A very significant post over at Dr. Curry’s blog ~ctm
From Climate Etc.
Posted on August 16, 2017 |
by Dr. Ronan Connolly & Dr. Michael Connolly
Satellite observations indicate that the average Arctic sea ice extent has generally decreased since the start of the satellite records in October 1978. Is this period long enough to assess whether the current sea level trend is unusual, and to what extent the decline is caused by humans?
This change in Arctic climate is often promoted as evidence that humans are causing drastic climate change. For instance, an April 29th 2017 article in the Economist (“Skating on thin ice”, pg 16) implied that the Arctic is melting unusually, dramatically and worryingly:
“The thaw is happening far faster than once expected. Over the past three decades the [Arctic sea ice extent] has fallen by more than half and its volume has plummeted by three-quarters… SWIPA estimates that the Arctic will be free of sea ice in the summer by 2040. Scientists previously suggested this would not occur until 2070.”
However, is the 1978-present satellite record really long enough to allow us to:
- a) Assess how unusual (or not) the recent trends are?
- b) Determine how much of the recent climate change is human-caused vs. natural?
Recently, we published a study in Hydrological Sciences Journal (HSJ) in which we extended the Arctic sea ice estimates back to 1901 using various pre-satellite era data sources (Abstract here).
HSJ have chosen this article as one of their “Featured Articles” which means that it is free to download for a limited time: here. But, if you’re reading this post after that offer has expired and you don’t have paywall access, you can download a pre-print here.
In our study, we found that the recent Arctic sea ice retreat during the satellite era actually followed a period of sea ice growth after the mid-1940s, which in turn followed a period of sea ice retreat after the 1910s. This suggests that the Arctic sea ice is a lot more dynamic than you might think from just considering the satellite records (as the Economist did above). So, in this post, we will review in more detail what we currently know about Arctic sea ice trends.
Sea ice trends during the satellite era
The Arctic and Antarctic sea ice extent satellite data can be downloaded from the US National Snow & Ice Data Center (NSIDC) here. In the graphs below, we’ve plotted the average annual sea ice extents from this satellite data for both the Arctic and the Antarctic. For comparison, we’ve also shown Arctic air temperature trends since 1900 (adapted from our HSJ article).
We can see that, yes, the average Arctic sea ice extent has generally decreased since the start of the satellite record. Although, interestingly the average Antarctic sea ice extent has generally increased over the same period. However, when we look at the much longer Arctic temperature record we can see that this is not surprising. The Arctic region has been warming since the late 1970s (when the satellite records began), but this followed a period of Arctic cooling from the 1940s to the early 1970s! In other words, if the satellite records had begun in the 1940s and if the Arctic sea ice extent is related to Arctic temperatures, we would probably have detected a period of Arctic sea ice growth.
Arctic sea ice changes during the pre-satellite era
One of the reasons there has been such interest in the satellite-based sea ice records is that the satellites are monitoring most of the planet and provide almost continuous coverage. But, people were also monitoring Arctic sea ice before the satellite era using various land, ship, submarine, buoy and aircraft measurements.
In the 1990s and early 2000s, Profs. Walsh and Chapman decided to try to combine together some of these pre-satellite measurements to extend the satellite record back to the early 20th century. You can see from the figure below, that their estimates implied that there was almost no variability in Arctic sea ice extent before the satellite era!
For many years, their “Walsh and Chapman dataset” was assumed to be fairly reliable and accurate, and it was widely used by the scientific community.
As can be seen from this clip, it even was shown in Al Gore’s 2006 An Inconvenient Truth film, although Gore seems to have been confused about what data he was showing and mistakenly claims that the Walsh & Chapman graph is based on “submarine measurements of ice thickness”.
https://www.youtube.com/watch?v=vTFuGHWTIqI
By the way, we suspect the submarine measurements Gore refers to are probably the ones from Rothrock et al. 1999 (Open access), but those measurements are a lot more limited than Gore implies, and it had already been published in 1999, so it’s unclear why Gore felt he needed to “persuade them to release it”.
However, when we looked in detail at the available pre-satellite data, we realised that there were serious problems with the Walsh & Chapman estimates.
The main problem is that the pre-satellite data is unfortunately very limited. If a ship travelled through a particular region in a given season, then they could have reported how much ice was in that region, or whether it was ice-free. But, what do you do if there were no ships (or airplanes, buoys, etc. ) in that region?
It seems that in a lot of cases when Walsh & Chapman didn’t have measurements for a given region they effectively ended up assuming that those regions were ice-filled!
