There has not been a period as universally warm as the current one in the last 2,000 years. That is the main result of an investigation that has reconstructed the evolution of the average annual temperatures from year 1 to 2000. Despite the natural variability of the climate, in these 2 millennia, there were 5 great periods; 3 warm and 2 dominant cold. But, according to this research, the previous 4 had a continental impact or did not occur in all regions simultaneously. Only the current warming is occurring in 98% of the planet and at the same time.

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The first official records of temperatures using thermometers do not go beyond the mid-19th century. Those are the years in which the Industrial Revolution, with its machines burning coal, becomes universal and its impact on the climate as well. In order to compare, it was necessary to reconstruct the temperatures of the past. This is what a group of scientists has done using more than 700 records from 5 very different origins: tree rings, mineral accumulation in coral exoskeletons, centenary mollusk growth rings, lake sediments or successive layers of ice at the poles and glaciers.

Thus they were able to estimate the annual thermal evolution since year 1, both regionally and globally. But it is moving the focus away until periods of one or several decades as 5 small climatic ages emerge; of course, we mean small in the geological sense of the term, but not on a human scale. For example, the so-called Little Ice Age extended from the 14th century until well into the 19h century. Before the Roman Warm Period had been produced -which coincided with the splendor of Rome-, the Cold Period of the Dark Ages (4 centuries later), and the Medieval Climate Optimum (from 900 to 1300).

“During the Medieval Optimum, the temperature was very similar to the current one”, says the physicist of the Regional Atmospheric Modeling Group of the University of Murcia and co-author of the study Juan José Gómez. But that warming was very localized in the northern continental regions and the North Atlantic.

The same happens with the Little Ice Age. Until now, scientists could discuss its duration, its intensity, or when it started. But no one denied his universal character. However, this work, published in the journal Nature, graduates that universality. The coldest years of this period did not occur at the same time everywhere: the minimum in most of the Pacific was in the 15th century, in northwestern Europe and much of North America in the 17th, and the rest of the planet they suffered it already in the 19th century.

“Yes, there was an ice age, but at different times”, explains Gómez. “That lack of coherence is repeated in the different climatic periods of the past: there were regions colder (or warmer) than others, but the cooling/heating did not occur in all parts at once”, adds this physicist who leads a project for young scientists of the Among the factors that influence the emergence and maintenance of a warm or cold period, solar activity and volcanism stand out. The Little Ice Age, for example, would be related to a series of sunspot minima. Volcanic eruptions such as Pinatubo (Philippines) or Eldgjá (Iceland) reduced the global average temperature by 0.3 ºC the first and up to 2 ºC the second, although only in the northern hemisphere. But, for Gómez, natural variability is not able to explain what has been going on since the 20th century.

“The current warming is globally synchronous: the warmest period of 51 years of the last 2 thousand years occurred during the 20th century in more than 98% of the planet”, recalls Raphael Neukom, lead author of the study. “The heating rates during pre-industrial times were around 0.6 °C per century, currently the heating rate is around 1.7 °C per century. This is much more than we could expect just for natural variability,” adds this researcher from the Oeschger Center for Climate Change Research at the University of Bern (Switzerland).

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The study shows, in effect, that the last 10 years of the 20th century, the last 30 or the last 50 are among the warmest since year 1. And this warming is occurring at the same time in the Amazon basin, the center of Europe, the Far Arctic or Southeast Asia: 98% of the planet’s surface is warming, leaving only Antarctica on the sidelines. Something, by the way, that the latest studies, with data after 2000, also start questioning.

In a comment also published by Nature, the paleo-climatologist at the University of Minnesota (USA), Scott St. George, writes: “The well-known maxim that the weather is always changing is very true. But even when we expand our perspective until the early days of the Roman Empire, we do not see any event that is even remotely similar, whether in degree or extent, to the warming of the last decades. Today’s climate is distinguished by its torrid global synchrony”.

Although the authors do not enter to attribute such globality and synchrony, everything points to human activities. “This research should silence the climate deniers at once who maintain that the current warming is part of a natural climate cycle”, says University of London climatology professor Mark Maslin, adding: “This work shows the raw difference between regional and localized changes in the climate of the past and the true global effect of anthropogenic greenhouse gas emissions“.