There are ancient secrets about our planet hidden where one would least expect: in tree rings, deep in ocean beds, in the shells of crustaceans.
Together, these form a vast archive of Earth’s “climate memories”. “These ‘climate proxies’ can be studied today, to learn about things like past temperature or composition of atmosphere, details of which are otherwise lost to us,” says Paul Pearson, a palaeoclimatologist, geoscientist and professorial research associate at University College London.
So what do they look like, and how do they work?
Such proxies generally fall into three categories: physical, chemical and biological.
Physical proxies include tree rings, ice cores and coral reefs, which preserve chemical evidence of past climates, and are helping researchers reconstruct timelines of temperature, precipitation and atmospheric conditions from millions of years ago. Chemical climate proxies include compounds found in shells and sediments, that serve the same purpose. Along similar lines, biological proxies such as pollen, spores and traces of algae found in lake or ocean sediment can offer clues to ancient ocean temperature, salinity and composition.
Tree rings in the southern hemisphere, for instance, have helped scientists learn more about climate transition roughly 11,000 years ago, when a severe cold snap was followed by extended spells of warm and wet weather, marking the end of the last Ice Age.
A powerful proxy for long-term changes in humidity, tree rings have also helped track droughts through the late 1200s, and explain why ancient settlements such as the Mesa Verde cliff dwellings in modern-day Colorado were abandoned.
In 1947, meanwhile, chemist Harold Urey (who would go on to win a Nobel prize for a different discovery) proved that ocean temperatures affect the chemical composition of crustacean shells.
He and his team at University of Chicago applied their findings to fossils of the squid-like belemnite from the Cretaceous period, and emerged with temperature data from approximately 100 million years ago. Their study indicated a surprisingly consistent climate (with estimated temperatures of 15 to 16 degrees Celsius) in present-day USA, England and Denmark. All in all, a warmer world than previously thought. Later studies have shown that Urey’s method underestimated ocean temperatures. It is possible that average temperatures in some of the regions he studied were as high as 20 degrees Celsius to 30 degrees Celsius.
The list goes on. Ice core records can famously hold molecules from 800,000 years ago, but new discoveries involving ocean sediment or deep-sea cores are revealing continuous records from as much as 200 million years ago, with microfossil shells providing information on dramatic climate shifts.
The break-up of the supercontinent Pangea was causing massive volcanic eruptions at this time. Today, studying the isotopes in these microfossil shells, researchers are able to glean literally granular data on how the volcanic activity and its fallouts caused sharp increases in average global temperatures, rising ocean temperatures, levels of carbon-dioxide and ocean acidification.
Given that Earth has experienced major climate shifts, often with sudden variability, through prehistory, such data “provides invaluable context to understanding the modern climate crisis. We’d be lost without this context,” says Pearson, of University College London.
DEEP, DARK BLUES
How exactly can this context help — beyond the obvious answer, that the more we know about the impact of temperature shifts, the better we can attempt to prepare for the shifts of our Anthropocene age?
Well, take just one example.
Data from deep-sea cores is offering new information on how pivotal ocean currents such as the Gulf Stream (which regulates global climate and ocean habitats by serving as a sort of conveyor belt of warm water) have behaved over time.
In a study by scientists from Germany’s Heidelberg University and University of Bern in Switzerland, published in Nature in July this year, material from deep-sea cores was used to estimate how such currents weakened about 8,000 years ago, as meltwater from glaciers and melting ice sheets flooded the oceans as the last Ice Age gave way to the Holocene (the epoch in which civilisations flourished, which has been succeeded by the current Anthropocene). The data from that research also shows how the current then began to stabilise 6,500 years ago. It is, of course, weakening all over again today, amid floods of meltwater from a warming Arctic icecap.
“One of the biggest uncertainties when it comes to the climate crisis is how ocean life — especially the biological processes that consume CO2 and help the ocean absorb carbon — will respond to climate change,” says Hemant Khatri, a senior researcher on climate mitigation with the UK Met Office. “Understanding these processes in minute detail will be critical if we are to improve climate models and make reliable long-term projections.”
READ-ONLY MEMORY
There is, literally and figuratively, a lot of ground to cover, when it comes to understanding how these memory systems work.
A recent study suggests, for instance, that ocean memory can (and does) “travel”.
To begin to understand this, think of the ocean as a slow-moving sponge. It absorbs heat and other elements from the atmosphere and holds on to them. Because the ocean and atmosphere are constantly interacting, the oceans cannot store this information indefinitely; it morphs or gradually fades over time.
Khatri’s research aims to accurately calculate the time between the generation and loss of these heat signals, to quantify ocean memory.
“The ocean has high heat capacity, much higher than air, and currents move much more slowly than atmospheric winds. This means that heat and climate signals can linger in the ocean for longer, influencing regional weather patterns for several years. We call this the ocean’s ‘long memory’,” says Khatri, who co-authored a paper on this in 2024.
