- Earth's temperature has
fluctuated wildly over its life. Maybe more importantly, during
the past 500 million years, it hit peaks of around 17
degrees Celsius hotter than pre-industrial temperatures. And even when dinosaurs roamed, it was much hotter than today. So what's the big deal about one or two degrees of warming today? Well, looking back into our
climate record provides some fascinating lessons and a
pretty dire warning about our current situation. But as it turns out, there is not much new about
the method of climate change. - Greenhouse gases and carbon dioxide in particular
is sort of the primary driving force over all of earth history. We are sort of at the precipice right now, and if we do not take dramatic action within
the next several years, then we will miss that
opportunity to stabilize warming below truly catastrophic levels. Holes. - In this episode, I'm going to walk you through our planet's climate history. Michael Mann just wrote a
book on this very topic called Our Fragile Moment. So we called him up to help us understand
our current situation. But before we start, let's
take a look at this chart of the geologic timescale so
you can get your bearings. As you can see, earth was formed around 4.6 billion years ago, but today we're gonna
start our discussion. When Earth was an ice house or snowball earth, if you will, about two and a half billion years ago. - Up until that point, the atmosphere had been
relatively oxygen free. The earlier earth atmosphere had very large amounts of methane. That's part of how the
planet was as warm as it was despite the fact that the
sun was 30% less bright. There was an innovation,
a biological innovation. Cyanobacteria developed a new
pathway for photosynthesis, a pathway that actually released oxygen. So these bacteria scavenged
all the methane, you know, lowering the greenhouse effect, cooling off the planet, forming ice. The ice reflects sunlight back to space so you get more cooling. And we got a runaway ice
house where Earth got trapped in this frozen state. Earth was literally a giant snowball. There was ice all the way down at the equator at sea
level covering the oceans. - Many scientists believe
that CO2 from volcanoes and other processes is what finally thawed the planet
300 million years later. But how did life survive?
Such extreme conditions? - We think it survived in
hydrothermal vents in the deep ocean, and perhaps shallow pools of melt water at the very surface of the frozen tropical oceans. - Fast forward a couple billion years, the dinosaurs have come and gone and it starts to get very hot. - So one of the sort of defining episodes in in
climate history is the so-called Paleocene Eocene Thermal Maximum,
an episode of rapid warming that came about 10 million years after the demise of the dinosaurs. The asteroid that struck the
planet was already fairly warm, and then it warmed even
more fairly rapidly. - Average temperatures
rose by as much as five to eight degrees Celsius, reaching as high as 22.8 degrees Celsius,
or 73 degrees Fahrenheit. And again, the reason for this change in temperature is important. - 90% of it was a rapid release of carbon dioxide in this case because of an unusually active episode of vulcanism in the North
Atlantic near modern day Iceland that put a lot of carbon dioxide
into the atmosphere over a relatively short period of time. And short period of time in
this case is like 10,000 years, not nearly as short as today, but what that episode tells
us is the warming was caused by the carbon dioxide, and
that's what's happening today. The difference is we can stop it. - Carbon dioxide is like
the Earth's thermostat. Changes in this key greenhouse
gas have been associated with almost every major temperature fluctuation In Earth's history. Methane plays an important but lesser role as well as aerosols from volcanoes and asteroids. But carbon dioxide is really
the driving force here. Now, let's zoom in on
the last 800,000 years. When we do that, we see this
fascinating saw tooth pattern where earth enters ice
ages, then the ice retreats and the earth warms. Amazingly, our hominid ancestors live through these giant saw tooth swings of temperatures in
relatively low populations, and looking at a comparison
of temperature in CO2, the relationship is striking. But about 10,000 years ago, something strange happened instead of slowly descending back into an ice age as one would expect we didn't. So for the last 10,000
years, humans experienced an unprecedented, warm, stable
period of climate history. And I know you might be wondering why. Well, there isn't a
scientific consensus on it, but one theory suggests that it was actually human agriculture and land use that decreased tree cover and increased atmospheric CO2, just enough to keep temperatures constant
when they might have dipped back down to ice H levels. During that time, the
