Saving Our World–Chapter Four–The History of Our Planet’s Climate

Earth’s climate has changed throughout history. Just in the last 800,000 years, there have been eight cycles of ice ages and warmer periods, with the end of the last ice age about 11,700 years ago marking the beginning of the modern climate era — and of human civilization. Most of these climate changes are attributed to very small variations in Earth’s orbit that change the amount of solar energy our planet receives.

A History of Earth’s Climate

Many dramatic changes to the Earth’s climate have occurred over the planet’s 4.5-billion-year history. Long periods of stability, or equilibrium, are occasionally disrupted by periods of change that vary in length and intensity. Climatic shifts are destructive, and some even caused mass extinction events that wiped out high percentages of species. Despite these extinctions, life has always rebounded, allowing new species to dominate the landscape.

Some examples include:

770 million years ago – Snowball Earth

Scientists believe that there may have been several times when the entire Earth was frozen over with ice. There is no consensus as to what exactly caused these frigid events. One theory holds that a number of large volcanic eruptions sent sulfur gas particles into the atmosphere that reacted with solar radiation to produce a cooling effect. Some scientists speculate that snowball conditions facilitated an explosion of multicellular organisms.

305 million years ago – Carboniferous Rainforest Collapse

The Carboniferous period was known for its marshy forest communities inhabited by the ancestors of reptiles, mammals, and amphibians. It was also an “icehouse” period, in which permanent ice caps sat at the Earth’s poles. But around 305 million years ago, levels of carbon dioxide, a greenhouse gas, increased. Greenhouse gasses prevent heat from escaping the atmosphere into space, insulating the Earth. This caused the planet to warm, dry out, and experience more intense seasonal fluctuations. Such a climate was intolerable for the Carboniferous rainforest plants, leading to a shift in the types of plant and animal communities and eventually the age of the dinosaurs.

66 million years ago – Cretaceous-Paleogene extinction event

The most well-known example of extreme climate change is the Cretaceous-Paleogene extinction event, the extinction of the dinosaurs. 66 million years ago, an asteroid collided with the Earth, sending a colossal cloud of ash and other debris into the atmosphere. This dense cloud blocked out the sun, creating an “impact winter” and halting the photosynthesis of plants and phytoplankton. The effects of the impact winter rippled throughout ecosystems, causing the extinction of the non-bird dinosaurs.

55 million years ago – Permian-Eocene Thermal Maximum

Over a period of about 100,000 years, the planet slowly warmed by between 5° and 8° Celsius (9°-14.4° Fahrenheit). What caused the warming? Some scientists point to a volcanic eruption that prompted marine sediments to release the powerful greenhouse gas methane into the atmosphere. Oceans across the globe reached tropical temperatures, causing the extinction of a significant percentage of marine life.

What do these dramatic shifts in Earth’s climate have in common?

In these examples, we saw that geological phenomena and natural cycles can drastically alter the Earth’s physical attributes. This includes the chemical composition of Earth’s oceans and atmosphere, the wind and ocean currents, the ice caps, and other factors that contribute to Earth’s climate. These shifts in climate – rainfall, temperature, sea level, and more – can in turn severely disrupt the ability of organisms and ecosystems to function.

What causes the Earth’s climate to change?

Geological records show that there have been a number of large variations in the Earth’s climate. These have been caused by many natural factors, including changes in the sun, emissions from volcanoes, variations in Earth’s orbit and levels of carbon dioxide (CO2).

Global climate change has typically occurred very slowly, over thousands or millions of years. However, research shows that the current climate is changing more rapidly than shown in geological records.

Strength of the Sun

Almost all of the energy that affects the climate on Earth originates from the Sun. The Sun’s energy passes through space until it hits the Earth’s atmosphere. Only some of the solar energy intercepted at the top of the atmosphere passes through to the Earth’s surface; some of it is reflected back into space and some is absorbed by the atmosphere.

The energy output of the Sun is not constant: it varies over time and this has an impact on our climate. 

Changes in the Earth’s orbit, axial tilt and precession

The three changes in the Earth’s orbit around the Sun — eccentricity, axial tilt, and precession — are collectively called ‘Milankovitch cycles’.

