we've all contributed to climate change.

Actions, choices and behaviors

will have led to an increase in greenhouse gas emissions.

And I think that that's quite a powerful thought.

But it does have the potential to make us feel guilty

when we think about decisions we might have made

around where to travel to,

how often and how,

about the energy that we choose to use

in our homes or in our workplaces,

or quite simply the lifestyles that we lead and enjoy.

But we can also turn that thought on its head,

and think that if we've had such a profound

but a negative impact on our climate already,

then we have an opportunity to influence the amount of future climate change

that we will need to adapt to.

So we have a choice.

We can either choose to start to take climate change seriously,

and significantly cut and mitigate our greenhouse gas emissions,

and then we will have to adapt to less of the climate change impacts in future.

Alternatively, we can continue to really ignore the climate change problem.

But if we do that, we are also choosing

to adapt to very much more powerful climate impacts in future.

And not only that.

As people who live in countries with high per capita emissions,

we're making that choice on behalf of others as well.

But the choice that we don't have

is a no climate change future.

Over the last two decades,

our government negotiators and policymakers have been coming together

to discuss climate change,

and they've been focused on avoiding a two-degree centigrade warming

above pre-industrial levels.

That's the temperature that's associated with dangerous impacts

across a range of different indicators,

to humans and to the environment.

So two degrees centigrade constitutes dangerous climate change.

But dangerous climate change can be subjective.

So if we think about an extreme weather event

that might happen in some part of the world,

and if that happens in a part of the world where there is good infrastructure,

where there are people that are well-insured and so on,

then that impact can be disruptive.

It can cause upset, it could cause cost.

It could even cause some deaths.

But if that exact same weather event happens in a part of the world

where there is poor infrastructure,

or where people are not well-insured,

or they're not having good support networks,

then that same climate change impact could be devastating.

It could cause a significant loss of home,

but it could also cause significant amounts of death.

So this is a graph of the CO2 emissions at the left-hand side

from fossil fuel and industry,

and time from before the Industrial Revolution

out towards the present day.

And what's immediately striking about this

is that emissions have been growing exponentially.

If we focus in on a shorter period of time from 1950,

we have established in 1988

the Intergovernmental Panel on Climate Change,

the Rio Earth Summit in 1992,

then rolling on a few years, in 2009 we had the Copenhagen Accord,

where it established avoiding a two-degree temperature rise

in keeping with the science and on the basis of equity.

And then in 2012, we had the Rio+20 event.

And all the way through, during all of these meetings

and many others as well,

emissions have continued to rise.

And if we focus on our historical emission trend in recent years,

and we put that together with our understanding

of the direction of travel in our global economy,

then we are much more on track

for a four-degree centigrade global warming

than we are for the two-degree centigrade.

Now, let's just pause for a moment

and think about this four-degree global average temperature.

Most of our planet is actually made up of the sea.

Now, because the sea has a greater thermal inertia than the land,

the average temperatures over land are actually going to be higher

than they are over the sea.

The second thing is that we as human beings don't experience

global average temperatures.

We experience hot days, cold days,

rainy days, especially if you live in Manchester like me.

So now put yourself in a city center.

Imagine somewhere in the world:

Mumbai, Beijing, New York, London.

It's the hottest day that you've ever experienced.

There's sun beating down,

there's concrete and glass all around you.

Now imagine that same day --

but it's six, eight, maybe 10 to 12 degrees warmer

on that day during that heat wave.

That's the kind of thing we're going to experience

under a four-degree global average temperature scenario.

And the problem with these extremes,

and not just the temperature extremes,

but also the extremes in terms of storms and other climate impacts,

is our infrastructure is just not set up to deal with these sorts of events.

So our roads and our rail networks

have been designed to last for a long time

and withstand only certain amounts of impacts

in different parts of the world.

And this is going to be extremely challenged.

Our power stations are expected to be cooled by water

to a certain temperature to remain effective and resilient.

And our buildings are designed to be comfortable

within a particular temperature range.

And this is all going to be significantly challenged

under a four-degree-type scenario.

Our infrastructure has not been designed to cope with this.

So if we go back, also thinking about four degrees,

it's not just the direct impacts,

but also some indirect impacts.

So if we take food security, for example.

Maize and wheat yields

in some parts of the world

are expected to be up to 40 percent lower

under a four-degree scenario,

rice up to 30 percent lower.

