A couple years ago, my friend sent me this photo from Ürümqi,

which is the capital of Xinjiang province in northwest China.

On this particular day, she couldn't believe her eyes.

Checking the quality of the air outside using this app on her iPad,

the numbers were telling her the air quality was good,

one on a scale of 500.

But when she looked outside, she saw something much different.

Yes, those are buildings in the background.

(Laughter)

But the data were simply not telling the truth

of what people were seeing and breathing,

and it's because they were failing to measure PM2.5,

or fine particulate pollution.

When PM2.5 levels went off the charts in 2012,

or "crazy bad," as the US Embassy once described it in a tweet,

Chinese denizens took to social media

and they started to question why it was that they were seeing this disconnect

between official air quality statistics

and what they were seeing and breathing for themselves.

Now, this questioning has led

to an environmental awakening of sorts in China,

forcing China's government to tackle its pollution problems.

Now China has the opportunity to become a global environmental leader.

But the picture that I'll paint for you today

is one that's mixed.

There are some signs that are very promising,

and there are other trends that are more troubling

that warrant closer attention.

But now let's go back to the story at hand.

I started to witness the beginnings of China's green evolution

when I was a PhD student conducting fieldwork in China in 2011.

I traveled all across the country seeking answers to the question

that I often got myself from the skeptical outsider:

What, you mean China is doing something on the environment?

They have environmental policies?

What policies?

At that time, PM2.5 data was considered too politically sensitive

and so the government was keeping it secret,

but citizens were becoming aware of its harmful human health effects,

and they were demanding greater transparency

on the part of the government.

I actually started to see some of this growing evolution and awareness myself

cropping up all over China.

Department stores, for example, started to market these air purifiers

that could filter out harmful PM2.5.

Citizens were also adopting PM2.5 as the title of musical festivals.

(Laughter)

And then I went to a golf course in Shenzhen, which is in southern China,

and you can see from this banner, they're advertising a retreat from PM2.5.

Golf sub-par, but don't breathe sub-par air.

And then Shanghai's Environmental Protection Bureau

decided to create a mascot named after the air quality index

to better communicate the air quality data to its people.

I call her AQI Girl,

and her expression and hair color changes depending on the quality of air outside.

Five years later and she's still the mostly smiling face

of Shanghai's air quality.

And then in 2015,

former CCTV reporter Chai Jing

created this documentary called "Under the Dome."

It would be likened to Rachel Carson's "Silent Spring."

And much like Rachel Carson

brought to attention the fact that pesticides were harming human health,

"Under the Dome" stamped into the popular consciousness

that air pollution was leading to one million premature deaths

every year in China alone.

This video garnered

more than a hundred million views in a single weekend

before China's government,

fearing that it might incite some type of social unrest,

pulled it from the internet.

But the damage had already been done.

Public outcry over air pollution

galvanized China's government,

perhaps in an act of self-preservation,

to think big and decisively about how it could tackle

the root of its air pollution

and many of its other environmental problems:

its energy system.

For you see, in China,

about two thirds of its electricity comes from coal.

China has more coal-fired power plants than any other country in the world,

about 40 percent of the global total,

and it's because of this fact that China's government

has decided since 2014 to wage a war on coal,

shutting down small coal mines,

setting limits on coal consumption,

even canceling an Australia's worth of coal-fired power plants.

They've also been making enormous investments

when it comes to clean and renewable energy,

like hydropower, wind and solar,

and the pace and the scale of this transformation

has been absolutely mind-blowing.

Let me give you a couple of statistics to show you what I mean.

China leads the world when it comes to hydropower,

with a third of total capacity.

There's enough for every Chinese citizen

to power two homes in a single year from hydropower alone.

You may have heard of the Three Gorges Dam,

pictured here,

which is the largest power station in the world,

and it's powered by water.

In terms of wind power,

China has a third of the global capacity.

This makes it the number one leader by far.

When we look at solar, China's also leading.

In fact, they crushed their 2020 target

of installing 105 gigawatts of solar power.

This is after the government already revised upwards

several times its solar energy target

between 2009 and 2015.

Last year, in seven months alone,

China was able to install a whopping 35 gigawatts of solar power.

This is more than half of what the US has combined in total

and China did this in just seven months alone.

We can verify this remarkable growth in solar power from space,

like the startup SpaceKnow has done in this slide.

By 2020, China is on track to generate Germany's entire electricity consumption

from just wind and solar power alone.

It's pretty darn remarkable.

And we see some evidence now

that China's efforts on clean energy

is actually having an effect,

not just on air pollution reduction,

but also on global climate change,

where China has the world's largest carbon footprint.

If we look at some of the data, we can see that China's coal consumption

may have already reached a peak as early as 2013.

This is a major reason why China's government announced

that actually they've already achieved their 2020 carbon reduction pledge

ahead of schedule.

This reduction in coal consumption

is also directly driving improvements in air quality

across the country,

as I've shown here in blue.

In most major Chinese cities,

air pollution has fallen by as much as 30 percent.

And this reduction in air pollution is actually leading people

to live longer lives in China,

on average two and a half years more than they would have in 2013.

In yellow, we can see the cities that have experienced

the greatest improvements in air quality.

But of course, as I mentioned at the beginning of this talk,

we have to temper some of this optimism

with a healthy dose of caution,

and that's largely because the data are still being determined.

At the end of last year,

after roughly three years of pretty steady global carbon emissions,

scientific projections suggest

that global emissions may be on the rise again

and that could be due to increases in China's fossil fuel consumptions,

so they may not have reached that peak that I showed earlier.

But of course, the statistics and the data are still murky

and that's because China regularly revises its coal statistics after the fact.

Actually, it's funny,

since I've been here I've been having a debate on Twitter

with other climate modelers,

trying to figure out whether China's carbon emissions

have gone up, gone down or whether they're staying relatively stable.

And of course, China is still a rapidly developing country.

It's still experimenting with a range of policies,

like dockless bike sharing,

which has been hailed as a possible sustainable transport solution.

But then we have images of this bicycle graveyard

that tell a more cautionary tale.

Sometimes, solutions can move too fast

and outpace demand.

And of course, coal is still king in China,

at least for now.

So why should we care about what China is doing on the environment?

Well, what China does at home on the environment

can have global implications for the rest of us.

To borrow a line from Chai Jing,

we're all under the same dome,