“Those who do not learn history are doomed to repeat it.” It’s an aphorism frequently cited for its relevance to social change—but it may apply to science just as well.
In a study published Monday in the Proceedings of the National Academy of Sciences (PNAS), scientists report that a planet-wide cooling trend that began nearly 50 million years ago is reversing due to climate change. According to the researchers, if current emission trends continue, then by 2030, Earth’s climate may resemble the mid-Pliocene epoch, which lasted from 3.3 million to three million years ago. During that period, there were no large ice sheets in the Northern Hemisphere and sea levels were about 60 feet higher than present. The epoch also marked the dawn of hominid evolution.
What’s more, the researchers found that—again, if current emission trends continue—Earth’s climate could plunge further into the past. By 2150, the planet could look something like the Eocene epoch, which began just after dinosaurs went extinct and was characterized by high carbon dioxide levels, gigantic volcanic eruptions, and the advent of the ancestors of modern mammals.
In short, if our planet’s history were a movie, we’re currently pressing the “rewind” button.
The study’s lead author, Kevin Burke, worked with paleoecologist Dr. John Williams of the University of Wisconsin-Madison to assess the climatic characteristics of several geologic time periods, including the Early Eocene (beginning 56 million years ago), the mid-Pliocene (beginning 3.3 million years ago), the Last Interglacial (beginning 130,000 years ago), the mid-Holocene (beginning 7,000 years ago), the pre-industrial era (beginning in 1750), and the early 20th century.
Using several climate scenarios put forth by the IPCC Fifth Assessment (the fifth in a series of reports from the United Nations and the World Meteorological Organization designed to assess scientific information on climate change)—including one known as “Representative Concentration Pathway 8.5” (RCP8.5), which paints a picture of a future sans climate mitigation attempts (what climate scientists call the “business-as-usual” scenario), and RCP4.5, which depicts a future in which we’ve moderately reduced emissions—Burke and Williams compared years-from-now conditions across every inch of the planet to the past.
“I think one of the real challenges of 21st century climate change is that it’s very hard for us to imagine what the world looks like when it’s 2 degrees warmer or 4 degrees warmer, or it’s outside of anything we’ve seen in our history,” Williams said. Having this data is therefore helpful—we can better visualize what our home will be like decades and even a century from now. From these past climate conditions, we can start to draw comparisons and analogies to help conceptualize the climates we can expect in the coming decades.
“One of [our goals] is to place future climates in better context of the past,” said Burke, a PhD candidate at the University of Wisconsin-Madison’s Nelson Institute for Environmental Studies. He says researchers have previously uncovered “climate matches,” but in a more informal, generalized way. In this study, Burke and Williams analyzed all projected decade-by-decade climate simulations for the future and juxtaposed them with simulations of past time periods included in the study. “We’re sort of extending the baseline of what we’re comparing the future to, and we’re doing it in a quantitative manner,” Burke said.
They found that under a “business-as-usual” scenario, Earth’s climate starts to become like the Eocene’s beginning around the year 2100. “That’s really striking,” Williams said. “That’s, of course, a long time back—back when whales and horses started to evolve.” However, under moderately reduced emissions, it stabilizes at mid-Piocene-esque climes. The researchers hope to investigate a greater variety of scenarios—beyond just RCP8.5 and RCP4.5—in the future.
"These are terrifying projections,” said Dr. Sarah Myhre, a research associate at the University of Washington’s School of Oceanography and a leading voice in the field of climate science communication. “They show that there are landscapes that are more vulnerable than others—risk is not equally distributed.” She also noted the study’s finding that deep-geological climate matches first appear in the centers of continents and then expand outward. In other words, Madison, Wisconsin warms up more than Seattle, Washington under these projections—despite the fact that they’re at the same latitude.
The study also identified that under the “business-as-usual” scenario, so-called “novel” climates with no close matches in the six past time periods would emerge across nearly 9 percent of the planet. Burke said these novel climates are “essentially so different from anything in the baseline that none of the time periods in our assessment provided a close analog for that place in the future.” Many of the climates Burke and Williams classified as novel tended to be in monsoon-prone areas of the globe, like the tropics or other regions that experience extremely high levels of precipitation. “You can imagine things either warming beyond comparison of the already warm climates, or things getting extremely wet or extremely dry, and just not having any similar climates in that baseline,” Burke said.
Myhre found that particular result compelling. “I think the ‘novel climate’ finding under the ‘business-as-normal’ emission trajectory is a significant finding.”
Many of the climates Burke and Williams classified as “novel” tended to be in monsoon-prone areas of the globe, like the tropics or other regions that experience extremely high levels of precipitation. “You can imagine things either warming beyond comparison of the already warm climates, or things getting extremely wet or extremely dry, and just not having any similar climates in that baseline,” Burke said.
Believe it or not, 9 percent is actually low compared to prior estimates; other quantitative assessments have compared climate projections to solely the modern era at the exclusion of deep geologic time. Such analyses, according to Burke, “saw a higher prevalence of novel climates for the same types of high-end emission scenarios” because the smaller sample size equated to a lesser chance of finding close matches. By studying a greater number of potential close matches, Burke and Williams saw the number of climates classified as “novel” go down.
Does the rarity of these novel climates make them scarier than ever before?
“Pliocene and Eocene are quite scary enough,” Myhre said. Plus, “changing temperature and precipitation are physical measurements, but what they produce are vast reorganizations” of biology and culture.
Burke agrees that the implications of this study are urgent. “This is a problem we’re facing now,” he said. “This paper is showing us that within the next couple of decades, if we continue to emit greenhouse gases at an unmitigated level, we’re going to be seeing climates that more commonly resemble time period like the Pliocene than the pre-industrial. We’re well on our way toward that future.”