population exploded from as few as 1 million to 8 billion people. - We built our entire infrastructure that now serves the needs of
more than 8 billion people. That infrastructure was
built is tied to a climate that existed for thousands of years, but is now changing very rapidly. And so that's the fragile moment. We probably leveraged the
carrying capacity of the planet by a factor of 10 through our technology. And if that infrastructure
collapses, we lose our ability to maintain the resources necessary for this global population
that exists today. - But we know the Earth's climate has made wild swings before. So what's different today? During the time of the dinosaurs, carbon dioxide levels were
much higher than today and the world was much warmer. But then nature buried that carbon in dead
plant matter in animals. And over time that dead animal and plant matter turned into
coal, natural gas and oil. That's why they're called fossil fuels. And now we're taking
all of that dead stuff that accumulated over a
period of 100 million years and we're burning it rapidly. And remember earlier
Michael Mann mentioned that the rapid warming of the Paleocene Eocene Thermal Maximum happened over a period of 10,000 years. We are currently burning
an exorbitant amount of carbon in just a couple of centuries. - The rapid warming of the
past century is unprecedented. There's nothing like it
in at least 2000 years. We know we're responsible
for that warming spike. - When Michael Mann and his
colleagues first published this, so-called Hockey Stick Graph
in 1999, it received a lot of backlash, but dozens of independent studies
verified the results. We are truly warming the climate
at an unprecedented rate. Just for an example, the
warming that happened after the last ice age
18 to 12,000 years ago, that was a hundred times slower than the warming we're seeing today. But perhaps more importantly,
there are a lot of us now and everything we've built is dependent upon a stable climate. - Now, the worst case
scenario where we fail to act, we continue to warm the planet
by, you know, seven, eight, or nine degrees Fahrenheit
by the end of the century. That's where we see
massive sea level rise, where we see hundred million or more people displaced
by global sea level rise - Based on current energy
and emissions projections. Eight or nine degrees of warming
is at the very high range of what's possible. But I think it's important
to look at sea level because it's the primary impact of climate change we're experiencing today that we can actually
measure from the past. And it also shows just how
long it takes for the effects of excess heat energy to
play out on earth's systems. The last 2,500 years of sea level mirrored the
earth's stable temperature with relatively minor ups and downs. And if you look at sea level
rise over a longer period, you can see that it follows the carbon and temperature graft closely. But let's zoom in. At the end of the last ice
age, around 20,000 years ago as the earth warmed sea
level rose around 130 meters. But check this out, there's a lag even after warming leveled off,
sea level rise continued, and that's important
for future projections. - Consequences are certainly dire, but we can prevent additional
warming of the planet and all of the extreme weather events and everything else that goes
with that from getting worse. If we solve this problem, if
we attack it at its source, geoengineering carbon
capture, no, we just need to stop burning fossil fuels. - Well, you heard him
according to Michael Mann. It comes down to us humans
cause this rapid warming, which means at least in
theory, we can stop it. And that's not something that only Michael Mann believes
human cause climate change is pretty much settled science. But regardless of that fact, and I'm speaking to those who
still may be a bit skeptical of all this, those who say that the climate has always changed. So why should we be concerned now? Perhaps that is in fact
why we should be concerned. Change is happening. Change
is literally constant. It happened before human civilization. It's happening now and it
will continue to happen. But as I digress, this
current rate of change is unprecedented and it's almost certainly
faster than 8 billion humans can adapt to. So maybe the question should be this, can humans adopt a nimble
mindset on a large scale to halt human caused warming? And can we adapt to the
change that's already baked in to continue to thrive on this planet? Let us know what you think in the comments below before you go. We want to hear your feedback every year. PBS Digital Studio surveys our audience. It helps us understand
what you enjoy on YouTube and what you want to see more of. You also get to vote on new show ideas. So it'd be great if
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