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10 years on

Filed under: — gavin @ 17 November 2019

I woke up on Tuesday, 17 Nov 2009 completely unaware of what was about to unfold. I tried to log in to RealClimate, but for some reason my login did not work. Neither did the admin login. I logged in to the back-end via ssh, only to be inexplicably logged out again. I did it again. No dice. I then called the hosting company and told them to take us offline until I could see what was going on. When I did get control back from the hacker (and hacker it was), there was a large uploaded file on our server, and a draft post ready to go announcing the theft of the CRU emails. And so it began.

From “One year later”, 2010.

Many people are weighing in on the 10 year anniversary of ‘Climategate’ – the Observer, a documentary on BBC4 (where I was interviewed), Mike at Newsweek – but I’ve struggled to think of something actually interesting to say.

It’s hard because even in ten years almost everything and yet nothing has changed. The social media landscape has changed beyond recognition but yet the fever swamps of dueling blogs and comment threads has just been replaced by troll farms and noise-generating disinformation machines on Facebook and Twitter. The nominally serious ‘issues’ touched on by the email theft – how robust are estimates of global temperature over the instrumental period, what does the proxy record show etc. – have all been settled in favor of the mainstream by scientists plodding along in normal science mode, incrementally improving the analyses, and yet they are still the most repeated denier talking points.

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Sensitive But Unclassified

The US federal government goes to quite a lot of effort to (mostly successfully) keep sensitive but unclassified (SBU) information (like personal data) out of the hands of people who would abuse it. But when it comes to the latest climate models, quite a few are SBU as well.

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Unforced variations: Nov 2019

Filed under: — group @ 1 November 2019

This month’s open thread.

Do you want to share your views on climate change and reading blogs?

Filed under: — rasmus @ 21 October 2019

A survey is conducted by researchers of Cambridge University and Wageningen University. They have asked us to post information about it. Please share your views on climate change and reading blogs by filling out this survey. The data will be used to get a better understanding of climate change blog audiences’ views on climate change and their blog reading behavior.

What’s in it for you?

  • You have a chance on winning a $20 gift card of Amazon;
  • You will get a sneak preview of the preliminary results;
  • You will contribute to research on climate change blogs.

Participation is anonymous, and your answers will be handled confidentially. The data is only used for research purposes.

The Cambridge University and Wageningen University team highly value your input. Please fill out the survey by following this link.

More than 500 people misunderstand climate change

Filed under: — rasmus @ 15 October 2019

A consensus is usually established when one explanation is more convincing than alternative accounts, convincing the majority. This is also true in science. However, science-based knowledge is also our best description of our world because it is built on testing hypotheses that are independently reexamined by colleagues.

It is also typical that there are a few stubborn people who think they know better than the rest. When it comes to climate science, there is a small group of people who refuse to acknowledge the facts that have convinced almost the entire scientific community. Most of these contrarians are not even scientists.

But there are also about 500 scholars who recently have come forward and signed a declaration at odds with the scientific consensus,  claiming “there is no climate emergency”. They represent a tiny fraction of the scholar community dismissing man-made climate change –  by comparison, there is about 20,000 participants on the annual meetings of the American Geophysical Union.

A press conferences has been scheduled on Friday October 18th in Brussels, Rome and Oslo in order to promote the declaration. The intention behind the declaration is to influence the EU and the UN.

Most of the academics who signed the petition have no or little experience within climate research (check Google Scholar). Some of the signatures also have connections with political think tanks.

The message of the declaration is the same that the contrarians have repeated over and over again – but repeating it doesn’t make it more true.

I and some colleagues have examined the most common contrarian papers on climate change and have found that all of them were based on flawed methods/analysis (see previous post Let’s learn from mistakes). Some of the people who signed this petition have demonstrated their incompetence – the proof is in the papers that I and my colleagues reexamined in that study.

