Thread by @climpeter

Having been asked by national media what matters to Ireland in the recent @IPCC_CH report one of the things I have mentioned is committed sea level rise. Apparently some choose not to believe what is said so lets unpack this further. First the SPM says the following ... (1/n)

@IPCC_CH So, we are committed to long term changes and those changes take time because the oceans and particularly the ice sheets (as a whole) take a long time to respond. We are bequeathing a problem to many generations hence (2/n)

@IPCC_CH But lets unpack a bit the observational change. This is a figure in the chapter I was CLA on. It shows changes over various historical timescales (3/n)

@IPCC_CH This is the first panel of that figure. It shows changes over the last 800,000 years and yes the scale really is between -150m and plus 50m. This is because earth has repeatedly cycled between huge ice sheets over N. America and Eurasia and not. Sea level is not stable (4/n)

@IPCC_CH The next panel in this figure shows changes over the last 2500 years where we have better chronologies. This has the all too familiar 'hockey stick' - small slow changes followed by a rapid change - that's us at the end there (5/n)

@IPCC_CH Coming forward to the age of tide guages we can reconstruct from direct observations. Several groups have done this - independently - and come to the same conclusions - its risen by about 0.25m since 1900 and its accelerating. (6/n)

@IPCC_CH Coming even closer in we have global measurements of sea level changes from satellites and these agree with the tide guage based reconstructions (7/n)

@IPCC_CH Right, enough about the past. What about the future? There is a really impressive figure in the Technical Summary that can talk to this. Here it is. (8/n)

@IPCC_CH The first panel is the classical timeseries to 2150. Different SSPs range from very low emissions (light blue) to very very high emissions (dark red). The shaded areas are ranges on 2 scenarios. Those dashed lines are low confidence things we can't rule out (9/n)

@IPCC_CH Those low confidence things are principally to do with marine ice cliff instability processes which are highly uncertain. Note that IPCC WGI uses the 5 illustrative scenarios without assessing their viability - that's WGIII (10/n)

@IPCC_CH The next panel speaks to the very long term commitment. It looks at where we might be in 2000 and 10000 years time and compares to cases in the past. Even at 1.5C we are committed to multimetre sea-level rise. I'll unpack this further over the next three tweets (11/n)

@IPCC_CH So, in our lifetimes, this century, even under the most horrible case that we are collectively dumb enough to commit to warming of 5C sea level rise will probably be a metre or so - catastrophic for some for sure. Particularly low lying island states (12/n)

@IPCC_CH But, it presages longer term changes as both the oceans and ice sheets play proverbial catch up. In 2000 years even at 1.5C we are looking at 2 or so metres and at 3C we are looking at 5-10m and at 5C we'd be looking at of the order 20m. (13/n)

@IPCC_CH And at 10,000 years at 1.5m we are looking at sea level somewhere around 7.5m higher, at 3C 10-25m higher and at 5C 30-35m higher. These estimates are fully consistent with the available evidence of prior sea-level temperature relationships (14/n)

@IPCC_CH Sea level processes truly put the multi-generational into the impacts of climate change. The island of Ireland we leave future generations truly is down to the choices we make in our generation. (15/n)

@IPCC_CH There is a hard upper-limit by the way in that if we were foolish enough to just keep emitting greenhouse gases a combination of ocean thermal expansion and ice sheet disappearance means about 70m of sea level rise. I guess the Dail could be hosted in St. Patricks college (16/n)

@IPCC_CH Final panel for completeness flips the normal view on its head and asks when different sea levl states could be reached in near term depending upon scenario and low confidence processes. This figure by the way is all thanks to @mpclimate and @bobkopp (17/n)

@IPCC_CH @mpclimate @bobkopp That'll do I think. If you want more read the very accessible Box TS.4 in the Technical Summary of the report. And if you still wish to not believe well, quite frankly, physics doesn't care about your beliefs. We need to act now to avoid bequeathing a very different island /fin