Transcript: Stefan Rahmstorf and Matt England - Climate science and Paris COP21

As part of an article for University of New South Wales magazine, Uniken, Alvin Stone sat down with Prof Stefan Rahmstorf and Prof Matthew England to talk about their careers, how climate science has changed and what the Paris talks means for action on global warming. Below you can find a full transcript of the interview session that goes beyond what appears in the magazine. 




Question: How did you get involved with climate research in the first instance? When I talk to students, for many of them, it appears to be they fell into this field almost by accident.

Stefan: I decided to move into ocean science during my physics studies. It basically started off by doing a year abroad in North Wales. I've always loved the oceans. I grew up in Holland by the North Sea. After I had taken one year of physical oceanography I decided this is the thing for me, I loved doing that. So, I very deliberately chose that and I have never regretted it.

Matt: I have to say my story is  crazily similar to those students you mentioned. I was doing a maths and physics major at Sydney University. I was about to finish my degree, and panicked. What was I going to do with this degree that didn't lead me to jobs I thought it would? Flicking through a handbook I saw a big ocean wave. It was an advert for a physical oceanography course, so I fell into it by accident. That was such a good discovery because it was a course that combined maths and physics, studying the ocean and going off in research vessels.

Within a year of that time I was experiencing trekking across the Pacific Ocean from Tahiti to Ponape for 30 days with 30 Japanese men on a Japanese ship. What an experience.

From very early on in my career, I got to see some of the wonderful aspects of what oceanography is all about. To study in a global community where there is a genuine need to branch across nations and discover aspects of ocean circulation.


Question: How did you become aware of each other's work?

Stefan: We met in person at a winter school for postdoctoral scientists…

Matt: Yeah, that's right.

Stefan: Les Houche in France, in the French Alps.

Matt: You were just out of your PhD from New Zealand. I was just about to finish my PhD from Sydney University. There were late PhDs and postdocs in there, so it was wonderful winter school in the snow, in the Alps. We got to play table tennis, drink beer and talk about all things climate.

But actually, Stefan may have not have realised it, but I put two-and-two together. I had seen him give a lecture about two years before that in Hobart. At the time I thought this guy is a fantastic presenter…

Stefan: That was my first conference, during my PhD. It was the first time I went abroad during my PhD - in fact it was the only time.

Matt: I seem to remember you gave a presentation on a very simplified model of climate change — like a one-dimensional radiation balance model — something like this. I went there to see Stuart Godfrey my adviser give his presentation. I think Stefan was either before or after. Stuart was a classic academic, he was all over the place, disorganised, his thoughts were going in six different directions… but he was a brilliant scientist.

Stefan gets up and he was a brilliant scientist but his presentation just flowed. I thought this guy knows his stuff. I didn't see him again for two years until the meeting in La Houche.

Question: How long ago was this?

Stefan/Matt: (talking over each other) 1990, 1991?

Matt: Les Houche was 1991.

ON THE ROCKS: Matt and Stefan during the Uniken photoshoot. Picture: Nick Cubbin


Question: Interestingly, you both came to this discipline before climate science really hit the headlines, which didn't start happening until 2001. But the science was pretty well-established. Give me a sense of how things have changed over the science in the period that you two have been involved with it.

Matt: Back then the very first things I looked at were simulations from a climate model that was used to project a response to greenhouse gases.

Way back then, papers were appearing in Nature documenting how the world's climate would change. The IPCC was formed… the first report came out in 1990. There was already a clear appreciation within the science community for the urgency of communicating this. The UNFCCC was signed off in 1994, so there was significant progress towards action on greenhouse gases. There was an awareness in our community.

But the hijacking of the science really kicked in post-2000. There may have been the seeds of it before then but the closer we got to real action on reducing emissions, it has meant there has been an absolute building up of this anti-science campaign. It has been quite revolting to look at both from within the community and sometimes as an outsider sometimes seeing colleagues being persecuted for doing their best to advance the science.

In the 90s, it was a fun field to be in and it didn't feel like there was that hijacking of the science that we were pushing out to the community. It was a cleaner process, I think. I don't know what you think Stefan, it was just…

Stefan: Well, while I was doing my PhD, it was in 1988 that Jim Hansan said famously, "global warming is here", in some testimony before the U.S. Congress. I quite clearly remember that because that was quite controversially debated in the scientific community, particularly whether that was premature to already be so sure about this or not.

