Thomas Stocker was born in Zürich and obtained a PhD in Natural Sciences of ETH Zürich in 1987. He held research positions at University College London, McGill University (Montreal), Columbia University (New York) and University of Hawai'I (Honolulu).
Since 1993 he is Professor of Climate and Environmental Physics at the University of Bern. From 2008 to 2015 he co-chaired Working Group I "The Physical Science Basis" of the IPCC, the United Nations Inter-governmental Panel on Climate Change.
Thomas Stocker has co-authored more than 200 publications; he holds an honorary doctorate from the University of Versailles and has received several awards for his work. He is a Fellow of the American Geophysical Union and a Foreign Member of the Accademia Nazionale dei Lincei.
© The Open University
Stories of Change project
Thomas Stocker interview
RH : = Roger Harrabin, Interviewer
TS: = Professor Thomas Stocker Intergovernmental Panel on Climate Change, Climate Scientist, Participant
RH: This is for the BBC and Open University, all our interviews start with the same question; when did you first get interested in energy?
TS: I got interested in climate and it's through the backdoor of climate that energy became an issue. But I must say my primary research interest and competence and activity has been around the question of climate change.
RH: But you're now drawn into energy de facto because it's so important with everything that happens to do with the climate.
TS: Absolutely, but I would not call myself an expert on this.
RH: There are a lot of people, you probably don't come across many of them, but there are still a lot of people in the world particularly in the USA, some in the UK, who think that climate change is all natural. How are you certain that it's not?
TS: Well, just look at the measurements that many societies, the World Metrological Organisation, but every reasonable Met service of every country around the world is taking. I have in particular a paleoclimatic perspective where we see how much the earth's atmosphere, every composition has changed in the past 800,000 years. Taken altogether it's very clear that climate is changing in many aspects over the last 50 years in an unprecedented manner, over decades to millennia. So it's the measurements that actually speak a very clear language.
RH: Another thing that makes people sceptical is the pause. Climate scientists predicted very confidently that the climate would continue to warm, but it hasn't, at least not surface temperatures anyway.
TS: Well, here you have to really take a very careful look and not gloss over the details that the climate system offers in terms of information from all components of the climate system. We have learnt in our recent assessment that the ocean is a fantastic integrator of climate signals and over 90 per cent of the energy absorbed by the earth's system is residing and integrated in the ocean. Clearly visible in the measurements, an ocean that actually integrates over the shorter term fluctuations in terms of climate change, 15 years are simply not enough to make a reasonable statement about long term climate change.
RH: But that wasn't known in the past. In the past scientists were really pretty confident that the temperature was going to continue to go up. They didn't realise the oceans would buffer climate in the same way. I remember going to several MET office presentations along those lines.
TS: That's not true. I mean if we look at the statements of previous assessments, for example, it was always clear that we are not making a statement on a year-to-year basis or on a 15 year basis. In fact, in the past assessment we made it very clear by quantifying what it means to consider a 15 year temperature trend in the global mean. If you start your analysis in 1998 to 2012 you get 0.05 degree Celsius warming per decade. If you had started your analysis only two years earlier, you would have received a trend that is almost a factor of three larger than that number I just quoted, indicating that conclusions based on 15 year trends are simply not reliable indicators of long term climate change, climate change over many decades, over centuries that we're talking about here in this debate.
Borrowing recent results that also give us a much better understanding on the physical processes on decadal climate change, which components are involved, the winds, the ocean, the absorption of heat, etc, etc. but also the quality of the data. Recent observations and re-analysis of data indicate that there are also uncertainties with the simple temperature measurements.
RH: The University of East Anglia Scientist Phil Jones said, when asked about the pause, five years is far too soon to tell any trend whatsoever. If after ten years we're still in a pause then we'll start wondering where our models have gone wrong. And we're now what, 18 years now?
TS: No, the pause has gone away to a large extent. If you believe the latest scientific papers in the peer review literature. But I would say …
RH: I'm sorry to interrupt you, so if you believe it, so we've had contradictory reports along the years, haven't we? And why should we believe this report rather than the others?
TS: What we have in front of us is an assessment that has carefully followed the procedures of IPCC which consist of multiple stages of review, in fact almost a public review over the internet. Otherwise we couldn't have received 54,677 comments in the working of one contribution. That's one thing.
The other thing is that we are basing our conclusions on multiple lines of independent evidence, so we would never make a conclusion in a summary for policymakers basing our conclusions on just one single peer reviewed paper. So you have to consider what was the outcome of the last assessment and put that into context with any individual scientist's statement regarding which is the number of years precisely that should be taken into account to assess climate.