We cannot expect every scientist to have the same understanding, especially when it comes to scientific disciplines other than those in which they have professional experience. When they dismiss evidence on matters in an unfamiliar discipline without a convincing explanation, then they demonstrate a lack of respect for both science and the wider public.

They obviously don’t care whether people get true facts of false ideas. Below, I’ll try to explain why their arguments still do not convince.

The following statement is misleading:

“The geological archive reveals that Earth’s climate has varied as long as the planet has existed, with natural cold and warm phases. The Little Ice Age ended as recently as 1850. Therefore, it is no surprise that we now are experiencing a period of warming. Only very few peer-reviewed papers even go so far as to say that recent warming is chiefly anthropogenic”

It is true that Earth’s climate has changed over the past, but such changes have had specific physical causes, which are reasonably well understood. 

There have been changes in the shape of the continents, formation of mountain ranges, changes in atmospheric composition, changes in Earth’s orbit around the sun (the Milankovitch cycles), changes in the sun, volcanic activity, and changes in ocean currents, all of which have influenced Earth’s climate. 

As for the “Little Ice Age”, it was very different to the present global warming. It had a more regional character and was not as synchronised on a global scale as the ongoing climate change. 

The scientific documentation of past changes in climate is one of the ways that we know that that the climate is sensitive to changed conditions. The Earth has never been as closely monitored as today, especially with the help of satellites and advanced modern instruments, giving unprecedented amounts of high-quality data. 

This monitoring shows that the conditions that caused climate change in the past are absent today, except for the increases in greenhouse gases. The IPCC reports provide lists of peer reviewed papers on the global warming. 

The following statement is incorrect:

“The world has warmed at less than half the originally-predicted rate, and at less than half the rate to be expected on the basis of net anthropogenic forcing and radiative imbalance. It tells us that we are far from understanding climate change.”

Indeed, comparisons between simulated and observed global mean surface temperatures indicate a good correspondence.  

I can believe that the people who signed the petition don’t understand climate change, but they should speak for themselves. The rest of the science community has a fairly good understanding. 

The fact that we can write computer code based on the fundamental laws of physics that is able to reproduce phenomena we observe on Earth, indicates that we do understand the climate system. See the description of climate models on both Carbonbrief.org and TED.com.  

The following statement is incorrect

“Climate models have many shortcomings and are not remotely plausible as policy tools. Moreover, they most likely exaggerate the effect of greenhouse gases such as CO2. In addition, they ignore the fact that enriching the atmosphere with CO2 is beneficial.”

The scientific knowledge underpinning climate policies is established both from observations as well as the laws of physics and climate models. 

The global climate models share common description of the atmosphere with weather forecast models used on a daily basis to provide operational weather warnings. 

All climate models have been evaluated and tested, and they do reproduce the observed global warming as seen with the observations. 

The concentration of greenhouse gases in the atmosphere is increasing. Their physical properties can be established accurately through lab studies.

Carbon Dioxide (CO2) is a greenhouse gas which is a byproduct from the consumption of fossil energy, and the increase in the atmospheric CO2 concentrations represents approximately 40% of the total amount produced from the exploitation of fossil fuels. 

The CO2 bears a fingerprint that connects the increased amount to coal, oil and gas, in terms of the isotopes carbon-13 and carbon-14, as well as the comparable concentrations of oxygen and nitrogen and ocean acidification.  

The climate models reproduce the observed sensitivity, as shown in Benestad and Schmidt (2009) and the figure below.

Fig 2 from Benestad & Schmidt (2014)

Observed 〈T〉 and “all” (thick curves), together with predictions based on equation (1) (open circles) and linear multiple regression models in equation (2) using all known forcings as input (solid circles). Source: Benestad & Schmidt (2009).

The following statement is irrelevant:

“CO2 is not a pollutant. It is essential to all life on Earth. Photosynthesis is a blessing. More CO2 is beneficial for nature, greening the Earth: additional CO2 in the air has promoted growth in global plant biomass. It is also good for agriculture, increasing the yields of crops worldwide”

Water too is essential to all life on Earth. Too much is not good, such as flooding or drowning.

