Enlarge / An oceangoing scientific drilling vessel may be needed to figure out how huge undersea aquifers formed. (credit: Credit: IODP)

One-quarter of the world’s population is currently water-stressed, using up almost their entire fresh water supply each year. The UN predicts that by 2030, this will climb to two-thirds of the population.

Freshwater is perhaps the world’s most essential resource, but climate change is enhancing its scarcity. An unexpected source may have the potential to provide some relief: offshore aquifers, giant undersea bodies of rock or sediment that hold and transport freshwater. But researchers don’t know how the water gets there, a question that needs to be resolved if we want to understand how to manage the water stored in them.

For decades, scientists have known about an aquifer off the US East Coast. It stretches from Martha’s Vineyard to New Jersey and holds almost as much water as two Lake Ontarios. Research presented at the American Geophysical Union conference in December attempted to explain where the water came from—a key step in finding out where other undersea aquifers lie hidden around the world.

Enlarge / Top: a look through the past 2,000 years of summertime temperatures, showing that 2023 is considerably warmer than anything earlier. Bottom: a bell curve of the typical temperatures, showing that the hot outliers are all recent years. (credit: Esper, Torbenson, and Büntgen)

Starting in June of last year, global temperatures went from very hot to extreme. Every single month since June, the globe has experienced the hottest temperatures for that month on record—that's 11 months in a row now, enough to ensure that 2023 was the hottest year on record, and 2024 will likely be similarly extreme.

There's been nothing like this in the temperature record, and it acts as an unmistakable indication of human-driven warming. But how unusual is that warming compared to what nature has thrown at us in the past? While it's not possible to provide a comprehensive answer to that question, three European researchers (Jan Esper, Max Torbenson, and Ulf Büntgen) have provided a partial answer: the Northern Hemisphere hasn't seen anything like this in over 2,000 years.

Tracking past temperatures

Current temperature records are based on a global network of data-gathering hardware. But, as you move back in time, gaps in that network go from rare to ever more common. Moving backward from 1900, the network shrinks to just a few dozen land-based thermometers, almost all of them in Europe.