I haven't decided whether to be embarrassed

about that or not.

But I was on Twitter one day and

a relatively prominent left of centre pundit,

piped up and said

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

why are so few people talking about it?

Why have so few thought leaders made it

their signature issue?"

And another reasonably prominent

left of centre pundit piped up

and said "Well, for my part,

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

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

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

and assert things publicly about it."

You know, anybody who has ever so much as mentioned

climate change on television or on the internet

will understand why this person thinks the way they do.

Any time you mention it,

the hordes descend, bearing complicated stories

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

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

borne by these climate sceptics

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

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

and be able to respond to them in detail,

and a lot of people just find that prospect dreary,

and so they don't bother.

And this, of course, drives me crazy,

so I piped up on Twitter and said

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

What you need to know to be able

to speak out publicly about it,

just about the basic structure of the problem,

is really not that complicated,

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

let this be a lesson to you:

don't go talk smack on Twitter,

unless you are willing to back it up.

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

with 15 minutes to explain climate change to you.

So, let's get started.

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

The reason is that it is enveloped by this tiny,

tiny thin layer of gases and chemicals

that we call our atmosphere.

So, the Sun's energy,

rather than just coming down and bouncing right back off,

it comes down and is held

close to the surface of the Earth for a while

and then bounces off,

and then this simple process is why we have evaporation,

and precipitation, and photosynthesis,

and life on our planet.

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

that the atmosphere and the systems on Earth

are in this dynamic relationship

and you can change the chemical composition of the atmosphere

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

The energy still has to escape, of course,

but in the meantime it will cause changes

in these biophysical systems of the Earth.

And, you know, you often hear people say,

"The Earth has always changed,

the climate has always changed",

and that's true, it has.

This relationship between the atmosphere and the systems,

they go through cycles,

but these cycles have typically taken

hundreds of thousand of years,

millions of years.

The key thing to know first is that

for the last 10,000 years on Earth,

the climate has been relatively stable, unusually stable,

and by stable I mean temperature has varied,

it's gone up and down,

but it's stayed on a fairly narrow band

of about plus or minus 1 degree Celsius,

and all of advanced human civilisation

has taken place during these 10,000 years,

the development of agriculture, the written word,

the wheel, the iPhone, everything we know,

everything we have built,

we have done in this period of relative climate stability.

So, what we have been doing

for the last couple of hundred years

is digging up carbon out of the earth,

and throwing it up into the atmosphere,

and changing the chemical composition of the atmosphere,

like has happened in the past

except for extremely faster.

In geological time, the blink of an eye,

we are substantially changing

the chemical composition of the atmosphere

and all of climate science has been about,

"What's going to happen?

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

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

we have measured, we have witnessed,

observed with our eyes and our thermometers

about a 0.8ºC rise in global average temperature

since before the industrial age,

since before we started digging all this carbon up.

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

but the thing to know about it is

these greenhouse gases we throw up

stay in the atmosphere for a very long time,

there are very long time lags involved here

so this 0.8º temperature rise is a response

to what we were doing 50-100 years ago,

and what we see in the first half of this century

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

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

will be a response to decisions we make today.

So the question is, "Temperature's rising,

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

before we're in danger,

before bad things start happening?"

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

Anyone who has followed climate change discussions knows

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

Typically, climate scientists who model impacts

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

typically economists who try to model what it would cost

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

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

So obviously, what counts as not dangerous vs dangerous,

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

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

mainly made by European climate negotiators

well over 10 years ago,

and it's just sort of stuck since then.

All the countries involved in climate negotiations

have basically signed on saying

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

The bad news on this 2C number is twofold:

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

has pointed to the conclusion that those impacts

we thought were going to happen around 2ºC

are in fact going to happen much earlier than that,

the climate is more sensitive

to these added greenhouse gases than we thought.

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

then the real threshold of safety

ought to be something like 1.5ºC.

James Hansen is the climate scientist

most famously known for raising these warnings,

but it's a growing scientific consensus that

2º is, in fact, dangerously high,

which is bad, because

we are almost certainly going to blow past 2ºC.

There's some reason to believe

a recent study said that

even if we stopped our carbon emissions tomorrow,

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

just from momentum from the previous emissions.

But stopping at 2º now would take a level

of global coordination and ambition

that is nowhere in evidence.

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

because they don't want to depress you,

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

2ºC is probably off the table.

So, then the question becomes

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

What would, say, 4ºC look like?"

Oddly, there hadn't really been

a lot of concerted scientific attention to that question

because climate scientists honestly thought

we wouldn't do that to ourselves,

but we are doing it to ourselves.

So, in 2009,

several climate change research groups in England

drew together a group of scientists,

commissioned some papers and

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

What would 4ºC look like?

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

a lot of talk and complexity I have hopefully paraphrased

here for you, to make it easier to grasp.

4ºC temperature rise would look ugly.

Among other things,

that would be the hottest the Earth has been

in 30 million years.

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

and this excludes some really tail end possibilities,

but 3-6 feet at least.

And persistent drought would cover about 40%

of the currently occupied land on Earth,

which would wreak havoc on agriculture

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

Well this combined will produce

hundreds of millions of people

who have been driven from their homes

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

by hunger or by all the attended ills

that come along with those things.

And, to boot, probably somewhere around half of

the known species on Earth would go extinct.

This question of pinning down the exact number of species

is very difficult, this is very much an approximation,

but some substantial chunk of life on Earth

would be wiped out.

The final bit of bad news...

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

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

by the International Energy Agency,

we are currently on track

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

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

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

something, 5-7, these are obviously estimations.

So, if 4º is hell on Earth,

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

but, one danger that comes up

when we contemplate going this high with our temperature

is the possibility that

climate change will become irreversable.

I think when people typically think about climate change,

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

circumstances will change, some places will get warmer,

some places will get wetter,

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

people will migrate from one city to another,

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

The really dangerous possibility is that what are called --

the Earth has several of what are called

positive feedback systems,

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

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

As it melts, it releases that methane,

the methane causes more warming,

which melts more ice, which releases more methane,

it's a self-sustaining process;

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

when it melts becomes dark blue and absorbs more energy,

which heats the oceans, which melts more ice,

which creates more dark surfaces.

You see, there's a number of these systems

that are self-perpetuating,

and the danger, the great danger of climate change,

that towers above all these other more specific dangers,

is that these positive feedback systems will take on

a momentum of their own that becomes unstoppable,

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

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