Ilulissat, Greenland, in the summer.
Climate records, like tree rings or ice cores, are invaluable archives of past climate, but they each reflect their local conditions. If you really want a global average for some time period, you’re going to have to combine many reliable records from around the world and do your math very carefully.
That’s what a group of researchers aimed to do when (as Ars covered) they used 73 records to calculate a global overview of the last 11,000 years—the warm period after the last ice age that’s called the Holocene. The Holocene temperature reconstruction showed a peak about 7,000 years ago, after which the planet slowly cooled off by a little over 0.5 degrees Celsius until that trend abruptly reversed over the last 150 years. That behavior mirrored the change in Northern Hemisphere summer sunlight driven by cycles in Earth’s orbit.
A new study published in the Proceedings of the National Academy of Sciences and led by the University of Wisconsin’s Zhengyu Liu delves into a problem with that pattern—and it’s not what climate models say should have happened.
The researchers used three different global climate models to run a series of computationally intensive simulations spanning the last 21,000 years. The simulations were responding to the orbital change in sunlight and the documented increase in greenhouse gases.
The global average temperature in the models did not peak and decline, however, unlike the Holocene temperature reconstruction. The models show that warming out of the last ice age slowed down markedly around 12,000 years ago, but still continued gradually—temperatures increased by about another 0.5 degrees Celsius before the last couple millennia. That puts the peak of the Holocene reconstruction about 1 degree Celsius higher than the temperatures in the models reach.
So, the models and reconstruction of historic temperatures don’t agree. Understanding why requires thinking about that orbital change in a little more detail. These orbital cycles don’t affect how sunlight reaches the planet uniformly. When looking at the ice age cycles, what matters is summer sunlight in the Arctic, where most of the world’s ice sheets were located and vulnerable to summer melt. Feedbacks translate the loss (or growth) of those ice sheets into global climate changes.
After they ended the most recent ice age, the orbital cycles decreased Northern Hemisphere sunlight in the summer over the course of the Holocene. Winter sunlight, conversely, saw an increase. Averaged over the whole year, the Arctic decrease was small, and the equator actually saw a slight increase.
But orbital cycles weren’t the only things changing. We also know that greenhouse gas concentrations increased a bit over the Holocene. In the climate model simulations, this overpowers the small changes in sunlight. The resulting warming causes further shrinking of the simulated ice sheets, which amplifies the warming, as the loss of reflective ice allows more sunlight to be absorbed.
The researchers say the cooling in the Holocene temperature reconstruction runs counter to what we should expect given what we know. But does this conflict arise from problems with the reconstruction or inadequacies of the models?
The researchers first point to potential issues with the temperature reconstruction. There are many different kinds of proxies for temperature, including organic compounds produced by photosynthetic plankton, ratios of elements in the shells of zooplankton, oxygen isotopes in ice cores, and identification of pollen grains. Some of these could provide recordings that are biased toward summer temperatures rather than an annual average, and the researchers believe this has had an outsized effect on the Holocene temperature reconstruction. That could exaggerate the impact of increased Arctic summer sunlight on the calculated global average temperatures.
To test this, the researchers created a virtual Holocene reconstruction by taking temperatures generated by the models from the locations where the proxy records used in the reconstruction were obtained. When summer values were used for the Arctic points rather than annual averages, the virtual reconstruction more closely resembled the real one, with a peak and decline instead of continual warming.
It didn’t look exactly like the real reconstruction, though, which left the researchers to consider what the models might be getting wrong. It’s possible that the models should be more sensitive to changing Arctic summer sunlight. Feedbacks like changes in reflectivity with snow cover or atmospheric dustiness responding to precipitation changes could be more potent than the models simulate. That would allow the summer Arctic Sun to have more influence over the rest of the planet.
Boston College’s Jeremy Shakun, one of the researchers behind the Holocene temperature reconstruction, told Ars that “any time there is a big data-model discrepancy like this, there’s a good opportunity to learn something about the world. My hunch is that both the data and models are a bit off.”
“Our [reconstruction] indeed shows the biggest cooling over the northern higher latitudes, and I have to imagine that many of the records here are biased toward summer,” Shakun said, but there are also questions about climate models. “[O]ne of the big underestimations of models with global warming so far has been that they were suggesting that summer Arctic sea ice wouldn’t fully disappear until the end of this century— whereas it has actually been slipping away much faster—so, is it possible that the models are similarly underestimating how much sea ice varied (and amplified summer [sunlight] forcing) over the Holocene?”
The new paper concludes, “If the [Holocene temperature] reconstruction is correct, it will imply major biases across the current generation of climate models. To provide a credible benchmark for future climate models, however, the proxy reconstructions will also need to be reexamined critically.”
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