Chapter 21: Global Climate Change

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Welcome to the Deem Dive.

We're here to help you get a real handle on the important stuff.

Today, let's start with an image.

Alaska's Marjorie Glacier.

It's this massive river of ice, but like so many others, it's shrinking, receding year after year.

It's a really striking visual, isn't it?

And these glaciers, they're more than just beautiful landscapes.

They're like Earth's own thermometers giving us vital clues about how our planet's climate is changing.

Exactly.

So our mission today is a deep dive into global climate change, but specifically looking at it through the lens of physical geology.

Think of it like we're walking you through the core ideas from a chapter, but without the textbook in front of you.

Right.

We want to unpack this really intricate dance between Earth's natural systems and, well, the significant role humans are now playing in shaping the climate's future.

Our goal is really just to give you clear, memorable insights, because understanding the stuff, it's key.

Whether you're thinking about natural hazards, resources, or just the planet's amazing history, grasping climate change is crucial for being well

Absolutely.

And we'll start by clarifying what we actually mean by climate, how it's different from just, you know, daily weather.

Then we'll journey into how scientists actually figure out past climates.

And explore the natural forces that have always changed Earth's climate long before us.

And finally, yes, look at the human impacts and what scientist's project might happen next.

Okay, let's get into it.

Earth's climate system.

You said it's more than just temperature.

What is it really?

Think of it as Earth's huge interconnected operating system.

It's all about how energy and moisture move around.

And it involves five major components or spheres, all interacting.

Five spheres?

Okay, what are they?

Well, first, you've got the atmosphere.

That's the air, the wind, all the weather patterns.

Pretty straightforward.

Got it.

Air.

Second, the hydrosphere.

That's all the water.

Oceans, lakes, rivers, even the water underground.

Okay.

Atmosphere, hydrosphere.

Third, the geosphere.

That's the solid Earth itself, the rocks, mountains, the land forms.

Right, the ground beneath our feet.

Fourth is the biosphere that's all life.

Plants, animals, microbes, everything living.

Makes sense.

And fifth, the cryosphere.

That's the frozen stuff.

Snow, glaciers, sea ice, frozen ground called permafrost.

Cryosphere.

Okay, the icy part.

So these five spheres, they're all linked.

Intimately linked.

It's really a two -way street.

Climate obviously impacts geology, rock weathering, how landscapes are shaped in deserts versus tropics versus glacial areas.

Like how rain carves canyons.

Exactly.

Or how huge floods or debris flows are often triggered by weather events.

But geology also hits back, influencing climate.

How so?

Well, think about massive volcanic eruptions, pumping gases like CO2 or sulfur dioxide into the atmosphere.

That directly changes its composition.

Oh.

Or over millions of years, mountain ranges rising up can totally alter regional wind and rain patterns, creating things like rain shadows.

Right.

So the key takeaway is everything's connected.

That's the bottom line.

A change in one sphere ripples through all the others, often in ways that aren't immediately obvious.

It's a complex dynamic system.

So to really understand what's happening now, we can't just look at recent history.

We need to understand the past, the deep past.

This is paleoclimatology.

That's exactly paleoclimatology, the study of past climates.

It's crucial because it gives us context.

It helps us see if current changes are within the range of natural variability Earth has seen before, or if something different is happening.

But how do we know what the climate was like, say, hundreds of thousands of years ago?

No weather stations back then.

Right.

So scientists rely on what we call proxy data.

These are indirect clues, natural archives that recorded climate conditions.

Natural archives?

Like what?

One of the most powerful tools is ice cores.

Scientists drill deep down into the ice sheets in places like Greenland and Antarctica, and they pull up these long cylinders of ice.

Some cores go back hundreds of thousands of years, trapping layers of ancient snow.

So what can you learn from that ice?

A lot.

The layers tell us about past snowfall rates.

Tiny air bubbles trapped in the ice are like little time capsules.

They contain actual samples of Earth's past atmosphere.

We can directly measure ancient CO2 and methane levels.

Actual ancient air.

That's incredible.

It really is.

And then there's the chemistry of the ice itself, specifically oxygen isotope analysis.

Okay, isotopes.

How does that work as a thermometer?

It's quite clever.

Water, HgO, is made of hydrogen and oxygen.

Oxygen comes in different forms or isotopes, mostly lighter oxygen, 16, and a bit of heavier oxygen, 18.