There was of course already back then a political discussion about climate science. We were all aware that climate is changing and it definitely has repercussions for humanity. I just want to remind everyone that the first expert report by a panel of the president's science advisers in the US on global warming was published in 1965, warning of the impending CO2 caused global temperature rise and sea-level rise etc.


Question: Do you think the fact that Al Gore was the prominent person bringing climate change into the public’s eye is why we see in the US and other conservative governments a resistance to climate change, because he is a Democrat? Had it been a conservative who did An Inconvenient Truth, would we be having this discussion?

Matt: I don't think Al Gore is the reason for the resistance. Maggie Thatcher had some very strong statements about climate change and the need to move away from fossil fuels when she was at the helm. I can't think of a more conservative politician than her in the last quarter of a century.

Stefan: It's the same in Germany. We have Angela Merkel, who now of course is our Chancellor but she was environment minister. She was very strong about global warming and she is from a Conservative party. I think in the US there is a certain polarisation around Al Gore as a person but in Germany that doesn't play a role at all.


Question: When did both of you recognise that climate change was a very serious issue and that we would have to readjust our society to combat it?

Matt: For me it was back in the 90s looking at the papers showing projected levels of warming, sea level rise… It was already very clear from those metrics that something had to be done.

But fast forward to when I was doing the Copenhagen Diagnosis, a lot of the work as a scientist is very focused in a narrow area.  The best scientists really just nail a problem. They really focus with myopia on the problem that they want to be expert in and they become expert in it. That is part of being a scientist to be narrow thinking.

To me doing this Copenhagen Diagnosis that Stefan was also involved with - it sounds silly because it would be in 2009, 20 plus years after I was on the field - but just seeing across the portfolio of sectors to me that was confronting.

And also by then, I guess, I had seen what it would take to act on this.  Back in the early 90s, I didn't realise the political hurdles of getting through policy to address climate change. I just thought this would happen because we tend to have law come in, or adjustments to policy, for the betterment of humanity. This has been absolutely ridiculously stalled.

Stefan mentioned the 1965 report. That's 50 years ago, right? Must be close to the 50th anniversary.

Stefan: It's the 50th anniversary next month of the report.

Matt: We had the UNFCCC signed off in 1992: committing to avoiding dangerous interference in the climate system. Yet carbon dioxide in the atmosphere is 30% higher since 1992. That's a catastrophic failure.

All of the science of saying we need to address climate change was actually well established in the 1960s already. To see such a glacial pace of policy change…  By 2009 when we did this Copenhagen Diagnosis, I guess I was much more alarmed by then because I saw the absolute lack of policy engagement.

When you have got feedbacks to the system… The track we are on by then was so well-established towards the business as usual scenario that it was a confronting couple of years for me. It was post-2007 optimism when everyone thought we were going to nail this and then Copenhagen kind of fell over. The political statements coming out of Copenhagen were as though we had an air conditioner that we could sort of switch on and off for 2°C plus warming. So, by then it was kind of scary to see a lack of action on something that was going to so fundamentally change the lives of the whole of humanity.

Stefan: I was definitely aware of the global warming problem since the early 80s. Just to give you an example, I spent four years in New Zealand doing my PhD and I never flew home because I didn't want to emit more CO2.

The only time in those four years that I was outside New Zealand was for that conference in Hobart. That entirely had to do with global warming and the aeroplane emissions.
SURF'S UP: Sometimes the most unexpected things happen during a photoshoot. Picture Nick Cubbin


Question: Both of you are very strong communicators, Stefan through RealClimate and the media you have done through Europe, while Matt is everywhere over here. Both of you have been attacked for being outspoken. That journey from being just a researcher to a communicator, what did it involve and why did you take it?

Stefan: I think it probably involves a certain desire and ability to explain things, to teach things. I am also of course a university teacher. From when I was a boy I loved reading popular science books and I just enjoy and am fascinated by science.

I like explaining things and I got into getting positively and actively engaged in the public information on global warming after we had the Elbe River flooding in 2002, in Germany.

Until then, I had perceived on the periphery that there were nonsensical climate sceptic articles appearing and some journalist writing a lot of nonsense about our science. But I had just thought, as other colleagues thought when you talk to them about it in a coffee break or so, saying, "that's so ridiculous, it's not even worth bothering with or responding to because it's just crazy".