The following statement is incorrect:

“There is no statistical evidence that global warming is intensifying hurricanes, floods, droughts and such like natural disasters, or making them more frequent. However, CO2-mitigation measures are as damaging as they are costly. For instance, wind turbines kill birds and insects, and palm-oil plantations destroy the biodiversity of the rainforests.”

CO2 has an indirect effect on extreme weather conditions through increasing the greenhouse effect and changing Earth’s hydrological cycle. It is well-established that increased surface temperatures lead to increased evaporation and water vapour in the atmosphere. 

Water vapour is the main fuel for weather phenomena such as storms and rainfall. Global warming is also accompanied by changes to the large-scale circulation pattern, such as the Hadley cell, affecting both extreme rainfall in the tropics and drought conditions in the sub-tropics. 

The observed number of record-breaking temperatures and rainfall provide statistical evidence for the weather becoming more extreme. One example is the increased probability of heavy precipitation.

The following statement is misguided:

“There is no climate emergency. Therefore, there is no cause for panic and alarm. We strongly oppose the harmful and unrealistic net-zero CO2 policy proposed for 2050. If better approaches emerge, we will have ample time to reflect and adapt. The aim of international policy should be to provide reliable and affordable energy at all times, and throughout the world.”

There is ample evidence of changing risks connected to weather, with more heatwaves and more extreme rainfall. 

The global mean sea-level is rising and coral reefs are dying. Glaciers providing predictable water supply are melting, such as in the Himalayas. The consequences for ecosystems and agriculture are dire. 

The insurance sector is already affected, and the consequences from climate change will increasingly disrupt new sectors such as agriculture, water management, transport, tourism, and trade. 

There will be regions where people no longer will be able to reside and there will be increased levels of migration and conflicts connected to climate change.

Rather than pushing a petition, the contrarians should present scientific evidence for their view. If such evidence exists, it needs to be transparent so that others can reexamine it and get swayed by the information. So far, the typical contrarians (and one of the signatures) have preferred not to disclose their work.

There have already been some reactions to this petition, e.g. on Climatefeedback.org. It was also preceded by a similar Italian “pro-fake-news” petition (signed by more or less the same Italian contrarians as this version) that prompted a response from Italian scientists.

The claims presented in the petition signed by 500 contrarians is the strongest case the contrarians can muster against climate science. In other words, the best shot from the majority of world’s supposedly prominent academics known to have an alternative opinion (i.e. the majority of a tiny minority).

Obviously, there is not much convincing evidence against anthropogenic climate change.

References

  1. R.E. Benestad, and G.A. Schmidt, "Solar trends and global warming", Journal of Geophysical Research, vol. 114, 2009. http://dx.doi.org/10.1029/2008JD011639

Forced responses: Oct 2019

Filed under: — group @ 1 October 2019

Bi-monthly open thread on climate solutions. Please try to be civil. Remember, climate science questions can be discussed on the Unforced Variations thread.

Unforced Variations: Oct 2019

Filed under: — group @ 1 October 2019

This month’s open thread. Please try to stick to climate science topics.

Unforced variations: Sep 2019

Filed under: — group @ 1 September 2019

This month’s open thread for climate science topics. A new two-part community assessment of tropical storms and climate change is online at BAMS: Knutson et al. (2019a ; 2019b). And for those interested in Arctic Sea Ice, there is always the NSIDC.

References

  1. T. Knutson, S.J. Camargo, J.C.L. Chan, K. Emanuel, C. Ho, J. Kossin, M. Mohapatra, M. Satoh, M. Sugi, K. Walsh, and L. Wu, "Tropical Cyclones and Climate Change Assessment: Part I: Detection and Attribution", Bulletin of the American Meteorological Society, vol. 100, pp. 1987-2007, 2019. http://dx.doi.org/10.1175/BAMS-D-18-0189.1
  2. T. Knutson, S.J. Camargo, J.C.L. Chan, K. Emanuel, C. Ho, J. Kossin, M. Mohapatra, M. Satoh, M. Sugi, K. Walsh, and L. Wu, "Tropical Cyclones and Climate Change Assessment: Part II. Projected Response to Anthropogenic Warming", Bulletin of the American Meteorological Society, 2019. http://dx.doi.org/10.1175/BAMS-D-18-0194.1

The Antarctic ice sheet is melting and, yeah, it’s probably our fault.

