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0:00
Welcome to Deep Dive. I'm Margaret
0:10
Leinen. I'm the director of Scripps Institution of Oceanography here at University of California San Diego and in this series I talk to scientists from Scripps and take a deep dive into their latest science. So today I'm joined by Dr Nick lutzko Nick is an assistant professor of oceans and climate here at Scripps he's a climate scientist whose primary focus is on understanding the basic physics of the climate system including topics like climate sensitivity which we'll hear
0:47
more about later temperature variability heat stress monsoons and even solar geoengineering. Dr Lutzko received his PhD in atmospheric and oceanic science from Princeton and he joined the Scripps faculty in 2020. With the upcoming United Nations climate conference in Egypt in November this is the 27th climate conference I thought it would be particularly interesting to talk to Nick about climate models and how they're used in climate negotiations thank you for joining me today Nick
1:27
Yeah thanks for having me Margaret it's great to see you so with COP27 coming up a lot of their focus will be on whether countries are able to cut their greenhouse gas emissions enough to keep the planet from warming by no more than 1.5 to 2 degrees Centigrade and its projections from climate models that provide information about what happens to our climate under different warming scenarios so can you tell us about the climate model do you use and how you develop them
2:03
yeah so climate models are really kind of the foundation of how we do climate science
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and they're basically collections of about you know several million lines of code
2:18
that try to simulate the our best understanding of the physics chemistry and biology of all the different parts of the climate system so the atmosphere and the ocean
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also for example land processes like for example gas exchange between
2:42
crop fields and the atmosphere and then they also include representations of ocean biogeochemistry and Atmospheric chemistry and there are somewhere between let's say 40 modeling centers um around the world who develop these kind of state-of-the-art climate models
3:07
the number seems to grow every few years so a lot of the models do share code
3:14
and every few years all the modeling centers run a coordinated set of experiments where they all run the same types of experiments for example one of them is to just quadruple the amount of CO2 in the atmosphere and this is part of What's called the climate model intercomparison project or CMIP and it's the simulations which are included in CMIP which form the basis for the projections which the IPCC puts out and also which policy makers that cop use to
3:52
determine how much warming or what the climate impacts would be under different emission scenarios okay and are there other tools that you use to look at climate yeah so
4:06
you know in some ways I think it's a little bit like in higher energy physics how they have these super colliders like the LHC where you have hundreds or thousands of people who build the the LHC or who built the climate models but then you also have lots of people who interpret their results you know to to make turn the signals which the colliders put out into data that we can read and similarly with climate models we have you know it's an incredible amount of data
4:43
and so we have lots of teams of people who use Big Data techniques like machine learning and other tools to interpret the results and to analyze the data and actually a lot of developments in Big Data like more broadly are driven by climate uses and then of course we also collect observations
5:08
and really here we just try to get anything that we can so you know satellite data or for example at Scripps right now there's a field campaign to make measurements of clouds Cloud properties where they're off Mount Soledad here they're launching balloons and probes and things
5:30
and so yeah so we incorporate all these different
5:34
sources of data to try to evaluate our models and also to see how the climate system is changing Oh, great. I know that another topic you are studying is climate sensitivity and you have a NOAA grant to study low cloud hot spots I was told and one of these low cloud hot spots are one of the largest uncertainties in climate change models and predictions So what is climate sensitivity? What are low cloud hot spots?