But then we had this very serious flooding that put Dresden underwater and other towns, major news magazines were writing articles blaming it on solar activity. They were showing a curve of solar activity that in the scientific community was long recognised as wrong. Some fundamental error with the original paper had seen it withdrawn and corrected. And there it was rolled out as saying, it's not CO2 it's due to solar activity. At that point I thought, we just can’t let this go on in the media as scientists if we know better. We have to actually stand up and explain this is wrong.

That is how I wrote my first newspaper article countering some of the disinformation that was circulating.

Matt: For me it is not too dissimilar. I had a really strong sense of injustice actually. I hated seeing the attacks on the incredibly hard work that went into proving scientific concepts were valid and disproving other crank theories. To see the airspace and print space given to these crackpot ideas from climate deniers… it just really angered me actually to see whimsical ridiculous comments from retired geologists being put forward as expert commentary in say a paper like The Australian.

On sea level research, Australia has John Church, a fellow of the Academy of Science, one of the world leaders in sea-level rise physics being ignored in an article about sea-level rise and going to, whoever it was, Bob Carter, I think, for expert opinion. I felt so angry the scientists were not being consulted.

The other thing I would say, in that anger I realised there was actually a readership of around hundred thousand people at this newspaper — or whatever it was. We get really excited when the great scientific papers might get downloaded 10,000 times while a really good paper might get downloaded five times. So, I had a sense that the scientific communication we had for our specialist journals was very specialist and not getting out to the public. That worried me. There was so much good science going on but the IPCC cycle was very infrequent, every five years or so. I had a sense that the science was just being buried in the literature that we read and the public was seeing none of that.

I wanted to get the science more into the mainstream media where the readership was so much greater.


Question: There is an argument that goes on in the community all the time about where communicating science ends and advocacy begins. The question is often asked, are you advocates or scientists? Some people are quite comfortable that the two go hand-in-hand while others say we should talk about science and just the science. Where do you two fit on that spectrum?

Stefan: I think this term advocacy is basically used to discredit scientists speaking openly about their science. When, say, a lung expert recommends not to smoke you would never say, "you are an advocate, I don't trust you any more".

Actually, we had a popular vote on new anti-smoking laws in Bavaria in Germany and the professional organisation of lung experts actually actively campaigned in this vote to make people vote to ban smoking in public places. That would be unthinkable for climate scientists to get in any campaign supporting renewable energy or cutting emissions or something like that.

I think there are double standards being applied here to climate scientists.

Matt: I was going to bring up the smoking analogy too. For me, I don't understand the obsession with this word as well. A couple of years ago, Kim Carr, when he was science minister, held a forum that I got to speak at. He more or less said, and I thought it was a brilliant speech from the Science Minister, he said I want you guys to advocate. It wasn't just climate scientists, it was a collection of scientists across a broad spectrum of disciplines. He said, "I don't want to hear this ridiculous comment that scientists shouldn't advocate. You are the most expert people in all these sectors that really impact society. Get out there and tell us what we should be doing. Don't hold back".

So, I agree with Stefan. It's a shame that people bring up this word time and again with climate scientists. How will people judge us in 100 years time if there is a perception that we didn't get out and advocate for the science to get policy changes in place to secure a safe climate future? Anything but championing the science would be ridiculous.

Having said that, I also think when it goes through to policy implications sometimes, because there has been such little uptake of the policy required, getting out and saying what the policy changes required is, again, completely appropriate. Why wouldn't we say that we have got to reduce emissions drastically? We have got to make that sort of comment.

And even advocating for political leaders who are pushing that kind of line, again I don't see any problem with doing that.

Stefan: You know, for eight years I was serving on a government advisory council for the German government appointed by the German government. We were specifically tasked to develop recommendations for the government and what they could be doing about certain issues. My rule is that I don't speak on things where I have no expertise. I'm not a renewable energy expert for example, so I can't give any specific recommendations in that area unless I am a member of the panel like that where you really develop those recommendations in an interdisciplinary way. We had economists on this panel, renewable energy experts, an expert engineer on the panel and so on.

Then when you jointly looked at an area thoroughly and developed recommendations for the government then of course I am going to advocate, in the sense that I inform the public that this is what we recommend.


Question: There is some discussion at the moment about whether or not the 2°C target is achievable or not. The two of you have slightly different positions on this. Stefan, you think it is still possible while Matt is more doubtful. Could you explain your points of view?