Filed under: — eric @ 14 August 2019

Glaciers in West Antarctica have thinned and accelerated in the last few decades.  A new paper provides some of the first evidence that this is due to human activities.

by Eric Steig

It’s been some time since I wrote anything for RealClimate. In the interim there’s been a lot of important new work in the area of my primary research interest – Antarctica. Much of it is aimed at addressing the central question in Antarctic glaciology: How much ice is going to be lost from the West Antarctic ice sheet, and how soon? There’s been a nearly continuous stream of evidence supporting the view that the West Antarctic Ice Sheet is in serious trouble – perhaps already undergoing the beginning of “collapse”, which John Mercer presaged more than four decades ago.

Yet showing that the ice sheet has changed doesn’t really address the question about what will happen in the future. To do that, we also need to answer another one: How much of the ice loss that has already happened is a response to anthropogenic climate change? A new paper in Nature Geoscience this week is one of the first to attempt an answer, and that is what has inspired me to get back to RealClimate blogging. Full disclosure: I’m a co-author on the paper.

In this post, I’d like to provide a bit of context for our new paper, and to emphasize some points about our findings that are generally going to be lost in popular accounts of our work.

The key finding is that we now have evidence that the increasing loss of ice from the West Antarctic Ice Sheet is a result of human activities — rising greenhouse gas concentrations in particular. Now, some may be surprised to learn that this wasn’t already known. But the argument that humans are responsible has rested largely on the grounds that there must be a connection. After all, why should melting have increased only in the late 20th century, precisely when the impacts of anthropogenic climate change were becoming more and more apparent? It seems an unlikely coincidence.

As Richard Alley* put it:

It has been hard to imagine that the ice sat around happily for millennia and then decided to retreat naturally just as humans started perturbing the system, but the evidence for forcing by natural variability was strong.

To be sure, there have been studies suggesting a discernible anthropogenic impact on Antarctic surface temperature, particularly on the Antarctic Peninsula.  And it’s known that the depletion of stratospheric ozone and the rise in greenhouse gases has caused the circumpolar winds to increase in strength. But there has been little direct evidence that what’s happening to the ice sheet itself can be attributed to human-induced climate changes. Consequently, there has been no paper published that makes a strong claim about this. Indeed, a formal solicitation of expert views in 2013 showed that opinion was pretty much evenly divided on whether observed changes to the Antarctic ice sheet were simply part of the natural variability of the climate/ice-sheet system.  In stark contrast, agreement among those same experts was (and is) unanimous that Greenland is melting because of anthropogenic global warming. 

Before getting into what is new in our paper, it’s worth starting with a bit of background on West Antarctica, and a review of the evidence for the role of natural variability. Since not everyone will want to read the play-by-play, I’ve put most of that in a separate post, here. I hope you’ll read it.

In short, glacier melt in West Antarctica has increased because more Circumpolar Deep Water (which is relatively warm) is getting from the ocean surrounding Antarctica onto the Antarctic continental shelf and reaching the floating ice shelves of the large outlet glaciers that drain the West Antarctic ice sheet into the ocean. As shown by Thoma et al. (2008) in a seminal modeling study in Geophysical Research Letters**, how much Circumpolar Deep Water (CDW) gets onto the continental shelf is strongly influenced by the strength and direction of the winds at the shelf edge. Essentially, stronger westerlies (or simply weaker easterlies) tend to cause more CDW inflow, and hence, more glacial melt.

