What 485 Million Years of Climate History Tell Us About Today's Crisis

New research pub­lished in Science has found that co-occur­rences of rises in Earth’s aver­age tem­per­a­ture and high lev­els of atmos­pheric car­bon diox­ide (CO2) are not exclu­sive to the mod­ern era but go back nearly half a bil­lion years in the planet’s his­tory.

In a land­mark study, researchers from American and British uni­ver­si­ties and the Smithsonian National Museum of Natural History looked into the past to map the vari­a­tions in Earth’s tem­per­a­ture.

The researchers col­lected over 150,000 tem­per­a­ture esti­mates from fos­silized shells and organic mat­ter and com­bined them with 850 cli­matic model sim­u­la­tions devel­oped by the University of Bristol in the United Kingdom.

Then, they used a method known as data assim­i­la­tion, which sta­tis­ti­cally inte­grates geo­log­i­cal data with cli­matic mod­els, to con­struct a global mean sur­face tem­per­a­ture curve that depicted in detail the fluc­tu­a­tions of Earth’s tem­per­a­ture over the past 485 mil­lion years.

The sci­en­tists noted that the avail­abil­ity of old rocks and fos­sils with pre­served tem­per­a­ture indi­ca­tors lim­ited their abil­ity to go back in time.

The curve revealed a con­sis­tent link between tem­per­a­ture shifts and atmos­pheric car­bon diox­ide, a long-lived green­house gas. Periods of extreme heat and increased lev­els of CO2 were often aligned.

“We found that car­bon diox­ide and tem­per­a­ture are not only really closely related but related in the same way across 485 mil­lion years,” said pale­o­cli­ma­tol­o­gist and co-author of the study Jessica Tierney from the University of Arizona.

“This research illus­trates clearly that car­bon diox­ide is the dom­i­nant con­trol on global tem­per­a­tures across geo­log­i­cal time,” she added. ​“When CO2 is low, the tem­per­a­ture is cold; when CO2 is high, the tem­per­a­ture is warm.”

The study also found that the Earth’s sur­face tem­per­a­ture has var­ied more over time than pre­vi­ously thought, from 11 ºC to 36 ºC com­pared to 14 ºC to 26 ºC as shown by pre­vi­ous sim­u­la­tions, par­tic­u­larly dur­ing the Phanerozoic eon.

The Phanerozoic is the most recent of the four eons in Earth’s geo­log­i­cal his­tory, stretch­ing back almost 540 mil­lion years. During this period, life on Earth pro­lif­er­ated, diver­si­fied and pop­u­lated new land.

Furthermore, the study indi­cated that the planet’s cur­rent aver­age tem­per­a­ture of 15 ºC is lower than the aver­age tem­per­a­ture dur­ing much of the Phanerozoic. However, the researchers pointed out that today’s lower aver­age tem­per­a­tures are not a rea­son for com­pla­cency.

“[It] has kept me awake at night,” said one of the researchers, Emily Judd. ​“I’m wor­ried that cli­mate deniers and cli­mate skep­tics and cli­mate delay­ers will point to this and say, ​‘See! We have noth­ing to worry about.’”

Judd added that the most impor­tant aspect of the cli­mate cri­sis is how fast CO2 and tem­per­a­ture change.

Scientists have long warned that green­house gas emis­sions from human activ­i­ties are warm­ing Earth at an unprece­dented rate, with some of the hottest years ever recorded on the planet occur­ring in the last ten years.

According to a 2023 study that reviewed atmos­pheric car­bon diox­ide lev­els and cor­re­spond­ing tem­per­a­tures from 66 mil­lion years ago to the present day, the cur­rent lev­els of Earth’s atmos­pheric CO2 – around 420 parts per mil­lion – are almost 50 per­cent higher than the CO2 lev­els before the start of indus­tri­al­iza­tion in the 18^th cen­tury.

The result is a rise of about 1.2 ºC in the global aver­age tem­per­a­ture com­pared to pre-indus­trial lev­els, close to the 1.5 ºC warm­ing thresh­old the world’s nations have pledged not to cross.

“Regardless of exactly how many degrees the tem­per­a­ture changes, it’s clear we have already brought the planet into a range of con­di­tions never seen by our species,” said Gabriel Bowen, one of the researchers. ​“It should make us stop and ques­tion what is the right path for­ward.”

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