English subtitles for clip: File:Climate change is simple- David Roberts at TEDxTheEvergreenStateCollege.webm

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Hi, this talk started out of a Twitter conversation.

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I haven't decided whether to be embarrassed

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about that or not.

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But I was on Twitter one day and

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a relatively prominent left of centre pundit,

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piped up and said

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"You know, climate change seems like a really big deal,

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why are so few people talking about it?

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Why have so few thought leaders made it

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their signature issue?"

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And another reasonably prominent

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left of centre pundit piped up

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and said "Well, for my part,

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the reason I don't talk about it is it seems really complicated,

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I don't feel like I have a good grasp on all the science

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and so I just don't feel qualified to go out

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and assert things publicly about it."

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You know, anybody who has ever so much as mentioned

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climate change on television or on the internet

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will understand why this person thinks the way they do.

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Any time you mention it,

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the hordes descend, bearing complicated stories

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about the medieval ice age, or sunspots, or water vapour,

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and, you know, there is a lot of myths about climate change

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borne by these climate sceptics

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but to debunk those myths you have to know, you know,

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you have to go online, and research, and read,

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and be able to respond to them in detail,

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and a lot of people just find that prospect dreary,

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and so they don't bother.

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And this, of course, drives me crazy,

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so I piped up on Twitter and said

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"You know, climate change is not actually that complicated.

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What you need to know to be able

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to speak out publicly about it,

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just about the basic structure of the problem,

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is really not that complicated,

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I could explain it to you in 15 minutes" so,

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let this be a lesson to you:

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don't go talk smack on Twitter,

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unless you are willing to back it up.

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So, one thing led to another, and here I am

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with 15 minutes to explain climate change to you.

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So, let's get started.

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Why is the Earth not a cold dead rock floating in space?

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The reason is that it is enveloped by this tiny,

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tiny thin layer of gases and chemicals

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that we call our atmosphere.

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So, the Sun's energy,

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rather than just coming down and bouncing right back off,

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it comes down and is held

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close to the surface of the Earth for a while

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and then bounces off,

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and then this simple process is why we have evaporation,

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and precipitation, and photosynthesis,

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and life on our planet.

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So, scientists discovered, well over a hundred years ago,

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that the atmosphere and the systems on Earth

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are in this dynamic relationship

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and you can change the chemical composition of the atmosphere

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and hold more of the Sun's energy for longer.

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The energy still has to escape, of course,

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but in the meantime it will cause changes

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in these biophysical systems of the Earth.

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And, you know, you often hear people say,

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"The Earth has always changed,

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the climate has always changed",

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and that's true, it has.

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This relationship between the atmosphere and the systems,

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they go through cycles,

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but these cycles have typically taken

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hundreds of thousand of years,

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millions of years.

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The key thing to know first is that

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for the last 10,000 years on Earth,

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the climate has been relatively stable, unusually stable,

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and by stable I mean temperature has varied,

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it's gone up and down,

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but it's stayed on a fairly narrow band

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of about plus or minus 1 degree Celsius,

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and all of advanced human civilisation

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has taken place during these 10,000 years,

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the development of agriculture, the written word,

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the wheel, the iPhone, everything we know,

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everything we have built,

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we have done in this period of relative climate stability.

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So, what we have been doing

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for the last couple of hundred years

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is digging up carbon out of the earth,

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and throwing it up into the atmosphere,

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and changing the chemical composition of the atmosphere,

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like has happened in the past

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except for extremely faster.

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In geological time, the blink of an eye,

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we are substantially changing

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the chemical composition of the atmosphere

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and all of climate science has been about,

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"What's going to happen?

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What is the Earth going to do in response to this?"

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And so, we've already seen that the process is underway,

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we have measured, we have witnessed,

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observed with our eyes and our thermometers

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about a 0.8ºC rise in global average temperature

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since before the industrial age,

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since before we started digging all this carbon up.

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And this may not seem like a lot -- less than 1ºC --

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but the thing to know about it is

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these greenhouse gases we throw up

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stay in the atmosphere for a very long time,

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there are very long time lags involved here

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so this 0.8º temperature rise is a response

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to what we were doing 50-100 years ago,

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and what we see in the first half of this century

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will be a response to what we've done in the last 50 years

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and what we'll see in the latter half of this century

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will be a response to decisions we make today.