For example, in the figure below, the map on the left shows the main data source used by Walsh & Chapman for August 1952. It’s an estimate of the Arctic sea ice extent that was compiled by the Danish Meteorological Institute (DMI). On the map, the red lines indicate the ice boundaries where the DMI actually had taken measurements – in this case, mostly around Greenland and eastern Canada. The white regions on the rest of the map indicate regions where “ice supposed, but no information at hand”. In other words, the DMI was guessing that there might be ice there, but didn’t know!
This period was in the middle of the Cold War and unfortunately there was very little data-sharing between the Soviet Union and western countries like Denmark. So, the DMI had almost no information for the Russian Arctic. However, as it happens, the Russians were making their own observations of the Russian sea ice using aerial reconnaissance, ships, buoys, etc. In the 21st century, some Russian scientists began digitizing this data and publishing it. The map on the right hand side shows the Russian observations for the exact same month (August 1952). The blue regions were ice-free, the white regions were ice-filled and the grey regions were regions they weren’t measuring.
Notice how all of the Siberian Arctic regions that the Russians could see were ice-free were assumed to be ice-filled by the DMI?
The Walsh & Chapman estimates assumed that the DMI’s guesses were accurate, but they weren’t!
Now, we must point out that while we were carrying out our study which used both the Russian data and the DMI data, Walsh and Chapman also updated their estimates. And, the new 2017 Walsh et al. dataset (Open access) uses the Russian dataset too.
However, as we discuss in the paper, their approach still ends up effectively assuming that most of the regions without observations were “ice-filled”! To us, this is a very unwise assumption, particularly for the earlier years when there were very few observations.
So, we realised that the pre-satellite data needs to be re-calibrated to account for the limited observations and also the changes in different data sources (airplanes vs. ships vs. buoys, etc.) for different regions and times. For a detailed discussion of our re-calibration procedure, we’d recommend reading our paper (Abstract here). But, essentially, we used Arctic temperature records from weather stations on land to ensure that the sea ice measurements from each of the data sources show a similar response to Arctic temperatures to that observed in the satellite era.
After re-calibration, we obtained the following result including error bars:
There are several points to notice:
- While Arctic sea ice has indeed been generally decreasing since the start of the satellite era, this coincidentally followed a period of Arctic sea ice growth from the 1940s to 1970s!
- Indeed, the Arctic seems to routinely alternate between periods of sea ice growth and sea ice retreat. This is quite different from the previous Walsh & Chapman estimates which implied that Arctic sea ice was almost constant before the satellite era!
- If we ignore the error bars, perhaps you could argue that sea ice extents since 2005 are lower than they have been since 1901. However, we shouldn’t ignore the error bars. We can see that the lower error bars for the pre-satellite era have been lower at several stages than the upper error bars for the entire satellite era. In other words, the recent low values are still consistent with our estimates for the pre-satellite era.
A useful test of the Global Climate Models used by the IPCC reports (called “CMIP5” models) is to see how good they are at “hindcasting” Arctic sea ice trends. A “hindcast” is a “forecast” that goes backwards in time.
Below, we compare our summer reconstruction with the average of the IPCC’s climate model hindcasts.
We can see that the IPCC climate models were completely unable to reproduce the different growth and retreat periods.
Arctic sea ice changes over the last 10,000 years
In recent years, several researchers have developed an interesting new “proxy” for Arctic sea ice cover, called “PIP-25”, which can be used for estimating long-term changes in Arctic sea ice extent. A “proxy” is a type of measurement which can be used to indirectly approximate some property – in this case, Arctic sea ice cover.
In 2007, Prof. Simon Belt and colleagues noticed that a type of algae which seems to only grow in sea ice produces a specific group of molecules called “IP-25” – see Belt et al., 2007 (link to abstract, link to Google Scholar). They found that if the sea ice in a region melts in the summer, some of this IP-25 will sink to the bottom of the ocean as part of the ocean sediment. However, if there is no sea ice, or if the sea ice remains frozen all year, then the ocean sediment for that year doesn’t contain any IP-25. They realised that if you drill an ocean sediment core for that region, you could use the presence of IP-25 as a proxy for “seasonal sea ice”, i.e., ice that only forms for part of the year.
Later, it was realised that if the IP-25 was absent you could also use the presence of certain species of phytoplankton to distinguish between periods with permanent ice cover (less phytoplankton growth because the sea ice reduces the amount of sunlight under the ice) and ice-free conditions (more phytoplankton growth). So, by combining the IP-25 and phytoplankton measurements in an ocean sediment core, you have a “PIP-25” proxy series (“P” for phytoplankton) which can distinguish between three types of sea ice cover:
- Permanent ice cover (low IP-25 and low phytoplankton)
- Seasonal ice cover (high IP-25)
- Mostly ice-free (low IP-25, but high phytoplankton)
In Stein et al., 2017 (abstract here, although the paper is paywalled), Prof. Rüdiger Stein and colleagues presented the results from two new PIP-25 ocean sediment cores (from the Chukchi and East Siberian Seas) and compared them with another two cores from earlier studies in different parts of the Arctic (one from the Laptev Sea and the other from Fram Strait).
