Of forests and oceans

00:00:02: Ahoy and welcome to the Gen Z podcast, a podcast by early career marine scientists.

00:00:10: Hi I'm

00:00:11: Leonard And i am currently writing my master thesis To finish up my Master in Environmental Modelling at The University of Oldenburg In Germany!

00:00:20: And hi ,I Am Charlotte .

00:00:22: I have recently graduated from the Masters in Marine & Coastal Systems At The University Of Algarve in Portugal And together we are going to record some episodes for this podcast over the next few weeks.

00:00:34: So, Lennart let's dive right in!

00:00:38: Hello Charlotte here again for another episode of Gen C. Good morning Lennard and today We're gonna be talking about forests.

00:00:49: so were leaving the marine topic just a slight bit but will come back to combine ocean & forest.

00:00:58: Maybe you can start by telling us a bit about how do we do reforestation and why did we talk about it in the first place.

00:01:07: Yeah, We are talking about reforestation a lot because its considered one of main tools for climate change mitigation, because of course when trees grow they take CO₂ out the atmosphere and this can help us to decrease CO₀ in the atmosphere.

00:01:26: And therefore hopefully decrease global warming.

00:01:31: There has been a lot initiatives.

00:01:33: I'm sure that people have heard about them on media.

00:01:38: For example France just recently committed planting one billion trees until twenty thirty two.

00:01:45: And there's also these, I want to say well famous tree planting campaign of one trillion trees which is also endorsed by the UN.

00:01:57: Of course like planting trees has a lot of advantages because forests they provide a lot ecosystem services such as biodiversity They can create livelihoods for people and help us mitigate climate change.

00:02:12: But then also if tree planting is not done correctly, it doesn't have all of these positive impacts.

00:02:20: If you plant a tree and it dies after five years, it's not going to help because it's sequestering carbon any longer.

00:02:27: Or if your planting trees in other ecosystems such as for example grasslands they like these grasslands can already store a lot of carbon.

00:02:37: And when you disturb this ecosystem by planting trees then we're actually having the opposite of the intended effect.

00:02:45: It will contribute too climate change even some cases.

00:02:50: And then also you have to plant diverse forests.

00:02:52: So one of the issues with these campaigns sometimes is that they're literally only counting trees, but if your are planting a monoculture than it won't provide as much biodiversity and benefits off course.

00:03:10: Okay yeah I see.

00:03:11: so i guess he could somewhat describe this again as geoengineering what we mentioned in last episode as we are altering ecosystems or restoring them maybe.

00:03:26: And yeah, I think... I don't know this anymore but it's a long time ago on my Bachelor, isn't that trees basically emit more CO₂ when they're young because still they're breathing?

00:03:39: So in the young age there really not sequestering that much.

00:03:44: Maybe you can say some more to that.

00:03:47: Yeah.

00:03:49: It depends on the case, but it's mostly the opposite of what he said.

00:03:54: Okay

00:03:55: No problem Because there's this difference between sequester and carbon, so you're actually like taking out CO₂ from the atmosphere compared to storing it.

00:04:04: So if you have a young forest that trees will sequester quite a bit of carbon whereas because they can grow relatively fast also depending on species in the environment off course.

00:04:16: but then the older the forest becomes Sometimes the sequestration slows down with age of trees because they don't grow so much anymore.

00:04:27: However, in old forests there is a lot of carbon already stored from all previous sequestrations and also more carbon in soil.

00:04:38: when you disturb these forests that can emit lots of carbon.

00:04:44: So it's like peatland maybe?

00:04:49: Yeah, it can if

00:04:50: you compare that.

00:04:51: Okay?

00:04:52: Yep That makes sense also.

00:04:53: what is true?

00:04:54: that sometimes Very old forests they can actually be a source of CO too as well due to like some processes happening but Nevertheless these old growth forest have so much other like biodiversity values, that should not be an argument to cut old growth forests whatsoever of course.

00:05:18: No I agree we have already not enough primary forest anymore so if you cut them down it's probably not the best idea.

00:05:27: yeah absolutely but um yeah It's sometimes like a little bit of controversially discussed topic, but I think one can say that planting trees is very good nature-based solution for trying to prevent the climate crisis from worsening.

00:05:50: However and this something interesting we're going talk about now... The answer obviously not as easy!

00:06:13: Now to come back, to the actually marine topic of this podcast.

00:06:18: So could you tell us how does planting trees affect the ocean in the end?