After all the traditional definition of 30 years is not a very bad time period. But even on multi decadal timescales the science has revealed that there is also variability, perhaps less in the global mean temperature than in the regional temperatures certainly.
RH: Would you think that looking back ten years ago that there was a certain degree of hubris about climate science? You didn't really understand enough about the oceans. You didn't really understand enough about natural variability. Do you think there should be a little bit of modesty from climate scientists who did maybe think they knew too much?
TS: I would contest this statement because if you look carefully at the summaries for policymakers as they have evolved over the course of five consecutive assessments, starting with the assessment in 1990 and now with the most recent one in 2013, an integral element of our communication was uncertainty. And we have spent hours, days and weeks to assess very carefully the uncertainty. This time around we even produced at the beginning of the assessment cycle a document that laid out how uncertainty is expressed across the three working groups so that we all speak on the same terms when we speak about uncertainty and try to quantify and assess uncertainties with indicators of likely, very likely, extremely likely, etc …
So I think the modesty has been there. The realisation that the climate system has always an inherent component of limited predictability was a guiding principle also in the design of the last assessment report where we had purposely designed an entire chapter on the question of short term climate prediction and predictability. And the scientists have actually proposed that in 2009, during this coping process, well before that highly hyped discussion of the warming pause.
RH: How long do you think the ocean will be able to absorb the excess warmth in the planet? In other words, how long will it buffer climate change?
TS: Well, the physical process is such that the ocean will warm up along with the atmosphere, there is no limit to that absorption of warmth. The much more important question is what does that warming, that pervasive warming of the world ocean right down to the bottom of the ocean eventually, cause to the functioning of the marine ecosystems, the coral reefs, etc.? What does it do physically, biogeochemically? If you compound that warming with the acidification that is ongoing.
RH: What I mean was really in the shorter term how many decades do you think we've got before we see a very strong warming trend re-emerging in the atmosphere? In other words, how long before the oceans stop absorbing the excess heat?
TS: Well, I wouldn't be surprised if we are already in the course of seeing it. We are in 2015 now in a year where Another El Niño ruffles the feathers. Whether it will fly is yet to be seen by the end of the year and the beginning of 2016, which could mark another record warm year over the entire observational period, which would equal if not surpass 1998, which was already marginally surpassed by 2014.
So one could argue that if you accept the definition of the beginning of the warming pause in 1998 then you also have to accept the fact that 2014 was the year that was even warmer than 1998, and therefore you have to accept that that is the end of the pause.
RH: If you look back over the past five years what would you say were the big advances in climate science. Obviously the pause has been a setback I'd say for climate scientists I think. Would you agree with that?
TS: The pause is not a setback. I think it was a bell that reminded us that there are still interesting effects and processes that we have not fully understood. If you look at the pause and the amplitude of the pause, the requirements for deep ocean temperatures for example to verify that the deep ocean is involved in this lessening of the temperature trend …
RH: So there was a hole in the science basically. Where has the science succeeded over the past five years? What have been the big successes in science do you think?
TS: I think there are several aspects of successes of the science. If you look at the pause assessment we have managed to find consensus on the projections of sea level rise. We have close to sea level …
RH: Well, it's a very broad consensus that, isn't it?
TS: Well, I think that is an achievement that is not to be underestimated. If you remember the situation in 2008 or 2009 when we talked about sea level rise, the published range was huge. And I think we have taken really a very careful approach to that extremely complex topic, because it involves four contributions to the sea level rise. And we've managed for the observational period to close the sea level budget, in other words the observations of the melting of the ice sheets, the melting of the glaciers, the change in the storage of water and land, and the thermal expansion of the ocean. All basically make up the sum that we observe in terms of sea level rise in the past.
RH: And what are the …
TS: And that's a great achievement in terms of …
RH: What are the projections on that? Remind me.
TS: The projections then of course are forward looking but first you have to establish the confidence in your understanding …
RH: Remind me of the numbers that we came out with in IPCC.
TS: Well, that depends on the scenario obviously. The upper end is around 98 centimetres for the likely range of the RCP 8.5 and 0.09 centimetres for the lower end of the massive mitigation scenario, RCP 2.6. So it is still a large range, but that range is dominated by the different choices of scenarios, and that’s the big point, that the uncertainties are of two natures. The uncertainty about the scenario on which path humankind will develop over the next 100 years. In other words, how much fossil fuels will be emitted to the atmosphere. And on the other side the inherent uncertainty regarding the model projections, the responses of the glaciers, of the ice sheets, etc … So that is certainly a big progress.
