How scientists calculate climate change | The Economist

published on July 2, 2020

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 15 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

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