The history of climate change provides insight into the planet's future

How can we research a past that has no written history? What information may hints and relics provide about the era in which they were created? And, more importantly, can this portrait of our past assist us in preparing for an uncertain future?

The Earth's past was not identical to the present. We now know that the Earth has been both much hotter and much colder than it is now. Additionally, we recognise that humans are altering the climate now, warming the earth and ushering in an uncharted future. However, how did we get to this knowledge? To be sure, in their drive to unearth the secrets of yesterday, academics have provided us with unprecedented glimpses into the future.

"It's depressing, because we have to be concerned about the future," Sidney Hemming said. However, from an intellectual one, it is tremendously stimulating and interesting. It's a tremendously enjoyable investigative exercise to attempt to piece together what happened in the past and why."

This is Sidney Hemming, a Columbia University historical geologist.

It's difficult to pinpoint when people initially proposed that vast ice sheets previously covered a much larger area of the Northern Hemisphere than they do today. However, as the nineteenth century progressed, the evidence accumulated. Once you look for them, evidence of these previous circumstances is everywhere: unexplained scouring of rock surfaces, boulders that could only have been carried and left behind by a glacier, and fossils of cold-adapted species. Consider woolly mammoths as an especially spectacular example.

Indeed, by the twentieth century, it was evident that there had been more than one ice age. What remained a mystery, though, was why.

Here is Pennsylvania State University geoscientist Richard Alley.

"As a result, ice ages have proven to be one of the most perplexing aspects of Earth's history." Richard Alley And there are so many moving elements that it's been quite difficult to disentangle. We know the ice grew larger and smaller throughout time, but when? How frequently?"

One idea — which you may have heard of — is based on what are known as Milankovitch cycles, which are named after the Serbian physicist Milutin Milankovitch, who perfected the necessary calculations about a century ago. The hypothesis postulated that minute changes in Earth's orbit around the Sun may result in periods of time when the Northern Hemisphere received less sunshine, resulting in the accumulation of ice over time. In other words, contributing to the onset of an ice age. However, by the mid-twentieth century, many scientists remained doubtful that such a fantasy notion adequately explained ice ages. In 1952, one researcher stated:

The hypothesis is incapable of accounting for historical changes. The consequences are too minor, and the timeline of... Because the occurrence of glaciation is so unknown, any link... appears to be coincidental.

In other words, while it appeared to be a plausible theory, there was no reason to believe that the Earth's orbit could be causing the planet to cool. And it didn't help that scholars were still grappling with the ice ages' timing. That, however, was about to change. Here's Richard once more, who began his profession in the 1970s peeking into Earth's past:

"When I entered the field, it was exploding with the opportunity to discover what happened and when it happened," Richard Alley explains.

To accomplish this, researchers needed to develop the ability to read the hints left by the past. Naturally, this requires the skills and insights necessary to comprehend what you're looking at. However, scientists must first determine where to look.

"Those who research the history of climate absolutely need a record," says Richard Alley.

The issue with ice age records was that there had been more than one ice age. Observing where ice grew and then receded could only disclose so much, as each new ice age trampled on the past of its predecessors. The researchers need a continuous record that caught the specifics and chronology of the latest ice age and those that preceded it — from a location where climatic history accumulated through time.

Richard Alley: "Almost everywhere in the ocean, things accumulate. They are not eroding. And things live in surface water, sink, and accumulate on the bottom. Additionally, you can embark on a ship. And then you take a glorified drill - it has a spinning pipe. Additionally, you can spin it into the muck and then retrieve it. And then you'll have a record."

Extraction of these muddled cores from the ocean's depths without disrupting the layers and therefore erasing the record was a significant technical barrier. However, the reward was a comprehensive climate history book. Each layer of silt represented a different time period in Earth's history - and the deeper the layer, the further back in time it was deposited. However, researchers had to acquire the ability to read the pages of these books.

One of the tricks was derived from radioactive dating. Certain atoms decay at a constant pace. Thus, determining the fraction of destroyed atoms helps researchers to date the sample. Thus, using this technique, a chronology for a core of sediment from the sea floor may be computed.