00:06:22: Or is there a fact or no effect

00:06:26: I was so fascinated by?

00:06:30: Trees have impacts on the natural environment beyond the ecosystem services and carbon sequestration.

00:06:36: So if you change from grassland to a forest, that for example changes the surface albedo of land which is basically the reflectiveness of the land has an influence.

00:06:51: how much heat will stay in atmosphere from radiation into the sun?

00:07:00: And then trees also change, or forests also change the surface roughness which can have influence on like how winds form and behave and move.

00:07:11: Of course because trees are breathing through this process of evapotranspiration The presence of forest has an influence in clouds and precipitation And all of these factors, they're changing the surface energy balance in our atmosphere on our globe.

00:07:32: Therefore those effects from trees are not only very local but can also be regional or even global because the energy balance has an influence on our climate system.

00:07:44: Of course we already know and heard in a podcast episode that the ocean is important component of this climate system and also drives it to a certain extent.

00:07:57: And therefore ocean circulation, and reforestation they can actually or could be connected with each other?

00:08:06: Okay I see...I mean maybe to draw comparison but the surface albedo is like in the Arctic when you lose your eyes um then you have darker soil which uh takes up more of the solar irradiation, so it heats warmer.

00:08:21: That's why we have this Arctic amplification right?

00:08:25: Exactly yes exactly!

00:08:27: It is the same with snow on top of sea ice because like snow has a much higher reflectiveness than if there are no snow in the ice and that also can actually amplify the melting.

00:08:39: then

00:08:39: yeah Do you know how?

00:08:46: like this reforestation or some Like this plant, one trillion trees.

00:08:52: How would that affect the climate in the end?

00:08:55: It's one of those questions That we don't know the answer to but We can use models to predict What

00:09:03: will happen

00:09:03: future.

00:09:04: once again model.

00:09:05: here we go.

00:09:07: There is no episode without

00:09:09: No.

00:09:12: And I'm bringing a study from Portman et al.

00:09:15: today, from two thousand twenty-two and it's one of the earlier studies on this that uses models to look into this question?

00:09:26: And i really liked this one because you refer to this in our very first episode when we talked about how or what we use models for for example, we can figure out what happens when we cut all trees in this planet.

00:09:46: Oh yeah I remember that!

00:09:47: Yes and... This is exactly what Portman et al did.

00:09:52: Ah funny i didn't know the study.

00:09:55: Yeah so they were working with um The Community Earth System model the CESM And they had like a fully coupled model setup.

00:10:04: So they have an atmosphere They had land ,they had the ocean .They also included sea ice actually included their size the CRO-PCIS model that I was working with, and then they also had river influx into the oceans.

00:10:19: And they spun up the model for a couple of hundred years... ...and then they were looking at simulations of three hundred year lengths for their analysis.

00:10:31: They had a control run which is when the planet had pre-industrial land cover of forests, which were approximately thirty percent of forest at that time.

00:10:44: And then they have one run called a Forest Run where all grassland, cropland, shrubs and also urban areas were reforested or afforested.

00:10:55: That led to eighty per cent of forest cover And they compared this to the control and a second run, which is called The Grass Run.

00:11:04: Which basically all trees are gone.

00:11:07: so forest cover was decreased by zero per cent.

00:11:12: One question!

00:11:13: Do you know what kind of forests in this model?

00:11:17: Because I suppose it's different if its like Siberian Tiger Forest or Tropical Rainforest.

00:11:26: Yes, that's a good question which I'm not sure if i read it in the paper or they made this specification.

00:11:33: But because we had supplementary materials also on the specifics of the model run and yeah...I can put those in the episode description.

00:11:44: Because yes, you're right.

00:11:47: Different types of forests will change the albedo differently and they'll also change like surface roughness And so on.

00:11:54: it will depend what type of forest is.

00:11:56: Yes that's a very good point.

00:12:09: Can you tell us what did actually find in the forest run?

00:12:12: Is eighty percent for something we should achieve?

00:12:16: First off all... What I found mostly looking into was how these different forest scenarios or deforestation scenarios change the radiated forcing due to changes in albedo.

00:12:32: And I will give you some insights on global analysis that they did.

00:12:39: So what they found is that for the forest scenario with the eighty percent of forest cover, The mean annual temperature increased by zero point five degrees Celsius.

00:12:49: And of course there was a found like regional differences.

00:12:52: so For example in some areas it also pronounced cooling such as over-the-north Atlantic and That's something which has been observed In other models before.

