Greenhouse gases are warming our planet, causing extreme weather and harming the health of populations across the globe

As we come ever closer to the 1.5°C global warming limit, set in the 2015 Paris Agreement, it's clear that it isn't enough to just limit the amount of new greenhouse gas emissions – we also need to remove existing greenhouse gases from our atmosphere and restore them to solid earth.

So just what are greenhouse gases, how do we know how much to remove, and what happens to them afterwards?

Experts from across Oxford have given their answers to some of the most pressing questions when it comes to removing greenhouse gasses from our atmosphere.

What are greenhouse gases?

The emission of greenhouse gases is the major driver of our changing climate, but what are they and why are they called that?

Philip Stier is Professor of Atmospheric Physics in the Department of Physics and Director of Intelligent Earth – Oxford's UKRI AI Centre for Doctoral Training in AI for the Environment.

As Professor Stier explains, greenhouse gases are gases that absorb infrared radiation.

Infrared radiation is emitted from the surface of the Earth, normally escaping to space, keeping Earth's radiation in balance with absorbed sunlight, and therefore Earth's temperatures stable.

As the amount of greenhouse gases in the atmosphere increases, the amount of absorbing gas in the atmosphere is raised.

The levels from which radiation can escape to space are also raised to levels higher than the atmosphere.

As the atmosphere is colder at higher levels, less radiation is emitted to space, therefore, as more greenhouse gases are introduced to the atmosphere, the amount of radiation that can be emitted to space is reduced.

This warms the planet, as we can't then balance incoming solar radiation.

The analogy with greenhouses, though, Professor Stier suggests, is somewhat misleading.

Greenhouses warm themselves via a different mechanism, keeping the heat absorbed from solar radiation, rather than letting it escape.

Aerosols differ from greenhouse gases, but also have a critical impact on our changing climate.

Essentially, aerosols are small particles in the atmosphere, basically air pollution, Professor Stier explains, but they can also come from natural sources.

They scatter sunlight back to space and make clouds brighter, compounding the scattering of light back to space.

Therefore aerosols cool the Earth, and quite substantially at the moment – around 0.5°C.

As we continue to clean up air pollution, we therefore reduce a cooling effect, accelerating global warming.

What is net zero?

The term net zero is a common part of our language when we talk about action towards climate change, but what does it mean, and how do we get there?

Thomas Hale is Professor in Public Policy (Global Public Policy) at the Blavatnik School of Government.

Net zero, he says, means reducing our emissions to about 80 to 90% less than what they are today.

Myles Allen is Head of Atmospheric, Oceanic and Planetary Physics in the Department of Physics, and Professor of Geosystem Science in the School of Geography and the Environment.

The reason net zero works, Professor Allen explains, is that if we reduce the rate at which we put carbon dioxide (CO₂) into the atmosphere to zero, and allow nature to continue to take carbon dioxide out of the atmosphere, that's enough to draw down CO₂ concentrations in the atmosphere just fast enough to stop temperatures rising any further.

To achieve net zero emissions, Professor Allen highlights the most important thing that we need to do – reducing the rate at which we are producing carbon dioxide from activities like the burning of fossil fuels and deforestation.

We also to scale up our ability to remove greenhouse gases like carbon dioxide from the atmosphere and put them back underground.

There are many innovative ways this can be done, Professor Allen says, but ultimately, net zero has to mean a like for like balance of carbon dioxide coming from fossil sources with carbon dioxide committed to permanent disposal.

This is what is called geological net zero – a balance at the Earth's crust.

What is a carbon budget?

Setting a budget for the amount of carbon that can be emitted into the atmosphere is a useful tool for global climate action. But what does the carbon budget entail, and who sets it?

Cameron Hepburn is Battcock Professor of Environmental Economics in the School of Geography and the Environment's Smith School of Enterprise and Environment.

If we want to stop the planet warming, the have to get our human-caused emissions down to zero in time to stop warming to a level like 1.5 or 2°C.

Thomas Hale, Professor in Public Policy (Global Public Policy) at the Blavatnik School of Government, highlights just how long our carbon emissions stay in the atmosphere for.

The carbon that powered the first steam engine, he says, is still up there in part. What we emit now will still be in the atmosphere for our children, and their grandchildren.

We need, therefore, to think of this as a long-term cumulative problem.

A budget in this case, helps you understand how much of a rise in temperature you will face, depending on levels of emissions.

The carbon budget, Professor Hepburn explains, is set by the Intergovernmental Panel on Climate Change, the IPCC.

At the moment, it is estimated that there are 2-300 billion tonnes of CO₂ left in the budget before we hit the 1.5°C temperature threshold, but the problem is that there is no global consensus on how to divide this budget out.

So, as Professor Hale explains, we need to find equitable solutions for a fair and effective distribution of the carbon budget, which as Professor Hepburn states, only has a single digit number of years left until it is exceeded.

How do we remove greenhouse gases?

In addition to reducing our greenhouse gas emissions, we're going to have to remove existing greenhouse gases, especially carbon dioxide (CO₂) from our atmosphere – this is called carbon dioxide removal.

Just how possible is this, and can we work with nature to find solution?

Nathalie Seddon is Professor of Biodiversity in the Department of Biology and the founding Director of the Nature-based solutions Initiative (NbSI).

There's an enormous amount evidence, Professor Seddon argues, to demonstrate the essential role that nature has to play in the removal of carbon dioxide from the atmosphere.

Cameron Hepburn, Battcock Professor of Environmental Economics in the School of Geography and the Environment's Smith School of Enterprise and Environment, highlights that there are also industrial ways to remove carbon dioxide from the atmosphere.

This would include the scaling up of chemical processes where CO₂ can be stored underground in rocks, where it won't end up back in the atmosphere any time soon.

Professor Hepburn also highlights the key role of nature in carbon dioxide removal, noting that the restoration of nature could also buy us more time to sort some the problem of climate change.

What happens once you remove greenhouse gases?

There are two main ways in which carbon dioxide (CO₂), can be removed from the atmosphere. But once it's removed, where does it go?

Nick Eyre is Emeritus Professor of Energy and Climate Policy and was the founding co-Director of the University's ZERO Institute.

As Professor Eyre explains, the first is neatly done by green plants, turning carbon dioxide into plant proteins.

The second is through a more geo-engineering approach.

This is where carbon dioxide is sucked out of the atmosphere and pumped underground into geological storage.

Once carbon dioxide is captured, and purified, it needs to be compressed. Under sufficient pressure, 15 atmospheres, it becomes a liquid.

As Professor Myles Allen, Head of Atmospheric, Oceanic and Planetary Physics in the Department of Physics, and Professor of Geosystem Science in the School of Geography and the Environment, explains, this makes it slightly more dense than hydrocarbon fuels, allowing it to be injected into rock formations.

This needs to be done safely though, and as Professor Allen points out, it's a racing certainty that we need to remove carbon dioxide from the atmosphere due to the continued emissions of greenhouse gases.

Source: https://www.ox.ac.uk/climate-and-environment/removing-greenhouse-gases