I don't think it was possible to measure them separately until we had good satellite data from the last two decades. If we're extending trends back to the beginning of the industrial revolution, they're going to be mixed.
Satellite data isn't that good unfortunately. Tide gauges say 1.5mm/year of rise, satellites say 3mm/yr, and they estimate satellite error to be +/- 0.5mm year, so the error bars are a significant fraction of the size of the overall change.
Also that assumes the scientists are completely accurate and don't make any mistakes. As recently as 5 years ago they discovered that the measured rise for almost all of the 90s was wrong and revised it by 3mm/year +- 1.7mm/year - the error was the same amount as the imputed level of rise!
This is no knock on the scientists, because measuring the height of a moving ocean from orbit to a level of accuracy that lets you see mm level changes is inevitably going to be very hard. But we should bear in mind that they're heavily biased towards wanting to believe the data is accurate. Their decade long inability to get the measurements correct didn't seem to have any impact on their confidence that they're currently getting it right, and why would it? Those who seize the data, seize the day.
> Also that assumes the scientists are completely accurate and don't make any mistakes. As recently as 5 years ago they discovered that the measured rise for almost all of the 90s was wrong and revised it by 3mm/year +- 1.7mm/year - the error was the same amount as the imputed level of rise!
Citation for this? There are always improvements to our understanding of past data, but you seem to be implying that the uncertainty exceeds the signal. That's simply not the case [1].
Ablain, "Uncertainty in Satellite estimate of Global Mean Sea Level
changes, trend and acceleration", p4
They have shown that there was a drift in the GMSL record over the period 1993-1998. This drift is caused by an erroneous on-board calibration correction on TOPEX altimeter side-A (noted TOPEX-A). TOPEX-A was operated from launch in
October 1992 to the end of January 1999. Then TOPEX side-B altimeter (noted TOPEX-B) took over in February 1999 (Beckley et al., 2017). The impact on the GMSL changes is -1.0 mm/yr between January 1993 and July 1995, 120 and +3.0 mm/yr between August 1995 and February 1999, with an uncertainty of ±1.7 mm/yr (within a 90%CL, (Ablain, 2017)).
Figure 1 from that paper didn't convince you that it was a minor issue? Their correction is about 7% of the 1993-2018 signal, and it resolves the discrepancy with respect to other estimates, including non-satellite ones. Nobody relies on estimates from a single satellite alone.
> The six main groups that provide satellite-altimetry-based GMSL estimates ...
1) You're restricting your attention to groups producing satellite-derived estimates.
2) Several of those groups, e.g. CSIRO, are using data assimilation to combine satellite estimates with gauges [1].
3) There are multiple satellites, often overlapping in time, often of totally different design/orbit/etc... [2]
4) Up to the most recent IPCC report (AR6), assessments were based on tide gauges alone [3]. For many of the reasons that you bring up. Hence "Nobody relies on estimates from a single satellite alone." Reconstruction of historical sea level is a huge scientific discipline and you would benefit from doing some more reading before beating a dead horse. The estimated uncertainty is large for a reason, but it does not encapsulate any scenarios where the observed change has been insignificant. If you think otherwise, I'd be happy to take your money for 2100 options on coastal real estate in about 90% of the world's coastline.
>> Nobody relies on estimates from a single satellite alone.
Yes they did. From the paper I just cited:
The six main groups that provide satellite-altimetry-based GMSL estimates (AVISO/CNES, SL_cci/ESA, University of Colorado, CSIRO, NASA/GSFC, NOAA) use 1 Hz altimetry measurements from the T/P, Jason-1, Jason-2 and Jason-3 missions from 1993 to 2018 (1993–2015 for SL_cci/ESA). The differences among the GMSL estimates from several groups arise from data editing, from differences in the geophysical corrections and from differences in the used method to spatially average individual measurements during the orbital cycles
The apparently independent time series are actually computed from the same raw data sources, as there isn't an abundance of redundant satellites measuring sea level.
But isn't this goalpost shifting? At first you said I was wrong that uncertainty has been the same size as the measurements in the past, citing the IPCC as proof which of course doesn't mention any of this, saying only that they have high confidence in these numbers (the IPCC is not trustworthy). Then it became that the error isn't big enough for you, and nobody relied on a single satellite. Now I show that they did indeed rely on a single satellite.
The core issue here remains the same: how do they know they're getting it right now? A calibration error so large it invalidated their entire time-series lasted for the entire lifetime of the TOPEX-1 mission, and it then took 20 years for the problem to actually be detected and corrected for. They've only been measuring sea level with satellites for 30 years, and the discrepancy between tide gauges and satellites has never been properly reconciled even though they theoretically measure the same thing.