Matt: I'm doubtful, but it is possible, in the sense that it is possible to give us a chance of keeping it below 2°C warming. But I believe the estimates at the moment are that if we stopped all emissions right now globally, the lag in the climate's response to even our current level of emissions would still bring us in at 1.5 to 1.6°C above preindustrial times. It could even be as high as 2.5°C with all emissions stopping today.

With drastic, drastic emissions reductions we can give ourselves a chance of staying below 2°C. My comment of pessimism was probably around seeing where all the trajectories are across all of the nations and seeing the political process that has to play out on the international stage. We have to get the developing nations to the point of not doing what we did and becoming so reliant on fossil fuels for their energy. All of those low carbon technologies need to go into those nations.

I know Stefan has used this analogy as well, it is like a war effort is required to address this problem. This problem will have the ramifications of a nasty war if we don't deal with it, so why not have a war effort to reduce emissions. Until that happens I think the 2°C target is highly unlikely to be achieved. Stefan may differ.

Stefan: I think that is the key point that you shouldn't mix up political pessimism with what is physically possible, what is technologically possible or what is economically possible. I think it is, from the point of view of geophysics and climate science, there is nothing stopping us from keeping warming below 2°C, or even below 1.5°C. I don't think we are locked into a warming above that level and I don't see any evidence for that.

The technological question; I would say technologically, yes, it is entirely possible because we already have the alternatives. We know what the alternatives are. We have the technologies for renewable energy to supply 100% of our energy needs. For that we just have to tap less than 1/10,000th of the solar radiation that reaches the Earth. Just harvesting a tiny amount of that will generously supply the whole of humanity with energy.

The economic question: how much would this cost to stay below 2°C has been very thoroughly answered by the last IPCC report. Most people would be very surprised by how low the estimated costs are at 0.06% of GDP each year, global GDP. If you assign that in billions of dollars it's a big sum of course but what it means is that rather than having 2% of growth you would have 1.94% of growth.

Matt: The other way I saw it framed was that global economic wealth at 2050 would come in eight months later, or something like that. It is barely indistinguishable and of course you have got to think about the costs of all of the impacts of climate change if we do nothing. They were added into those calculations but I think the error bars on those estimates are probably much more uncertain at the high-end. So, there could be significantly higher costs from the impacts of climate change.

Stefan: I would say it is almost certain that if we don't do anything the impacts of climate change will be far more expensive than making that transformation now. These calculations with limiting global warming to 2°C, we will reach the same level of wealth may be one year later. So, in 2100 — and this is, by the way, this is about an eightfold increase in global GDP according to what economists expect. It's not like we're getting poor, we are getting very rich but maybe not in 100 years but in 101 years — everything slightly slower. These calculations do not account climate damages. If you factored in the climate change damages then on pure economic grounds it would make sense to limit global warming to below 2°C.

So I would say, it is physically feasible, technologically feasible and economically feasible. It's comparable to other economic efforts that we have had in the past.

If people are pessimistic it is usually because they are pessimistic about the political process. We shouldn't mix that up because even if we resigned ourselves to start saying, “we won't make it, it isn’t possible”, this is just defeatism. It gives politicians an excuse not to get their act together and act responsibly because politicians can then say, "oh well, even the scientists are saying it's impossible, so we're let off the hook".

Matt: It's a very good point. It's natural for a scientist to get pessimistic. They are working in the field for 30 years seeing no political action on something they have been communicating for about three decades. Of course some pessimism is going to creep in.

I think Stefan is entirely right, this is so dangerous because politicians just want the smallest excuse not to deal with this problem.

Stefan: I find that, for example, colleagues from the United States are far more pessimistic about this because they experience everyday this highly polarised debate and no progress. Whereas we in Germany are rather more optimistic about the opportunities and possibilities to limit warming because last year German greenhouse gas emissions were 27% below the 1990 level and our economic output, GDP, has almost doubled since 1990. So, we have doubled GDP with 27% reduction of greenhouse gases. That just shows you can decouple the growth of emissions from economic growth and prosper.


Question: Now that you have been in Australia and had a chance to compare the two countries, why do you think there has been this very different approach to reducing greenhouse gases when it seems that Australia has far more opportunity for the use of renewable energy sources?

Stefan: Well, first of all, when I look back at when I did my PhD in the 1980s in New Zealand, I looked across to Australia and there were some very good and enlightened things coming out of Australia about global warming. This seemed to be a good awareness. So, I think it is changed since the late ‘80s.