Once the dates were determined, scientists required a technique to determine the growth and contraction of ice sheets within the core. And, once more, it came down to an atomic trick. Of course, water is the molecule H2O. However, O – oxygen — can exist in a variety of "isotopes." There are two in particular that geologists are interested in, one slightly heavier than the other. The lighter colour evaporates faster from the waters. When a result, snowfall is disproportionately composed of lighter isotopes, and as ice sheets increase, they effectively remove the lighter isotopes from the ocean, leaving the heavier isotopes concentrated in the seas.

This ratio of heavier to lighter isotopes in the ocean makes its way into the shells of small plankton, which sink to the ocean floor when they die, gradually accumulating in the layers of sediment that researchers may dig up as cores.

"And you can look back through the ice ages, ice ages, ice ages, and ice ages, and see the ice growing larger and smaller, the ocean growing smaller and larger, and the temperature changing as the ice grows larger, to the point where the entire world becomes colder."

Such compelling data from a core taken from the bottom of the ocean was first published in 1973, and the timing was unusual. The ice eras looked to correspond to aspects of the Earth's orbit. This added to a growing body of evidence suggesting Milankovitch cycles were actually responsible for the onset and termination of Earth's ice ages. Here's Sidney once more:

"To discover evidence in continuous records that could be proved to be approximately paced as predicted by orbital variations – without a doubt, that was a game changer." Sidney Hemming

However, the exact time did not totally dispel reservations about Milankovitch's theory. As noted in a 1978 review:

However, the mechanism by which these fluctuations in solar radiation affect the atmosphere and cryosphere remains to be demonstrated in detail.

The title of the review was "Glacial Inception and Disintegration During the Last Glaciation," and it was published in the Annual Review of Earth and Planetary Sciences. One of the most striking aspects of this assessment, however, is what it does not include – one of the first things that comes to mind when discussing climate change: carbon dioxide.

By the 1970s, the importance of carbon dioxide to the climate had been widely established for decades. It was obvious that without some of the gas in our atmosphere, the Earth would be far too cold for human comfort. However, the concept of "global warming" as a result of fossil fuel use and CO2 emissions was not generally debated.

"So, honestly, 50 years ago, that would have been before general realisation that there was a problem." Sidney Hemming

Indeed, several academics had already concluded — and warned policymakers — that burning fossil fuels may severely increase global temperatures. However, the full omission of CO2 from this 1978 study demonstrates that the relevance of this greenhouse gas was not emphasised, even by some climate specialists.

However, another analysis published the same year and in the same journal, "Temporal Fluctuations of Atmospheric 14C: Causal Factors and Implications," did acknowledge the critical role of CO2, emphasising the need of accurately calculating...

... the ramifications of increasing atmospheric CO2 concentrations for the future climate of the Earth.

Additionally, this assessment included significant insights on how we could come to comprehend this future climate:

In earth science, forecasting the future is highly dependent on knowledge of the past.

Thus, in the 1970s, carbon dioxide was recognised as critical to the current climate, but this recognition was not necessarily shared within climate scientists, let alone in wider society. And, as the 1978 study of glaciation demonstrates, this significance was not widely discussed during the last ice ages. Why, after all, should it? How does CO2 fit into the jigsaw puzzle of Earth's orbit and ice ages?

Indeed, to some academics researching the Earth's cooling and warming cycles, these dug-up cores hinted at a conclusion that was diametrically opposed to our current anxieties.

"The initial thinking — and this was in the 1970s, and if you have listeners from my generation, they may recall small warnings — was that perhaps the next ice age was approaching."

These concerns — which were bolstered by fears about aerosols' cooling effects — were widely seized upon by journalists. However, we should make a couple of caveats here. To begin, the term "pretty close" has a different meaning for geologists than it does for the rest of us — in this case, it refers to tens of thousands of years. And, secondly, even in the 1970s, there was far more research on the hazard of global warming than there was on the possibility of an impending ice age, demonstrating that scientists were becoming concerned about global warming.

