When countries pledge to lower their emissions under the Paris Agreement, how do we count which emissions belong to which countries?

LHF: Hello, and welcome back to Today I Learned: Climate. I’m Laur Hesse Fisher from the Massachusetts Institute of Technology. All this season we have been answering climate questions from our listeners and readers, and in this episode, our season’s final episode, we thought we’d ask a pretty basic question of our own.

Why do we hear so much about the dangers of the Earth warming by 1.5 or 2 degrees Celsius? Who picked those nice, round numbers, and why, and what do they mean?

To answer these questions, we invited in an expert in international environmental policy.

MI: My name is Maria Ivanova and I'm the director of the School of Public Policy and Urban Affairs at Northeastern University.

LHF: So, you may already know that these numbers—and in particular, 2 degrees Celsius—are an agreed-upon goal for limiting climate change, shared by pretty much every country in the world. But before we dig into where they came from, we should be really clear about what these numbers mean.

MI: We have two climate goals. We have a climate goal that limits the global temperature at two degrees Celsius above pre-industrial levels and an aspirational climate goal of 1.5 degrees Celsius.

LHF: Okay, let’s take a minute to unpack that. These numbers relate to what is called the world’s “pre-industrial temperature.” That means, basically, how warm the Earth was, on average across the year, before we started burning massive amounts of fossil fuels. Scientists usually use the baseline of the 50 years between 1850 and 1900 for those numbers, mainly because we have enough temperature records from that time. And those records tell us that the Earth’s surface temperature on average across the year was about 13.7° C, which is about 57° Fahrenheit.

So the goal is not to get 2° higher than that. And every day, organizations like NASA are using a massive network of weather stations and satellites to track the world’s average annual temperature and see where we stand.

MI: A global temperature goal is like a speed limit. It gives you a sense of, oh, here is the boundary, here is what I should not go beyond. Also, think about it as the speed limit that flashes at you. When you are driving toward a sign, you can see, oh, the speed limit is 30 miles an hour and you are approaching it at 50. You hit those breaks.

LHF: Right, a global temperature limit doesn’t tell us how to slow down, just what we need to slow down to. So that’s what our 2° C climate goal means. 

But why two degrees? Well, that story begins around 1975.

MI: So in the 1970s, Professor William Nordhaus, an economist at Yale University, suggested that global warming had to be limited to two degrees Celsius on average, otherwise the global conditions would be pushed past any point that human civilization had experienced.

LHF: Yeah, 2 degrees didn’t come from a climate scientist. It came from an economist who wanted to explore what climate change meant for the economy and for public policy.

Prof. Nordhaus looked up what scientists knew about the world’s temperature over the few hundreds of thousands of years that humans have existed. And he learned that, in this time, the great ice ages and the very warmest periods between those ice ages were only separated by about 5° Celsius. And since we’re in one of the warmest periods now, it would take only 2° of warming to push us off the top of the chart.

That nice, round number was pretty memorable. So, as scientists and policymakers grew more concerned about climate change, they kept returning to that 2 degree number and exploring what could happen if we crossed it.

MI: And I think that that resonance of a very clear number, of a very clear target, honestly, this is what made it stick as a concept. And this is what has led politicians to hear it, to see it. And then that has led to scientists saying, oh, okay, if there is political attention, if there is need for it, we better produce some studies.

LHF: Those studies increasingly found that the consequences of 2 degrees of warming really are alarming, from more frequent and intense storms and heatwaves to whole habitats erased by thawing tundras and rising seas.

And policymakers were following that science with a growing level of concern. Enough that in 1992, over 160 countries, including the United States under President George H. W. Bush, joined an agreement creating what we call today the UNFCCC—that’s the United Nations Framework Convention on Climate Change.

MI: Countries began meeting in what we now affectionately call as COPs, and it stands for conferences of the parties. Annually, countries, non-governmental organizations, scientists, businesses, meet at these climate conferences and they reaffirm a global goal and individual and collective commitments.

LHF: The first COP happened in Berlin in 1995, and now we call that COP1.

The UNFCCC called on its members to prevent dangerous human interference with the climate system. But at the beginning, what “dangerous interference” meant went undefined.

MI: But in 1996, the European Union proclaimed that it will limit its emissions so that we would globally achieve a goal of two degrees Celsius.

LHF: Which is how that suggestion by William Nordhaus became a clear goal adopted by an entire region of the world.

Now, it wasn’t smooth sailing to go from the European Union adopting a 2 degree warming limit, to the whole world pledging to keep below this limit. In fact, there were over 20 years of negotiations before there was a global agreement.

The UNFCCC came pretty close to a global climate agreement in COP3 in 1997, with the Kyoto Protocol, which included quotas for how much member countries were allowed to pollute the climate. But the Kyoto Protocol wasn’t ratified by every country. Most notably, it wasn’t ratified by the U.S.

