The papers build on a major global effort to reconstruct past climate using a range of data sources, such as tree rings and coral cores, that was published in 2017.
Dr Henley said the new research revealed “incredible consistency” across different methods, adding to the confidence that current models can predict the future climate as greenhouse gas emissions continue to rise.
The work also further debunks the claims of climate change deniers who often point to periods such as the Little Ice Age as evidence the climate is in constant flux. Rather, unusual conditions were typically confined to regions.
For example, while north-western Europe experienced a cold spell in the 17th century – as widely depicted in paintings of frozen rivers such as England’s Thames – the central and eastern Pacific experienced the chill in the 15th century.
“By contrast, we find that the warmest period of the past two millennia occurred during the 20th century for more than 98 per cent of the world,” another of the papers said.
Interestingly, volcanoes were found to be the dominant influence for most of the Common Era, potentially masking the start of the impact of the Industrial Revolution on global conditions.
Volcanic particles pumped high into the atmosphere typically cause widespread cooling followed by a warming rebound as they dispersed over time.
Several large tropical volcanic eruptions within three decades of the first half of the 19th century triggered “substantial drops of summer temperatures over the Northern Hemisphere land areas”, the third paper found.
“Only after the 1850s did the transition into the period of anthropogenic warming start,” it stated.
For Michael Mann, the director of Penn State University’s Earth System Science Centre, the papers offer fresh vindication of work he led two decades ago in the so-called “Hockey Stick” studies. These revealed the relatively recent ramping up of global temperatures.
“We’re pleased that decades after our original work, independent, international teams of scientists using entirely different approaches, and more widespread now-available paleoclimate data, have come to virtually identical conclusions to those we offered in our original work,” Professor Mann told the Herald and The Age.
These included that past climate episodes such as the Medieval Warm Period of about three centuries after 950 CE and the Little Ice Age “were far more regional in nature than the globally-pervasive warming of the past century”, he said.
A second conclusion reaffirmed was that the current warmth at global and hemispheric scales “is unprecedented as far back as the estimates go – now more than 2000 years”, Professor Mann said.
Heatwaves only part of the problem
The papers’ release coincided recent heatwaves that have baked Europe – including setting records on Wednesday in several countries, with more expected on Thursday – and North America during their current summers.
Global temperatures in June were the hottest in more than a century of data, the US National Oceanic and Atmospheric Administration reported last month. July is on course to set global records too.
For Dr Henley, though, heatwaves are temporary and, while among the clearest signals of global warming, are still weather-related.
“The far bigger concern is the long term and the far bigger changes we are making to the climate system,” he said, noting effects ranging from more severe flooding and droughts, and rising sea levels.
“It’s more the cumulative impacts on the human system and the natural system,” Dr Henley said. “Many of the ecosystems we have on earth won’t be able to handle the pace of change.”
Peter Hannam writes on environment issues for The Sydney Morning Herald and The Age.