However, while the timing of ice ages had been conclusively established, the precise explanation for this time remained unknown. How could such minute variations in sunshine cause such huge shifts? As it became evident that the entire planet cooled throughout ice ages, the storey intensified. According to Milankovitch's cycles, when the Northern Hemisphere received more sunshine, the Southern Hemisphere received less, and vice versa. As a result, it makes sense that the South would warm while the North cooled. However, the data indicated that both poles were warming and chilling concurrently.

"And this is quite strange," Richard Alley continued. You're merely redistributing sunlight, but the entire planet becomes warmer or colder; what the hell is going on?"

To address this question, researchers would require another core, where the layers would once again trace a history into the past. However, these were not mud layers.

"However, the truly critical relationship is the ice core evidence," Sidney Hemming stated.

As with ocean sediment cores, ice cores excavated from ice sheets or glaciers include clues that enable researchers to date them and determine historic temperatures. However, they conceal a secret that sediments cannot: air bubbles. As new snow accumulates on top of previous accumulations, it progressively fills in minute crevices, trapping air inside. Scientists can directly quantify ancient air by carefully excavating and defrosting this ice.

The Soviet Union's Vostok Station, located deep in Antarctica, took the lead on this mission. This distant station braved freezing temperatures to drill kilometres into the East Antarctic ice sheet, revealing hundreds of thousands of years of Earth's history. And by the mid-1980s, research of the air bubbles in these cores revealed a startling truth: the carbon dioxide concentrations in the Earth's atmosphere rose and dropped in lockstep with the planet's temperatures.

This helped to complete the picture of the ice ages' driving force: variations in the Earth's orbit cause seasonal shifts in the Northern Hemisphere's sunlight, which causes it to cool. This can result in a decrease in the atmospheric concentration of carbon dioxide, for example, as the gas dissolves into the ocean. And thus diminishes the effectiveness of the atmosphere's insulation of the earth via the greenhouse effect, so cooling the entire world. And all of these procedures reverse to bring the ice ages to a close. We already had indirect evidence of temperature variation throughout time, referred to as "proxy data," via shells in sediment cores and other evidence. The discovery of these CO2 oscillations explained why the Milankovitch cycles were so powerful and capable of cooling the entire world in parallel.

"The data from Antarctica when the temperature proxy is compared to the gas composition are incredible, right?" Sidney Hemming It demonstrates the extremely close relationship between global temperature and greenhouse gas concentrations. Additionally, it establishes the context for how out of whack the carbon dioxide concentration is now."

That is because if carbon dioxide can play such a critical part in such dramatic climatic shifts as the onset and end of ice ages, it must definitely play a role in the planet's present and future as well. And, as Sidney pointed out, it was obviously out of whack. CO2 levels are almost 50% greater today than they were at any point in these ice cores – during ice ages or mild periods in between.

As previously stated, the notion that carbon dioxide trapped heat and so had the ability to govern the Earth's temperature was well established long before all of this. However, observing CO2 levels and temperatures rise and fall in lockstep over Earth's history painted a vivid picture of how close this relationship was.

"So I truly believe that this understanding of the ice ages, the role of carbon dioxide, has been a critical step toward a complete understanding of carbon dioxide's involvement in our climate."

This is demonstrated in a critical 1990 article in the Annual Review of Energy titled "Energy, Greenhouse Gases, and Climate Change."

... seeks to give an in-depth examination of the relationship between energy consumption and climate change.

This review utilises historical climate records to demonstrate how extraordinary human emissions — and their potential effects — are. For instance, ice core evidence...

... demonstrates that CO2 concentrations now are 20-25 percent greater than they have ever been in the last 160,000 years...

Although the review author expressed concern about political inertia, he indicated that paleoclimate data could provide one line of evidence to lawmakers. Indeed, the review expresses concern that if left unchecked...

... warming may result in the highest average global temperatures in the last 160,000 years.