According to Milankovitch’s theory, these three cycles combine to affect the amount of solar heat that reaches the Earth’s surface and subsequently influences climatic patterns, including periods of glaciation (ice ages). The time period between these changes can be tens of thousands of years (precession and axial tilt) or more than hundreds of thousands of years (eccentricity). 

The Earth’s orbit

The Earth’s orbit around the Sun is an ellipse (an oval shape), but it isn’t always the same shape of ellipse. Sometimes, it is almost circular and the Earth stays approximately the same distance from the Sun throughout its orbit. At other times, the ellipse is more pronounced, so that the Earth moves closer and further away from the Sun in its orbit.

When the Earth is closer to the Sun, our climate is warmer and this cycle also affects the length of the seasons. The measure of a shape’s deviation from being a circle, in this case the Earth’s orbit, is called ‘eccentricity’. 

The Earth’s axial tilt

The tilt in the axis of the Earth is called its ‘obliquity’. This angle changes with time, and over about 41 000 years it moves from 22.1° to 24.5° and back again. When the angle increases the summers become warmer and the winters become colder. 

Obliquity. BGS © UKRI.

The Earth’s precession

The Earth wobbles on its axis, much like a spinning top that is slowing down. This is called ‘precession’ and is caused by the gravitational pull of the Moon and the Sun upon the Earth. This means that the North Pole changes where it points to in the sky. Currently the Earth’s axis points at Polaris, the North Star, but over thousands of years the axis moves around in a circle and points at different parts of the sky. It impacts on the seasonal contrasts between hemispheres and the timing of the seasons. 

This image has an empty alt attribute; its file name is Precession-960x779.jpg

Quantity of greenhouse gases in the atmosphere

Greenhouse gases include carbon dioxide (CO2), methane (CH4) and water vapour. Water vapour is the most abundant greenhouse gas in the atmosphere, but it stays in the atmosphere for a much shorter period of time: just a few days. CHstays in the atmosphere for about nine years until it is removed by oxidation into CO2 and water. CO2 stays in the atmosphere much longer, from years to centuries, contributing to longer periods of warming. These gases trap solar radiation in the Earth’s atmosphere, making the climate warmer.

Changes in ocean currents

Ocean currents carry heat around the Earth. As the oceans absorb more heat from the atmosphere, sea surface temperature increases and the ocean circulation patterns that transport warm and cold water around the globe change. The direction of these currents can shift so that different areas become warmer or cooler. As oceans store a large amount of heat, even small changes in ocean currents can have a large effect on global climate. In particular, increases in sea surface temperature can increase the amount of atmospheric water vapour over the oceans, increasing the quantity of greenhouse gas. If the oceans are warmer they can’t absorb as much carbon dioxide from the atmosphere.

Ocean currents during the cretaceous
Ocean currents during the Cretaceous. BGS © UKRI.
Ocean currents present day
Ocean currents present day. BGS © UKRI.

CO2 content of the oceans

The oceans contain more CO2 in total than the atmosphere and exchanges of CO2 occur between the oceans and the atmosphere. CO2 absorbed in ocean water does not trap heat as it does in the atmosphere.

The world’s oceans absorb about a quarter of the CO2 we release into the atmosphere every year. As atmospheric CO2 levels increase so do the ocean’s CO2 levels. 

Plate tectonics and volcanic eruptions

Over very long periods of time, plate tectonic processes cause continents to move to different positions on the Earth. For example, Britain was near to the equator during the Carboniferous Period, around 300 million years ago, and the climate was warmer than it is today. The movement of the plates also causes volcanoes and mountains to form and these can also contribute to a change in climate.  Large mountain chains can influence the circulation of air around the globe, and consequently influence the climate. For example, warm air may be deflected to cooler regions by mountains.

Volcanoes affect the climate through the gases and particles (tephra/ash) thrown into the atmosphere during eruptions. The effect of volcanic gases and dust may warm or cool the Earth’s surface, depending on how sunlight interacts with the volcanic material. During major explosive volcanic eruptions, large amounts of volcanic gas, aerosol droplets and ash are released.

Ash falls rapidly, over periods of days and weeks, and has little long-term impact on climate change. However, volcanic gases that are ejected into the stratosphere stay there for much longer periods. Volcanic gases such as sulphur dioxide (SO2) can cause global cooling, but CO2 has the potential to cause global warming.