Because of the important role played by the winds, many have assumed that there must be a link between the melting glaciers and the ozone hole. But the greatest control on wind variability along the coast of West Antarctica is the state of the tropics. Just as El Niño event causes widespread climate anomalies in the Northern Hemisphere — such as increased rainfall in southern California — it also causes changes in the West Antarctic. Indeed, the Amundsen Sea, where the largest West Antarctic glaciers are, is one of the areas on the planet that is most strongly dependent on the El Niño-Southern Oscillation (ENSO) (e.g. Lachlan-Cope and Connolley, 2006). In 2012, we published a paper showing that changes in the winds in this region in the last few decades, which correspond well with variations in the glaciers, are very well explained by changes in ENSO, and very poorly by changes in ozone. We also noted that because big ENSO events had occurred in the past, it was quite plausible that wind conditions not that different than those of today had also occurred in the past. Indeed, we have very good evidence from ice cores that climate conditions in West Antarctica in the 1940s were not very different than those in the 1990s.

It is clear from this work, and much other recent research, that ENSO plays a dominant role in determining the climate conditions in West Antarctica that are relevant to the ice sheet. And since there is little evidence for a long-term anthropogenic change in ENSO, this implies that natural variability in Amundsen Sea winds (driven by natural variability in ENSO) may be the primary driver of observed ice-sheet change in West Antarctica in the last few decades. This is what Richard Alley is referring when he says that the evidence for forcing by natural variability was strong, and it throws a lot of cold water (no pun intended) on the purported link with human activities. But that’s not very satisfying. It doesn’t answer the question of why glacier retreat is occuring now. This is where our new paper comes in.

The new work is led by Paul Holland of the British Antarctic Survey (BAS), with help from Tom Bracegirdle, Adrian Jenkins (also of BAS), Pierre Dutrieux (now at LDEO) and myself. What we argue, in brief, is that although ENSO does indeed dominate the wind variability in the Amundsen Sea on timescales from interannual to multi-decadal, there is also a longer-term trend in the winds, on which the ENSO-related variability is superimposed.

The graph below (Figure 1) summarizes the key finding. What is shown are the winds in the key sector of the Amundsen Sea, centered on ~71°S and ~108°W, with observations in blue, and model results in black and gray. The model results are from an ensemble of simulations, referred to as the “tropical pacemaker” or “PACE” runs, of the CESM climate model. Details are given in Schneider and Deser (2017). Briefly, what has been done is to adjust an otherwise free-running climate model (forced by greenhouse gas emissions) so that it follows the actual history of sea surface temperature in the tropics, but is otherwise left unconstrained by data. We use these experiments as an estimate of how winds have varied over the last century in the Amundsen Sea, a) given what we know happened in the tropics and b) given what the climate model’s physics dictates about how conditions in the tropics affect the Amundsen Sea. Critically, there is nothing done to make the model match observations outside the tropics. Yet the results are in superb agreement with the observed Amundsen Sea winds. While we can never know exactly what happened prior to the advent of satellite observations in the late 1970s, the PACE ensemble provides a set of histories that is plausible, and compatible with modern data. This is probably the best current estimate of how winds have in fact varied in this region.

Figure 1. Zonal wind speed (positive = westerly, negative = easterly) over the continental shelf edge in the Amundsen Sea, Antarctica, since 1920. Observations (ERA-interim data) are in blue. Model results in black (average) and gray (individual ensemble members) are from the tropical pacemaker (PACE) experiments with the CESM climate model, from Schneider and Deser (2017). The dashed line shows the average trend. From Holland et al., 2019.

What Figure 1 suggests is that the winds in this region have varied between easterly and westerly from decade to decade, throughout the 20th century. This is the natural variability associated with ENSO, and is no surprise. But in addition, there is a long-term trend. When averaged over several decades, the winds can be seen to have shifted from mean easterly in the 1920s through 1980s, to mean westerly thereafter.

