I wasn't worried about climate change. Now I am.

YouTube has removed the dislike counter but the numbers are still available backstage and I can tell you that my most disliked videos by far are those on climate change doesn't matter if it's good news or bad news some people it seems reflexively dislike anything about the topic every time and to be honest I can kind of understand that it's a little tiresome isn't it climate change extreme weather heat records blah blah blah we've heard this for so long and look we're still here stop talking about it already I get it and I'd rather just talk about some fun new physics stuff but I feel like I need to tell you about this because the lives of hundreds of millions of people depend on it climate scientists are having an argument about a number one single number called the climate Sensitivity I don't like what I've read it really worries me and I think you should know I know you expect me to be funny haha not funny peculiar but I'm afraid this video will be more on the peculiar side why does Sabina worry about climate change and why now that's what we'll talk about today this video comes with a quiz that lets you check how much you remember 2023 was the hottest year on record since the beginning of Records in the mid 19th century not just the average temperature increased to neverbe seen levels in many places heat waves were also longer and hotter than ever before in February Antarctic sea ice reached an absolute record low since the beginning of satellite measurements in 1979 and Global ocean temperatures reached a new record too I don't know about you but to me that sounds pretty bad now it's possible that 2023 was somewhat of an outlier and average temperatures will somewhat decrease in the next for years there are several reasons for this first there's just regression to the mean but second there's also that in 2023 we switched from a linia to an ELO phase the linia ELO phases are Quasi periodic Global Climate patterns these phases switch somewhat irregularly but roughly every 2 to three years and the AL n phase that we just switched to is typically somewhat warmer so next year might break more records because it will still be alinu but in 2 to 3 years we might see a slight Cooling and third some researchers have speculated that part of this year's warming might have to do with the decrease in pollution over the oceans caused by new regulations of ship exhausts it's somewhat unclear how large this effect is but we do know that air pollution does indeed have a cooling effect so maybe that's part of the reason be that as it may I worry that even if 2024 is not a new record breaker the overall trend in the next few years will be steeply up and the situation is going to deteriorate rapidly the reason is to do with a quantity called climate sensitivity climate sensitivity contrary to what you might think is not what makes people hit dislike on climate change videos it's a property of climate models it's the temperature change that one finds in a model when one doubles atmospheric carbon dioxide over the levels of pre-industrial times and then waits for the system to come into equilibrium in the literature it's called the equilibrium climate sensitivity ECS for short this equilibrium climate sensitivity isn't something we directly observe because no matter how much they dig in Saudi Arabia in reality carbon dioxide levels don't suddenly Jump by a factor two however it's a useful quantity to gauge house strongly a model will react to changes in carbon dioxide and this climate sensitivity is the key quantity that determines the predictions for how fast temperatures are going to rise if we continue increasing carbon dioxide levels up to 2019 or so the climate sensitivity of the world's most sophisticated climate models was roughly between 2 and 4. 5 de C these big climate models are collected in a set that's called the coupled model interc comparison project seip for short that's about 50 to 60 models and is what the ipcc reports are based on so until a few years ago we had a climate sensitivity of 2 to 4. 5 de or so and that's what we came to work with that's what all our plants rely on if you can even call them plants youve probably all seen this range in the ipcc projections for the temperature increase in different emission scenarios it's this shade region around the mean value Loosely speaking the lowest end is the lowest climate sensitivity the highest end is the highest climate sensitivity then this happened in the 2019 model assessment 10 out of 55 of the models had a climate sensitivity higher than 5° C that was well outside the range that was previously considered likely if this number was correct it would basically mean that the situation on our planet would go to hell twice as fast as we expected okay you might say but that was 5 years ago so why haven't we ever heard of this what's happened is that climate scientists decided there must be something wrong with those models which gave the higher climate sensitivity they thought the new predictions should agree with the old ones in the literature they dubed the hot moders problem and climate scientists argue that these hot models are unrealistic because such a High climate sensitivity isn't compatible with historical data this historical data covers many different periods and reaches back to a few million years ago back then when we still used D up modems it's called the paleoclimate data of course we don't have temperature readings from back then but there's lots of indirect climate data in Old samples from rocks eyes fossils and so on in 2020 a massive study compiled all this paleo climate data and found that it fits with a climate sensitivity between 2.

6 and 3. 9 de C and this means implicitly that the hot models the ones with the high climate sensitivity are not compatible with this historical data as a consequence the new ipcc reports now weigh the relevance of climate models by how well the models fit the historical data so the models with the high climate sensitivity contribute less to the uncertainty which is why it has barely changed and that sounded reasonable to me at first because if a model doesn't match with past records there's something wrong with it makes sense then I learned the following the major difference between these hot models and the rest of the pack is how they describe the physical processes that are going on in clouds a particular headache with clouds is the super cool phase of water that's when water is below the freezing point but remains liquid the issue is that the reflectivity of the clouds depends on whether it's liquid or not and that super cooling makes the question just exactly what influence the clouds have very complicated but how much data do we have about how clouds behaved a million years ago as you certainly know the dinosaurs forgot to back up their satellite images so unfortunately all that mil year old cloud data got lost and we don't have any to use the argument from historical data therefore climate scientists must assume that a model that is good for clouds in the current climate was also good for clouds back then under possibly very different circumstances without any direct data to check that seems to me a very big if given that getting the clouds right is exactly the problem with those models wouldn't it be much better to to check how well those models work with clouds for which we do have observations like you know the ones that we see in the sky in principle yes in practice it's difficult that's because most climate models aren't any good as weather models while the physics is the same they're designed to run on completely different time scales you make a weather forecast two weeks at most but climate models you want them to run a 100 years into the future there's one exception to this there's one of the hot climate models that can also be used as a weather model with only slight adaptations it's the one from the UK Met Office so a small group from the UK Met Office went and used this hot model to make a 6-hour weather forecast they compareed the forecast from the hot model with a forecast from an older version of the same model that didn't have the changes in the cloud physics and was some what colder they found that the newer model the hotter one gave the better forecast And just so we're on the same page when I say the forecast was better I don't mean it was all sunny I mean it agreed better with what actually happened and that model with the better predictions had a climate sensitivity of more than 5° C I know this all sounds rather academic so let me try to put this into context the climate sensitivity determines how fast some regions of our planet will become uninhabitable if we continue pumping carbon dioxide into the atmosphere the regions to be affected first and most severely are those around the equator in central Africa India and South America that's some of the most densely populated regions of the world the lives of the people who live there depend on that scientist get this number right so we need this number to get a realistic idea of how fast we need to act that the climate sensitivity might be considerably higher than most current policies assume is a big problem why wasn't this front page news Well quite possibly because no one read the paper I didn't either as of date it's been cited a total of 13 times I only know about this because my friend and colleague Tim Palmer wrote a comment for nature magazine which he drew attention to this result he also asked other researchers to try and do similar tests with other models to see how well they perform with the short-term weather forecast unfortunately no one listened to him that includes me because I have better things to do than read all of Tim's comments sorry Tim and in all honesty I had pretty much forgotten about this but then late last year a new paper with Jim Hansen's lead author appeared which remind Ed me of this the new Hansen at all paper is a reanalysis of the historical climate data in a nutshell they claim that the historical data is compatible with the climate sensitivity of 4. 8 plusus 1.