees Celsius, an enormous range of uncertainty. In the meantime, the researchers have calculated it to be probably between 2.5 and four degrees Celsius; the best estimate remains unchanged at around three degrees. This is also due to the better climate models of the new generation, called CMIP6. Some of them show higher sensitivities, but these results have been excluded as unrealistic for the new report, also due to the knowledge about earlier warming periods in the earth’s history.
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. How quickly something happens or how bad it gets is up to us.” 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 and beyond, the differences between the scenarios are still dramatic.