The trend in the winds is small, and easily lost within the variability of individual model ensemble members, but it is robust (it occurs in all the ensemble members) and statistically significant. Moreover, we know its cause (at least in the model experiments): radiative forcing. Although these experiments also include radiative forcing changes resulting from the ozone hole, it’s clear that the trend in the winds begins well before ozone depletion begins in 1970s. Thus, the key forcing is greenhouse gases.

These results show that variations in the winds that have occurred at the same time as we have been observing the glaciers retreat (i.e., since the 1970’s) are largely attributable to ENSO, as we had thought. But at the same time, the prevalance of strong westerlies in the Amundsen Sea has gradually increased throughout the 20th century. That is, although anomalous westerlies tend to occur most often during an El Niño, the long-term underlying trend means that the likelihood of strong westerlies in any given year is increasing, regardless of whether there is an El Niño or not. Thus, the radiatively-forced change (the trend) accentuates the effect of the natural variabilty (ENSO). As we wrote in the paper, the recent wind anomalies of the last few decades “…reflect Pacific variability that is not at all unusual…. However, when superimposed on the anthropogenic trend, this variability produces periods of absolute westerly winds that are sufficiently anomalous to account for much of the current ice loss.”

Now, a couple of caveats:

First of all, our finding are “simply” the result of looking at climate model simulations. We don’t know exactly what happened in the Amundsen Sea in the last century. On the other hand, Figure 1 looks very much like the data from ice cores from West Antarctica: variability that can be related to ENSO, superimposed upon a long-term trend. (See e.g. Schneider and Steig, 2008 and Steig et al. 2013 for details.)

Second, we are assuming that the Amundsen Sea shelf-edge winds are indeed the most relevant aspect of the system to consider. Again, this is based on the body of work showing that the inflow of CDW onto the Amundsen Sea continental shelf is strongy controlled by these winds. But the physics linking wind variability and CDW inflow is complex, and not everyone agrees with our view on this. Indeed, it is most certainly an oversimplification. Furthermore, as many authors has emphasized, there are complex feedbacks and internal ice-sheet and glacier dynamics involved, and it’s not as if there is a one-to-one relationship between changing winds and glacier retreat. For an excellent discussion of this, see the paper by Christianson et al. (2018).

Third, even without the first two caveats, we are far from proving that the ongoing ice loss from Antarctica can be attributed to human-induced climate change. The challenge here is that the natural component of the wind variabilty is so large that actually detecting (with direct observations) the trend inferred from the model results is not likely to be possible for some time. As we say in the paper, “Decadal internal variability therefore dominates ice-sheet and ocean variability during the modern observational era (since 1979), and will continue to dominate observations for decades to come.” We are not likely to find the smoking gun any time soon.

That all said, our findings are supported by other experiments. It is not only CESM, which is the main focus of the paper, that shows a long-term trend in the winds. In fact, most climate models (i.e., “CMIP5” — see details in our paper) show the same thing. Also, we find that the better the agreement between a given model and observations, the stronger the trend. (Note that the wind speeds shown in the figure above are not anomalies. These are the actual modeled and observed wind speeds. As it happens, CESM has unusally low bias in comparison with observations.)

Finally, our findings provide an important opportunity to glimpse into the future. We examined additional results with CESM, from the so-called “Large Ensemble” (LENS) and “Medium Ensemble” (MENS) set of experiments. These are identical to those of the PACE set-up but without the constraint to follow the observed tropical sea surface temperature. The results are illustrated in Figure 2, below.

Figure 2. Zonal wind speed (positive = westerly, negative = easterly) over the continental shelf edge in the Amundsen Sea, Antarctica, since 1920, and projecting through to 2100. Results are from the “Large Ensemble” (LENS, in black), using known greenhouse gas and other radiative forcing for the past, and “business as usual” RCP 8.5 radiative forcing scenario in the future. Also shown are results from the “Medium Ensemble (MENS) (in red) which uses lower (RCP 4.5) radiative forcing for the future. Gray shows the individual ensemble members from LENS. From Holland et al., 2019. Error bars show the standard deviation of wind anomalies (solid) and the magnitude of historical and projected trends (dashed).