Now, when water evaporates from the ocean, the water molecules with the lighter ILO evaporate slightly more easily.

During cold periods,

when massive ice sheets grow on land, a lot of that IO gets locked up in the ice.

So the oceans are left with relatively more of the heavy isle.

Exactly.

And during warmer periods, when ice melts, that lighter isle flows back into the oceans.

Okay, I think I follow.

So scientists can measure the ratio of HgO2O in the ice layers.

They can also look at tiny marine organisms, like foraminifera, that build shells out of calcium carbonate using the ocean water.

Ah, so their shells record the ocean's isotope ratio at time they lived.

Precisely.

These shells sink to the seafloor, creating a sedimentary record.

So analyzing that ogre ratio in either ice cores or seafloor sediments gives us a direct indicator of past temperatures and how much ice was on the planet.

It's like a paleo thermometer.

Amazing.

Okay, so ice cores and deep sea sediments give us a global picture.

What about clues on land?

On land, tree rings are incredibly useful.

In temperate regions, trees add a growth ring each year.

Right, I know about counting rings for age.

Well, the width and density of those rings tell you about the conditions that year.

Wide rings usually mean good growing conditions, enough water, warmth.

Narrow rings suggest stress, like drought or cold.

So you can reconstruct past regional climates.

Yep.

By matching patterns from different trees, including very old ones, like bristlecone pines, scientists can build chronologies stretching back thousands of years.

High resolution records for specific regions.

What else?

Any other natural recorders?

Sure.

Corals also build skeletons with growth bands, kind of like trees underwater.

Their chemistry changes with water temperature and even rainfall patterns, so they act as ocean thermometers.

Corals too.

And fossil pollen.

Different plants thrive in different climates.

Pollen grains are really tough and preserve well in lake sediments or bogs.

By identifying the types of pollen, scientists can reconstruct past vegetation patterns, which tells you about the climate.

So using all these proxies, ice, sediments, trees, corals, pollen, we've learned that Earth's climate isn't static.

It swings naturally.

Absolutely.

We know Earth is cycled between warm greenhouse periods, think no polar ice, maybe tropical forests way up north, and colder icehouse periods like the one we're technically in now with ice sheets at the poles.

You mentioned the Cenozoic era, the last 66 million years.

Right.

Over that time, Earth generally cooled, transitioning from a warmer greenhouse state into our current icehouse world.

And within this recent icehouse phase, especially over the last 800 ,000 years or so, we've seen these dramatic cycles of major ice ages followed by warmer interglacial periods happening roughly every 100 ,000 years.

Okay.

So before we get to why things are changing now, let's quickly cover the stage for all this.

The atmosphere itself, what's it made of?

Good idea.

Most of the air we breathe, if it's clean and dry, is nitrogen about 78 % and oxygen about 21%.

Argon is most of the rest.

But we always hear about carbon dioxide, CO2.

Right.

CO2 is actually a tiny fraction, only about 0 .04 % or 400 parts per million, but it plays a huge role because it's a greenhouse gas.

Meaning it traps heat.

Exactly.

It's really good at absorbing the heat energy that Earth radiates outwards.

Other things vary too, like water vapor, which can be up to 4 % of the air.

It's also a powerful greenhouse gas and the source of clouds and rain.

And ozone, that's important too, right?

Very important.

Ozone, O3, is concentrated way up in the stratosphere.

It's vital because it absorbs most of the harmful ultraviolet, or UV,

radiation from the sun.

Without the ozone layer, life on the surface would be very difficult.

Okay.

So that's the composition.

How does this atmosphere actually get heated up by the sun?

Well, the sun radiates energy across a spectrum -visible light, UV, infrared heat.

This is electromagnetic radiation.

When this energy hits Earth's surface and gets absorbed, it makes the molecules vibrate faster, which we feel is heat.

So the ground heats up first.

Primarily, yes.

About half the incoming solar energy is absorbed by the land and water.

Some gets absorbed by the atmosphere and clouds, and about 30 % is just reflected straight back to space.

That reflectivity is called albedo.

Albedo.

Like snow reflects a lot, asphalt absorbs a lot?

Precisely.

Snow has a high albedo, dark forests or pavement have low albedo.

This matters for how much energy is actually absorbed.

So the surface heats up, then what?

Then Earth re -radiates that energy back outwards.

But as longer wavelength infrared radiation basically heat.