In the present day, the contribution of volcanic emissions of CO2 into the atmosphere is very small; equivalent to about one per cent of  anthropogenic (caused by humans) emissions.

Changes in land cover

On a global scale, patterns of vegetation and climate are closely correlated. Vegetation absorbs CO2 and this can buffer some of the effects of global warming. On the other hand, desertification amplifies global warming through the release of CO2 because of the decrease in vegetation cover.

A decrease in vegetation cover, via deforestation for example, tends to increase local albedo, leading to surface cooling. Albedo refers to how much light a surface reflects rather than absorbs. Generally, dark surfaces have a low albedo and light surfaces have a high albedo. Ice with snow has a high albedo and reflects around 90 per cent of incoming solar radiation. Land covered with dark-coloured vegetation is likely to have a low albedo and will absorb most of the radiation.

Meteorite impacts

Nowadays, most of what is on the Earth stays on the Earth; very little material is added by meteorites and cosmic dust. However, meteorite impacts have contributed to climate change in the geological past; a good example is the Chicxulub crater, Yucatán Peninsula in Mexico.

Large impacts like Chicxulub can cause a range of effects that include dust and aerosols being ejected high into the atmosphere that prevent sunlight from reaching the Earth. These materials insulate the Earth from solar radiation and cause global temperatures to fall; the effects can last for a few years. After the dust and aerosols fall back to Earth, the greenhouse gases (CO2, water and CH4) caused by the interaction of the impactor and its ‘target rocks’ remain in the atmosphere and can cause global temperatures to increase; these effects can last decades. 

Everything You Ever Wanted to Know About Earth’s Past Climates

They have a lot to tell us about our future

In Silent Spring, Rachel Carson considers the Western sagebrush. “For here the natural landscape is eloquent of the interplay of forces that have created it,” she writes. “It is spread before us like the pages of an open book in which we can read why the land is what it is, and why we should preserve its integrity. But the pages lie unread.” She is lamenting the disappearance of a threatened landscape, but she may just as well be talking about markers of paleoclimate.

To know where you’re going, you have to know where you’ve been. That’s particularly true for climate scientists, who need to understand the full range of the planet’s shifts in order to chart the course of our future. But without a time machine, how do they get this kind of data?

Like Carson, they have to read the pages of the Earth. Fortunately, the Earth has kept diaries. Anything that puts down yearly layers—ocean corals, cave stalagmiteslong-lived trees, tiny shelled sea creatures—faithfully records the conditions of the past. To go further, scientists dredge sediment cores and ice cores from the bottom of the ocean and the icy poles, which write their own memoirs in bursts of ash and dust and bubbles of long-trapped gas.

In a sense, then, we do have time machines: Each of these proxies tells a slightly different story, which scientists can weave together to form a more complete understanding of Earth’s past.

In March, the Smithsonian Institution’s National Museum of Natural History held a three-day Earth’s Temperature History Symposium that brought teachers, journalists, researchers and the public together to enhance their understanding of paleoclimate. During an evening lecture, Gavin Schmidt, climate modeler and director of NASA’s Goddard Institute for Space Studies, and Richard Alley, a world-famous geologist at Pennsylvania State University, explained how scientists use Earth’s past climates to improve the climate models we use to predict our future.

Here is your guide to Earth’s climate pasts—not just what we know, but how we know it.

How do we look into Earth’s past climate?

It takes a little creativity to reconstruct Earth’s past incarnations. Fortunately, scientists know the main natural factors that shape climate. They include volcanic eruptions whose ash blocks the sun, changes in Earth’s orbit that shift sunlight to different latitudes, circulation of oceans and sea ice, the layout of the continents, the size of the ozone hole, blasts of cosmic rays, and deforestation. Of these, the most important are greenhouse gases that trap the sun’s heat, particularly carbon dioxide and methane.

As Carson noted, Earth records these changes in its landscapes: in geologic layers, fossil trees, fossil shells, even crystallized rat pee—basically anything really old that gets preserved. Scientists can open up these diary pages and ask them what was going on at that time. Tree rings are particularly diligent record-keepers, recording rainfall in their annual rings; ice cores can keep exquisitely detailed accounts of seasonal conditions going back nearly a million years.

