Earth at Its Hottest in 6,500 Years, and Humans Are Mostly to Blame

(Victor O. Leshyk, Northern Arizona University)

(CN) — The planet has not seen temperatures as high as they are now since the last ice age, due largely to human-induced factors such as increasing levels of greenhouse gases.

Those gases, including carbon dioxide and methane, are produced by everything from automobiles and factories to the bovine that may be on your dinner plate. Aside from contributing to the increasingly poor air quality in many regions, they trap heat from the sun like a blanket.

These gases have helped upend an extended period of global cooling that began 6,500 years ago following the last ice age and maintained a respectable run until the middle of the 19th century when industrialization took off, according to a new study from Northern Arizona University’s School of Earth and Sustainability published Tuesday in the journal Nature.

A team of researchers, led by Darrell Kaufman and Nicholas McKay, along with Cody Routson and Michael Erb, worked to piece together a complete picture of the average global temperature in the Holocene period — the age we currently inhabit. They found the current global average across the planet to be warmer than any time in the past 125,000 years.

“Before global warming, there was global cooling,” said Kaufman. “Previous work has shown convincingly that the world naturally and slowly cooled for at least 1,000 years prior to the middle of the 19th century, when the global average temperature reversed course along with the build-up of greenhouse gases. This study, based on a major new compilation of previously published paleoclimate data, combined with new statistical analyses, shows more confidently than ever that the millennial-scale global cooling began approximately 6,500 years ago.”

Mean temperatures back then topped out around 1.3 degrees Fahrenheit warmer than those of the mid-1800s. Greenhouse gas emissions have since contributed to the mean temperature rising 1.8 degrees Fahrenheit (1 degree Celsius) above the 19th century average and led to record temperatures around the world, increased wildfires, rapidly melting arctic sea ice and a host of other problems.

Kaufman, McKay, Routson and Erb led an international team of 93 paleoclimate scientists from 23 countries who compiled data records based on samples from hundreds of sites worldwide to publish an extensive set of climate data covering the last 12,000 years. 

Researchers analyzed ecological, geochemical and biophysical evidence such as lake deposits, marine sediments and peat and glacier ice from each site. It took countless scientists working for decades to compile the raw data needed to put together this picture of the world’s steadily transforming climate, according to the study released Tuesday.

The National Science Foundation recently awarded the team $678,000 in grants to continue their work through 2023.

According to McKay, who developed some of the statistical approaches to wrangling these vast stores of data, individual decades are not resolved in the team’s 12,000-year-long temperature scale reconstruction, which makes comparing historical temperatures to the recent shifts in our climate particularly challenging. 

“On the other hand, this past decade was likely cooler than what the average temperatures will be for the rest of this century and beyond, which are very likely to continue to exceed 1°C above pre-industrial temperatures,” McKay said.

Anthropogenic climate change, the kind caused by humans, isn’t the only factor to be concerned about either, according to the scientists. Earth’s climate naturally varies over time, which makes it difficult to model. However, these predictions can be improved by continuing to study the data. Volcanic activity, changes in solar intensity, plate tectonics and shifts in the planet’s orbit can all have a substantial — and as of yet largely unpredictable — effect on climate. 

“Investigating the patterns of natural temperature changes over space and time helps us understand and quantify the processes that cause climate to change, which is important as we prepare for the full range of future climate changes due to both human and natural causes,” Routson said.

“Our future climate will largely depend on the influence of human factors, especially the build-up of greenhouse gases,” he added. “However, future climate will also be influenced by natural factors, and it will be complicated by the natural variability within the climate system. Future projections of climate change will be improved by better accounting for both anthropogenic and natural factors.”

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