And this is where those greenhouse gases like water vapor and CO2 come in.

Ah, the natural greenhouse effect.

Exactly.

The atmosphere is mostly transparent to the incoming sunlight, but greenhouse gases are very good at absorbing the outgoing infrared heat radiated by the Earth.

They trap some of that heat, warming the lower atmosphere.

It's like a blanket.

And this is essential for life.

Absolutely essential.

Without this natural greenhouse effect, Earth's average temperature would be about negative 18 degrees C, well below freezing.

Instead, it's a much more comfortable 15 degrees C on average.

It makes the planet

Okay, so the greenhouse effect is natural and vital.

But we know climate changes naturally, too, over long time scales.

What drives those big shifts?

Several things.

Over millions of years, plate tectonics is huge.

As continents drift, they change ocean currents and atmospheric circulation patterns, which fundamentally alters global heat distribution.

Moving a continent towards the pole obviously makes it colder.

Makes sense.

What about those cycles related to ice ages?

Those are likely linked to orbital variations, or Milankovitch cycles.

These are subtle, slow changes in Earth's orbit around the sun, its shape, the tilt of Earth's axis, and the wobble of the axis.

And these small changes are enough to trigger ice ages.

They change the amount and distribution of solar energy reaching Earth, particularly at high latitudes in different seasons.

It's believed these cycles act as a pacemaker for the glacial, interglacial cycles of the recent ice age,

although feedbacks within the climate system likely amplify the effects.

Interesting.

Now, what about volcanoes?

You mentioned they can impact climate.

They definitely can.

Explosive eruptions, the big ones, can cause short -term cooling.

Cooling?

I thought they released CO2.

They do release CO2, but the immediate dominant effect of a large explosive eruption comes from sulfur dioxide, SO2, gas shot way up into the stratosphere.

Up there, SO2 reacts with water to form tiny droplets of sulfuric acid.

These effectively shading the planet and causing a temporary cooling, maybe for a year or three.

We saw this after Mount Pinatubo erupted in 1991.

Global temps drop by about half a degree Celsius.

Ah, so it's the sulfur, not the ash.

Mostly the sulfur aerosol effect, yes.

Volcanic ash falls out much quicker and has less impact on global climate.

But you're right about the CO2 over very long time scales.

Massive, prolonged volcanic activity can release enough CO2 to cause significant long -term warming.

Like during the time of the dinosaurs?

Exactly.

The Cretaceous period was exceptionally warm, probably due in large part to an enhanced greenhouse effect from immense volcanic outpourings associated with things like large alias provinces releasing vast amounts of CO2 over millions of years.

It shows how deep Earth processes connect to climate.

Okay, so we have plate tectonics, orbital cycles, volcanoes.

What about the sun itself?

Does its output change?

It does slightly.

The sun has sunspots, which are magnetic storms on its surface.

They follow roughly an 11 -year cycle.

And more sunspots mean more energy?

A tiny bit more, yes, about 0 .1 % variation between sunspot maximum and minimum.

Generally, this cycle is considered too small and too short to have a major impact on long -term global climate trends.

There was a period called the Maunder Minimum in the 1600s with very few sunspots that coincided with the Little Ice Age, leading to some debate, but it's not seen as the driver of current warming.

Right.

Which brings us to the modern era and the human fingerprint.

We've always changed the environment, but things really shifted with industrialization, didn't they?

Dramatically.

While humans have long impacted local environments through things like deforestation or agriculture,

the Industrial Revolution starting around 1750 marked a turning point.

There's a fossil fuels.

Precisely.

Burning coal, then oil and natural gas released and continues to release enormous quantities of carbon dioxide, CO2, that have been locked away underground for millions of years.

Deforestation adds to this, too, because trees store carbon, and burning or decay releases it.

And we can measure this increase?

Oh, absolutely.

Since 1958,

continuous measurements at Mauna Loa, Hawaii, the famous Keeling Curve, show this undeniable steady rise in atmospheric CO2, superimposed on a seasonal wiggle caused by plant's breathing.

How does today's level compare to the past?

That's where the ice cores come back in.

Those trapped air bubbles show us that for at least the last 800 ,000 years, CO2 levels naturally cycled between about 180 and 280 parts per million.

Today, we're over 400 ppm.

Wow.

Significantly higher than the natural peaks.

Significantly.

About 40 -50 % higher than pre -industrial levels.