Ice cores reveal annual layers of snowfall, volcanic ash and even remnants of long-dead civilizations. 

What else can an ice core tell us?

“Wow, there’s so much,” says Alley, who spent five field seasons coring ice from the Greenland ice sheet. Consider what an ice core actually is: a cross-section of layers of snowfall going back millennia.

When snow blankets the ground, it contains small air spaces filled with atmospheric gases. At the poles, older layers become buried and compressed into ice, turning these spaces into bubbles of past air, as researchers Caitlin Keating-Bitonti and Lucy Chang write in Scientists use the chemical composition of the ice itself (the ratio of the heavy and light isotopes of oxygen in H2O) to estimate temperature. In Greenland and Antarctica, scientists like Alley extract inconceivably long ice cores—some more than two miles long!

Ice cores tell us how much snow fell during a particular year. But they also reveal dust, sea salt, ash from faraway volcanic explosions, even the pollution left by Roman plumbing. “If it’s in the air it’s in the ice,” says Alley. In the best cases, we can date ice cores to their exact season and year, counting up their annual layers like tree rings. And ice cores preserve these exquisite details going back hundreds of thousands of years, making them what Alley calls “the gold standard” of paleoclimate proxies.

Wait, but isn’t Earth’s history much longer than that?

Yes, that’s right. Paleoclimate scientists need to go back millions of years—and for that we need things even older than ice cores. Fortunately, life has a long record. The fossil record of complex life reaches back to somewhere around 600 million years. That means we have definite proxies for changes in climate going back approximately that far. One of the most important is the teeth of conodonts—extinct, eel-like creatures—which go back 520 million years.

But some of the most common climate proxies at this timescale are even more miniscule. Foraminifera (known as “forams”) and diatoms are unicellular beings that tend to live on the ocean seafloor, and are often no bigger than the period at the end of this sentence. Because they are scattered all across the Earth and have been around since the Jurassic, they’ve left a robust fossil record for scientists to probe past temperatures. Using oxygen isotopes in their shells, we can reconstruct ocean temperatures going back more than 100 million years ago.

“In every outthrust headland, in every curving beach, in every grain of sand there is a story of the earth,” Carson once wrote. Those stories, it turns out, are also hiding in the waters that created those beaches, and in creatures smaller than a grain of sand.

How much certainty do we have for deep past?

For paleoclimate scientists, life is crucial: if you have indicators of life on Earth, you can interpret temperature based on the distribution of organisms.

But when we’ve gone back so far that there are no longer even any conodont teeth, we’ve lost our main indicator. Past that we have to rely on the distribution of sediments, and markers of past glaciers, which we can extrapolate out to roughly indicate climate patterns. So the further back we go, the fewer proxies we have, and the less granular our understanding becomes. “It just gets foggier and foggier,” says Brian Huber, a Smithsonian paleobiologist who helped organize the symposium along with fellow paleobiologist research scientist and curator Scott Wing.

How does paleoclimate show us the importance of greenhouse gases?

Greenhouse gases, as their name suggests, work by trapping heat. Essentially, they end up forming an insulating blanket for the Earth. (You can get more into the basic chemistry here.) If you look at a graph of past Ice Ages, you can see that CO2 levels and Ice Ages (or global temperature) align. More CO2 equals warmer temperatures and less ice, and vice versa. “And we do know the direction of causation here,” Alley notes. “It is primarily from CO2 to (less) ice. Not the other way around.”

We can also look back at specific snapshots in time to see how Earth responds to past CO2 spikes. For instance, in a period of extreme warming during Earth’s Cenozoic era about 55.9 million years ago, enough carbon was released to about double the amount of CO2 in the atmosphere. The consequentially hot conditions wreaked havoc, causing massive migrations and extinctions; pretty much everything that lived either moved or went extinct. Plants wilted. Oceans acidified and heated up to the temperature of bathtubs.

Unfortunately, this might be a harbinger for where we’re going. “This is what’s scary to climate modelers,” says Huber. “At the rate we’re going, we’re kind of winding back time to these periods of extreme warmth.” That’s why understanding carbon dioxide’s role in past climate change helps us forecast future climate change.

How does a climate model work?