00:13:05: There even were names for

00:13:07: them Yeah the cold blob.

00:13:09: In this case they are talking about the North Atlantic warming

00:13:12: hole.

00:13:14: Okay, maybe not!

00:13:16: I'm not sure that's the same.

00:13:17: but basically what happens like with increasing surface temperatures in this specific area on the north atlantic?

00:13:24: you'll see a very pronounced decrease in surface temperature.

00:13:29: and what is interesting in comparison between this simulation from the study In comparison to these greenhouse gas emission scenarios, this is like something you see in both.

00:13:45: So this pronounced cooling over the North Atlantic we also see in the warming and climate models that simulate the warming because of green house conditions.

00:13:55: Interesting!

00:13:57: On the contrary they saw in the grass scenario with zero forest cover but the temperature decreased by one point six degrees Celsius And that's of course a much larger amplitude for the cooling compared to warming in forest scenario.

00:14:16: They discuss this is likely due to feedback effect from albedo, snow and ice.

00:14:31: This is sheltering sort of the warmer ground underneath from atmosphere, so if you have more ice in place for longer there's less heat flux from soil or ocean into the atmosphere and that kind decreases warming.

00:14:57: And then they also saw some associated changes in the cloud cover and in the precipitation patterns.

00:15:03: They have a very detailed description of what they saw, their model runs but I'm not going to go into so much detail here because we want focus on ocean right?

00:15:13: I think so yeah!

00:15:15: That was super fascinating.

00:15:17: What they found is that through these changes in Albedo The overall heat transport in climate system changed and they found that this change in heat transport was mostly associated with the ocean-heat transport.

00:15:33: So, on the forest scenario where we see the overall warming of the atmosphere... They also determined that the heat transport from the tropics towards the poles actually decreased.

00:15:45: And then there's also the Atlantic Marigional Overturning Circulation so AMOC weakened.

00:15:53: Okay!

00:15:54: That is super interesting.

00:15:56: That's a point that is really debated right now, or one of the tipping points as well.

00:16:05: It is, yes.

00:16:06: Because this heat transport towards the poles and then warm water like from the Gulf Stream travels toward North Atlantic and Arctic Ocean And it cools in sinks.

00:16:17: We've talked about that previously.

00:16:19: I think This really what drives the conveyor belt of our global ocean circulation.

00:16:26: So with this process slowing down, or expected to have quite some implications for our climate system as well.

00:16:41: The grass scenario also like the zero percent forest cover they found in their model is exactly opposite.

00:16:47: so there was an increased heat transport towards the poles and we had a strengthened AMOC.

00:16:54: Of course, this doesn't mean that we should stop planting trees because you know... We need trees to mitigate climate change for sure.

00:17:02: But I think it really points at especially when we're planning very large scale reforestation.

00:17:10: It's important to consider all of these like climate ocean feedback processes and trying to estimate how how useful it actually will be.

00:17:24: So I think this is like the main message as well here, that we don't know a lot about yet and should really try to find out more because in sort of worst case scenario all our reforestation efforts for climate change mitigation may lead to little bit opposite or at least somehow take away some positive impacts.

00:18:03: That's very interesting.

00:18:05: And I think there are probably more studies like this that go a bit deeper into it, no?

00:18:12: Or do you know anything?

00:18:15: They're further studies.

00:18:17: There is the recent one i found from Van Wille et al and they released a preprint in twenty-twenty five.

00:18:27: From my perspective ,they had similar research question.

00:18:33: However, they were running their model under different deforestation scenarios.

00:18:39: So I think there had like different percentages of forest cover in the model runs and then also They were running at once with a dynamic ocean that changes like sea surface temperature And heat content in the ocean as In this study of Portman et al.

00:18:57: And then they also ran it with a model that had fixed sea surface temperature.

00:19:05: That was actually quite interesting because through this, there could show these heat dynamics and sea surface temperatures.

00:19:14: changes in the ocean were really transmitting non-local effects of warming much more compared to when you had it run with like a fixed ocean.

00:19:26: So then these non-local effects were of much smaller magnitude and the geographical extent was also much smaller, so that kind of points at the really important role in this space.

00:19:37: Super interesting!

00:19:39: Without the ocean basically there is just no effect... or

00:19:46: a

00:19:47: smaller effect?

00:19:47: It

00:19:47: seems like much smaller effects yes And I thought what was also an interesting addition from this study here is they really emphasized the importance of time scales.

