Everyone knows that emissions are the main culprits of climate change and the raising global temperature, which leads to biodiversity loss, sea-level changes and more extreme weather conditions. But how exactly do climate scientists know this? And what do climate scientists really do? 

We sat down with Kasia Tokarska, a climate data scientist, postdoctoral researcher at ETH Zürich and a member of Cooler Future's Sustainable Committee, to talk about just this — and more. 

No time to listen? Then keep on reading!

Climate science explained to non-scientists

The Speed Read:

1. Emissions are gases (e.g carbon dioxide, methane, etc) released into the air and are the main drivers of climate change.

2. There is a finite amount of carbon in different forms on Earth, which is stored in the land, the ocean and the atmosphere. 

3. About half of emissions emitted into the air is taken up by the land and the ocean, while the other half remains in the atmosphere.

4. Climate models are physical and mathematical representations of the earth system and are used in  research to provide a better understanding of the interactions between different components of the climate system (such as carbon-climate interactions). 

5. If humans don’t do anything about climate change and continue to burn all remaining fossil fuel resources, the global mean temperature could rise up to 10°C (with the Arctic warming even up to almost 20°C).

6. Negative emissions are technologies that remove emissions from the air and store it permanently (for example, by solidifying CO2 and storing it underground in geological structures).

To answer this question, let’s start at the beginning and answer what emissions really are.

Emissions are gases (e.g. carbon dioxide (CO2) or methane (CH4)) that are released into the atmosphere, mostly through processes like burning of fossil fuels and, to a smaller extent, from agriculture. Since the beginning of industrialisation in the 1850s, humans have started to emit a lot of CO2 and we haven’t really stopped since. 

Greenhouse gas (GHG):

A molecule which absorbs solar radiation, warms itself up and then emits additional radiation. The greenhouse gas emitted into the air traps heat from the Sun and causes the warming of the atmosphere, which ultimately leads to global warming. 

Related read: What is CO2e and how is it calculated?

Because Earth has an atmosphere, emissions stay in the earth system and can’t escape into space. Instead, they remain in the atmosphere for a really long time — from centuries to millennia. 

Some of it the earth can store — but with the increasing amount of carbon added into the atmosphere, the natural capacity of the earth system to store it is running out. Natural carbon sinks, such as the ocean and land, are like a sponge — once they are saturated with CO2, they cannot store any more of it.

Carbon sink:

Any reservoir, natural or not, that absorbs more carbon than it releases and therefore lowers the concentration of CO2 in the atmosphere. The biggest carbon sinks are the world's oceans and forests that absorb large amounts of carbon dioxide from the earth’s atmosphere. 

Related read: 28 essential climate change terms you should know

When CO2 goes into the atmosphere, about half of it gets redistributed to the land and the ocean, while the other half stays stored in the atmosphere. These “storage places” (i.e. carbon sinks) are interconnected and always in balance with each other. This means that if CO2 is suddenly taken from the atmosphere, then CO2 stored in the land and the ocean is released back into the atmosphere in order to maintain the perfect “storage” balance between the three components. Such process is called the carbon cycle

Take a look at the diagram below, which shows the movement of carbon between land, atmosphere, and oceans. Yellow numbers are natural fluxes, red are human contributions of carbon per year (in gigatonnes), and white numbers indicate stored carbon:

Carbon cycle explained

Source: NASA

To be able to predict the future, scientists need to understand how Earth works. And to understand how Earth works, they rely on climate models that help research the effect of emissions on the climate system. 

Climate models can simulate what happens when a certain amount of CO2 is emitted into the atmosphere. How much ends up in the land, how much in the ocean, how much does sea level rise, what happens to the vegetation and where? Climate models help us understand better the interconnected feedbacks in the climate system and the behaviour of the carbon cycle.

In a very basic climate model, there is energy coming in (from the Sun towards the earth’s surface) and energy going out (away from the earth’s surface back to space). The existing temperature on Earth is ultimately the result of this balance between incoming and outgoing energy (kind of like a balance in your bank account between savings and expenditures). 

