At the end of the 20th century, temperatures on 98 percent of the earth's surface were at their highest levels ever seen in the 2000 years before.
This near-universal warming, which occurs in lockstep around the world, is unique in that time, say scientists. Other well-known cold and warm snapshots of the past, such as the Little Ice Age or the Medieval Warm Period, were indeed regional rather than worldwide.
In addition, the rate at which temperatures rise now far exceeds any previous temperature fluctuations measured in the last two millennia. These are the conclusions of a trio of recent papers examining the evolution of temperature over the last 2,000 years and published online on July 24 in Nature and Nature Geoscience. These past climate variations were primarily due to natural causes, including violent volcanic eruptions, rather than man-made greenhouse gas emissions.
The findings, based on newly available global paleoclimate data, support an inescapable conclusion, says Michael Mann, a climatologist at Penn State University, who was not involved in the new studies: "The current warming period is unprecedented in his global reach in the last 2000 years.
In the study Nature a team of Raphael Neukom, a climate researcher at the University of Bern in Switzerland, used many different types of temperature recordings. These data were collected by an international group of scientists called PAGES 2k Consortium collected. These include proxies for temperatures resulting from tree rings, glacial ice cores, lake and ocean sediments, cave deposits such as stalactites and stalagmites, and historical documents ( SN: 1
0/28/17, p. 29 ).  Using this data, as well as direct temperature measurements in some places since the 19th century and some climate simulations, the team created more than 15,000 different climate reconstructions of past global temperatures. The researchers then examined the exact time of warming or cooling within four previously identified "climatic periods" – the Roman warm period of about 1 to 300 AD, the dark glacial period of 400 to 800 AD, and the medieval warm period of about 800 to 1200 and the small ice age from 1300 to 1850.
Hot and cold
Well-known climatic fluctuations of the last 2,000 years were once considered global. However, studies using newly available temperature proxy data from around the world show that these variations were regional. In each map, different colors represent the warmest (or coldest) century for a particular location within the illustrated time period. For example, it is believed that the cold period of the dark age ranges from about 400 to 800 AD. But in the Pacific (white to pale violet), some hundred years earlier, and in East Asia and other areas (darker) a few hundred years later, colder temperatures appeared on purple). Similarly, the medieval warm period (or medieval climatic anomaly) lasted from about 800 to 1200, with peak heat in northwestern Europe in the 11th century (orange). But Central and South America experienced peak heat (brown) a few hundred years later. For every two thousand years that have been included in the new research, temperatures towards the end of the twentieth century were highest for 98 percent of the planet (see the third chart).
cold and hot shots of the past were different regional current global warming