We have adapted the maps below from Figure 2 of Stein et al., 2017, with some editing to make the locations easier to see. The maps show the location of the four cores relative to the maximum and minimum Arctic sea ice extents in 2015. The four cores are quite well distributed throughout the Arctic and so should give us a reasonable estimate of how sea ice has varied throughout the Arctic over longer time scales.
Notice that all four locations were ice-free during the summer minimum (06 September 2015), but three of the locations (the Chukchi Sea, East Siberian Sea and Laptev Sea cores) were ice-covered during the winter maximum. In other words, these three locations currently experience “seasonal sea ice cover”. The remaining location (the Fram Strait core) was still outside the ice extent even during the winter maximum (17 March 2015). So, currently that location is “mostly ice-free”. However, as we will see, the PIP-25 sediment cores suggest that these conditions have changed over time.
For the four plots below, we have digitized the PIP-25 results for the four sediment cores from Figure 10 of Stein et al., 2017. Roughly speaking, PIP-25 values below about 0.5 indicate that the region was mostly ice-free at the time (Stein et al., 2017 use the term “reduced sea-ice cover”), while values above about 0.7 indicate that the region was permanently ice-covered, i.e., it remained ice-covered throughout the entire year (Stein et al., 2017 use the term “perennial sea-ice cover”). Values between 0.5 and 0.7 indicate that the region experienced “seasonal ice coverage”, i.e., it was usually ice-covered during the winter maximum, but ice-free during the summer minimum.
As we discussed above, three of the locations (the Chukchi Sea, East Siberian Sea and Laptev Sea sites) currently experience “seasonal ice coverage” and the Fram Strait site is currently “mostly ice-free”. However, according to the PIP-25 data, over the last 10,000 years, all four of these sites have gone through extensive periods with less ice coverage as well as extensive periods with more ice coverage. In particular, all four locations seem to have experienced much less ice coverage 6,000-8,000 years ago (i.e., well before the Bronze Age) than they do today.
This suggests two points particularly relevant to our discussion:
- Arctic sea ice extents have shown a lot of variability over the last 10,000 years (at least), so we shouldn’t be too surprised that the extents have substantially changed since the start of the satellite records in 1978.
- Despite the widespread belief that the current Arctic sea ice coverage is “unusually low” (based on a combination of the 1978-present satellite records and computer model results), it seems that the coverage was actually a lot lower 6,000-8,000 years ago.
Summary
- After re-calibrating the pre-satellite data, it now transpires that Arctic sea ice has alternated between periods of sea ice retreat and growth. The satellite record coincidentally began at the end of one of the sea ice growth periods. This has led to people mistakenly thinking the post-1978 sea ice retreat is unusual.
- The results from new sea ice proxies taken from ocean sediment cores suggest that Arctic sea ice extent has varied substantially over the last 10,000 years. They also suggest that Arctic sea ice extent was actually less before the Bronze Age than it is today.
- The current Global Climate Models are unable to reproduce the observed Arctic sea ice changes since 1901, and they seem to drastically underestimate the natural sea ice variability
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Several commenters have pointed out that there were some satellites that monitored the sea ice before 1978 and wondered why these were not included in the standard satellite era datasets.
We provide a discussion of these pre-1978 satellites in Section 3.2 of the paper. But, essentially, the reason why people focus on the 1978-present satellites is that these had multi-channel passive microwave detectors while the 1972-1977 NIMBUS-5 satellite only had a single-channel passive microwave detectors and the early satellites used different detectors such as visible light. If the NIMBUS-5 satellite had lasted a bit longer than 1977 or the NIMBUS-7 satellite had launched a bit earlier than October 1978 then it might have been possible to directly compare them. Walt Meier actually made an attempt at this in a 2014 paper.