Another big progress is the carbon budget. That is now firmly engrained in the Working Group One contribution in the summary for policymakers, and made it all the way up into the synthesis report and became a unifying principle for the joint messages of the working group, one, two and three in the synthesis report summary for policymakers. The carbon budget is essentially the sum of all the carbon emissions since industrialisation. That means in 1750 roughly and the insight, the scientific result that that amount of carbon since the beginning of the industrialisation is related linearly to the increase of temperature in the 21st century is of primary importance. Because by limiting the temperature in the 21st century you automatically limit the total amount of carbon emitted since the beginning of industrialisation.
So you're transporting a temperature target into a target that is intimately related with industrialisation as we know it traditionally based on the fossil fuel technology and deforestation. Together that is the big new result of AR5.
RH: Any other big advances we've made?
TS: I think we now have a much better grasp of regional climate change, changes in precipitation, changes in extremely event statistics, although we appreciate that the uncertainties there are still considerably high and that the future development of research in particular in the physical science base is consisting of improvement of the models plus an increase of the grid resolution of these models. The granularity with which the models can inform us will increase, which will provide even a better information on these key processes that determine the vulnerability and the impact on regional and local communities and ecosystems.
RH: So tell me, when you look at the scenarios now, which are the areas of the climate that give you greatest concern?
TS: Everything that concerns resources. The two most fundamental resources of humankind and ecosystems are water and land. Both are challenged by climate change. Both are threatened by climate change. Water through the changes in the hydrological cycle where we have an indication that on very large scales the hydrological cycle becomes more intensive which means those areas that are dry today already will become dryer, and therefore the risk of drought and water scarcity will increase; whereas on the other side, where you are in zones of already abundant precipitation, we will experience more rainfall and a higher risk of flooding. So that's the first resource to which humankind and ecosystems have to adapt. Question, can they adapt? And that is the question of the limited capacity for adaptation.
And the other resource which is of central importance is the access to land, land on where we can live and where we can produce our food. Land is clearly threatened by sea level rise on the long term, so coastal communities are threatened in an exceeding way through the slow increase of sea level.
RH: Do we know more about the Arctic and Antarctic than we did before?
TS: Yes, for the first time in the past assessment we could project for the business as usual scenario the indication from the available climate models that the Arctic will become ice free in summer, around the mid of the 21st century, which is …
RH: Free of sea ice?
TS: Free of sea ice. This is an environmental condition which is fundamentally different from what we now know and what we have known for many millennia where we had a clear seasonal cycle of sea ice coverage in the Arctic. So for the ecosystems in that vulnerable reach we can only start assessing the consequences of such a fundamental change. In Antarctica our assessment I believe has triggered an intensification of research regarding the stability of ice sheets, the interaction of ice streams with a warming ocean where we have indication that instabilities may actually already be underway. But the definitive assessment is to be awaited in the sixth cycle of the IPCC.
RH: Your predictions are based on global circulations models, highly complicated computer models. There's a group of scientists that call themselves the Lukewarmers who say that based on recent observations of the real temperature that actually probably the climate sensitivity to CO2 is rather less than the IPCC thinks, and that maybe a doubling of CO2 which we'd get possibly around the middle of the century would bring us maybe one and a quarter or one and a half degrees of warming, which we'd be able to cope with perfectly well. What's the chances that they're right?
TS: I regard personally and based on my knowledge of the past assessment and the great care we have taken to evaluate this question from many angles, as very low indeed. The chances are even lower recently with the advent of some new research that takes another look at the observations over the past 50 years. I think this has triggered research that needs to re-evaluate all the studies that have made an argument about climate sensitivity based on observations of temperatures over the past 50 years. I would not be surprised if some of the conclusions that were advertised very heavily would actually have to be revised.
RH: People who are sceptical about climate change will say well look, they're just going back and fiddling the data. They didn't like the data so they're going back and changing it.
TS: Well, that's not true. I have an extremely convincing counterargument. The Berkley Group just started out six years ago about it was, with reassessing under exactly those premises, the global temperature dataset, very carefully, completely independent scientists; in fact they come from different strands of the sciences. And they have come up with exactly the same conclusion as the climate science community, giving an interesting testimony that the science that we present certainly through the course of an assessment under the procedures of the intergovernmental panel on climate change is extremely robust because it bases itself on multiple lines of independent evidence.
RH: You were talking earlier on about the commitments, INDCs you called them, I'm calling them commitments in my report because I think it's a bit more accessible. And the sense of hope you had that they were unlocking the stalemate. Can you tell me about that?
