Climate change: unprecedented coherence

No climate change of the last 2, 000 years was globally as synchronous as the current warming

The current warming affects 98 percent of the Earth's surface - it is thus as comprehensive and consistent as no other climate change of the last 2, 000 years. © bischy / thinkstock
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Global instead of regional: the pattern of current warming is unique in the last 2, 000 years, as revealed by two studies. Accordingly, previous climatic fluctuations such as the medieval warm period or the "Little Ice Age" never synchronized on the entire earth. The situation is different with the current climate change: The warming of the last decades is detectable on 98 per cent of the earth's surface - this coherence was unprecedented since the antiquity, so the researchers in the journal "Nature".

The climate varies by nature: Again and again there are decades to centuries of periods in which it is colder or warmer before. Such fluctuations include short cycles such as El Nino / La Nina, but also longer climatic variations such as the "Little Ice Age" from 1300 to 1850 or the medieval warm period of 800 to 1200. The more difficult, but also more important, to determine if and to what extent Current global warming differs from such natural fluctuations.

Climate fluctuations on the test bench

At this point Raphael Neukom from the University of Bern and his team start with their new study. On the basis of almost 7, 000 series of tree rings, ice cores, lake sediments and corals, they have analyzed five pre-industrial climate epochs of the last 2, 000 years and compared them with the warming since the 20th century. Using six different statistical methods - more than ever before - they obtained a detailed geographical and climatic picture of the past 2, 000 years.

The result confirms that climate fluctuations in pre-industrial times were mainly influenced by random fluctuations within the climate system. Increased volcanic eruptions led to a cooling of the northern hemisphere during the Little Ice Age. However, the new data also shows that this cold phase was geographically much less extensive and consistent than many people have anticipated. "Although it was generally colder during the Little Ice Age around the world, but not everywhere at the same time, " explains Neukom.

Earlier climatic phases were never in sync

The same was true for other climatic fluctuations: "The peak periods of pre-industrial hot and cold periods occurred at different times in different places, " the researchers said. "The minimum and maximum temperatures were spatially very differently distributed." At each of these phases, it was therefore at that same time particularly cold or warm to less than 50 percent of the global earth's surface. display

According to the researchers, this could also solve two problems of climate research with these climatic fluctuations. Because the regional nature of the warm and cold phase explains why there is so little agreement on the exact dating of these climatic phases. "And it explains the discovery of climate data that does not fit the standard assumptions, " said Neukom and his team.

"Unprecedented Coherence"

The pattern of the current warming is quite different: "The most likely warmest phase of the last 2, 000 years is in the late 20th century, " the researchers report. "And it is found almost everywhere on 98 percent of the global earth's surface." Only in the Antarctic, the current warming can not be observed on the entire continent.

"The climate system is now in a state of temperature cohesion unique in the last 2, 000 years, " states Neukom and his colleagues. "This provides strong evidence that anthropogenic warming is unprecedented not only in terms of absolute temperatures, but also in terms of their spatial uniformity."

According to the researchers, this demonstrates once again that humans have intervened in the climate system and that the current climate change can no longer be explained solely by coincidental fluctuations. (Nature, 2019; doi: 10.1038 / s41586-019-1401-2)

Source: University of Bern

- Nadja Podbregar