Great question! In science, you can’t make a model unless you understand the basic principles underlying the system. So the mere fact that we’re able to make good models means that we understand how this all works. A model is essentially a simplified version of reality, based on what we know about the laws of physics and chemistry. Engineers use mathematical models to build structures that millions of people rely on, from airplanes to bridges.

Our models are based on a framework of data, much of which comes from the paleoclimate proxies scientists have collected from every corner of the world. That’s why it’s so important for data and models to be in conversation with each other. Scientists test their predictions on data from the distant past, and try to fix any discrepancies that arise. “We can go back in time and evaluate and validate the results of these models to make better predictions for what’s going to happen in the future,” says Schmidt.

Models aren’t very accurate

By their very nature, models are always wrong. Think of them as an approximation, our best guess.

But ask yourself: do these guesses give us more information than we had previously? Do they provide useful predictions we wouldn’t otherwise have? Do they allow us to ask new, better questions? “As we put all of these bits together we end up with something that looks very much like the planet,” says Schmidt. “We know it’s incomplete. We know there are things that we haven’t included, we know that we’ve put in things that are a little bit wrong. But the basic patterns we see in these models are recognizable … as the patterns that we see in satellites all the time.”

So we should trust them to predict the future?

The models faithfully reproduce the patterns we see in Earth’s past, present—and in some cases, future. We are now at the point where we can compare early climate models—those of the late 1980s and 1990s that Schmidt’s team at NASA worked on—to reality. “When I was a student, the early models told us how it would warm,” says Alley. “That is happening. The models are successfully predictive as well as explanatory: they work.” Depending on where you stand, that might make you say “Oh goody! We were right!” or “Oh no! We were right.”

To check models’ accuracy, researchers go right back to the paleoclimate data that Alley and others have collected. They run models into the distant past, and compare them to the data that they actually have.

“If we can reproduce ancient past climates where we know what happened, that tells us that those models are a really good tool for us to know what’s going to happen in the future,” says Linda Ivany, a paleoclimate scientist at Syracuse University. Ivany’s research proxies are ancient clams, whose shells record not only yearly conditions but individual winters and summers going back 300 million years—making them a valuable way to check models. “The better the models get at recovering the past,” she says, “the better they’re going to be at predicting the future.”

Paleoclimate shows us that Earth’s climate has changed dramatically. Doesn’t that mean that, in a relative sense, today’s changes aren’t a big deal?

When Richard Alley tries to explain the gravity of manmade climate change, he often invokes a particular annual phenomenon: the wildfires that blaze in the hills of Los Angeles every year. These fires are predictable, cyclical, natural. But it’d be crazy to say that, since fires are the norm, it’s fine to let arsonists set fires too. Similarly, the fact that climate has changed over millions of years doesn’t mean that manmade greenhouse gases aren’t a serious global threat.

“Our civilization is predicated on stable climate and sea level,” says Wing, “and everything we know from the past says that when you put a lot of carbon in the atmosphere, climate and sea level change radically.”

Since the Industrial Revolution, human activities have helped warm the globe 2 degrees F, one-quarter of what Schmidt deems an “Ice Age Unit”—the temperature change that the Earth goes through between an Ice Age and a non-Ice Age. Today’s models predict another 2 to 6 degrees Celsius of warming by 2100—at least 20 times faster than past bouts of warming over the past 2 million years.

Of course there are uncertainties: “We could have a debate about whether we’re being a little too optimistic or not,” says Alley. “But not much debate about whether we’re being too scary or not.” Considering how right we were before, we should ignore history at our own peril.

What’s the hottest Earth’s ever been?

Our 4.54-billion-year-old planet probably experienced its hottest temperatures in its earliest days, when it was still colliding with other rocky debris (planetesimals) careening around the solar system. The heat of these collisions would have kept Earth molten, with top-of-the-atmosphere temperatures upward of 3,600° Fahrenheit.

Even after those first scorching millennia, however, the planet has often been much warmer than it is now. One of the warmest times was during the geologic period known as the Neoproterozoic, between 600 and 800 million years ago. Conditions were also frequently sweltering between 500 million and 250 million years ago. And within the last 100 million years, two major heat spikes occurred: the Cretaceous Hot Greenhouse (about 92 million years ago), and the Paleocene-Eocene Thermal Maximum (about 56 million years ago).

Welcome to greenland cartoon.