TS: Yes, I think it's a completely different philosophy. It doesn't supplant any of the previous efforts. But it gives it a completely new way of looking at it by handing back the responsibility to the individual countries who all face individual challenges regarding climate change in terms of exposure but also in terms of how to confront climate change in the future. Asking these countries to bring forward their contributions to reduce emissions over the course of the next few decades and beyond, and at the same time explain why their proposal is actually ambitious. This is the additional qualifier which I think is very important in that process, a so-called self-declaration of ambition. Why …
RH: And it's given you some more hope I think?
TS: It is giving me some more hope because I already now see a process where in the recent past two countries, I'm referring to China and the United States of America, a developing country with rapid economic development and a developed country who is responsible in terms of cumulative carbon emission for more than a quarter of the total emissions, making a statement collectively how to confront climate change, G7 has issued a statement, many countries have now filed their commitments. And while we know that the numbers that are on the table now are not sufficient to meet the two degree ambition, it is a new mechanism that makes me hopeful that by raising and continuously raising the ambition in the light of the ongoing climate change and the challenges, will actually move us more rapidly forward than what we have seen in the past 20 years.
RH: You mentioned the two degrees. The IPCC scenarios which look at a 66 per cent chance of keeping temperature rise within two Celsius, there are 113 of them according to Kevin Anderson who's been looking at them. He says 107 of those scenarios assume widespread use of geoengineering to suck carbon out of the atmosphere. And the other six involve a peak of emissions in 2010 which involves time travel which I think we both agree is somewhat improbable.
TS: That's right.
RH: I mean how can we trust those sorts of projections when they rely on a technology which is highly uncertain?
TS: These are scenarios, these are assumptions that are being made informing decision makers. Some of these studies actually go back to a time where 2010 was the starting year of these studies. So we should not judge back in time when the scientists prepared their scenarios. But I agree with you that some of these scenarios tell us actually in very clear words how ambitious a climate target of two degrees has become in the meantime.
RH: Well, the scenarios are fanciful, aren't they, if they agree to the idea of burning huge amounts of biomass, woody mass or plant mass, and capturing the CO2 from it in order to reverse climate change. I mean this is just fanciful, there isn't enough land on the planet to provide that sort of biomass, is there? So in other words all these projections are based on a fiction.
TS: I think some of them are certainly less realistic than other ones. But there is a whole research community that takes very careful account of what their projection of technological advance is. I cannot comment on the quality as a physical climate scientist, that's certainly not in my …
RH: That would be something for the Chair of the IPCC to do.
TS: No, for that you have a working group and it's very clear where that expertise is. And I know clearly that there is a scientific community out there that is growing, that is taking a much more encompassing look at the different possibilities of scenarios. And one thing is clear; the two degree target has requirements and these requirements were spelled out in the third working group very clearly. And some of these requirements indicate to us that if you freeze the current technology, yes, they will not bring us on the pathway of two degrees, that's for sure.
RH: One last question; the moral benefit of coal, the coal industry is now saying you want to put us out of business because we're high emissions. The whole global economy has grown up on coal, people have grown rich on coal, people have been dragged out of poverty by coal. The whole of modern development is based on coal. It is immoral to try to clamp down on coal. In fact the Pope was accused of being evil for trying to deny people development through coal. What's your take on that?
TS: Well, it's the first time that I hear the use of the word moral from that business community. So I'm rather surprised. I think there are larger questions regarding the moral. If you look at poverty, if you look at the development, if you look at differential development on this planet and if you look at the intergenerational equity, here there are real question of moral. And it's almost cynical in my personal view to invoke the term morality when it comes to using one specific technology, which arguably has been very successful and has brought us a lot of development, but is simply no longer the technology of the future where we have to live on the planet in numbers that are much greater than the numbers of population for example, and the requirements of resources, where that technology has been invented.
So I think these are the questions that we have to look at and the encyclical of the Pope has actually looked at that in a much more encompassing way than just focusing on one technology which will become soon outdated.
RH: Are you optimistic?
TS: Yes, I am, because the alternative to be pessimistic is not really an alternative if you're in front of a large problem, of a problem of global scale. And of the indication that through technological development through an industrial revolution, the force after mechanisation, electrification and digitalisation, the industrial revolution of decarbonisation carries the same amount of hope that any other industrial revolution has carried before, in which wealth was created, jobs were created and power structures have shifted. I have no indication that the fourth industrial revolution will not do anything different from that.
RH: Tom Stocker, thank you very much.
<End of recording>