This will be absolutely devastating for global food security.

So all in all, the kinds of impacts anticipated

under this four-degree centigrade scenario

are going to be incompatible with global organized living.

So back to our trajectories and our graphs of four degrees and two degrees.

Is it reasonable still to focus on the two-degree path?

There are quite a lot of my colleagues and other scientists

who would say that it's now too late to avoid a two-degree warming.

But I would just like to draw on my own research

on energy systems, on food systems,

aviation and also shipping,

just to say that I think there is still a small fighting chance

of avoiding this two-degree dangerous climate change.

But we really need to get to grips with the numbers

to work out how to do it.

So if you focus in on this trajectory and these graphs,

the yellow circle there highlights that the departure

from the red four-degree pathway

to the two-degree green pathway is immediate.

And that's because of cumulative emissions,

or the carbon budget.

So in other words, because of the lights and the projectors

that are on in this room right now,

the CO2 that is going into our atmosphere

as a result of that electricity consumption

lasts a very long time.

Some of it will be in our atmosphere for a century, maybe much longer.

It will accumulate, and greenhouse gases tend to be cumulative.

And that tells us something about these trajectories.

First of all, it tells us that it's the area under these curves that matter,

not where we reach at a particular date in future.

And that's important, because it doesn't matter

if we come up with some amazing whiz-bang technology

to sort out our energy problem on the last day of 2049,

just in the nick of time to sort things out.

Because in the meantime, emissions will have accumulated.

So if we continue on this red, four-degree centigrade scenario pathway,

the longer we continue on it,

that will need to be made up for in later years

to keep the same carbon budget, to keep the same area under the curve,

which means that that trajectory, the red one there, becomes steeper.

So in other words, if we don't reduce emissions in the short to medium term,

then we'll have to make more significant year-on-year emission reductions.

We also know that we have to decarbonize our energy system.

But if we don't start to cut emissions in the short to medium term,

then we will have to do that even sooner.

So this poses really big challenges for us.

The other thing it does is tells us something about energy policy.

If you live in a part of the world where per capita emissions are already high,

it points us towards reducing energy demand.

And that's because with all the will in the world,

the large-scale engineering infrastructure

that we need to roll out rapidly

to decarbonize the supply side of our energy system

is just simply not going to happen in time.

So it doesn't matter whether we choose nuclear power

or carbon capture and storage,

upscale our biofuel production,

or go for a much bigger roll-out of wind turbines and wave turbines.

All of that will take time.

So because it's the area under the curve that matters,

we need to focus on energy efficiency,

but also on energy conservation -- in other words, using less energy.

And if we do that, that also means

that as we continue to roll out the supply-side technology,

we will have less of a job to do if we've actually managed

to reduce our energy consumption,

because we will then need less infrastructure on the supply side.

Another issue that we really need to grapple with

is the issue of well-being and equity.

There are many parts of the world where the standard of living needs to rise.

Bbut with energy systems currently reliant on fossil fuel,

as those economies grow so will emissions.

And now, if we're all constrained by the same amount of carbon budget,

that means that if some parts of the world's emissions are needing to rise,

then other parts of the world's emissions need to reduce.

So that poses very significant challenges for wealthy nations.

Because according to our research,

if you're in a country where per capita emissions are really high --

so North America, Europe, Australia --

emissions reductions of the order of 10 percent per year,

and starting immediately, will be required for a good chance

of avoiding the two-degree target.

Let me just put that into context.

The economist Nicholas Stern

said that emission reductions of more than one percent per year

had only ever been associated with economic recession or upheaval.

So this poses huge challenges for the issue of economic growth,

because if we have our high carbon infrastructure in place,

it means that if our economies grow,

then so do our emissions.

So I'd just like to take a quote from a paper

by myself and Kevin Anderson back in 2011

where we said that to avoid the two-degree framing of dangerous climate change,

economic growth needs to be exchanged at least temporarily

for a period of planned austerity in wealthy nations.

This is a really difficult message to take,

because what it suggests is that we really need to do things differently.

This is not about just incremental change.

This is about doing things differently, about whole system change,

and sometimes it's about doing less things.

And this applies to all of us,

whatever sphere of influence we have.

So it could be from writing to our local politician

to talking to our boss at work or being the boss at work,

or talking with our friends and family, or, quite simply, changing our lifestyles.

Because we really need to make significant change.

At the moment, we're choosing a four-degree scenario.