History of hot

Temperature records from thermometers and weather stations exist only for a tiny portion of our planet’s 4.54-billion-year-long life. By studying indirect clues—the chemical and structural signatures of rocks, fossils, and crystals, ocean sediments, fossilized reefs, tree rings, and ice cores—however, scientists can infer past temperatures.

None of these techniques help with the very early Earth. During the time known as the Hadean (yes, because it was like Hades), Earth’s collisions with other large planetesimals in our young solar system—including a Mars-sized one whose impact with Earth likely created the Moon—would have melted and vaporized most rock at the surface. Because no rocks on Earth have survived from so long ago, scientists have estimated early Earth conditions based on observations of the Moon and on astronomical models. Following the collision that spawned the Moon, the planet was estimated to have been around 2,300 Kelvin (3,680°F).

Artist's conception of the collision between early Earth and the planetisimal that created the moon

Even after collisions stopped, and the planet had tens of millions of years to cool, surface temperatures were likely more than 400° Fahrenheit. Zircon crystals from Australia, only about 150 million years younger than the Earth itself, hint that our planet may have cooled faster than scientists previously thought. Still, in its infancy, Earth would have experienced temperatures far higher than we humans could possibly survive.

But suppose we exclude the violent and scorching years when Earth first formed. When else has Earth’s surface sweltered?

Thawing the freezer

Between 600 and 800 million years ago—a period of time geologists call the Neoproterozoic—evidence suggests the Earth underwent an ice age so cold that ice sheets not only capped the polar latitudes, but may have extended all the way to sea level near the equator. Reflecting ever more sunlight back into space as they expanded, the ice sheets cooled the climate and reinforced their own growth. Obviously, the Earth didn’t remain stuck in the freezer, so how did the planet thaw? 

Ray troll cartoon depiction of the geologic periods of Earth as an eroded bluff

Even while ice sheets covered more and more of Earth’s surface, tectonic plates continued to drift and collide, so volcanic activity also continued. Volcanoes emit the greenhouse gas carbon dioxide. In our current, mostly ice-free world, the natural weathering of silicate rock by rainfall consumes carbon dioxide over geologic time scales. During the frigid conditions of the Neoproterozoic, rainfall became rare. With volcanoes churning out carbon dioxide and little or no rainfall to weather rocks and consume the greenhouse gas, temperatures climbed.

What evidence do scientists have that all this actually happened some 700 million years ago? Some of the best evidence is “cap carbonates” lying directly over Neoproterozoic-age glacial deposits. Cap carbonates—layers of calcium-rich rock such as limestone—only form in warm water.

Photo of cap dolostone overlying jumbled sedimentary rock in Namibia

The fact that these thick, calcium-rich rock layers sat directly on top of rock deposits left behind by retreating glaciers indicate that temperatures rose significantly near the end of the Neoproterozoic, perhaps reaching a global average higher than 90° Fahrenheit. (Today’s global average is lower than 60°F.)

The tropical Arctic

A Smithsonian Institution project has tried to reconstruct temperatures for the Phanerozoic Eon, or roughly the last half a billion years. Preliminary results released in 2019 showed warm temperatures dominating most of that time, with global temperatures repeatedly rising above 80°F and even 90°F—much too warm for ice sheets or perennial sea ice. About 250 million years ago, around the equator of the supercontinent Pangea, it was even too hot for peat swamps!

Graph of Earth temperature over 500 million years

Geologists and paleontologists have found that, in the last 100 million years, global temperatures have peaked twice. One spike was the Cretaceous Hot Greenhouse roughly 92 million years ago, about 25 million years before Earth’s last dinosaurs went extinct. Widespread volcanic activity may have boosted atmospheric carbon dioxide. Temperatures were so high that champsosaurs (crocodile-like reptiles) lived as far north as the Canadian Arctic, and warm-temperature forests thrived near the South Pole.

Another hothouse period was the Paleocene-Eocene Thermal Maximum (PETM) about 55-56 million years ago. Though not quite as hot as the Cretaceous hothouse, the PETM brought rapidly rising temperatures. During much of the Paleocene and early Eocene, the poles were free of ice caps, and palm trees and crocodiles lived above the Arctic Circle.