However, because of the nearly two year gap between NIMBUS-5 and NIMBUS-7 as well as the change in detector type, there are considerable challenges in combining the two datasets. So, this is the reason the satellite-era dataset begins with NIMBUS-7 in late 1978. At any rate, several of the so-called “pre-satellite era” datasets actually incorporate information from these earlier satellites. So, this earlier satellite data is included in our 1901-2015 reconstruction. And our re-calibration process should be equally applicable for making the pre-1978 and post-1978 satellite records more comparable.
seaice1 said at August 18, 2017 at 4:13 pm:
“Look at the graph from paper. we know this is not evidence from alarmists – it is the interpretation that is most charitable for the skeptics. Yet it still shows a rapid decline since 1979 to levels lower than in the previous century. There is just about room to argue that if the error bars overlap, but to have the recent lows not different from the previous lows we need the error to be in the very bottom of the previous and the very highest of the recent. This is very unlikely. We can say that the recent drop – even in this evidence – is very likely to be real and only has a tiny probability of being due to error.
If we look at other reconstructions (as this is a reconstruction) we see an even greater difference. So if we look at all the evidence it is an act of supreme bias to conclude that sea ice is not at its lowest (over the last decade) over the last 150 years.”
Unfortunately, the pre-satellite era data is very limited and so we really cannot ignore the error bars. This is why in Section 5, we point out that our reconstruction is unable to rule out the possibility that post-2004 sea ice extent is the lowest since 1901 or the possibility that it was just as low in the 1940s as it is today.
At any rate, the Stein et al. (2017) PIP-25 sea ice proxies suggest that the recent decline is not unusual compared with the variability of the last 10,000 years.
Everyone of us is prone to confirmation bias, and ironically a good test for whether you might be affected by it is if you believe you are unaffected and the people who disagree with you are! So, when you say “…it is an act of supreme bias to conclude [the opposite of what you happen to believe]”, I’d recommend a bit of self-reflection!
Both Griff and Walt Meier have pointed out that as well as finally introducing the Russian dataset, the new 2017 version of the Walsh dataset also incorporates several sub-Arctic datasets for some of the sub-Arctic seasonal sea ice regions, such as the Bering Sea and Nova Scotia.
As we discussed in Section 2 of the paper, we also looked at the same sub-Arctic datasets. However, aside from the Russian dataset, all of these datasets were taken from sub-Arctic regions, and because our reconstruction was confined to the Arctic Ocean and Seas, none of the other extra datasets were suitable for our analysis.
In the figure below, the yellow, pink and blue regions correspond to the three sectors of the Arctic Ocean and Seas, while the white regions are sub-Arctic (according to the IHO’s “Names and Limits of Oceans and Seas”). The map on the left corresponds to the average winter maximum, while the map on the right corresponds to the average summer minimum
As can be seen from the figure, the sub-Arctic regions do indeed contribute to some of the Northern Hemisphere sea ice during the winter, however they mostly just contain seasonal sea ice. The extra datasets Griff and Walt Meier are referring to provide some extra data for a few of these sub-Arctic regions. But, the Arctic regions comprise the vast majority of the sea ice extent during the winter (~80-82% during March) and almost all of the summer sea ice extent (~98-99% during September).
By the way, thanks Walt, for clarifying that Al Gore was indeed referring to the Rothrock et al. 1999 paper and not the graph he actually showed in the film, and for providing that extra information on his role.
Don Easterbrook said at <a href=” https://wattsupwiththat.com/2017/08/17/what-do-we-know-about-arctic-sea-ice-trends/#comment-2584152”August 17, 2017 at 4:28pm,
“Comparing extent of sea ice now (at the end of a warm period) with sea ice extent in 1979 at the end of a cool period) makes no sense at all. What should be compared is extent of sea ice now with the extent of sea ice at the end of the 1915 to 1950 warm period. Unfortunately, we don’t have good data for the earlier time period. We would also like to compare the sea ice extent in 1979 at the end of the 1950 cool period with the extent in 1915, the end of the 1880 to 1915 cool period. But again, we don’t have the data.
Until we can make these kinds of comparisons, conclusions about trends since 1979 don’t mean much.”
I agree. This was one of the main motivations for our study.
“History of sea ice in the Arctic,” Leonid
Polyak et al, Quaternary Science Reviews
29 (2010) 1757–1778.
http://research.bpcrc.osu.edu/geo/publications/polyak_etal_seaice_QSR_10.pdf
Gloateus said at August 17, 2017 at 4:41 pm:
” If global warming be the alleged cause of Arctic sea ice decline since 1979, then explain please why the highest Antarctic sea ice year in the satellite record was 2014, after steady increases since 1979.
If air temperature be the cause, then why the stark difference, 180 degrees out of synch, between the two poles?”
This is indeed a bit of a head-scratcher as we alluded to in our post. There have been some post-hoc attempts to explain the general increase in Antarctic sea ice extent over the satellite era, but it was certainly not predicted by the climate models in the 1980s, 1990s and 2000s.