6:09
And what do you hope you're going to be ble to understand with this research
6:15
Yeah so these are really great questions. So climate sensitivity is just a measure of how much our surface temperature will warm up in response to increases in CO2 concentrations and normally we Define it per doubling of CO2 so let's say we went from like roughly 400 parts per million today to 800. parts per million how much would the climate system warm up
6:46
um and what I love about this is or the reason I think it's important is because so many of the impacts of climate change scale with global mean surface temperature
6:57
you know we already see this in cop or we know it's much worse to be to have two degrees of warming versus 1.5 degrees of warming and so we think that earth's climate sensitivity is somewhere between let's say two and four and a half degrees Celsius
7:14
There's still quite a lot of uncertainty and so again a four and a half degree world where we the climate system would be really sensitive would be a lot worse in two degree world
7:26
another reason why I think this is important of course is it's really easy to communicate there's you know the climate system is so complex and climate change is so sort of all-encompassing and the impacts that it has that I feel just to get people to understand climate change it's so useful to have one number that you can give or that you can argue about or that you can you can share
7:48
so I said climate sensitivity oh sorry go ahead well I wanted to make sure I understood exactly what you were saying so you said that that people judge this by doubling of CO2 and so if we doubled CO2 to 800 PPM that's going to result in a four degree increase to four degrees centigrade increase that's anywhere between maybe two and a half and four and a half degrees okay two and a half to four and a half and you're trying to figure out which one it's closer to yeah perfect
8:25
okay so in 1979 a group of scientists met at Woods Hole in Massachusetts and made the first estimate of climate sensitivity which just came from sitting around in a room basically for a few days and their best guess was somewhere between two and four and a half degrees
8:46
and since then we've learned a lot about the climate system but the range of our estimate hasn't changed that much and so that's one reason why this is such an important question and has motivated so much research and it turns out that one of the main sources of uncertainties or low clouds like we see off the coast of San Diego here actually these clouds have a huge impact on the climate system and just small changes in low cloud cover so you know if June Gloom
9:14
was a little bit worse or a little bit less gloomy could have a huge impact on warming
9:21
and so Joel Norris Knight Scripps we call these hot spots because if you look at maps think of cloud cover they kind of pop up as being important and so that's something that we've been working on
9:36
and yeah I I guess one thing I love about being at Scripps is both like I get to look out of my office every day and see these clouds which is pretty inspiring
9:46
but also our Emeritus Professor Ramanathan was the first person who identified the importance of these low clouds because he led a satellite mission in 1989 which quantified the effects of different clouds and clouds in general had on the climate system
10:05
before that we knew that clouds were important but we didn't know how important they were and he wrote the first paper which really put numbers on everything
10:13
and kind of set these targets for us to work on how fascinating so
10:21
you you said that that the the group from Woods Hole made their estimate of climate sensitivity without models so and you said you know basically they got together and they were they were
10:38
you know they were right in the right range so what did they use to make those estimates yeah so there are bits and pieces of information that they had um so for example there were
10:54
um very simplified climate models so Syukuro Manabe who won the Nobel Prize in physics last year yeah so here on some models those models um you know in many ways for example they would just model the Earth as a single grid box so they just is much simpler than what we have today but they were kind of roughly in that range and people had very early satellite observations and they were kind of looking at the variations in
11:27
radiation emitted by the Earth and again it's much worse data than what we have today but
11:35
yeah people were still able to come up with this range which is is pretty plausible
11:40
That's fascinating. So I understand that you recently had a paper on clouds that came out of discussions with a group of students and other faculty that you call the Climate Journal Club. Yeah so over the pandemic you know we were worried about keeping our students engaged and having a sense of community and the climate science community at Scripps and so we revived the Climate Journal Club which I think was started before I got to Scripps
12:18
and at first we just met on Zoom and now we've transitioned to being in person and I have to say thank you for a small grant which will allow us to bring in some speakers now but yeah so as part of this we were reading papers on clouds and just having lots of conversations about about some things we were unclear about and we realized that one thing which people haven't looked at very much is the effect of CO2 on clouds
12:48
so when I said that when I was talking about these low clouds what people are have mostly looked at is if you warm the sea surface how does that affect the clouds above
13:01
and similarly if you wanted to land how does that affect the clouds but the just having more CO2 in the atmosphere even if you keep the temperatures the same as an impact on clouds and so um together with these students Matt Longo and also a postdoc KC wall
13:19
we found that this has been pretty neglected and actually that the climate models show very different behaviors in the direct effective CO2 on clouds and that this changes quite a lot between different models so this is something we've been digging into and yeah we just had a paper on this interesting and what does the CO2 do that changes the clouds yeah so basically the CO2 sits above the clouds and it absorbs radiation that the clouds emit and so you can think of
14:00
it as there's this little bit of warming right above the clouds where there's or extra warming where these the CO2 is absorbed in radiation so it's a little blanket on top of it exactly yeah
14:13