There has been an analysis of media coverage of climate sceptic misinformation — an international analysis done by a British media expert. He found that this is a particular Anglo-Saxon phenomenon, that you get this large volume of sceptics all over the media. I think they have really slowed down the progress and polarised the debate, especially in the Anglo-Saxon countries. This is true in Britain and the US and Australia and New Zealand as well. In other countries that were studied like Germany or Brazil and some Asian countries, that was not the case at all. It was not split along party lines for example.

In Germany we have a Conservative party and the social Democrats all agreeing on the problem and everybody agrees on reducing warming below 2°C. Basically they are just arguing that the other party isn't doing enough to reach that goal and maybe how you should do it. Will you put the costs more on the energy utilities or the consumers, this kind of thing.

Matt: The Brits just came out and said this is no longer a political issue. The three parties came in and said we see climate change as something that needs to be dealt with.

In terms of the question, my first thought was in regards to the US is how do you answer the question of the different approach to the debate for the US? Whereas in Australia the fossil fuel sector that has been so strong and so vocal that they have effectively campaigned by getting behind some of these well-known Australian climate change sceptics — people like Ian Plimer. These people have made a lot of money out of being climate change deniers. They get flown around the world to give talks. Their careers are ending and suddenly they are revitalised by this process. Money comes into fund that.

There has been some wonderful analyses by different people on how this works. The newspapers get on board.

I have always seen it as this fossil fuel centre in Australia that has really led to us being a hotspot for climate change deniers.

Stefan's point is interesting because you also see this in the US.

Stefan: You also have a problem in the US that money plays a very large role in politics and in the media. There is also this corporate ownership of media.

In Europe, we have more public media and you don't have to be a millionaire to become German Chancellor. You can just be the son or daughter of some priest like Angela Merkel. We don't have this great influence of money influencing policy. It's bad enough but it is not as bad as in the US.

Matt: In America it is also a free marketeer campaign, with the Heartland Institute and places like that that really advocate small government and lack of intervention in business. There is a perception that climate change will put constraints on the free markets.

Ironically, the whole fact that they have been slowing down action on climate change means the chances of this government intervention having to happen go up. The amount governments will have to intervene will scale up with every year of inaction.



Question: in the six months leading up to Copenhagen it was very apparent that those talks were going to fail. That doesn't seem to be the case as we move towards Paris. How do you compare the two talks ad do you see any differences that should give us more hope?

Stefan: The are a lot more optimistic signs now especially for example because the United States and China have agreed on what they are going to do to reduce emissions. There is now a commitment from countries representing 80 or 90% of emissions. Although these commitments are not enough yet to make a 2°C limit, I think that also wasn't really to be expected given we have more than 190 countries and a consensus decision process. This is almost impossible.

If you tell somebody 190 countries with very diverse interests by consensus agree on some pretty strong measures that will affect their economies and everything, it's not just peanuts it's a real transformation of the energy system. That sounds like a recipe for something that is totally impossible, so in a way it's amazing that we have come this far and I think we will reach a meaningful agreement in Paris. I'm quite optimistic.

But Paris alone will not deliver enough to stay below 2°C. I think it would be utopian to believe that.

I think number one it is critical that Paris has mechanisms to tighten up the commitments later on, just like the Montréal protocol. The original Montréal protocol to reduce CFCs was far too weak but it had a mechanism and it was tightened up in subsequent years.

Doubtless we will see a similar thing with carbon dioxide.

Secondly, I think, after Paris a strong coalition of pioneers, decarbonisation pioneers, has to form because we need a strong group of countries to go ahead and set an example and move this whole transformation forward. We can't wait in the consensus way for the slowest in the whole process, we have to have a committed group that surges ahead.


Question: Do you think China has a big role in that process?

Stefan: I think China does have a big role, yes. They really understand the problem. They are not as paralysed in polarised politics as the United States and they are already the biggest investor in renewable energies.

By the way, the renewable energy story also gives me real sense of optimism because we are much further along in renewable energy development now than we were in Copenhagen in 2009. Last year, already more than half of all the investments in energy infrastructure went into renewables.

Matt: That is despite many nations not having the policies in place to encourage growth of that sector.