The ensemble mean trend in the LENS experiments is nearly identical to that of the PACE experiments, which further demonstrates that the trend is not part of the natural variability. Comparison between the LENS experiments, which uses the “business-as-usual”*** RCP 8.5 IPCC scenario for the future, and MENS, which uses RCP 4.5, shows that reducing greenhouse gases reduces the future trends.

This is a big deal! Although we humans have evidently caused a long-term increase in westerly winds along the Amundsen Sea coast (which is bad for the West Antarctic ice sheet), the future is not yet written (which is an opportunity). Lowering greenhouse gases to a more modest rate of increase might be enough to prevent further changes in those winds.

Of course, many glaciologists believe we have already passed the point of no return for West Antarctica. I personally think the jury is still out on that. But that’s a discussion for another time.

*The quote from Richard Alley is from a National Geographic article about our paper.

**Not all the most important papers are published in Nature or Science.

***Some people think calling RCP 8.5 “business as usual” is misleading. Hence the quotes.

Background on the role of natural climate variability in West Antarctic ice sheet change.

Filed under: — eric @ 14 August 2019

This is a summary of some of the key details that underpin the discussion of anthropogenic vs. natural forcing in driving glacier change in West Antarctica. This is useful background for the paper by Holland et al. (2019), discussed in another post (here).

We’ve known for some time that Pine Island Glacier (PIG) and Thwaites Glacier, the two largest of several fast-moving outlet glaciers that drain a large fraction of the West Antarctic ice sheet (WAIS) into the Amundsen Sea are critical to the stability of the ice sheet as a whole. Way back in 1979, Terry Hughes argued that these glaciers make the WAIS susceptible to large-scale collapse, which almost certainly occurred during some previous interglacial periods and contributed several meters to sea level rise. In the mid-1990s it was discovered that melt rates under the floating portion of the glaciers was orders of magnitude greater than previously assumed (Jacobs and others, 1996). Shepherd and others (2002, 2004) showed that this melting at the margin had resulted in thinning upstream, and retreat of the grounding line (the point at which the glacier goes afloat). It quickly became obvious that melt rates must have increased in the preceding few decades. Otherwise these glaciers would already have retreated even further. The culprit was suspected to be the increased inflow of Circumpolar Deep Water (CDW) on the Antarctic continental shelf, where it contacts the floating margins of the glaciers.

These ideas were validated in 2010 by direct observations made by an autonomous underwater vehicle under the PIG ice shelf (note: an ice shelf is the floating portion of a glacier; it should not be confused with the continental shelf). The submarine observations (Jenkins and others, 2010) showed that CDW was flooding the cavity below PIG, >30 km upstream of areas that were at least partially grounded as recently as the early 1970s. Although CDW is just a few degrees above freezing, it provides enough heat to melt the ice from below at rates in excess of 50 meters (vertical) per year. Independent estimates derived from satellite observations of ice speed and thinning rates (e.g. Rignot and others, 2008) agreed well with such numbers, sealing our basic undestanding of what was going on.

Now, the reason that glacier melt in West Antarctica has increased is not because Circumpolar Deep Water itself is getting warmer (although it probably is). Instead, it’s clear that more CDW is getting from the ocean surrounding Antarctica onto the Antarctic continental shelf and reaching the glacier margins. As shown in a seminal modeling study in Geophysical Research Letters (Thoma et al., 2008), how much CDW gets onto the shelf is strongly influenced by the strength and direction of the winds at the edge of continental shelf. It is useful to picture this as wind-driven upwelling (Ekman pumping). Westerly winds (blowing fromthe west) along the edge of the continental shelf divert cold surface waters northward because of the Coriolis effect. This surface water is replaced by the upwelling of warm water from below. The upwelled CDW then makes it’s way along the continental shelf and up to (and below) the floating ice shelves. While this picture is greatly oversimplified*, the essential insight is that stronger westerlies (or merely weaker easterlies) along the shelf edge should tend to cause more CDW to get onto the shelf. Numerous modeling studies since the original Thoma et al. work have supported this. Perhaps more important, it’s been verified by observations (more on that below).