at least that's what we think is happening in most models because I think most modeling most people who build models haven't thought about this before we saw a much wider range of behaviors and we typically see for cloud properties and climate models because no one had thought to look into this and and to see what the models were actually doing well it sounds like they were very interesting discussions yeah I'm fortunate to have really fantastic students at Scripps that's great
14:46
so clouds I know are a very complex part of our climate and understanding future climate and I also understand that you have been looking at how clouds might respond to aerosols so what are aerosols what kind of aerosols do they respond to and what have you found out good um so I I think this is one of the most important questions in climate right now so aerosols are basically air pollution which is not CO2 so you know when you think about like a
15:23
a smoke stack by a a chemical plant or something it's just spewing aerosols into the atmosphere and we think that aerosols cool the climate system and so they actually kind of counteract the effects of CO2 and so if I go back to climate sensitivity if we think about the warming that we've experienced over the 20th century on the one hand we've been emitting CO2 which raises temperatures and on
15:48
the other hand we've been emitting aerosols which cool temperatures but we're very uncertain about the size of the aerosol cooling effect so if they didn't have much of an effect and it was all CO2
16:03
we think that the climate system might be have a relatively low climate sensitivity but if the aerosols have done a lot of cooling and we've still managed to get all this warming then that tells us that the climate system is actually more sensitive to CO2 than we thought yeah and so one reason that we're uncertain about the aerosol forcing is that aerosols have uh impacts on clouds
16:28
and these are in quite complicated ways but they affect things like how long clouds hang out in a life in the atmosphere and also for example how easily clouds rain out
16:44
and maybe one well-known thing is they make Cloud droplets smaller
16:51
but so we call this the indirect effect of aerosols on clouds
16:56
because it's affecting our climate system indirectly
17:00
and again it's very uncertain but recently a few of us at Scripps managed to develop to develop a new method for quantifying um this effect this indirect effect of aerosols on clouds and um the postdoc who was working on it Casey Wall came up with a really clever way of validating his this technique which was to look at volcanic eruptions so if you remember a few years ago over Iceland for example there were some volcanic eruptions which disrupted our air travel over Europe
17:34
um and so we have um satellite data from them that we can use to try to quantify the effects of aerosols and we also have aerosol data and it's like emission data from the volcanoes and so he showed that this method really accurately reproduced the climate impacts of those volcanoes
17:54
and so once we felt confident that the method was was reliable we went back and used historical sulfate emissions which we think are are relatively reliable and use those to quantify the historical aerosol forcing and we think that it's actually on the high end of uh for example the range that the IPCC gives we think that the historical historically aerosoles have provided more cooling than was previously thought and the implication of course is that
18:25
the climate system is more sensitive than maybe people thought so if we clean up more pollution
18:34
in the atmosphere and remove that cooling that that we would see a much more sensitive climate response yeah I have thought about before yeah this is a big irony it's you know of course we want to clean up all this air pollution but that's also going to lead to more warming Right interesting. So I also heard that you have started another avenue of inquiry and that you're really interested in heat in cities and extreme heat events yeah so thanks for bringing that up
19:21
this work was motivated by some um pieces that I read in the New York Times and NPR about how
19:29
within cities during heat waves temperatures can vary by up to 20 degrees Celsius between different parts or Fahrenheit between different parts of the city so you know if you're in an area that's really green and lush it tends to be cooler than in an area that's full of concrete and this this is one of the potentially one of the most impactful
19:51
aspects of the climate crisis is extreme heat events and the stress that this puts on people
20:00
and so yeah I've been working on a couple different projects related to this so one thing that I was interested in is heat stress depends both on temperature and on humidity so when the humidity is very high it's it's difficult for our bodies to sweat and we feel hotter And so I showed in a paper that actually humidity increases faster than temperature in a warmer world especially in summer and so future heat stress events are are going to be
20:34
much more affected by humidity than by temperature it turns out
20:41
and then another project with our graduate student Anthony Wilson we've been just having a work in a fun little project thinking about how how hot our surface can get so the warmest temperatures which we see in the Middle East or also actually in Death Valley are about 55 degrees Celsius
21:04
and so the question Anthony's been asking is why is it 55 and not 65 or 35 or 85
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and so we've been playing around with some simple explanations for this
21:18
that I'm pretty excited about and it doesn't look like there's really a limit to the the temperatures that we would see so what Anthony did is he tracked down data from these from the atmospheric data from these extreme heat events and then he did calculations to see given the properties the atmosphere how warm could it actually get
21:44
and it's it turns out that we're right at that limit so the limit that he found I'm trying to remember off the top of my head but it was right around you know 55 or 57 degrees and he's also of course been thinking about how this will change in the future and we think that the warmest temperature increases faster than the mean temperature so if we have let's say two degrees of global warming the warmest temperatures might go up by three or four degrees
22:20
in the very warmest temperatures and I just did the calculation that 55 degrees centigrade is 131 degrees Fahrenheit yeah for those of us who think in in Fahrenheit when we think about the temperature so now we've talked about models and we've talked about climate sensitivity and clouds and how hot it can get are there other areas of research that you're working on