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So the question is, "Temperature's rising,

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how high does it have to rise before we need to worry,

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before we're in danger,

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before bad things start happening?"

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The typical answer to this question has been "2ºC."

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Anyone who has followed climate change discussions knows

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that this 2º number has taken on a kind of iconic quality.

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Typically, climate scientists who model impacts

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of what's going to happen, model 2ºC rise,

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typically economists who try to model what it would cost

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to do something about climate change or what it's worth

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or what various policies would cost, model 2º centigrade.

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So obviously, what counts as not dangerous vs dangerous,

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is not a hard scientific question, it's a political question,

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and this was a political decision to take this 2C number,

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mainly made by European climate negotiators

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well over 10 years ago,

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and it's just sort of stuck since then.

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All the countries involved in climate negotiations

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have basically signed on saying

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"Yes, this is what we want to avoid, 2ºC temperature rise."

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The bad news on this 2C number is twofold:

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first of all, all the latest science done in the last 10-15 years

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has pointed to the conclusion that those impacts

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we thought were going to happen around 2ºC

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are in fact going to happen much earlier than that,

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the climate is more sensitive

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to these added greenhouse gases than we thought.

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So, if those were the impacts we were worried about,

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then the real threshold of safety

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ought to be something like 1.5ºC.

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James Hansen is the climate scientist

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most famously known for raising these warnings,

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but it's a growing scientific consensus that

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2º is, in fact, dangerously high,

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which is bad, because

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we are almost certainly going to blow past 2ºC.

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There's some reason to believe

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a recent study said that

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even if we stopped our carbon emissions tomorrow,

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we're still going to get more than 3º this century

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just from momentum from the previous emissions.

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But stopping at 2º now would take a level

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of global coordination and ambition

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that is nowhere in evidence.

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So, a lot of climate scientists don't really want to tell you this

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because they don't want to depress you,

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but I am just a blogger, so I am happy to depress you:

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2ºC is probably off the table.

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So, then the question becomes

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"Well, what would it look like if temperature goes higher than that?

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What would, say, 4ºC look like?"

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Oddly, there hadn't really been

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a lot of concerted scientific attention to that question

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because climate scientists honestly thought

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we wouldn't do that to ourselves,

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but we are doing it to ourselves.

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So, in 2009,

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several climate change research groups in England

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drew together a group of scientists,

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commissioned some papers and

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had them really take a hard look for the first time.

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What would 4ºC look like?

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There are a lot of papers, a lot of equations,

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a lot of talk and complexity I have hopefully paraphrased

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here for you, to make it easier to grasp.

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4ºC temperature rise would look ugly.

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Among other things,

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that would be the hottest the Earth has been

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in 30 million years.

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Sea-levels would rise at least 3-6 feet,

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and this excludes some really tail end possibilities,

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but 3-6 feet at least.

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And persistent drought would cover about 40%

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of the currently occupied land on Earth,

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which would wreak havoc on agriculture

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in East Asia, Africa, South America, Western US.

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Well this combined will produce

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hundreds of millions of people

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who have been driven from their homes

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either by their cities being swamped by sea-level rise or

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by hunger or by all the attended ills

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that come along with those things.

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And, to boot, probably somewhere around half of

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the known species on Earth would go extinct.

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This question of pinning down the exact number of species

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is very difficult, this is very much an approximation,

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but some substantial chunk of life on Earth

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would be wiped out.

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The final bit of bad news...

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that's not true, there's more bad news to come,

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a middle bit of bad news is that, according to a recent paper

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by the International Energy Agency,

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we are currently on track

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-- if we keep doing what we are now doing,

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if we go on with business as usual, as it's called --

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we are now on track for 6ºC temperature rise this century;

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something, 5-7, these are obviously estimations.

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So, if 4º is hell on Earth,

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I'll let your imaginations filling the blanks on 6º

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but, one danger that comes up

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when we contemplate going this high with our temperature

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is the possibility that

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climate change will become irreversable.

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I think when people typically think about climate change,

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they think, "Oh, temperature is going to rise X amount,

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circumstances will change, some places will get warmer,

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some places will get wetter,

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we'll adjust, we'll move our farms around,

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people will migrate from one city to another,

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we'll get resettled and we'll go on with life.

