The speed and regional extent of global changes in temperature towards the end of the 20th century is far greater than other climate fluctuations over the Common Era (the past 2,000 years). These findings are reported in two papers in Nature and Nature Geoscience that examine temperature trends over the past two millennia.

Climate variability over the past two millennia has been subject to debate. Notable periods include the Medieval Climate Anomaly, the Little Ice Age, and rapid warming over the past 150 years in response to the human influence on climate. Determining the extent of these periods, along with gaining a clear understanding of the factors that caused variability in the past, has been challenging.

Raphael Neukom and colleagues assess the global patterns of climate variability during the Common Era, using data compiled from nearly 700 proxy records of temperature changes. In their Nature paper, they report that before the 20th century, climate epochs did not occur simultaneously across the globe as previously thought. For example, the coldest temperatures over the Common Era occurred in central and eastern Pacific regions in the 15th century, in northwestern Europe and southeastern North America in the 17th century, and elsewhere during the 19th century. Similarly, no pre-industrial period experienced globally coherent, long-term warmth. By contrast, the warmest period during the Common Era occurred in its final decades for more than 98% of the globe.

In their Nature Geoscience paper, Neukom and co-authors examine rates of surface warming and driving forces averaged over decades. Their analyses reveal that rates of warming over periods of at least 20 years were fastest during the later 20th century. Pre-industrial fluctuations were primarily driven by volcanic activity. Agreement between reconstructions and simulations suggests that climate forecasts for the next few decades may be realistic, the authors propose.

The role of volcanic eruptions in the early 19th century climate is probed further in another related Nature Geoscience paper by Stefan Bronnimann and colleagues. They report that a cluster of eruptions was followed by sustained cooling and climatic upheaval, including droughts in Africa and weak monsoons. The recovery from this cooling resembled transited into a period that coincided with early influences from the industrial revolution, which makes it more difficult to determine the relative impacts of these factors.

Together, these studies help to paint a detailed picture of how climate variability changed between the pre-industrial period and the 20th century.