And the rate of increase is unprecedented in the record.

Okay.

So CO2 is rising rapidly.

What's the atmosphere's response?

The clearest response is warming.

Global average surface temperatures have risen by about 0 .8 degrees Celsius or 1 .4 degrees Fahrenheit over the past century or so.

And most of that warming is recent.

Yes, about two -thirds of it, roughly 0 .6 degrees Celsius, has occurred since the mid -1970s.

The scientific consensus reflected by the IPCC is that this warming is unequivocal.

And it's extremely likely, meaning 95 -100 % probability that human emissions of greenhouse gases are the main driver.

And the warmest years on record.

Almost all have occurred since the year 2000.

Each decade tends to be warmer than the last.

Climate models project further warming, potentially 2 to 4 .5 degrees Celsius, if CO2 doubles from pre -industrial levels.

It's not just CO2, though, right?

There are other greenhouse gases humans release.

Correct.

Methane, CH4, is another potent one, about 20 times more effective per molecule at trapping heat than CO2 over shorter time scales.

Its levels have also risen dramatically due to agriculture, things like rice patties and livestock digestion and fossil fuel extraction.

Then there's nitrous oxide, N2O, largely from agricultural fertilizers and industrial processes.

And fluorofluorocarbons, CFCs, those are the chemicals damaging the ozone layer.

But they're also very powerful greenhouse gases, although their use is now heavily restricted.

Okay, so more greenhouse gases mean more warming.

But do humans release anything that might cool things down?

Yes, we also release aerosols.

These are tiny particles suspended in the air, think sulfate particles from burning coal or soot from fires.

And these have a cooling effect?

Generally, yes.

Most aerosols, especially sulfates, tend to reflect sunlight back to space directly.

They can also make clouds brighter and more reflective, so they have a net cooling influence that partially offsets some of the greenhouse warming.

Black carbon, or soot, is an exception.

It absorbs heat and can warm the atmosphere, especially if it lands on snow or ice.

But aerosols don't stick around like CO2 does.

Exactly.

Aerosols typically stay in the lower atmosphere for only days or weeks before being washed out by rain.

CO2 persists for centuries.

So the aerosol cooling effect is more localized and temporary, while the CO2 warming effect is global and long -lasting.

The precise magnitude of the aerosol cooling effect is still one of the larger uncertainties in climate science, though.

This all sounds incredibly complex, especially with things that warm and things that cool interacting.

And then there are feedback loops.

Right.

Feedbacks are crucial.

They can either amplify or dampen the initial warming.

A positive feedback reinforces the change.

Like the melting ice example?

Yes.

The ice albedo feedback is a classic positive feedback.

Warmer temperatures melt ice.

Less ice means less reflection and more absorption of solar energy by the darker ocean or land surface.

This leads to more warming, which melts more ice, and so on.

Uh oh.

What's another major positive feedback?

Water vapor feedback?

Warmer air can hold more moisture.

So as the planet warms, evaporation increases, putting more water vapor into the atmosphere.

Since water vapor itself is a powerful greenhouse gas, this amplifies the initial warming caused by CO2.

So warming leads to more water vapor, which leads to more warming.

Exactly.

Another worrying one involves thawing the Arctic.

As the frozen ground thaws,

organic matter tracked inside starts to decompose, releasing CO2 and methane, which are greenhouse gases, leading to further warming.

Another positive feedback.

Are there any negative feedbacks, things that might counteract the warming?

That's the big question, especially regarding clouds.

Clouds are complicated.

They reflect sunlight, which cools the planet, but they also trap outgoing heat, which warms it.

So which effect wins?

It depends on the type of cloud, its altitude, time of day.

The overall net effect of clouds is still uncertain, but most evidence suggests it's likely a small positive feedback, meaning they probably amplify warming slightly overall, rather than counteracting it significantly.

But it's an area of active research.

Okay, so given all this complexity, greenhouse gases, aerosols, feedbacks, how do scientists project future climate?

They use sophisticated computer models, global climate models, or GCMs.

These are based on the fundamental physics and chemistry of the atmosphere and oceans.

They simulate the interactions between the different spheres.

But they must involve assumptions, right?

Of course.

They're simplifications of the real earth system.

And crucially, they require assumptions about future human activities, how much greenhouse gas will emit, land use changes, etc.

That's why projections usually come as a range of possibilities under different emission scenarios.

