[Narrator] Earth is warming, caused by the burning of fossil fuels like oil, coal, and natural gas, filling the atmosphere
with heat-trapping gases like carbon dioxide,
methane, nitrous oxide at levels humans
have never seen before. As the world debates how much
more warming the planet can take— one and a half degrees Celsius? Two degrees Celsius?— the climate crisis escalates. [Woodwell] The problems are that the world
is becoming too hot for the present distribution of people, agriculture, human welfare,
and human interests, and it's getting worse. [Narrator] But it's more than our emissions
heating the globe. Something else is at work here. The rising temperatures
are setting in motion Earth's own natural
warming mechanisms that then feed upon themselves. George Woodwell,
a distinguished scientist and a lion of
the environmental movement, has been sounding the alarm
about them for the past 50 years. In a 1989 Scientific American article, he wrote that warming
caused by human activity, "rapid now, may become
even more rapid as a result of the warming itself." Thirty years later, climate activist Greta Thunberg
repeated his warning, calling them "irreversible
chain reactions." [Thunberg] The popular idea
of cutting our emissions in half in ten years only gives us a 50% chance of staying below 1.5 degrees and the risk of setting off
irreversible chain reactions beyond human control. 50% may be acceptable to you, but those numbers do not
include tipping points, most feedback loops, additional warming hidden
by toxic air pollution, or the aspects of equity
and climate justice. [Narrator] So what exactly are
irreversible chain reactions, what scientists refer to
as feedback loops? [Emanuel] A feedback that
everybody is familiar with is an audio feedback, where if you put a microphone
too close to a speaker, you get this terrible
high-pitched screaming. And that happens because
the sound comes out of the speaker and it goes back into the microphone. That's called a positive feedback because it amplifies the loop. [Narrator] Instead of the guitar, emissions from
fossil fuels are the input which add heat-trapping
gases to the atmosphere, raising Earth's temperature,
and setting in motion self-perpetuating warming loops. Warming as a result
of the warming itself. That ever-growing screeching noise is an apt analogy
for the damage that human-caused
feedback loops are wreaking on the planet. Scientists have identified
dozens of feedback loops already in motion. It's imperative that
we understand them if we're going to solve
the climate crisis. As the climate warms, forests, once removers of carbon, release it back into the atmosphere as carbon dioxide, or CO2. Frozen ground in
the Northern Hemisphere thaws and emits CO2 and methane. These are the kinds of feedback loops
that lead to further warming, triggering the release of
even more heat-trapping gases, and raising the temperature
even higher. In this series, we will highlight
four of the major feedback loops impacting climate: the melting of sea ice
in the Arctic Ocean, increased drought and fires
in the world's forests, the decay of organic matter
from permafrost thaw in the Northern Hemisphere, and disruptions to the jet stream
and our global weather. Each amplifies warming and, combined, they are
spinning out of control. If we take action now, we could slow, halt,
or even reverse them before it's too late. If we don't, the planet
will reach a tipping point, where we will lose
the world as we know it. And what does that
tipping point look like? [Francis] A simple example of that would be if you push a ball up a hill, at some point you're going
to get to the top of the hill, and it's going to roll
down the other side. There are some things
like that in the climate system where it's really hard to get it back once it's rolled down to that other side
and now it's sitting in a new valley. [Narrator] With more than
7.5 billion people on Earth, a change this great
would spell catastrophe. Humans are well-suited
to the world we've known. The sun's radiation passes
through atmospheric gases, primarily oxygen and nitrogen,
to the Earth's surface. Some radiation is
absorbed by the planet, and the rest would
bounce back to space if it weren't for a tiny
percentage of gases, like carbon dioxide, methane,
water vapor, and others, that trap heat
and warm the atmosphere. These heat-trapping
greenhouse gases make up less than 1%
of the total atmosphere, but they are essential
in regulating the temperature. Without them, we would freeze, but too much of them
would overheat the planet. Both are possible
and have happened in Earth's geologic past. [Emanuel] So, the kinds of things that keep
climate scientists awake at night stem from their familiarity
with the geological record that shows that the climate
is capable of very abrupt changes. There are feedbacks
operating in the climate system that may temporarily
go out of control, if you will, and drive the climate
to a different state. We cannot rule out that we may be in for
such an abrupt change. So, we don't fully understand them, but we worry about that--a lot. [Narrator] These radical changes include
