Before the Industrial Revolution (roughly), the Earth’s climate had actually become slightly cooler. The sudden reversal to relatively rapid warming gave rise to graphs often described as “hockey sticks” because of their shape. But that stick only goes back about 2,000 years in climate records based on tree rings. So what happened between the end of the last “ice age” about 11,000 years ago and the beginning of the hockey stick?
Ice core records gave us the general idea decades ago: Temperatures were relatively stable. But to more accurately understand what the Earth’s temperature was doing, researchers need to collect many individual records from around the world. One such effort, published in 2013, showcased the slow cooling trend that started 5,000 years ago, creating a super-long hockey stick that followed a warm plateau that was also about 5,000 years long.
But in 2014, another study showed that this pattern didn’t quite fit in climate model simulations. Instead, the 2014 study showed a very gradual warming of about 0.5 degrees Celsius over the past 11,000 years. But a new study published this week compiled a separate climate record for the Northern Hemisphere — with slightly different results more akin to that model simulation.
The new study, led by Jeremiah Marsicek, used 642 existing climate records based on pollen in sediment cores from North America and Europe. By measuring the amount of pollen from different types of plants over time, you can monitor climate change according to the change in the local plant community.
This “reconstruction” (as it’s called) of temperatures in the Northern Hemisphere shows warming up to about 6,000 years ago, after which temperatures remained fairly stable until you got to the handle of the traditional hockey stick 2,000 years ago. This is comparable to the climate model simulations, in which the climate responds to rising CO2the loss of sunlight-reflecting ice sheets and changes in sunlight patterns caused by slow cycles in Earth’s orbit.
So why the difference? The researchers think it has to do with seasonal shifts. Many climate records are most sensitive to conditions at a particular time of year, such as the length of the growing season or how severe the winter gets. By analyzing pollen from multiple species, the researchers can also track winter and summer separately. In fact, in the Northern Hemisphere, summer temperatures drop during this period due to sunlight patterns, while rising winter temperatures cause a net annual warming.
The authors think the 2013 reconstruction — which couldn’t include pollen records because the data wasn’t sufficiently available at the time — relied on climate records leaning toward summer temperatures. It was also heavily influenced by records of seafloor sediments from the North Atlantic, which show a regional cooling trend.
That 2013 reconstruction was led by Shaun Marcott of the University of Wisconsin-Madison (with whom Marsicek currently works). Marcot told Ars he was a little surprised to see the prolonged warming in this new reconstruction, but he wasn’t surprised to see anything other than the cooling in his previous work.
“If you were to ask me if there is a sustained cooling of global temperatures, I wouldn’t bet the farm on that,” he said. “I’d say the thing was more or less flat.”
That’s because there are other climate records that seem to show a minimal cooling trend — records Marcott and Marsicek are currently adding for more complete coverage of the world.
Marcott compared the current state of this research with the time shortly after noted climate scientist Michael Mann published his first “hockey stick” tree ring reconstruction. Later work did not contradict that first reconstruction, but it added much more spatial detail and a deeper understanding.
“If I had to guess where we are as far as full Holocene reconstructions go, we’re somewhere in the early 2000s, in the context of hockey stick work,” Marcott said. “Just take a little look, and there will be other reconstructions that will add to everything, and I think things will get clearer here over the next few years.”
In terms of the big picture, there’s also an interesting thing to note about where modern temperatures have ranked over the past 11,000 years. Marcot’s 2013 research angered those who reject climate science’s central conclusions because it noted that we were on track to rise above the temperature range of that period before the year 2100.
“[Marsicek’s new reconstruction] would basically indicate that we left it a long time ago [and] that the temperatures we see today have no analogy within our current interglacial, at least,” Marcott said.
Because the new reconstruction essentially shifts earlier temperatures down a bit and more recent temperatures up a bit, it’s easier for current temperatures to stand out. And so the researchers conclude that the most recent decade at their reconstruction was already warmer than any 100-year average. Our warming trend isn’t stopping anytime soon, so if our picture of the past 11,000 years stays the same, the line marking our current position will only climb further into the unknown.
Nature2018. DOI: 10.1038/nature25464 (About DOIs).
Go to discussion…