It would take until COP21, held in Paris in 2015, before the UNFCCC put together a framework for overcoming climate change that virtually all of its members were ready to adopt. In Paris in 2015, almost every member government ratified an agreement that 2 degrees of warming was dangerous, as well as a framework for avoiding it.

MI: By the international agreement that we have, which is the Paris Climate Agreement, the goal is two degrees Celsius above pre-industrial levels with an aspirational climate goal of 1.5 degrees Celsius.

LHF: Wait – what? Where did that 1.5 degrees come from?

MI: AOSIS, the Association of Small Island States, was the one that really ran with this 1.5 degree goal in the Copenhagen COP in 2009. It was AOSIS that pushed for it and they didn't let go.

LHF: AOSIS is an alliance of countries like Fiji, the Maldives, Haiti, Papua New Guinea—39 of them in all. And they are tiny polluters. I mean, especially compared to the big drivers of climate change like the United States and China and the European Union. And, most importantly, they are islands and coastal countries, many with almost all of their land very close to sea level.

MI: The countries that make AOSIS, have been the most vocal proponents of the 1.5 degree because of their particular vulnerabilities to climate change. Because of sea level rise, because of the very existential risk of losing their territory. Where will these people go? When? And how will they survive, not just economically, but culturally, and what happens to their identity? There's a lot of discussion with New Zealand, with Australia, will you take these populations, but what happens to their language? What happens to their culture?

I remember being in the Copenhagen COP in 2009, waiting in that line for days to be let into the venue in the cold and hearing the chants again and again, 1.5, staying alive, staying alive.

LHF: And that advocacy, by some of the most vulnerable nations in the world, would go on to shape both the policy and the science of responding to climate change.

MI: What is really fascinating about the 1.5 degree goal is how much media attention it received when it was originally proposed. But the IPCC decided to do a separate report on a 1.5 goal.

LHF: The IPCC is the Intergovernmental Panel on Climate Change. It’s a group of hundreds of scientists around the world convened by the United Nations to summarize everything we know about our changing climate.

MI: And for the IPCC to do any report, it needs peer reviewed literature. Well, there was none on a 1.5 goal. And when the IPCC says we need it, a lot of scientists jumped in and produced that peer reviewed literature. So the demand from the policy mobilized the science, and science has really stepped in and has now shown that there is a difference between two degrees Celsius and 1.5. And that if you have global warming over 1.5 degrees, the ecosystems upon which we depend will suffer tremendously compared to even that two degrees Celsius target.

LHF: And that’s how we wound up with two global goals for halting climate change.

The very fact that there are two goals tells us something else that’s important. 

MI: The existence of goals can create the perception of a red line, that irreversible step into the abyss. And yet what we are living through right now is a layered reality of more and more intense consequences, whether these are fires or droughts or floods or increases in temperatures.

LHF: What we know from climate modeling is that 1.5° of warming brings some real consequences, 2° is worse, and 2.5 would be even worse than that. And that means that every tenth of a degree of warming that we prevent saves lives.

But let’s ask one more question before we end the episode. What does it even mean to say that the world has adopted these targets? I mean, are they enforceable if we miss them?

MI: We have a global agreement that is binding. But there is no global police, there is no global court where countries can be taken to and penalized if they promise and do not deliver.

LHF: You might have heard that we are fast approaching our 1.5° speed limit.

You could choose to look at that cynically. You could say, well, since there’s no penalty for going over this 1.5° speed limit, of course the world is. Or you could look at the impact that the 1.5° goal has had in less than a decade since these goals were adopted.

MI: And this is where global goals are important because they inspire certain action. And while we cannot necessarily enforce a particular type of behavior, we can incite it.

Because of these goals, the growth in renewable energy—it has been tremendous across the world. You see it in the United States, you see it in China. You can look at the regulatory measures and the policies and you see change across various levels of governance. So to me, we cannot negate the importance of so many countries, of so many companies, having started climate plans and changing their fundamental behavior. And I do not think that would have happened had we not had this international agreement and the goals that it articulated.

LHF: That’s our episode today—and the end of season six of Today I Learned: Climate. But as we work on our next season, we want to hear from you. We want to hear what your questions are about climate change, whether simple or sticky. So visit climate.mit.edu/ask or leave us a voicemail message at 617 253 3566, and ask us. We might just answer your question on the air.

TILclimate is the climate change podcast of the Massachusetts Institute of Technology. Aaron Krol is our Writer and Producer. David Lishansky is our Audio Producer. Michelle Harris is our fact-checker. The music is by Blue Dot Sessions. And I’m your Host and Executive Producer, Laur Hesse Fisher. 

A big thanks to Prof. Maria Ivanova for speaking with us, to Lindsay Fendt who did the original reporting for this episode, and to all of you for listening. Thank you for your climate curiosity.