Carbon fingerprints: The science of climate attribution

In May 2021, a Dutch court ruled that Royal Dutch Shell must cut the carbon emissions from both its operations and the oil and gas it sells by 45% by 2030, relative to 2019 levels — a global first, but likely not the last, thanks to rapid advances in the fields of attribution science and climate litigation. L Delta Merner leads the Science Hub for Climate Litigation at the Union of Concerned Scientists, a non-profit science advocacy organization based in the US. She provides evidence to inform legal cases that hold fossil fuel companies accountable for climate-related damage. UCS evidence has been cited in more than 70% of US climate damages and fraud cases to date.

What are the scientific advances that allow us to quantify the impact of climate change with such confidence?

Climate attribution has been around for about two decades now, which makes it an incredibly new science. But while the science is new, the concepts are not. There are two major areas — event attribution and source attribution — and there have been really important advances in both over the last five years. 

The area that’s generally talked about most is extreme event attribution, which can tell us how climate change may have added to the severity of an event. It’s about understanding the role of climate change versus natural weather patterns or random climate variability. Studies can now tell us how much hotter a heatwave is or how much greater the intensity of a downpour is during a hurricane or any other rainfall event because of climate change. We also see connections by understanding sea-level rise. Sea level is impacted by things external to climate change as well, but you can isolate the role of climate change and then understand how that’s changed a storm surge from a hurricane, for example. So these all fall under event attribution.

One of the things that’s been exciting is that we can now attribute changes to specific emission sources. This work started in 2013 with a publication by Richard Heede, which found that nearly two thirds of industrial carbon emissions from the Industrial Revolution onwards can be traced to the 90 largest fossil fuel and cement producers — so ExxonMobil, BP, Chevron, Royal Dutch Shell, to name a few. One of the first studies to make the connection with global warming was published in 2017 by some of my colleagues at UCS. They found that emissions from these 90 major fossil fuel producers and cement manufacturers contributed to around 42-50% of all global temperature rise. 

I think so often we get caught in thinking of climate change as something that’s global, that’s abstract, that’s hard to address. But here we see that there’s clear connections between specific actors. Global temperature rise is scientifically the easiest to look at, but another more recent UCS study found that 26-32% of global sea-level rise was tied to the products of just these 90 carbon producers.

Right now, the majority of the models that we’re looking at are global. So the next steps are about better connecting attribution work to local models. This is a scale that is much more beneficial for people, communities and businesses, so that they can better understand the impacts for their local environment. A study that was released in May looked at economic damage from Hurricane Sandy related to human-caused sea-level rise. Again, they did extensive scientific work to isolate the human-caused sea-level rise that was from climate change, and used that number to estimate that there was approximately US$8.1 billion of damages that were just attributable to climate-change-induced sea-level rise. This is just one small sliver of climate change impacting one individual storm, but these events are happening globally and impacting cities across the world.

How do you actually do all this — what kind of science is attribution science? Is it data analysis, or modelling, or…?

It is modelling. So in the very basics, we’ve built out worlds for what the global climate looks like today, with current levels of carbon in the atmosphere, and we can build out worlds without those emissions in them. Then we can do comparisons between events and run those thousands of times, and then carry out statistical analysis to see what we expect the differences to be. It’s much more complicated than that, and it’s a very delicate process, but essentially attribution science is comparing those two models.

You’ve described attribution science as a game-changer for addressing climate change. Could you spell out how?

I think that it’s a huge game-changer because, without it, fossil fuel companies could sow some uncertainty about the role of climate change in certain events, and they could distract the media by debating what caused something or didn’t. But it’s not about whether or not an event was impacted or caused by climate change. With attribution science, we can say, this percentage of this impact is because of climate change. We can talk about it much more precisely and stop that problematic discourse that was much further behind the science, where we were saying whether or not climate change was real. Now we can say, it is; these are the impacts.

From a city planning perspective, historically there’s been so much uncertainty in so much of the work. This can help us to make better decisions.

Attribution science is also informing climate litigation. What are the advantages and disadvantages of using litigation to address climate change, compared with other forms of advocacy or action?

It’s important to know that climate litigation is just one very specific tool in the toolbox for addressing climate justice issues and hopefully reducing carbon emission in the future. I think a lot of the reason that it’s come around is because policy has moved so slowly in the face of a concerted campaign of deception by the fossil fuel industry. For decades, science has been talking about the realities of climate change and the sources of climate change — you can cite back to the 1980s, where it was very clear that the companies themselves knew what climate change was and what their contribution was, and ultimately, what the impact was going to be for people and communities.

Despite all of this information, despite the really indisputable science around climate change and climate impacts, we haven’t seen policy overcome fossil fuel industry obstruction to address this global problem. So litigation has become such an important tactic and something that’s grown so quickly because it’s another way to address the urgency of climate change.