Stefan: At the moment, 19% of global electricity supplies are renewable. It has really come out of a small niche. It has been growing exponentially and now it has reached a level where business comes in. Exponential growth is very subliminal for many doublings and doublings and doublings. Solar electricity doubles every two years, I think. Initially you hardly notice that space below the 1% margin and then it really takes off. That's what renewables are doing right now.

I think, basically investment in fossil fuels will collapse over the next few years or so because investors will realise there is no future.

Matt: Yes, that is the big hope for Paris. As Stefan said, the cuts aren't going to be enough on their own but sometimes the business sector will respond. My hope is that the business sector will rush away with this because a lot of the investment in energy infrastructure is not to make money for the next year, it is to make money over the next 20 or 30 years.

Big investments like that, it's too risky to invest in fossil fuels. I hope that all the right signs come along from the business sector to run way ahead of the policy. To be dumping fossil fuels and investing in renewable infrastructure.

Stefan: At the moment it is actors like cities, universities, pension funds that are in this direction of movement and getting out of fossil fuels. Number one, for ethical reasons but also because this famous carbon bubble, explained by Lord Nicholas Stern, which is going to burst sooner or later. These fossil fuel investments will turn out to be bad investments


Question: Suppose we were to go to Paris and come out with emissions reductions that increased warming by 3°C. What would a world 3°C warmer than preindustrial times look like?

Stefan: I'm very concerned about the very large-scale functioning of our earth systems. This includes things like the global ocean circulation system, the monsoon system. There are many non-linear systems here that often have a critical point, when you cross that they fundamentally change. Some of these tipping points are in the 1 to 3°C range.

If we go to 3° then we are likely to really destabilise some of the key systems, like the Greenland ice sheet. The Greenland ice sheet is likely to be destabilised if we go to 3°C. West Antarctica probably already has crossed its tipping point but there are further huge ice masses in East Antarctica. The Wilkes basin for example can be destabilised if we get too much warming.

We have in our work recently documented that there is already a significant decline in the North Atlantic conveyor belt Gulf stream system. We don't know how far away that is from a tipping point.

When we get closer to 3°C it gets more and more risky and we may destabilise the fundamental workings of our climate system compared to how it has worked throughout the Holocene. I think, we should not move out of that relatively benign stable climate regime that we have enjoyed for the past 10,000 years.

Matt: I have the same kind of comments. With 3°C the Greenland ice sheet is almost certainly gone, locking in 7m of sea-level rise.

Stefan: The IPCC reckons the threshold for Greenland is between 1°C and 4°C above preindustrial but the upper limit is unfortunately based on a relatively simplistic and weak study. I would more likely say it is between 1°C and 3°C as the limit for the Greenland ice sheet.

That implies 7 m of global sea level rise and therefore the loss of large coastal cities and small island states. This has to be said very clearly.

Question: A recent animation by NASA shows that under those circumstances China may lose half of its land mass…

Matt: Half of its populated land mass.


Question: If the Atlantic overturning circulation stops, my impression is that this will stop the transfer of warm water to the northern hemisphere and bring about much colder conditions across Europe. But what is the reverse of that if you have more heat staying around the southern part of that circulation?

Stefan: Typically you get a cooling in the northern sub polar Atlantic, which is already what we observe. There is a cooling trend seen over the last hundred years and it has been the coldest on record the last winter, which is quite surprising when the whole rest of the planet is warming.

The heat that isn't transported up there comes from the South Atlantic, so what you get is some additional warming in the South Atlantic and Southern Ocean. It is fairly diffuse and spread out so it is not a very concentrated blob of heat like the cold blob you see in the northern Atlantic because it spreads over a very large area. You do get additional warming in the southern hemisphere.


Question: There appears to be a debate in the community as to whether the warming occurs gradually and consistently or whether it goes up in abrupt steps. We've seen this in global average temperatures but is it also true of global warming more generally?

Matt: It really depends on how close you look at the graphs of temperature change. If you step back and look at the last century, it is a kind of steady, large-scale warming. Zoom in close and you see these decades of accelerated warming or relatively slow warming. Year-to-year there are bumps and kinks because of El Niño but I think it is true to say that as we go towards say 4°C or 7°C warming by the end of the century, where ever it gets to, all of the knowledge we have of the climate system says that that can be achieved in fits and bursts.

If you go through the last centuryyou can probably select seven or so year-to-year variations that bump things up. That's a lot of the warming in those. 1998 was an iconic year because of that very strong El Niño. But what has been happening in the climate system, all the time there have been those steps up occurring due to natural variability on top of climate change, we are seeing plateaus that follow. So the analogy is that after bursts of warming you are not seeing a return to normal conditions. It is actually like a new normal is established. There is a relative flat-lining in the record and then it accelerates again.

