New Study Shows (Again): Deniers Wrong, NOAA Scientists Right
January 4, 2017
Once again, Science FTW.
I interviewed UC Berkeley’s Zeke Hausfather in December, and one of the things we discussed was his latest research study, now public.
When the National Oceanic and Atmospheric Administration (NOAA) updated its global temperature dataset to include revisions to the underlying databases, US House Science Committee Chair Lamar Smith (R-TX) somehow knew that the scientists had done it wrong. In fact, he accused them of having “altered temperature data to get politically correct results.”
How could he know that? Well, Rep. Smith rejects the conclusions of climate science—like the fact that the Earth’s climate is warming. Despite the slight up and down wiggles of the estimated annual temperatures, the warming of the last couple decades increased ever so slightly in NOAA’s new analysis. To Smith, the result was a red flag. Suddenly he wanted to see the researchers’ e-mails and echoed the accusations of contrarian blogs about scientists’ supposedly nefarious adjustments to sea surface temperature measurements.
I described the Anti-Science Committee’s witch hunt against NOAA and NASA a few months ago, in this must-see vid.
Once again, science is shown to work. The laborious process in which scientists check and recheck their work and subject their ideas to peer review has led to another success. An independent test of global warming data has confirmed a groundbreaking 2015 study that showed warming was faster than prior estimates.
Because of its inconvenient findings, the study’s lead author Thomas Karl was subjected to harassment by Lamar Smith (R-TX), chair of the House Science Committee, in an effort to impugn his credibility. But now Karl and his co-authors have been vindicated.
Let’s take a step back and discuss the science. Measuring the temperature of the Earth is hard. There are many locations to measure and many choices to make. Should we measure the temperature of the ground? Of the ocean waters? How deep in the water? If we measure air temperatures, what height should the measurements be taken? How many locations should we make measurements at? What happens if the instruments change over time or if the location changes? What happens if a city grows near a measurement location and the so-called urban heat-island effect grows? How do we estimate the temperatures in areas where no measurements exist?
These and many other questions make measuring global warming challenge. Different groups of scientists make different decisions so that depending on the institution, they will get a slightly different temperature result.
But this diversity is also a good thing. It turns out that it doesn’t matter whose results you use – NASA, NOAA, The Hadley Centre in the UK, the Japanese Meteorological Agency, or the Berkeley Earth group – they all report a warming world. However, the rates are slightly different. So, one persistent question is, which group is most accurate? Whose methods are best?
The new study looks into just this question. The group focused on perhaps the biggest differences among the groups – how they handle ocean temperatures. Specifically, global temperature values typically use a combination of near-surface air temperatures in land regions and sea surface temperatures in ocean regions. Since oceans cover approximately 70% of our planet, the way ocean temperatures are dealt with can separate the warming rates between these groups.
Ocean temperatures can be measured by ship-based temperature sensors, by special floating measuring instruments, or by satellites. Prior to the advent of satellites and floating sensors, ships were the main temperature sensing platforms. Ship sensors, which measure engine intake water, are known to be slightly warmer than the actual water. So using them introduces a warm bias in the measurements.
Also, as ships have gotten larger, the depth of the engine intakes have increased – meaning the tested water was further from the actual ocean surface. Since the temperature results from buoys differs from ship measurements, the various scientific groups have tended to try to perform corrections between the different sensors. The way the correction is done affects the reported warming rate.
The authors recognized that one of the biggest questions is how to stitch together different temperature results from different sensors. Therefore, the broke the temperature data up into groups according to the measurement device (buoys, satellites, ARGO floats, ships, etc.) and they evaluated warming rates separately for each group. The authors also used advanced statistics to handle areas where no data were recorded.
After applying their tests, the authors found that the results promoted by Karl at NOAA are the best, and other groups, in particular the Hadley Centre in the UK and the Japanese agency, are too cold.
So what does this all mean? A few things. First, this study is a nice reminder that the proper way for science to work is for publications to be scrutinized and checked by others. This process leads the entire scientific community to a deeper understanding of the science.
Second, this validates the scientists who were originally attacked by political non-scientists and in some cases by contrarian scientists. For instance, Judith Curry, a well-known critic of mainstream climate science was quoted as saying:
The new NOAA dataset disagrees with a UK dataset, which is generally regarded as the gold standard for global sea surface temperature datasets … The new dataset also disagrees with ARGO buoys and satellite analyses … Color me unconvinced.
I actually study ocean temperatures so I knew this statement by Judith Curry was complete nonsense. It is nice to see a team actually take the time to prove it. Perhaps she and others will finally admit they were wrong.
Not only do different measurement techniques have the potential to give slightly different temperature readings, but practices change over time. At one point, wooden buckets were thrown over the side and hauled up. Then it was canvas buckets, which allowed a little more evaporative cooling on the way up. Then it was mainly sensors at the engine’s coolant water intake. And in recent years, the number of scientific buoys and autonomous floats has exploded. Getting all the data collected over the years linked up on an apples-to-apples basis takes careful analysis.
NOAA had been relying on version 3b of a marine temperature dataset called ERSST. A 2015 paper described a version 4 update, which included important adjustments to ship-based measurements made since 1940. The adjustment corrected for the offset between ship-based and buoy-based measurements in more recent years.
In order to see how well these adjustments linked up different types of measurements over the last 20 years, Hausfather and his colleagues took advantage of some modern datasets with fewer complications. There have been enough buoy and satellite measurements to provide decent coverage back to 1997, and the fleet of autonomous floats in the Argo network can go back to about 2005. Taken separately, each dataset can be used as an independent test of NOAA’s effort to stitch together multiple types of measurements.
In addition, we can ask whether, say, satellite measurements confirm that version 4 of that ERSST dataset (which doesn’t include satellite measurements) really is more accurate than version 3b was.
The comparison has to be made carefully because each dataset includes measurements from different locations that don’t add up to perfect global coverage. So among other tests, the researchers broke the map into grid cells and used only those cells where there were measurements in both datasets being compared.
As it happens, the Argo float data, the buoy data, and the satellite data each hew closer to the updated dataset that NOAA used. The older version (3b) gives a global average that is too cool in recent years, growing to an offset of about 0.06 degrees Celsius. The researchers repeat this same analysis for two more major sea surface datasets that are used by the UK Met Office and the Japanese Meteorological Agency for their global temperature records. Both of those datasets also drift cooler than the comparison data, but less so than NOAA’s old dataset.
That cooling drift might be caused by a shift in the mix of ship-based measurement techniques. In addition to finding the average offset between buoy and ship measurements, the updated dataset NOAA used also weighted buoy measurements more strongly in its averaging, which would minimize any problems in the ship measurements.
While this study is certainly useful for researchers, it’s important to note that we’re actually talking about very small differences between global temperature datasets—too small to have real implications for our understanding of global warming. But it does confirm that NOAA and the scientists who revised the sea surface temperature dataset were just doing good science. The data don’t lie.
My video from last year explains in more detail what the new study proved.