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The really dangerous possibility is that what are called --

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the Earth has several of what are called

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positive feedback systems,

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so, for instance, in Siberia there is this permanent ice,

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the permafrost and it contains a bunch of methane in it.

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As it melts, it releases that methane,

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the methane causes more warming,

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which melts more ice, which releases more methane,

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it's a self-sustaining process;

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or sea ice melts, ice is white, it reflects energy,

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when it melts becomes dark blue and absorbs more energy,

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which heats the oceans, which melts more ice,

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which creates more dark surfaces.

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You see, there's a number of these systems

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that are self-perpetuating,

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and the danger, the great danger of climate change,

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that towers above all these other more specific dangers,

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is that these positive feedback systems will take on

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a momentum of their own that becomes unstoppable,

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and human beings will lose any ability to control it at all,

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even if we'd stop all our climate emissions on a dime.

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Will that happen at 2º?

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Probably not though there is a real chance of it

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and there is a lot of debate about that;

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will it happen at 4º?

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Well, it looks a lot more likely at 4º.

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Will it happen at 6º?

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Almost certainly.

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So, if we continue on our present course,

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climate change will probably take on a life of its own,

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spiral out of control and,

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according to a recent paper,

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by 2300, we could see temperature rise of up to 12ºC.

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Now if that happened,

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something like half the Earth's currently inhabited land

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would become too hot to survive on;

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and when I say too hot to survive on

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I don't mean it's difficult to grow beans

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or air conditioning bills are inconveniently high,

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I mean if you go outside you die of hotness.

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I mean, places that were an average of 80ºF

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would be now an average of 170-180ºF,

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literally too hot for human beings to go outside and survive.

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So, will there still be human civilization

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under those circumstances?

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Who knows, I mean,

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maybe we'll live in underground climate controlled caves,

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maybe we'll grow food in test tubes,

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but that wouldn't look anything like Earth as we now know it,

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it would look a lot more like Newt Gingrich's moon colony,

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assuming any human beings,

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or at least enough to make a civilization

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survived in those circumstances.

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So, when I say "Climate change is simple."

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-- I know this has been bugging you,

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you are not used to thinking in Celsius,

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those strange European metric temperatures,

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so here is good American Fahrenheit, it's just as ugly.

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So this is what I mean by climate change being simple:

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There are many complicated and fascinating discussions

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to be had about what to do about it,

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or about what effect our actions might have on the climate

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and when, or which policies are best

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based on cost benefit analyses.

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There is complexity, plenty of complexity,

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for those of you who like complexity,

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but we now know to a fair degree of certainty

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that if we keep doing what we are now doing,

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we will face unthinkable catastrophe;

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that's the bumper sticker,

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that's the take home message,

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and that, you know, saying

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"I don't want to talk about that because I don't know the ins and outs"

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is like saying, "I don't want to raise alarms

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about Hitler's army being a hundred miles out,

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because I don't know the thread count of their uniforms,

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or, I don't know the average calorie intake of a German soldier."

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You don't need to know those things

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to be scared that the army's on the march

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and to raise alarms about it.

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Similarly, if we keep doing what we are now doing,

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we are screwed,

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this we know now.

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To stabilize temperature,

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and I don't mean stabilize temperature at 2º, or 4º, or 6º,

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I mean to ever have a hope of

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ever again having a stable temperature, of any kind,

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global climate change emissions need to peak,

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stop growing, peak and start falling

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rapidly in the next 5-10 years.

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Every year we do not get started on this,

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we add, according to the International Energy Agency,

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an extra 500 billion, with a B, dollars

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to the price tag of what it is going to cost us to do this,

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eventually, every year we wait.

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That's $500 billion down the drain.

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Now, you and I look around at current politics,

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particularly US politics,

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and massive coordinated intelligent ambitious action

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does not strike us as particularly plausible.

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In fact, it might strike us as impossible,

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but that is where we are,

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stuck between the impossible and the unthinkable.

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So, your job, anyone who hears this,

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for the rest of your life, your job is

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to make the impossible possible.

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Thank you!

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(Applause)