Photo of a fossilized palm frond

During the PETM, the global mean temperature appears to have risen by as much as 5-8°C (9-14°F) to an average temperature as high as 73°F. (Again, today’s global average is shy of 60°F.) At roughly the same time, paleoclimate data like fossilized phytoplankton and ocean sediments record a massive release of carbon dioxide into the atmosphere, at least doubling or possibly even quadrupling the background concentrations.

Graph of global temperature over past 65 million years

It is still uncertain where all the carbon dioxide came from and what the exact sequence of events was. Scientists have considered the drying up of large inland seas, volcanic activity, thawing permafrost, release of methane from warming ocean sediments, huge wildfires, and even—briefly—a comet.

(A) Changes in Earth’s carbon dioxide (CO2) (left) and temperature (right) over the last 400 million years. This is only a small section of Earth’s 4.5-billion-year history. Notice how the x-axis changes. The red line shows the amount of CO2 in the atmosphere before humans started burning fossil fuels. Future scenarios are dependent on how much CO2 we release. (B) A more detailed view of temperature changes throughout the ice ages over the past 1 million years, constructed from Antarctic ice cores. These show how much climate has changed naturally throughout Earth’s history.

Like nothing we’ve ever seen

Earth’s hottest periods—the Hadean, the late Neoproterozoic, the Cretaceous Hot Greenhouse, the PETM—occurred before humans existed. Those ancient climates would have been like nothing our species has ever seen.

Modern human civilization, with its permanent agriculture and settlements, has developed over just the past 10,000 years or so. The period has generally been one of low temperatures and relative global (if not regional) climate stability. Compared to most of Earth’s history, today is unusually cold; we now live in what geologists call an interglacial—a period between glaciations of an ice age. But as greenhouse-gas emissions warm Earth’s climate, it’s possible our planet has seen its last glaciation for a long time.

What Controlled Past Climates?

So what caused these big changes in climate? This is complicated because there are many different reasons, and they occurred over different timescales.

One major long-term controller of climate is the amount of greenhouse gases.

These are gases that are in the atmosphere and trap heat from the sun. They include carbon dioxide and methane. in the atmosphere. These gases include carbon dioxide (CO2) and methane (CH4), and they act like a greenhouse around the Earth by trapping heat energy from the Sun. More greenhouse gases trap more heat, so temperatures rise. The amount of greenhouse gases has changed slowly and naturally throughout Earth’s history. There are lots of reasons for these changes, including the amount of volcanic activity, changes in ocean circulation, the types of vegetation, and complicated processes like weathering of rocks. More recently, greenhouse gases have been increasing dramatically due to human activity such as burning fossil fuelsNatural fuels that have formed underground over a long period of time from the remains of living organisms.. A rapid increase in greenhouse gases is playing a major role in the climate change that is happening today [3].

Another major controller of climate is the position of Earth’s continents (Figure 2A). Continents move on very long timescales because blocks of land sit on a layer of molten lava called the mantle, which is moving them very slowly. Every 300–500 million years, Earth’s continents join together into one massive continent. For example, around 175 million years ago, all of Earth’s land was joined together in one supercontinent called Pangea. When Pangea broke apart, it changed wind and ocean currents, eroded land, and created big volcanoes. All these things had significant impacts on Earth’s climate, partly by changing the amount of greenhouse gases in the atmosphere.

Figure 2 - Natural processes that control climate.
Figure 2 – Natural processes that control climate.(A) The position of Earth’s continents can change. In the past all of Earth’s land was joined together in one super continent called Pangea. (B) The Earth’s orbit around the Sun changes through time. These are called Milankovitch cycles, and they impact the amount of heat that reaches the earth, which can influence climate.

Earth’s vegetation also influences climate. The first land plants evolved about 470 million years ago and began to suck CO2 out of the atmosphere. These early plants may therefore have cooled the climate leading to ”snowball Earth.” Later, vegetation with dark green leaves (like ferns and trees) evolved. The dark colors absorbed the Sun’s energy, which may have helped warm the planet.

The Sun’s energy is the most important factor keeping the planet warm enough for life to exist—but the amount of energy we receive from the Sun is not constant. The way the Earth travels around the Sun changes in cycles of hundreds of thousands of years. The amount of energy reaching Earth is controlled by how close the Earth is to the Sun, how much the Earth’s axis tilts, and how much the Earth wobbles as it spins. These cyclic changes are called Milankovitch cycles.