The graphs depicting CO2 and temperature lowering and rising in lockstep with the arrival and departure of ice ages have been frequently duplicated and shared far beyond the confines of scholarly journals. Perhaps most notably in former Vice President Al Gore's 2006 film "An Inconvenient Truth":

Although the relationship is somewhat convoluted, there is one that is significantly more potent than the others: When there is more carbon dioxide in the atmosphere, the temperature rises because it traps more heat from the sun.

However, evidence from previous climates offers scientists with more than merely compelling proof that carbon dioxide has the ability to regulate the planet's temperature. Additionally, these data enable researchers to assess one of the most critical quantities in terms of global warming: the much mentioned "climate sensitivity."

The climate sensitivity index indicates how much the globe warms in response to a given increase in carbon dioxide. Understanding this is critical for imagining what our planet's future might look like. Researchers assess climate sensitivity in a variety of ways, from examining how climate processes will amplify or inhibit the effects of additional CO2 to examining the warming witnessed over the last few decades.

However, historic evidence linking CO2 and temperature fluctuations provides a valuable tool for calculating the climate's sensitivity to CO2.

Nonetheless, deriving reliable estimates of climate sensitivity from historical climates is not straightforward. It is one thing to understand that temperatures fluctuated in previous climates, but quite another to know precisely what those temperatures were. Alternatively, as Sidney puts it:

"The difficulty with utilising the paleoclimate record to do that is that we can never put a thermometer back in the paleoclimate record," Sidney Hemming explained.

However, even while our measurements of historical climates — whether of temperature or timing — remain imprecise, they continue to give scholars with crucial information, providing another line of evidence for deciphering the climate. As explained in a 2018 analysis published in the Annual Review of Marine Science titled "Comparing Climate Sensitivity, Past and Present,"

... estimations of climate sensitivity based on paleoclimate data have the advantage of being based on genuine data...

And studying historical climates remains a critical tool for scientists to forecast how hot the Earth's future will be. Apart from climate sensitivity, historical climates can teach us a great deal about our route. Constructing a more detailed picture of these historical climates could shed light on the future we face if we continue to use fossil fuels.

"As we collect more and more diverse types of data, we're gaining a clearer understanding of the types of changes that can occur and how quickly they can occur," says Sidney Hemming. The true rewards will come from developing global viewpoints. Thus, if we can locate sufficient records, such as those from previous periods with CO2 concentrations comparable to today's, And to see what was happening globally throughout that particular period."

Richard concurs that this evidence has the potential to have a major effect on our projections.

"So there is this wide universe of understanding history that informs understanding: what will happen where you live as a result of our decisions, what does it imply for living things, including other species as well as us."

We've gone a long way since the Earth's climatic history was cloaked in mystery just a few decades ago. Numerous researchers have dug up hints from previous eras and countless more have learned how to read these remains.

"It's been amazing for me," Richard Alley said. I've had a minor involvement - I've had the opportunity to be a part of it and observe what other people are doing. And it's been just amazing work: concentrated, focused, multinational, interdisciplinary, individuals who are leading in so many different ways, people who are striving to diversify science in order to include more perspectives and thus have a better understanding of what's happening. And I'm just sitting here with a big smile on my face, thinking about what's transpired and how wonderful my coworkers have been."

However thrilling this excursion into the past has been, it has also contributed to the development of a pessimistic view of the future. Carbon dioxide levels in the atmosphere are now higher than they have been for millions of years, according to data. As projected, this greenhouse gas is increasing global temperatures, intensifying extreme weather events, raising our oceans, and destabilising human and ecological systems worldwide. And our emissions continue to climb, despite scientists' warnings that they must be rapidly reduced.

So how will the climate of the future look? Exploring the Earth's past has provided us with some clues. However, it will ultimately be up to humanity to determine its course.

"It's a depressing picture," Sidney Hemming says. "I believe that the collective wisdom we've accumulated over these years has resulted in an unavoidable understanding that humans are having a significant impact on the earth." While I still like studying the past, I am concerned about the implications for the future."

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