Many scientists have assumed that there must be a link between the melting glaciers and the ozone hole. In fact, I got into this area of research partly in response to a press conference given by a well-known glaciologist who made such a claim in response to a reporter’s question, around 2010. We know that ice in West Antarctica is melting from below because it is bathed in warm Circumpolar Deep Water, and that more Circumpolar Deep Water gets onto the continental shelf when the local continental-shelf-edge winds are more westerly. We also know — as I noted above — that the strength of the westerly circumpolar winds around Antarctica has increased, in part because of the depletion of stratospheric ozone. It’s easy to link these separate ideas, but this links largely falls apart under scrutiny. The problem is that these are not the same winds! The circumpolar wind belt is centered around 52°S, very far north of the area of shelf-break winds that Thoma et al. (2008) wrote about, which are centered on about 70°S in the Amundsen Sea. Moreover, there is no correlation between the winds in the Amundsen Sea region and the Southern Annular Mode (SAM) index, a widely-used measure of the strength of the circumpolar westerlies. And the seasonal timing is wrong — the Amundsen Sea winds have increased largely in winter and fall, whereas the influence of the ozone hole is limited to spring and summer.

If it’s not the ozone hole, then what has caused the local winds to change, and to bring more CDW onto the continental shelf (if indeed this is what has happened)? Well, that’s where much of my own work, and that of my coauthors on the new paper, has focussed in the last few years. In 2012, we published a paper articulating the problems with the ozone-hole argument, and pointing out that a much better explanation for the recent glacier changes in West Antarctica was forcing from the tropics. The greatest control on wind variability in the Amundsen Sea is the state of the tropics, which can be characterized roughly by the state of the El Niño-Southern Oscillation (i.e., whether it is a neutral, El Niño, or La Niña year). Just as El Niño event causes widespread climate anomalies in the Northern Hemisphere — such as increased rainfall in southern California — it also causes changes in the West Antarctic. Indeed, the Amundsen Sea is one of the areas on the planet that is most strongly dependent on ENSO (e.g. Lachlan-Cope and Connolley, 2006). Our work showed that the changes in Amundsen Sea winds that had occurred over the last few decades were very well explained by changes in ENSO. We also noted that because big ENSO events had occurred in the past, it was quite plausible that wind conditions not that different than those of today had also occurred in the past.

A number of other papers have supported these findings. Dutrieux et al. (2014) showed that CDW flow onto the shelf, and ice-melt rates under the PIG, decreased during a major La Niña event. Smith et al. (2017) showed evidence that the PIG ice shelf retreated right around the time of really big El Niño event of 1941 (as we speculated in our 2012 paper), and Hillenbrand et al. (2018) showed that CDW may have first begun to flood the Amundsen Sea at about the same time. Finally, Paolo et al. (2018) showed that the influence of El Niño events on West Antarctic glaciers could be measured by satellite observations: El Niño events tend to be correlated with both increased melting from below, and increased snowfall above, and the variations in the altitude of the ice sheet surface (varying by a few tens of cm) can be detected by satellite altimetry.

In short, a lot of research has demonstrated the importance of ENSO in determining conditions in West Antarctica. This has meant that we cannot rule out the idea that natural variability in Amundsen Sea winds, driven by natural variability in ENSO, as the primary driver of observed glacier retreat in West Antarctica.

Our new paper makes the case that while ENSO dominates there is a significant anthropogenic component as well. See the main post on our new paper in Nature Geoscience, here.

*Ekman pumping is actually too weak to account for the observed flow and the reality is quite a lot more complex. For more details on this, see e.g. Arneborg et al., 2012, and Nakayama et al., 2018.