22:54
yeah so one a big question that I'm interested in is there's this sort of bias in the climate models where well climate models you know are incredibly accurate in sort of 99 of things that they simulate but people have noticed that there's this discrepancy where off the coast of the East Pacific a little bit further south than we are in San Diego so more off the coast of Central America or or South America we've seen cooling over the last uh 30 or 40 years or its climate models
23:30
predict that we should have seen warming and you know I said that these areas with low clouds are these kind of hot spots which are really important for climate change and because these low clouds
23:44
and so if the models show warming and then and the observation to show Cooling in these regions
23:52
it says that there's something potentially important that we're missing in the climate models and this could have implications for low cloud changes in the future
24:01
so I've been working with colleagues to try to think up different reasons uh for this potential this discrepancy between the models and the observations and I think last time I saw we had a list of 12 different possible explanations for it so it's a bit of a
24:20
yeah it's a it's a very difficult question so as you think about where things will be in
24:31
say let's say maybe 15 years 10 seems like a little bit close
24:37
um what do you think that the big issues in climate will be at that time I'm not so much talking about you know will we be on well we have gotten emissions under control or whatnot but what do you think that scientists like you will be looking at in climate what will we have discovered and what will be remaining to be looked at yeah that's a really great question
25:09
I guess a few different things come to mind um the first thing is uh I think there's a little bit of a disconnect sometimes between the type of work that climate scientists like I do like me do and the kind of actionable data and information that policy makers and planners and people who think about infrastructure need and you know you could look at this as a as a an issue or as an opportunity because I think you know we're scientists and so we try to make everything you
25:45
know the problems that we're working on kind of as simple as possible so that we can tackle them
25:50
and in a way you know turning our our theories and our models um are into something that we can give to communities outside science I think forces us to think about the some of the difficult questions that we don't know how to answer yet
26:08
so you know maybe we might have an idea for how extreme precipitation changes in a very idealized world without any let's say without any topography for example but when you add in all the messiness and complexity of the real world it's a lot harder to to sort of transfer that knowledge that we've gained into into something that we can actually use I don't know if that makes sense
26:41
so do you think that that climate scientists will be doing more work on trying to make their models actionable? Yeah I think it's not just the models but it's also kind of our understanding um you know because one people one thing that people worry about is that models are just black boxes uh you know how do we know that they're not getting the right answer for the wrong reason
27:11
um and so we always want to tie that to understanding but then you know if we're thinking for example about the California Current of the coast off the coast here you know I might know about how the perfect idealization of that current works but then when you add in all the bathymetry and stuff and things like seasons and um passing weather systems it's a much more difficult question and so we need to learn how to incorporate all these
27:39
complexities into our existing knowledge so that we can give people both the results of the models and say well we understand what's going on and we can explain that to you you know I've been hearing a lot about the a lot of the term digital twin applied to models what's a digital twin and are the models that you are working on or the big models that are in the climate model inner comparison project are those digital twins yeah this is a fascinating issue and it kind of gets
28:19
almost to the philosophy of climate modeling so I said we have about 40 different modeling centers all right and that gives us a wide diversity of models to work with but some people think that's a good thing whereas other people think that we should pull resources to build one supermodel that uses the resources that could otherwise go into let's say having five or ten different models
28:46
because right now a big limit on our climate models such as computational resources we can only run them at certain resolutions given the computers we have available to us but again if we pulled resources maybe we could run a model which has a resolution horizontal resolution of a couple hundred meters or a kilometer which would really seem like a twin additional twin of the Earth so I mean and again it's sort of a philosophical question is is it worth
29:18
doing that or is the diversity of models that we have now is it worth keeping that diversity
29:27
these digital twins yes there's a lot of talk about them so I'm sure we're going to see some of that and then and then we'll have to make some judgments about whether we learned more from the digital twin than it took resources to in terms of compute resources to use for that so that may be one of those questions that everybody's talking about or debating in Texas yeah
29:59
well it's been wonderful to talk to you and I really appreciated all of your answers to some simple questions about the the work that you do and it's wonderful to see the diversity of of projects that you're working on and I I know that that well I should say from my own work it was always fun to have different things to play around with so if I got stuck on one for a while I could sort of work on another one until I figured out how to move forward with the first
30:39
yeah for sure that's one of the great things about climate I think is there's so much to work on and you get to use different parts of your brain for different problems. Thank you Nick Lutsko for letting us take a deep dive into your science and thanks to all of you who have joined us today. I hope you enjoyed the inside look at climate models the way they're used in climate negotiations and some of the fascinating
31:04
problems that scientists have to solve in order to make these models better foreign
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