What about the risk of anti-climate litigation, where fossil fuel producers use the courts to frustrate attempts to tackle climate change? How can we defend against it being hijacked in that way?

We certainly see a lot of pushback. Even within the US, most of the litigation has been stalled now for a couple of years, as the fossil fuel defendants tie the cases up in procedural debates about whether they should be considered in state courts, where they were filed, or moved to federal courts.

But I think it’s more important to focus on the successes that we’re seeing and the power that’s coming from it. So what we see in the Netherlands, where cases are getting through and historic rulings are happening. This is a space where we can finally talk about human rights and climate change, especially internationally. In the US, we see cities and municipalities pursuing litigation to inform decisions and to help understand the responsibilities of the fossil fuel companies. That’s really encouraging. When there’s success, there’s always going to be pushback. But I think that the science is able to inform these powerful decisions, and as we get decision-makers to better understand the science, I’m really hopeful about what those outcomes could be.

How could city leaders and designers use attribution science or climate litigation to serve their communities?

This is a great question to think about. In terms of adaptations for cities, planning for infrastructure is increasingly difficult with the realities of climate change. The models that we used to use to understand the chances of a 50-year flood — that you would base your overpass design on — don’t work any more, and we can no longer rely on historical data to make infrastructure decisions. Attribution science can help planners to understand the increased role of climate change in weather events, and it’s a starting place for thinking about what the new steps in planning for city infrastructure might look like.

As cities are looking at adaptation to climate change, there’s also the question of who should be paying for it. For example, I live in the state of Maryland in the US, and right now there are three lawsuits against different fossil fuel companies. Our capital, the City of Annapolis, became the 25th jurisdiction in the US to file litigation against them. It sits on the water, and there’s been a documented increase in the number of flooding days due to sea-level rise. Analysis predicts that there will be some degree of flooding for the majority of the year as the sea level increases. So the city’s planners have talked about what infrastructure looks like in this new reality, and in order for the city to mitigate the sea-level rise that’s happening, they estimated that they’ll need to spend more than US$45 million on just four miles of sea walls by 2040. Often, when a city needs to raise money like that, it comes from the taxpayers, but they sat back and said, ‘who should be paying for this? Where is the responsibility for these damages?’ The science strongly shows that most of the sea-level rise impacts are from the actions of the fossil fuel companies, and therefore, they should play some role in funding the changes that are needed.

This is very new. It was just in the last year that this litigation got filed, so there’s still a lot to understand. But I think it’s important to be making these connections. The Annapolis lawsuit also talks a lot about the inequity of damages from climate change. The lawsuit itself says that climate change will disproportionately impact people of colour, people living in poverty, and other vulnerable communities. Litigation can play a role to highlight and address those inequalities.

Given the amount of adaptation and mitigation that will be required around the world, is litigation is a viable way to pay for it all, or is it more about winning a few headline cases to get people to start really doing something about climate change?

Litigation is not purely about having those responsible pay for everything. The reality is that we’re going to be dealing with climate change for the rest of our existence as a human race. So I think the hopeful impacts are twofold. Where people are being impacted now, there needs to be adaptation. But we also need to be changing behaviours, and that’s why you see in the Netherlands case against Shell that it wasn’t just about payouts for damages; it was very much about reducing their own emissions now.

The Netherlands case ruled that Shell should be in compliance with the Paris Agreement, which requires them to make massive reductions in emissions moving forward. Those are the same requirements that we have of every country that signed on to the Paris Agreement. So these are changes that are in line with societal needs, and it’s saying that these companies should be held accountable for doing their part.

There’s a legal precedent for this kind of approach — this is very much what we saw with the tobacco industry, when litigation helped hold the industry accountable for tobacco-related illness, death and associated healthcare costs. You were able to talk about what the health impacts were from tobacco, and it shifted the industry in a lot of ways.

Climate litigation is expanding to look at supply chains, and it’s a global phenomenon — a French supermarket chain was sued by Indigenous peoples from the Brazilian and Colombian Amazon because of how it sourced its beef. Is there a risk that, say, municipalities in the Global North find themselves the target of litigation by communities in the Global South because of their previous contribution to climate change?

What we’re seeing right now is less about jurisdictions or governments being sued by other individuals, but we are seeing acknowledgement of the different contributions between the Global North and the Global South, and what that looks like. So one example, in 2015 a Peruvian farmer filed a letter of complaint against a German energy company, because the farmers in that area were experiencing massive flooding due to glacier melt.

They essentially said that this company was responsible for X percentage of carbon emissions and therefore should be liable for that percentage of the costs of adaptation and mitigation efforts in this community and to pay for repairs to their flooded homes. That court case is moving forward, and it’s a big win that it’s considered viable for litigation. It very much comes back to these important equity questions. Having polluters pay validates the experience of frontline communities and begins to address the inequitable impacts of climate change.