Understanding how this “long memory” works will be crucial, because it can tell us how signals absorbed by an ocean in one location can travel to another (which we have known for decades) and also persist there for several years (which we are only now beginning to gauge).
Khatri and Richard G Williams, professor at the School of Environmental Sciences at University of Liverpool, are finding that the North Atlantic Ocean has a memory of approximately one to two decades, much longer than previous estimates of eight to 10 years. “This suggests that current temperatures in this ocean could influence climate and weather patterns over Europe and North America for that long,” Khatri says.
That’s not all.
Key physical processes could alter ocean memory itself.
“With climate change, processes may change, and this could either increase or decrease the ocean’s memory. Any changes in ocean memory magnitudes could lead to shifts in climate variability, which could affect how often giant storms take shape, and various other climate extremes,” Khatri says.
We cannot have an accurate sense of coming climate variability, over time, as he points out, without an understanding of such phenomena.
COLD TRUTHS
Over time, climate models themselves could need to be redrawn.
As new data emerges, and influential factors are understood better, there will be a need to optimise climate models to improve their accuracy.
“Already, with our new understanding of ocean memory, it is possible to assess climate models differently, and we are seeing how there is a need to investigate processes that are not properly simulated in existing models. The more analysis we have, the more accurately we can represent specific processes that affect climate, and the more we can strengthen our predictive models,” he says.
Pearson would agree.
As historians of the planet’s climate history, we are still barely scratching the surface, he says.
“We are in the middle of an age of discovery in this field. For example, we are still trying to figure out why the Earth started to glaciate in the northern hemisphere about 3.6 million years ago. We don’t know exactly what caused the massive Antarctic glaciation and its resultant End-Eocene Extinctions about 34 million years ago.”
Altered ocean circulation patterns like that of the Gulf Stream are believed to have played a more key role than earlier thought.
CORE INSIGHT
As more data is uncovered, the picture that emerges is becoming far clearer.
“Massive upheavals in Earth’s geology and biology due to climate events have caused major shifts and extinctions, but what we’re doing today by burning fossil fuels is happening more than ten times faster than previous such events,” says Peter Kalmus, a climate scientist with NASA’s Jet Propulsion Laboratory. “And, the effects of those previous shifts lasted for hundreds of thousands of years. It’s crucial to factor this into our understanding of what is happening today. The heat, the sea-level rise, the reduced biodiversity, with change occurring on the scale it is occurring at today, could last for longer than humans have existed.”
Big events are not rare in Earth’s history, Pearson adds. “But modern climate change has no analogue in the past. Nothing similar to the development of human civilisation has ever occurred on the planet. The impacts of it are proving to be very much faster and more profound than most past changes., so the future is in our hands.”
Kalmus suggests that part of the solution could be to appreciate the science, but also fight our way back to a collective sense of wonder and gratitude. “We are living under the warmth of a star, on a rocky planet that’s vibrantly, abundantly alive. How truly incredible is that,” he says. “That’s how I think we need to view our situation as well, in order to understand it better.”
.
CLOUD COMPUTING: HOW ‘MEMORIES’ IN THE AIR IMPACT MONSOON PATTERNS
Think about what you know about how a monsoon system works: moisture evaporates from the oceans; it accumulates in the air; wind and weather systems move this accumulated moisture; pressure systems and heat cause it to rain.
Traditionally, this has been seen as a rather smooth and gradual process.
So far, so familiar.
Except now, it turns out that the air itself has memory, of a kind. And that memory exists in the form of moisture or water vapour that can persist far longer than was previously thought — and exist and persist entirely independent of the oceans.
This “memory” can impact a monsoon system in ways that are entirely independent of the monsoon system too.
It can even serve as a tipping point, precipitating downpours and extending dry spells.
These rather pathbreaking findings were published by climate scientists Anja Katzenberger and Anders Levermann of Germany’s Potsdam Institute for Climate Impact Research, in the journal PNAS, in May this year.
“In physics, memory refers to how a system can actually sustain a (physical) state,” Levermann says. “Memory and stability are strongly interlinked in physics. While if you have a dissipative system like the atmosphere which has weather systems pushing it out, variability is very common. So, atmosphere was traditionally considered to be a system without ‘memory’. This is turning out to not be the case.”
The potential implications are immense.
Atmospheric memory is now being viewed as a factor with the ability to influence the behaviour of a monsoon system. Atmospheric memory could, under the right conditions, serve as an “on” or “off” switch for such a system.
And it can do all this independent of the oceans because the moisture retained as “memory” in the atmosphere typically accumulates during the thawing of Spring.
Amid the climate crisis, these new findings could also mean that prediction models (which have been struggling more than usual, amid changing monsoon patterns) will need to reassess the significance of a host of smaller variables: local evaporation rates, the presence of aerosols, etc.
“My hope is that this study can contribute to a better understanding of when a monsoon system is likely to turn ‘on’ or ‘off’,” says Levermann. “Potentially, we could also learn to better understand the internal dynamics of these massive weather systems.
