Let's not beat around the rapidly melting iceberg here: climate change is happening,

and we're causing it. The evidence is overwhelming.

Scientists usually reserve this level of agreement for claims like “Earth is a planet” and

“air is real” yet here we are, the climate change ship has now left the dock, and lots

of people on shore are still debating whether boats can actually float.

BUT, maybe you're a person who trusts and accepts what climate scientists are telling

us, it's just sometimes it's hard to explain why. I mean we've all been there…

I mean I care about the environment. I figure with the polar bears and everything we might

as well try electric cars. What do we have to lose?

And then they go CAPS LOCK SERIOUS saying they have proof that climate change is a hoax

perpetuated by scientists paid off by the Polar Bear Lobby as part of a plan to install

Al Gore as supreme world polar bear emperor. WAKE UP SHEEPLE!

To keep that from happening, we put together this handy reference.

The sun is the source of warmth on Earth, so thanks for that, Sun. Ice and clouds reflect

some of its light away, and the rest is absorbed by land and water and re-emitted as heat.

Some heat escapes to space, and some is held in by the atmospheric greenhouse effect.

The insulating effect of Earth's greenhouse gases are the reason that life exists as we

know it, but human activities have increased the concentration of one of em, carbon dioxide,

40% since the Industrial Revolution.

We know the sun's output has varied during history, but since 1970s, the period when

global temperatures increased in the fastest, temperature and solar activity have moved

in opposite directions.

If the sun was to blame, it would cook the upper and lower layers of atmosphere together.

Instead, we only see warming in the lower layers, the same place that human greenhouse

gases like carbon dioxide are piling up.

Since 1870, with fossil fuels, cement production, and land use combined, humans have put about

2,000 gigatons of CO2 into the atmosphere, that's two million million tons, and about

40% has stayed there.

Studying gases trapped in ice cores have let us see what Earth's atmosphere was like

in the past. At more than 400 parts per million, today's CO2 levels are the highest they've

been for almost a million years. That's before humans even existed, totally uncharted

territory for us.

More carbon dioxide in atmosphere means average temperatures across the globe are increasing,

and fast. Right now, Earth is warming about ten times faster than at the end of an ice age

Ok, so CO2 is increasing. How do we know it's our fault? The best evidence comes from looking

at what isotopes, or different kinds of carbon, are in the atmosphere.

Fossil fuels come mainly from old plants. Plants prefer to use the lighter isotope carbon-12

over the heavier carbon-13, so they contain a higher ratio of 12 to 13 than the atmosphere

does.

When more fossil fuels get burned, the percentage of carbon-12 in the atmosphere should go up,

and that's exactly what we see.

And it's not because of volcanic activity. Volcanoes only emit about 1% as much CO2 as

we do. Normally that CO2 is balanced and exchanged

between the atmosphere, plants, and animals, but eliminating carbon sinks has released

centuries worth in just a few years.

Other greenhouse gases are also increasing, like methane from farm animals and natural

gas processing, or nitrous oxide from fertilizers.

If we run simulations just using natural causes of climate change, they predict no change,

or even cooling in 20th century and that is not what's happening.

[OMINOUS MUSIC] It's still gonna get cold in some places,

but in 2000s there were twice as many record highs as record lows. Each of the past three

decades has been warmer than any other decade since we started measuring in 1850.

Since 1900, actual temperatures around the world increased almost a full degree, and

most of that has happened since the 1970's.

Looking at data from tree rings and ice cores, the past 30 years is probably the warmest

in eight centuries. Of course, not every place on Earth warms

equally.

Oceans cover more than 70% of Earth, and they absorb more than 90% of the heat added to

the planet. Naturally, that's where we see most extreme changes.

Around the world, oceans are rising a tenth of inch per year, and they're up 8 inches

since 1901. This is because water expands as it warms,

and when ice sheets and glaciers melt in Greenland and Antarctica, water that's normally on

frozen land gets put in the ocean.

The oceans are Earth's largest carbon sink. As more CO2 enters atmosphere, more of it

dissolves in the ocean, which makes the water more acidic.

This doesn't mean that the oceans will be made of acid, but animals with calcium shells

are super-sensitive to pH. We're on course for the oceans to hit pH 7.8 in 100 years,

which could wipe out one-third of species in the ocean.

We also know that levels of summer sea ice in the Arctic have decreased 40% since 1978,

they might be the lowest levels in 1400 years.

That white sea ice usually reflects the sun's energy back into atmosphere, but the dark

ocean is soaking it up like a black shirt on a sunny day, which feeds the cycle forward.

If CO2 emissions continue on their current trends, Earth is on course to be 2.5-5 degrees

warmer, and the oceans could be up to a meter higher, by the end of this century

Is that a big deal? Yeah! It's the biggest deal. This is by far the greatest issue facing

our species. The last time the Earth averaged a few degrees colder, most of North America

was covered in a mile-thick sheet of ice. That many degrees warmer? We're gonna have

a bad time.

So now you're armed with the facts. Why we know climate change is happening, and why

we're causing it. Please, share this information with the people you know, and let me know:

Did it change any minds? Did it change your mind?

I mean, are facts enough? If not, then why do so many people continue

to NOT believe in climate science? We'll answer that question in our next video.

Stay curious.