Thousands of years ago, a warm Arctic made mid-latitudes drier

Paleoclimate records show precipitation trends that could be repeated.

The Arctic is the fastest-warming region of the planet. Credit: NASA

Lessons from the past

One way to study patterns like this is to look to past climate changes. That’s what a team led by Northern Arizona University’s Cody Routson did, compiling paleoclimate records of rainfall in the Northern Hemisphere over the last 10,000 years.

This spans most of the period known as the Holocene—the warmer “interglacial” that has followed the end of the last ice age. The clockwork timing of the ice ages has been driven by cyclical wobbles in Earth’s orbit, which slightly alter the strength of summer sunlight in the high-latitude north—where most of the great ice sheets were located. That summer sunlight cycle peaked around 10,000 years ago, and Arctic temperatures started to slowly decline shortly after that.

That means that the trend from 7,000 years or so up to the Industrial Revolution was the polar opposite (if you will) of our current human-caused warming trend: as the sensitive Arctic cooled more quickly than the equator, the pole-to-equator temperature difference grew. The question the researchers set out to answer was how this changing pole-to-equator temperature difference affected the amount of precipitation in the mid-latitudes.

To answer, they compiled as many published temperature and precipitation records as possible for the last 10,000 years, based on everything from tree rings to insects found in lake mud. These records were separated into latitude bands from the equator to the Arctic. Based on the temperature data, the researchers were also able to calculate the pole-to-equator temperature difference as it grew over time.

In the mid-latitudes (between 30° and 50° north of the equator), the early part of this time period was actually significantly drier than today, with precipitation increasing over the millennia that followed.

graphs of temperature and precipitation trends — Over the last 10,000 years, the temperature difference between the Arctic and the equator got larger (down on this graph), and precipitation increased. Credit: Routson et al/Nature

Turning to models

To investigate how these two things were related, the team turned to climate model simulations of the last 10,000 years. The simulated pole-to-equator temperature difference matched the paleoclimate records pretty nicely, and the same pattern of increasing precipitation showed up, too, though the increase appeared to be smaller in the models.

The mid-latitude precipitation increase turned out to be driven by atmospheric circulation changes. In the drier early time period, the weaker pole-to-equator temperature difference made for a weaker jet stream and westerly winds and weaker versions of the spinning storm systems that typically account for the biggest rainstorms in the mid-latitudes. Running up to recent centuries, all those things strengthened, leading to more precipitation.

diagram showing a relationship between Arctic vs. equator temperatures, atmospheric circulation, and rainfall — Conceptual diagram of the basic processes in play. Credit: http://dx.doi.org/10.1038/s41586-019-1060-3

Projecting the future is not as simple as running these simulations in reverse, with a rapidly warming Arctic weakening winds and reducing rainfall in the mid-latitudes. The climate patterns of the Holocene were driven by influence of orbital cycles on sunlight, whereas humans are warming Earth’s climate in a different way. And while the researchers looked at the mid-latitude average here, there could be important regional differences that are hard to see with local paleoclimate records from a limited number of spots. Still, the physical process they’re identifying is something that can be studied more closely to understand how it will behave in the future.

“Currently,” the researchers write, “the northern high latitudes are warming at rates nearly double the global average, decreasing the Equator-to-pole temperature gradient to values comparable with those in the early to middle Holocene. If the patterns observed during the Holocene hold for current anthropogenically forced warming, the weaker latitudinal temperature gradient will lead to considerable reductions in mid-latitude water resources.”

Nature, 2016. DOI: 10.1038/s41586-019-1060-3 (About DOIs).