Measuring changing sea level precisely enough to say something interesting about it is an extremely difficult task. Many places have tide gauges that have been making measurements for ages, but regional sea level patterns are variable, in part because the coastline itself can be rising or subsiding in some locations.

Satellites sound like the ideal technological solution, but we need to measure the change in distance between the sea surface and a satellite hurtling by over 1,300 kilometers overhead very accurately. Getting accurate enough to detect an average change of about three millimeters over the course of a year has its challenges.

Even if a satellite holds its orbit perfectly, you still have to worry about subtle shifts in the measurements reported by your electronics as they bounce radio waves off the planet. Those shifts are called “bias drifts”—gradually increasing errors that mess with the trend you’re trying to reveal.

By teasing out these bias drifts, a group of researchers led by the University of Tasmania’s Christopher Watson has provided the most exact accounting of sea level rise yet. The researchers pulled together about a hundred of the best tide gauge records from around the world, tossing out those where land surface elevation changes could not be determined or where shorter-term tectonic changes muddied the data. Most of the tide gauges had GPS stations nearby directly measuring changes in land elevation, but almost a third did not.

For those locations without GPS data, the researchers used models that calculate the “rebound” of the crust. Rebounding has occurred in areas that were depressed by a thick, heavy ice sheet during the last ice age. Modern measurements from the GRACE satellites were also included.

The researchers found slowly growing mismatches with the tide gauge data that started as soon satellites began measuring sea level in 1993. The gap persisted even if they tried various subsets of tide gauges to rule out extraneous explanations.

The satellite record isn’t the product of one satellite. TOPEX/Poseidon operated until late 2002 when Jason-1 took over, which was itself replaced when Jason-2 launched in 2008. Within TOPEX/Poseidon’s run, measurements were also shifted to a secondary sensor when the first one died.

You wouldn’t expect the bias drift to be constant through all that, and it wasn’t. With each transition, the drift shrank. For TOPEX/Poseidon’s first sensor, the troublesome one that was in use for about six years, that drift was a whopping (relatively) 1.5 millimeters per year added to the real sea level rise trend. In contrast, the Jason-2’s drift was essentially zero.

The end result is that the rate of sea level rise measured by satellites in the 1990s gets adjusted downward a bit, with little change over the last decade.

This gets interesting for a couple of reasons. The first is that instead of a slight deceleration in the measured rate of sea level rise of -0.057 ± 0.058 millimeters per year per year, you get a slight acceleration of +0.041 ± 0.058 millimeters per year per year. The error bars of each obviously overlap with zero, but it’s a significant shift. Given that sea level rise accelerated over the 20th Century, a deceleration over the last 30 years would be a bit hard to understand, although natural variability can mess with trends over shorter time periods like this. Acceleration like this is what we expect to see, particularly as the contribution of Greenland and western Antarctica, which are both losing ice, has also accelerated over this time period.

The second reason this is meaningful relates to efforts to account for the various factors affecting sea level. Before adjustment, the average rate of sea level rise since 1993 measured by the satellites was 3.2 ± 0.3 millimeters per year. The adjustment drops that to about 2.6 ± 0.4 millimeters per year because of the slower rise in the 1990s. If we add up all our estimates of deposits to and withdrawals from the sea level bank account (things like melting glaciers, the expansion of warming seawater, and changes in the amount of water on land) we get 2.8 ± 0.5 millimeters per year— which suddenly looks a little better. (The tide gauge record comes in at about 2.7 millimeters per year, by the way.)

If this seems like much ado about some decimals, it’s because those decimals make a big difference. The Intergovernmental Panel on Climate Change “business-as-usual” projection that reaches about one meter of sea level rise by the end of the century involves an acceleration of “just” 0.07 millimeters per year per year over the first few decades. In order to get this stuff right, you really have to squeeze your instruments until all the decimals come out.

Nature Climate Change, 2015. DOI: 10.1038/nclimate2635 (About DOIs).