Before the Industrial Revolution (roughly), Earth’s climate had actually been trending slightly cooler. The sudden reversal toward comparatively rapid warming gave rise to graphs frequently described as “hockey sticks” for 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” around 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 understand what global temperature was doing more precisely, researchers have to compile lots of individual records from around the world. One such effort, published in 2013, showed the slow cooling trend beginning 5,000 years ago, making a super-long hockey stick following a warm plateau that was also about 5,000 years long.

But in 2014, another study showed that this pattern didn’t quite fit climate model simulations. Instead, the 2014 study showed a very gradual warming of about 0.5 degrees Celsius over the last 11,000 years. But a new study published this week compiled a separate climate record for the Northern Hemisphere—with slightly different results that look more like 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 abundance of pollen from different species of plants over time, you can track climate change by the change in the local plant community.

This “reconstruction” (as it’s called) of Northern Hemisphere temperatures shows warming all the way until about 6,000 years ago, after which temperature held fairly steady until you get to the handle of the traditional hockey stick 2,000 years ago. This is similar to the climate model simulations, in which climate responds to rising CO 2 , the 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 certain time of year—the length of the growing season, for example, or how harsh the winter gets. By analyzing pollen from multiple species, the researchers can also track winter and summer separately. In the Northern Hemisphere, summer temperatures actually drop throughout this time because of sunlight patterns, while rising winter temperatures produce a net annual warming.

The authors think the 2013 reconstruction—which wasn’t able to include pollen records because the data wasn’t suitably available at the time—relied on climate records that leaned toward summer temperatures. It was also heavily influenced by seafloor sediment records 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 is currently working). Marcott told Ars he was a little surprised to see the long-term warming in this new reconstruction, but he wasn’t surprised to see something different from the cooling in his previous work.

“If you were to ask me whether there was a long-term cooling in global temperature, I wouldn’t bet the farm on it,” he said. “I would say that more or less the thing was probably flat.”

That’s because there are other climate records that seem to show a minimal cooling trend—records that Marcott and Marsicek are currently working on adding for more complete coverage of the globe.

Marcott compared the present state of this research to the time shortly after well-known climate scientist Michael Mann published his first “hockey stick” tree ring reconstruction. Later work didn’t contradict that first reconstruction, but it added much more spatial detail and a deeper understanding.

“If I was to guess where we are as far as the full Holocene reconstructions, we’re somewhere in the early 2000s, in the context of the hockey stick work,” Marcott said. “Just sort of getting a peek at things, and there’s gonna be other reconstructions that’ll add to everything, and I think things will become more clear here in the next few years.”

As for the big picture, there’s also an interesting thing to note about where modern temperatures rank within the last 11,000 years. Marcott’s 2013 study drew the ire of those who reject the central conclusions of climate science because it noted that we were on track to rise above the temperature range of that time 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’re seeing today have no analog within our current interglacial, at least,” Marcott said.

Because the new reconstruction essentially shifts earlier temperatures downward a little and more recent temperatures upward a bit, it's easier for present day temperatures to stand out. And so the researchers conclude that the most recent decade was already warmer than any 100-year average in their reconstruction. Our warming trend isn’t stopping any time soon, so if our picture of the last 11,000 years stays similar, the line marking our current position will only climb further into the unknown.

Nature, 2018. DOI: 10.1038/nature25464 (About DOIs).