This is a conversation where we have had a lot of focus on global average temperatures but it is only one tiny metric of the global climate system. Actually, if you want to best measure global warming, that progression in time would be much more monotonic if we take the global integrated heat content of the oceans.

Yet, it's hard to measure so we have never tracked that and instead focus on global average temperatures.

Stefan: We also live at the surface of the Earth, so we are naturally most interested in the surface temperature. To me, the surface record, if you start the warming since 1970 for example, it is entirely consistent with a steady linear warming trend superimposed by random variability be that Interannual, some decadal etc. I see no physics in this idea that there is step and plateau and step and plateau. This kind of staircase idea is around I know but I haven't seen any convincing evidence to support that.

If you just generate a random time series with a linear trend plus noise, just white noise, by the eye you can subjectively pick out steps, selectively pick things where a plateau is and then goes up again, but there is no physics behind it. It is just the appearance you get from random variability.

Matt: I think a bit of physics does come into play. With some with some of these big El Niño events, basically the warm pool of water that is in the West Pacific, spreads across the whole of the Pacific and warms the lower atmosphere. Then the ensuing La Niña comes along and pushes up the warm water in the West Pacific.

La Niña events are much cooler but you're not seeing a return to the temperatures before the El Niño occurred. You are tending to see these steps maintained, so maybe there is a bit of physics in the El Niño process.

Stefan: Of course the El Niño process has physics but we would call the El Niño a classic example of quasi-normal variability. I wouldn't say El Niño by itself would cause a step like change, it is just randomly varying. Every few years you get an El Niño and then you get a La Niña year again. When you superimpose that on a linear warming trend it just looks like steep steps and plateaus.

After the ‘98 hot year you got two cool years in 99 and 2000. It is just variability caused by El Niño.


Question: What is the key issue that people generally overlook when they think about climate change?

Matt: To me it is the life of carbon dioxide in the atmosphere. It is the biggest thing people don't appreciate and they need to. What I mean by that is a lot of these gases in the atmosphere, like methane, their lifetime is not painfully long.

The moment you burn coal, this process is so non-linear. It takes millions of years to form coal but you burn it in an instant. Unfortunately, once it is in the atmosphere it stays there for an awfully long time, because the removal process — like forming coal — is incredibly slow. People keep thinking, like any problem humanity faces, if we don't solve it this year we can get to it a year later or maybe a decade later. Unfortunately with carbon dioxide it stays in the atmosphere for so long that it is completely impossible and impractical or too costly to remove it.

People don't get this. They don't get that delaying action on climate change means we are just jumping up the amount of CO2 in the atmosphere. It can take thousands and thousands of years to remove it naturally.

Stefan: I was going to say exactly the same. The key issue that people don't understand is that this is irreversible. We cannot afford to make a mistake. You can't decide 20 years down the line, oh well, we should really do something about global warming because it is going to be too late. Many of these effects we are going through now are simply irreversible due to the long lifetime of CO2. Then due to triggering tipping points like the Greenland ice sheet, sea-level rise, they act on such a slow timescale that from our emissions now we are causing a committed sea-level rise of metres unfolding over many hundreds of years into the future, for many generations.

This irreversiblility is the key thing.

Since you already mentioned that, I will mention another thing that people don't appreciate. CO2 emissions also cause ocean acidification, which, by itself, would be enough reason to stop emitting this because it is destroying the ocean systems.


Question: Should we be even having a conversation about geo-engineering?

Stefan: No. I think it is a desperate measure. We are not at the point where we need to implement desperate measures because we still have much better alternatives at our disposal. The whole geo-engineering debate simply has the effect that the general public start to think that maybe there is some kind of technological fixes down the line. So, it just is, as Lomborg uses it, another excuse not to reduce emissions now.

Matt: I completely agree. It shouldn't be discussed by virtue of the fact that it ruins the momentum of addressing the problem. It gives us a completely false sense that we can solve this with other approaches.

Every single technology that has been put up either has impacts that are worse than the ones we are trying to solve, way more costly than the renewable approach and decarbonising or are incredibly risky to ecosystems.

I think there is a lovely summary in one of the special journal articles on it. It is basically either too costly or too risky. 

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