Humans’ dependence on fossil fuels is heating up the planet...
...at an unprecedented rate
Governments and policymakers must take urgent action
That action is guided by what science says...
...about the impact of climate change on the Earth
This is how it’s calculated
This is Vilhelm Bjerknes
At the end of the 19th century...
...he devised formulas which linked the flow of heat, water and air
50 years later his ideas were the basis...
...of the first computer model of the atmosphere
And modern-day weather forecasting was born
Today’s climate models are the highly evolved descendants...
...of that original, except the computers are some 25trn times faster
Here’s how it works
Scientists divide the Earth’s atmosphere into a grid...
...of hundreds of thousands of cells, in stacks like a skyscraper
The model used by the Met Office, Britain’s national weather service...
...for example, uses stacks which are 85 cells high
The models then calculate how energy, air and water vapour...
...flow through each cell over a given time period
While weather forecasting takes a snapshot of the atmosphere...
...at a given time and predicts what will happen over the next few days
Climate models consider the atmosphere’s behaviour...
...over years and years...
...simulating either the world as it is...
...or the world as it might be
They sketch out the sort of processes...
...and the sort of results that you might see
And that allows you to inform policy to some extent
One of the classic things that climate models have always done is just...
...look at the world if the carbon-dioxide level...
...were twice as high as it is today
That was one of the first ways that climate models began...
...to illustrate the fact that greenhouse warming really was...
...likely to be a problem in the 21st century
For all their complexity, these models have limitations
Although you’re cutting the atmosphere into millions of cells...
...the atmosphere is a very big thing, and...
...a millionth of a big thing is still a pretty big thing
So, the cells tend to be...
...they may be hundreds of kilometres on a side
This means the models often struggle to capture details like cloud cover
And clouds play a key role in climate
As greenhouse gases make the atmosphere warmer...
...they change the amount of water vapour it can hold...
...and how air rises and falls
That in turn changes the amount and character of cloud cover
Depending on where clouds form, they can either trap sunlight...
...which warms the planet, or reflect it back into space, cooling it
And some processes which govern cloud formation...
...work on a very small scale
So, you can’t actually model those precise processes in the computer models
So, you have to build various different rules of thumb in...
...for what would you expect the clouds to be doing
How you do that means that your model will behave differently...
...compared to someone else’s model where they do that in a different way
Dozens of these models exist, run by teams all over the world
The modellers all want to capture the Earth...
...as close as possible to how it is
But their assumptions about how that actually works...
...differ from model to model and the ways that they...
...implement those assumptions change
Some models tend to, people say, run hot
So, they tend to provide more warming for...
...a given amount of carbon dioxide
And some models tend to run a little cold
Every few years the models are brought together...
...fed standardised questions and then their results are compared
This helps scientists understand the strengths and weaknesses...
...of different climate models, and improves them
Of course, it’s impossible to assess the models’ future projections
But what researchers can do is compare the success...
...of models from previous decades
A recent report compared the models from the 1970s to the 2000s
And it found that by and large, they were reasonably good
That the warming that we’d actually seen sat within...
...the error bars for most of them
So, it gave you a like general sense that these things...
...weren’t completely out of the park
The climate models are getting increasingly complicated...
...capturing more aspects of the Earth
But there is one element that is impossible to model
And it is the biggest contributor to climate change
The thing about human activity is that you can’t get it inside the model
The model deals with sort of like physical laws...
...of chemistry and physics and biology
So, there’s no way the model can say, well, in 2050...
...America will have stopped emitting carbon dioxide
Instead, scientists take simplified results from these climate models...
...and run them through economic models
In 2013 scientists used these models...
...to examine the effect of different climate policies...
...on the future temperature of the planet
The first looked at what would happen...
...if there was continued large-scale use of coal
The second, if there was continued use of fossil fuels...
...but some use of renewable energy
The third, if there was a much higher uptake of renewable energy
And the fourth if there was a lot of use of carbon-capture technology...
...and more land to grow biofuels
The striking thing about those comparisons...
...and by other comparisons that have come out later...
...is that only really tough climate policies...
...get you the sort of trajectories which keep the temperature...
...in the words of the Paris agreement...
...well under two degrees above the pre-industrial
What these models tell us is that...
...the current level of emissions reduction is not remotely enough...
...to have any assurance that you will stay under two degrees...
...even less, or like close to 1.5 degrees
Climate models don’t predict the future and they’re not perfect
There is a long way to go before they fully represent...
...all of Earth’s intricate processes
But for now they are the only way that scientists have...
...of understanding how damaging an increase in carbon dioxide...
...will be for the planet
Ultimately, the solution to climate change won’t lie inside models...
...but rather what humans choose to do with the information they provide
I’m Oliver Morton. I’m the briefings editor at The Economist
We’ve written a series of climate briefs to cover...
...the basics and a bit more than the basics...
...on all sorts of aspects of the climate crisis that’s facing the Earth
You can read them all at the link opposite. Thank you for watching