Even seemingly low levels of global warming can have serious consequences. For example, if Earth heats up by 1.5 degrees Celsius compared to preindustrial levels, 70 to 90 percent of the world’s coral reefs will likely die. At two degrees, practically all of them will die (98 to 99 percent).

Greenland’s ice sheet is shrinking by more than 250 billion metric tons every year. Since 2006, the melting of this ice sheet has contributed more than 7 millimeters per decade to the rise in the average global sea level.²³ The rate of ice loss on Greenland has accelerated drastically in recent years. Between 1981 and 2010, some 15 percent of Greenland’s ice surface experienced melt during the months of June and July; in June and July 2020, that figure had already ballooned to around 25 percent.²⁴ Parts of the Antarctic ice sheet are also exhibiting heavy losses, with an annual ice-mass loss of around 150 billion metric tons since 2006 (contribution to sea-level rise: around 4 millimeters per decade).²⁵

One key way for natural scientists to understand complex processes is to reproduce them in computer models. In recent decades, climate research has developed increasingly detailed models of the Earth’s climate system, which correctly predicted the current warming trend as early as the 1970s and 1980s.

The sea ice at the North Pole is shrinking. Both the volume of ice in the Arctic and the area of the ocean’s surface covered by ice there have been steadily decreasing since satellite measurements began in 1979 – by more than ten percent per decade on average. Whereas the extent of Arctic sea ice totaled some 7.3 million square kilometers between 1980 and 1989, the same figure only amounted to approximately 4.2 million square kilometers in the period from 2001 to 2019. The especially thick perennial sea ice is dwindling very rapidly; as a result, the remaining ice cover is reacting ever more sensitively to warming.¹

Since the 1980s, every decade has been warmer than the one before it and warmer than any previous decade since 1850.¹ Data collected for the current decade so far indicates that the decade 2011 to 2020 will set a new record as well. All of the ten warmest years since record-keeping began have occurred since 1998 (refer to the box).² According to data from the US agencies NASA and NOAA, 2019 was the second warmest year worldwide on record and already the 43rd consecutive year in which the mean temperature on the Earth’s surface was above the average for the 20th century.³

Since the beginning of systematic, nationwide weather records in 1881, the average near-surface air temperature in Germany has already warmed up considerably. According to data from Germany’s National Meteorological Service, the current decade was around 1.9 degrees Celsius warmer than the first decades (1881 to 1910) on record. That means temperatures in Germany have risen much more sharply than the global average.

Since 1900, sea levels have already risen worldwide by some 16 centimeters on average, increasing by about 9 centimeters alone since global satellite measurements began in 1993. Since 2006, the rate of sea-level rise has come to around 3.6 millimeters annually (i.e., approximately 3.6 centimeters per decade), more than twice as much as before. Behind this acceleration is the melting of the ice sheets in Greenland and Antarctica, which is proceeding at ever faster rates.

Global warming is affecting the lakes in the Alps and in the Alpine foothills, as well as in Germany’s lower-lying mountain ranges and the North German Plain. The rise in temperature is causing the biodiversity in lakes to change. Fish die-offs and algal blooms, which can pose a health risk to people who go swimming in lakes, are also occurring more often.

Warmer summers and longer dry spells are increasing the risk of forest fires, and the number of days with high fire danger levels has already gone up in recent decades. Across Germany, there were some 27 days per year on average with a high or very high risk of forest fire in the period from 1961 to 1990. From 1981 to 2010, there were about 33 such days per year, increasing yet again to some 38 days for the period from 1991 to 2019.