 will be Jaap Nienhuis, Jaap is a professor at the University of Utrecht and he will talk today about a global morphodynamic response of deltas to sea level rise in the 21st century and yeah if you can yes yes okay your screen is on so go ahead. Okay thank you thank you Albert and thank you Brendan as well for your earlier talk was very insightful. So for something completely different I'm going to talk about river deltas and sea level rise and projections of how they might change in the next century. This is work that I've been doing at Utrecht together with Roderick van der Waal funded by NSF and the Dutch Science Foundation. So how will how will deltas respond to sea level rise? Well there there are a lot of projections out there and this is part actually what inspired us to do our work and a lot of the current projections that are out there are what we are what we call bathtub models and they simply assume some sort of static DEM a digital elevation model that has been flooded depending on its elevation above mean sea level. And one of the big problems with a lot of these sea level rise projections that use static DEMs is that actually when you look at what has happened in the past 20 or 30 years and take into account historic sea level rise and also observations of what coastlines and deltas have done is that these bathtub models do not fit observations. So we've seen sea level rise but at the same time over these past decades we've actually seen coastlines expand. And of course a big component of that and that's something I'm going to discuss is that a lot of coastlines have been able to expand because of sediments. So what I want to talk about is a new model that that Roderick and I have made that includes sediments as well as sea level rise in a model and that can project how deltas will change and also of course how deltas have changed into the past. And this model probably many of you have seen a model like this before it's very very simple looking at a river delta from the side with water and sediments coming in slowly equilibrating to some sort of base level. And the big insight in this model that has been validated by numerous of experiments and also field work is that the response of the delta to sea level rise depends on two major things. One is that the rate of sediment input and the other is the rate of sea level rise and depending on the balance of these two rates if you have sea level rise a delta might still be able to expand just if there's enough sediment to fill up that that accommodation space. So you might have a situation of delta growth for sufficient sediment supply but if sediment surprise is diminished you might also have diminished delta growth or even delta retreat. Yes and this is that this is there the shoreline response of that delta to sea level rise in these three scenarios. You could probably imagine this is a really simple model that's going to be fairly inaccurate applied to just one river delta so to make our projections more accurate what we do instead is we look at all 10,000 river deltas at the same time and it allows us to look at river deltas for a variety of deltas morphologies a variety of sediment supplies and most importantly also a variety of relative sea level rise rates so we can then contrast deltas with very low sea level rise to deltas with very high sea level rise and improve our model fit. So what we've done for all these 10,000 deltas we have sediment supply from WM SET and we have sea level from the past 30 years from a recent paper by by Sankadangandor. We also have subsidence that of course can change the relative sea level rise in river deltas and to compare our delta model we also have landsat observations of what deltas have done over the past 30 years and when we do a simple model testing of the past 30 years based on these landsat images we find that there's a reasonable agreement without any fitting or tuning. So here I plot the predicted delta change using that model against observations from that landsat data set and it's fairly rough because this is still a very simple model and 30 years is not very much looking at the rate of sea level rise. Most importantly however is also that when we look at only a fraction of only the bigger deltas our model fit is improved. So going forward what does this delta tell us about future relative sea level rise? Well we can simply use the model for different or higher amounts of relative sea level rise and we can see that we're currently still in this world where we see net delta land gain so between the two and four millimeters per year right around here. If we go into a world of increased rates of sea level rise of four or five we'll see net delta loss. To give you some idea this is about the area land area of New York City that will lose per year. That's about 700 square kilometers per year for a rate of about 14 millimeters per year. Another cool thing with this model of course is that we can change the fluvial settlement supplies to all these deltas. So that red line was modern values but if we look at what settlements used to be like before land use change or river damming we see that deltas become slightly more resilient to sea level rise. We can also just eliminate the settlement supplying the baths of those baths of projections that I talked about before and we see that those greatly exaggerate the amount of land loss for the same relative sea level rise rate. Of course not all deltas are going to experience the same amount of relative sea level rise so we can look at different climate scenarios where different deltas experience different amounts of relative sea level rise and we can see for RCP 8.5 the deltas in total so all deltas together will lose about 800 square kilometers per year. Where this error bars is produced by doing a Monte Carlo analysis using all the known uncertainties that we have in the data and the model. When we sum up all the land area change that we might expect from 2007 to the end of the century we see that under RCP 8.5 so the most extreme climate scenario from the SROC report we see that deltas will have lost about 32,000 square kilometers of land and that's equal to about four percent of the entire river delta land currently on earth. What we can of course also do is not talk about bulk change for all deltas together but projections per delta we see that the mixed image we have now where some deltas gain land and others lose land is going to change rapidly for increased rates of sea level rise by the end of the century but I do want to know that these projections per delta as I said before are going to be fairly uncertain and last thing I wanted to talk about before my time is up is that with this model we can also make a quantitative assessment of how important different drivers are for delta change so we can simply say well let's do a sediment supply analysis only considering the dams or only considering subsidence or sea level rise. What we see in the past 30 years is that the dams and subsidence and sea level rise have had an about an equal negative effect on delta land loss so going from pristine high pristine land gain rates to what we currently observe all these three components have had about an equal negative effect. However if we go into the future with RCP 2.6 4.5 and 8.5 we see that again if we look at the worst-case scenario that climate change driven sea level rise is going to have the major a major effect on on delta land area change far exceeding what we have to see now because of dams and subsidence. So to conclude we have made a sea level rise impact assessment so land area change for deltas that are validated based on on landsat observations. We can make global projections with changing sediments or sea level rise rates and we can also quantify the net effect of these different components on delta land change. There's a preprint out there on earth archive and there's also some of this data is in an earth engine app that you can kind of scroll through and if you have any questions there's email and Twitter as well. Thank you. Thank you. Yeah that was wonderful. So let's see are there any questions for you about this delta study. So and just to point out if you have a question click on the blue hand and the participants it will raise the hand and then unmute. Don't be shy. If no questions for oh and there we go. Let's see you can unmute yourself I think you can ask the question. Yeah so great talk. On slide 14 where you're showing that land gain versus land loss there's sort of like a tipping point at a specific sea level rise. So I'm going to take a talk a little bit about what that number might actually tell us. This one you mean? Yeah. You mean the small kinks in the in the curve or? No where it crosses from delta land. From land gain to land loss. Well this is yeah if you look at all deltas together it's a good question. If you look at all deltas together it's a gradual change or more and more deltas will not have enough sediment to make it to the coastline. So the amount of sea level rise implies that you need to capture sediment on the delta top itself to prevent drowning but for higher sea level rise rate you need to capture more and at some point you cannot prograde anymore and you'll get into a stage of land loss. So that will happen with more and more deltas for higher and higher amounts of relative sea level rise.