So what are the main projected consequences if warming continues as expected?

The general picture includes things we're already starting to see intensify.

Warmer average temperatures globally, more frequent and intense heat waves, changes in precipitation patterns, some areas getting wetter, others drier.

Continued shrinking of glaciers and ice sheets.

And sea level rise?

Yes, sea level rise is a major consequence.

It's driven by two main factors.

Thermal expansion, warmer water simply takes up more space, and the melting of land -based ice, meaning glaciers and the huge ice sheets on Greenland and Antarctica.

How much has it risen already?

Globally, average sea level has risen about 25 centimeters, or nearly 10 inches, since 1870.

And the rate has accelerated in recent decades, now running at over 3 millimeters per year.

Doesn't sound like much, but I guess it adds up.

It really does, especially for low -lying coastal areas.

Even seemingly small rises dramatically increase the frequency and severity of coastal flooding during storms, cause erosion, and can contaminate freshwater sources with saltwater.

Places like Bangladesh, the Maldives, and parts of the U .S.

Gulf and Atlantic coast are particularly vulnerable.

You mentioned the Arctic is changing rapidly.

What's happening there?

The Arctic is warming much faster than the global average, a phenomenon called Arctic amplification.

The most visible sign is the dramatic decline in Arctic sea ice extent and thickness, especially in summer.

We're on track for potentially ice -free Arctic summers, perhaps as early as the 2030s.

And that feeds back into more warming.

Exactly, the ice albedo feedback.

Plus, the permafrost thaw we talked about is widespread, causing infrastructure damage, altering ecosystems, and releasing greenhouse gases.

We're even seeing shifts in vegetation, with shrubs moving into former tundra areas.

There's one more consequence you mentioned earlier, ocean acidification.

How does that fit in?

Right.

The oceans have absorbed about a third of the CO2 humans have emitted.

When CO2 dissolves in seawater, it forms carbonic acid.

This process is lowering the pH of the ocean surface waters, making them more acidic.

And that's bad for marine life.

It's particularly bad for organisms that build shells or skeletons out of calcium carbonate, like corals, oysters, clams, and many types of plankton which form the base of the marine food web.

Increased acidity makes it harder for them to build and maintain their structures.

It's a serious threat to marine ecosystems globally.

So warming, sea level rise, Arctic changes, ocean acidification, it's a lot.

Is there also a risk of just sudden changes?

Surprises?

That's a real concern.

Earth's climate system is highly complex and non -linear.

It means we could potentially cross tipping points, leading to abrupt and possibly irreversible changes that happen much faster than the gradual warming trend.

Like what kind of changes?

Things like a rapid collapse of the Greenland or West Antarctic ice sheets, which would cause much faster sea level rise, or major shifts in ocean circulation patterns, like the Atlantic meridional overturning circulation,

or widespread forest dieback.

The paleo climate record shows that abrupt climate shifts have happened naturally in the past.

And we might trigger one.

The further and faster we push the climate system away from its historical state, the greater the risk of triggering such surprises, while predicting them precisely as difficult.

The potential consequences could be enormous, far exceeding the impacts of gradual change.

Okay, let's try to wrap this up.

We've covered a huge amount of ground from Earth's basic climate system, these interconnected spheres.

The ingenious ways scientists read Earth's deep climate history, using proxies like ice cores, tree rings, and ocean sediments.

We looked at the natural forces, plate tectonics, orbital cycles,

volcanoes that have always driven climate change over geological time.

And then we focused on the modern era, the unprecedented rise in greenhouse gases, mainly CO2 from fossil fuels, and the resulting observed warming, especially in recent decades.

Plus the consequences, the melting ice, rising seas, the changing Arctic, ocean acidification, and those potentially dangerous feedback loops and tipping points.

It really highlights how interconnected everything is from the deep Earth to the atmosphere and oceans, and now increasingly human activities.

It leaves us with a big question, doesn't it?

Thinking about Earth's long history of climate swings between ha houses and ice ages, how is this current human -driven change fundamentally different?

And what does that mean for us?

That's definitely the provocative thought to ponder.

What are our unique challenges and responsibilities in the face of changes driven not just by nature, but primarily by ourselves and happening at such a rapid pace?

Thank you for joining us on this deep dive into the geology of global climate change.

We really hope this has given you some valuable insights and maybe sparked some further curiosity.