extreme temperature swings. During periods known as
Snowball Earth, it got so cold the planet was covered
completely in snow and ice. These alternated with
hothouse climates, when virtually all the ice melted. Dinosaurs lived at the poles where forests
and swamps flourished. These abrupt shifts were caused
by complex global processes. This is the first time that humans have been responsible
for an abrupt change. Today, Earth would
naturally be in a cooling trend. But because of human activity, it's not. [Emanuel] We know from paleoproxy records that the Earth has been cooling
for about 7,000 years. We recovered from the peak
of the last ice age, which was about 22,000 years ago,
it went up, and it's been slowly cooling, until about the time of
the Industrial Revolution. [Narrator] At that time, humans began emitting vast amounts of carbon dioxide. Since then, the atmospheric
content of CO2 has gone from 280 parts per million to over 400 today, and could approach 800 by the end of the century. Of all the carbon dioxide
humans emit each year, oceans absorb about one-quarter, plants take up another quarter, and the other half
stays in the atmosphere, accumulating over time
and raising Earth's temperature. But the percentage nature removes
is shrinking every year, as forests are destroyed
and oceans warm. Scientists estimate that doubling CO2 from pre-industrial levels could
produce an increase in temperature of up to eight degrees Fahrenheit, resulting in the deaths
of millions of people and the loss of countless species. It's this human-caused warming that's kicking off Earth's
natural feedback loops and heating up the planet further. National Medal of Science recipient
Warren Washington, a groundbreaking climate pioneer, began creating computer models
in the 1960s to predict the future
of atmospheric warming and the role feedback loops play. [Washington] The question always was, how do we see
the feedback mechanisms working realistically? We really don't know if
we've got the right amount of feedback in our models. Because they're so complicated
in many cases, we had to do a lot of experimenting. [Narrator] That experimenting,
coupled with observations, paid off. Because of that groundwork, today's models have
more accurately predicted what we're seeing today
and what our future holds. This kind of research is crucial
to understanding the policies we need to implement to avoid disaster going forward. [Duffy] Climate policy
really should be designed to avoid crossing
the important thresholds, avoid crossing
the important tipping points, and avoid setting off
these important feedbacks because we need to
essentially ask the question, you know, how much warming is okay? Is two degrees okay?
Is one and a half degrees okay? We really don't know
the answer to that very well. [Narrator] And yet, we continue to add
more heat-trapping gases to the atmosphere, setting off irreversible chain reactions. [Coe] Where we are now is like
driving in a car in a dense fog, and you know there's
a cliff out there somewhere, but you don't know where. Do you want to be going
60 miles per hour, or should you be going
about 10 miles an hour? [Narrator] Today we have a choice. If we take our foot
off the accelerator, stop deforestation
and regreen the Earth, we can reverse the feedback loops and begin to cool the planet. [Woodwell] And the solution is
to make that transition away from fossil fuels and into a new green Earth, and it really does require
a green Earth. [Emanuel] I'm encouraged by the fact that other countries have
decarbonized their electricity sector in 10 to 12 years. So, I know we can do it. And those countries that did it,
by the way, grew their economies rapidly while doing it. But we have to
put the incentives in place. [Narrator] Even with meaningful incentives,
it's going to take a while for the climate to recover. [Emanuel] Even if we stopped emitting
carbon right now, it will take thousands of years
for the system naturally to go back to what it was
before we started messing around with it. If we keep going industrially
at the same rate we are now, we will have succeeded
in roughly tripling the carbon dioxide content
of the atmosphere over its pre-industrial volume
by the end of the century. [Narrator] We have the technology and knowledge to solve the problem, to stop and reverse
the feedback loops, but we need leaders
who understand the urgency of getting it done, and an energized public
to advocate for change. It's not going to be easy. We've let the problem
go on too long. [Francis] That isn't to say, though, that we shouldn't do
everything we can possibly do to slow down our emissions
because it will make it that much better for our kids
and grandkids in the future. The problems won't be
quite as big. The warming won't be
quite as strong. [Emanuel] The Earth will be just fine. We may take along with us, unfortunately, lots of different species, but there have been catastrophic
extinctions of species in the past. I'm not worried about the planet,
I'm worried about us. We, the people. Thank you.
