Guest post: On Baselines and Buoys

One of the key criticisms of Karl et al. (2015) is that it used a dataset that adjusted buoy data up to ship data – the suggestion being that, in doing so, they produced more apparent warming than if the ships were adjusted down to the buoys.  In a guest post below, Zeke Hausfather shows how it makes no difference if you adjust the buoys up to the ships, or the ships down to the buoys.

Guest post: On Baselines and Buoys

Much of the confusion when comparing the different versions of NOAA’s ocean temperature dataset comes down to how the transition from ships to buoys in the dataset is handled. The root of the problem is that buoys and ships measure temperatures a bit differently. Ships take their temperature measurements in engine room intake valves, where water is pulled through the hull to cool the engine, while buoys take their temperature measurements from instruments sitting directly in the water. Unsurprisingly, ship engine rooms are warm; water measured in ship engine rooms tends to be around 0.1 degrees C warmer than water measured directly in the ocean. The figure below shows an illustrative example of what measurements from ships and buoys might look like over time:

Buoys only started being deployed in the early-to-mid 1990s. Back then about 95 percent of our ocean measurements came from ships. Today buoys are widespread and provide over 85 percent of our total ocean measurements, so it’s useful to be able to combine ships and buoys together into a single record. One option would be to ignore the temperature difference between ships and buoys and simply average them together into a single record. This is what the old NOAA dataset (version 3) effectively did, and we can see the (illustrative) results in the figure below:

Now, this approach of simply averaging together ships and buoys is problematic. Because there is an offset between the two, the resulting combined record shows much less warming than either the ships or the buoys would on their own. Recognizing that this introduced a bias into their results, NOAA updated their record in version 4 to adjust buoys up to the ship record, resulting in a combined record much more similar to a buoy-only or ship-only record:

Here we see that the combined record is nearly identical to both records, as the offset between ships and buoys has been removed. However, this new approach came under some criticism from folks who considered the buoy data more accurate than the ship data. Why, they asked, would NOAA adjust high quality buoys up to match lower-quality ship data, rather than the other way around? While climate scientists pointed out that this didn’t really matter, that you would end up with the same results if you adjusted buoys up to ships or ships down to buoys, critics persisted in making a big deal out of this. As a response, NOAA changed to adjusting ships down to match buoys in the upcoming version 5 of their dataset. When you adjust ships down to buoys in our illustrative example, you end up with something that looks like this:

The lines are identical, except that the y-axis is 0.1 C lower when ships are adjusted down to buoys. Because climate scientists work with temperature anomalies (e.g. change relative to some baseline period like 1961-1990), this has no effect on the resulting data. Indeed, the trend in the data (e.g. the amount of warming the world has experienced) is unchanged.

What the folks at the Global Warming Policy Forum have been trying to do is to compare “Up to Ships” and “Down to Buoy” records without accounting for the fact that they are on separate baselines (e.g. they are not both showing anomalies with respect to a common climate period). The graph they show, using our illustrative example, looks something like this:

However, when we put both on the same climatological baseline, we see there is in fact no difference between the two lines:

Similarly, here is what the actual graph comparing ERSSTv4 (which adjusts buoys up to ships) and an early draft version of ERSSTv5 (which adjusts ships down to buoys) looks like. When put them on the same baseline, however, we see that the new version 5 is nearly identical to the old version 4:

Here the old NOAA record is shown in blue, while the new NOAA record is shown in red. Its clear that the difference between the two is quite small, and in no way changes our understanding of recent warming.

As Peter Thorne, one of the authors of the upcoming version 5 of NOAA’s ocean dataset told Carbon Brief:

 “It’s worth noting that the ERSSTv4 and ERSSTv5 series are virtually indistinguishable in recent years and that the comparison does not include the data from 2016. The recent changes that were made for ERSSTv4 are largely untouched in the new version in terms of global average temperature anomalies. Therefore, as currently submitted, ERSSTv5 would not change the bottom-line findings of Karl et al (2015)… The change in long-term global average time series in the proposed new version is barely perceptible when the series are lined up together with the same baseline period, and much smaller than the uncertainties we already know about in the existing dataset.”

He continues:

 If ever there was a storm in a teacup, this was it. There is no major revision proposed here and anyone who tells you otherwise fundamentally misunderstands the submitted paper draft (which at this juncture should be the sole provenance of the editor and reviewers per the journal’s policy).

We should let peer review complete its course. Then, and only then, we can discuss this new analysis in more depth.

In the Daily Mail last week David Rose quoted John Bates as saying that “They had good data from buoys. And they threw it out and “corrected” it by using the bad data from ships.” This statement is patently false. Not only did NOAA not “throw out” any buoy data, they actually gave buoys about 7 times more weight than less reliable ship data in their new record. As we discussed in our recent Science Advances paper, relying on the higher quality buoy data removed some bias in recent years due to the changing composition of the global shipping fleet.

At the end of the day what matters is not that ships were adjusted down to buoys or buoys up to ships, what matters is that the offset between ships and buoys was effectively removed. This is now done by all groups producing sea surface temperature records, including NOAA, the U.K.’s Hadley Centre, and the Japanese Meteorological Association.

 Author: Zeke Hausfather is a climate/energy scientist who works with Berkeley Earth and is currently finishing a PhD at the University of California, Berkeley.