Refer to natural changes in Earth’s orbit around the sun that occur over long periods of time. These alter the amount of heat that reaches the Earth which impacts climate. These cycles impact Earth’s climate and are responsible for the ice ages that occurred over the past 2 million years.

There are also short, explosive events that can impact climate, including meteorites, which are big rocks from outer space that hit the Earth. For example, the dinosaurs went extinct when a huge meteorite hit the Earth 65 million years ago. The impact released ash and soot high into the atmosphere, which reflected some of the Sun’s energy back into space, away from the Earth. This cooled the planet; meaning plants and dinosaurs could no longer survive. However, seeds buried in the soil were preserved and grew again when the climate started to recover and warm.

How Do We Measure Past Climates?

Because we do not have a time machine to go back and measure Earth’s paleoclimate, scientists must use creative methods to understand what climate used to be like. One way to look into the past is to drill and extract ice from the north and south poles. These ice cores

A long cylinder of ice that is drilled out of an ice sheet or glacier. They can give scientists information about past climates. can be up to 3,000 meters long! Scientists directly measure small air bubbles in the ice that still contain CO2 from when they were formed, some as long as 800,000 years ago.

As we discussed, there were many times during Earth’s history when there was no ice on the Earth. So, what do scientists do then? One method is to use proxies

Preserved physical materials, such as fossils, minerals or molecules, which record past conditions and can be used to reconstruct past climate.. Proxies are physical materials that record past conditions. Proxy materials can be fossils, molecules, or minerals found in ancient sediments (Figure 3). For example, fossils of animals and plants (including plant pollen) can tell us about the climate of the past. If sediments contain palm tree fossils, for example, the climate was probably hot and tropical in that location in the past.

One of the most useful proxies is found in the oceans—the remains of tiny, shelled organisms called plankton. During their lives, these tiny organisms built their shells from molecules in the surrounding water. When they died, their shells floated to the ocean floor and were buried. When scientists dig up these tiny shells, they can calculate the number of organisms and analyze the molecules in the shells. This helps them understand what the environment was like when those organisms were alive, including climate factors like CO2 and temperature.

Another method to reconstruct the past is to use computers to build model worlds that simulate both past and future climate. These models use mathematical equations to represent the complex processes that make up the climate system. Scientists set up these models using information about the world today, which can then be changed to match the conditions found in the past. These models are very complex, so they need to be run on big supercomputers. Scientists can run simulations with the models to provide information about what the climate might have been like in the past.

Combining climate models with results from proxies gives scientists a powerful tool to more accurately understand paleoclimate.

Why Is Paleoclimate Important and How Can It Help Us Understand the Future?

Studying paleoclimate is important for understanding Earth’s past. It explains why the present-day Earth is the way it is, such as why certain animals and plants live where they do. Studying paleoclimate shows us how the Earth (and life on Earth) responds to change.

As you have seen, Earth’s climate has changed a lot in the past, but these changes usually take many thousands of years. Right now, we are seeing climate change happening over just a few decades, making it difficult to know what will happen next. Therefore, it is also important to study paleoclimate to understand the future of our planet.

One of the most important things for scientists to understand is exactly how changes in greenhouses gases (like CO2) affect temperature. This is known as climate sensitivity

A measure of how sensitive the climate is to a change in greenhouse gases.. Paleoclimate information can be used to help scientists better understand climate sensitivity. This information can be used to improve climate models, so scientists can more accurately predict how current changes in greenhouse gases might impact future climate. Climate affects all life on Earth, including plants, animals and us—so, understanding our past will help us prepare for our future.

Resources, “What causes the Earth’s climate to change?”;, “A History of Earth’s Climate.”;, “Everything You Ever Wanted to Know About Earth’s Past Climates: They have a lot to tell us about our future.” By Rachel E. Gross;, “What’s the hottest Earth’s ever been?” By Michon Scott and Rebecca Lindsey;, “66 Million Years of Earth’s Climate History Uncovered – Puts Current Changes in Context.”;, “Understanding the Climate of Ancient Earth.” By Edward Armstrong, Alexander Farnsworth, Victtoria Lauretano and Caitlyn Witkowski;