In more complex models, scientists can add different components — such as the atmosphere, greenhouse gases in the atmosphere, winds, vegetation on land, marine ecosystems, etc. — which will further impact this energy balance. The components are all interconnected and impact one another in different ways through exchanges of carbon, water, or energy. These complex climate models are representations of the existing climate system based on physical and mathematical equations. 

Of course, the world is very complex and there are unknowns. When scientists don’t have the exact physics of a component, the missing information can be approximated by looking at the observations, e.g. noticing how certain plants behave in changing conditions. 

Once the model has been built, scientists can then experiment. What happens in the model if a certain amount of CO2 is emitted? How does the global temperature change? What regional effects are there? How much does the sea-level rise, what vegetation changes happen? There are endless amounts of topics that can be explored in response to different emission scenarios. And there are also many different kinds of models used by scientists around the world which support one another and give them more confidence in the results.

Did you know?

Climate models respond to both CO2 and non-CO2 emissions differently, so in climate modelling these two types of emissions are treated separately.

Climate models are physical and mathematical representations of our world, but of course they can’t predict future human behaviour. Even though science has shown how the earth system will respond if a certain amount of CO2 is emitted, scientists cannot easily predict human behaviour and socio-economic choices of governments, companies, and individuals. The political decisions and individual behaviours are up to humanity. Scientists can make some assumptions and best-guesses of how the socio-economic system will behave, but that still is one of the largest sources of uncertainty in the future projections of climate change.

In 2015, Kasia and her colleagues looked at climate models to understand what would happen if humans continue to emit all remaining fossil fuel resources, which is as high as five trillion tonnes of carbon emitted in total. The more CO2 is emitted, the warmer it gets, it’s an approximately linear response. The result is alarming: the global mean temperature could rise as much as 10° C. And as regions are affected differently, the Arctic temperature could even rise above 20°C.

Reversing emissions, i.e. capturing carbon from the atmosphere and storing it permanently is possible. There are several existing technologies that can do this, they are also called negative emissions.

For example, using direct air capture, CO2 can be captured before it enters the atmosphere and then stored long term in geological structures underground. This way, the emitted CO2 would neither reach the atmosphere nor contribute to climate change. There are also some nature-based solutions, such as planting forests or certain plants (though they release the CO2 again when they die), creating biochar, or certain technologies that enhance the amount of carbon stored in the ocean, for example.

Biochar:

Biochar is a type of charcoal which is made by burning plants that are very effective at storing carbon and capturing the CO2 from the combustion.

If negative emissions would be implemented, climate models have shown that the global temperature can be lowered. So theoretically, a “reversing” of the global mean temperature is possible, if enough CO2 is captured. However, climate change has many and complex effects on other components of our climate as well, such as vegetation change, melting ice caps and sea-level rise. These other components are much more difficult to reverse.

According to scientists, negative emissions can act to slow down climate change, but they are not the answer to everything. First of all, the long-term risks of some of the technologies are unknown, e.g. the ones affecting the ocean or organisms living in algae. The usage of these technologies could be dangerous, causing irreversible changes in the ocean. Secondly, negative emissions are extremely expensive, and, at present, emitting CO2 is free, so the incentives to use carbon capture technologies are low.

On top of predicting certain scenarios in the future, climate models are also useful to determine the status quo. How much CO2 have we emitted since the Paris Agreement for example, and how close is the planet to the 1.5°C temperature rise? The amount of carbon humanity has left to emit is also called the carbon budget.

Climate models have shown that if humans keep emitting at present rates and emissions don’t start declining, from 2020 onwards there are about 6-11 years left before the 1.5°C target is hit. However, as Kasia told us in the interview, focusing too much on the years humans have left can be counterproductive and misleading, as the driving question, instead, should focus on what can be done to reach net-zero emission levels faster.  

Conclusion

Scientists have known for many years that there is a finite amount of CO2 that can be emitted before exceeding a given temperature target, and the more CO2 is emitted, the higher the temperature will rise. With the help of climate models, scientists have been able to look at the different components of the climate system, and explore what the impacts of temperature rises will be. The priority now needs to be in reducing emissions and reaching net-zero emission levels as fast as possible.

Miia Laine
Miia is a radio presenter and podcast producer living in Helsinki.