00:20:00: Because on the one hand like all these heat take-up and heat content, and sea surface temperature in the ocean.

00:20:08: This is a much larger time scales than what we see in the atmosphere.

00:20:12: so if you have an increase in sea surface temperatures it will take very long to transmit this into deeper water layers of course.

00:20:22: So they are pointing at the fact that these feedbacks, which are caused or driven by the ocean may be working on longer timescales compared to what we see and measure in this atmosphere.

00:20:38: This is also one of things they found when looking at the AMOG.

00:20:45: So for their first five hundred years of model simulation... ...they also found out that the AMOG was slowing down.

00:20:55: But then, for another five hundred years because they were running the simulation for a thousand years.

00:21:02: They found that the AMOG was actually strengthening again and there discussing this is likely associated with higher evapotranspiration therefore an increase in salinity of ocean and therefore increasing mixing.

00:21:19: but these points at importance on time scales.

00:21:24: If you only look at three or five hundred years, the picture might be completely different.

00:21:28: Or very the opposite of what you see when you have simulation over a thousand year assist?

00:21:34: Yeah I think especially the AMOG is really somewhat difficult.

00:21:41: The new studies now from Potsdam Institute for example they show that we are likely to reach this critical slowing down but i'm not sure on which time scales.

00:21:54: But I think they said it's supposed to slow down this century already.

00:22:03: So we are quite close to this point, but then there is also always the question if its transient so if it's recoverable or if that really irreversible and i think thats very hard.

00:22:17: Yeah,

00:22:19: I think it is.

00:22:21: And again this what we need models for to get a little bit an idea of whether or not This going only one way Or can be changing back.

00:22:30: But at the same time as you say It's already...I also read somewhere that its actually observed That its slowing and that this happening in this century From the impacts this will have for us right now, as like the world and humanity.

00:22:54: It doesn't really make a difference whether or not it's going to strengthen again in five hundred years from now because of the effects that is going on our immediate climate.

00:23:07: This what we are going deal with which will be pretty disruptive if we're not managing to mitigate climate change much better

00:23:18: For sure.

00:23:23: It's something that we should really be aware of.

00:23:37: So to conclude this, then you would say a forest station?

00:23:43: Yes or no?

00:23:45: Oh yes I'd say yes for sure!

00:23:48: I hope also our listeners don't get the impression that reforestation is not good thing now.

00:23:55: This isn't my intention Because as I said before, growing forests if you do it correctly has so much other benefits.

00:24:06: Absolutely like the ecosystem services.

00:24:10: and yes... It's just really important that we have healthy forests full stop.

00:24:16: this is the statement here!

00:24:20: And then also as we talked about before models there They're helping us to predict the future and they can be very good, but also not a hundred percent certain.

00:24:34: And I think especially in these cases... ...also the authors of those studies are pointing out that it needs more research in this direction,... ...that is needed for different model simulations using different models,... forestation, reforestation scenarios.

00:24:54: Also looking a little bit more onto like different regions for example.

00:24:58: so I feel...I had the impression that this is still very much in the beginning and i think over the next years we'll have a lot of work on this and we will see um maybe some uh much more precise predictions if i may phrase it like this?

00:25:17: So

00:25:17: yeah

00:25:19: overall Global means and like the global average is

00:25:24: yeah, I think at least a portman et al.

00:25:26: It seems very idealized Because you have no emissions And it's either full forest or no forests.

00:25:36: so it's two extremes.

00:25:37: and um i guess then if You run with the emission scenarios?

00:25:42: So on you have combined effects that could change again.

00:25:46: Yes There's definitely more work to be done.

00:25:52: Yes, absolutely.

00:25:53: And I think it's important to keep these potential effects and feedbacks in mind because also the whole reforestation climate... party or a group of people is kind of disconnected from the climate ocean group, at least in my perception.

00:26:15: So I think it's very

00:26:16: important to recognize

00:26:17: that there are these synergies and feedbacks when you're planning large-scale reforestation initiatives trying to determine how impactful they will be for climate change mitigation.

00:26:31: but yes!

00:26:31: There certainly more work to do.

00:26:39: Okay then, thank you very much for presenting us those two studies about forestation and how the forests can actually in quotation marks slow down our AMOG.

00:26:51: But it's to be seen so... Thank You!

00:27:13: Florentine Seifert, Production and Graphic Design Annabel von Jakowski Management Eva Rolfe.