{"id":22250,"date":"2019-01-28T07:52:03","date_gmt":"2019-01-28T12:52:03","guid":{"rendered":"http:\/\/www.realclimate.org\/?p=22250"},"modified":"2019-01-28T13:46:24","modified_gmt":"2019-01-28T18:46:24","slug":"what-the-2018-climate-assessments-say-about-the-gulf-stream-system-slowdown","status":"publish","type":"post","link":"https:\/\/www.realclimate.org\/index.php\/archives\/2019\/01\/what-the-2018-climate-assessments-say-about-the-gulf-stream-system-slowdown\/","title":{"rendered":"What the 2018 climate assessments say about the Gulf Stream System slowdown"},"content":{"rendered":"
\n\n

Last year, twenty\nthousand peer reviewed studies on \u2018climate change\u2019 were published. No single person\ncan keep track of all those \u2013 you\u2019d have to read 55 papers every single day.\n(And, by the way, that huge mass of publications is why climate deniers will\nalways find something to cherry-pick that suits their agenda.) That is why\nclimate assessments are so important, where a lot of scientists pool their\nexpertise and discuss and assess and summarize the state of the art.<\/p>\n\n\n\n

So let us have a quick look what last year\u2019s climate assessments say about the much-discussed topic of whether the Atlantic Meridional Overturning Circulation (AMOC, a.k.a. Gulf Stream System) has already slowed down, as predicted by climate models in response to global warming.<\/p>\n\n\n\n

\"\"<\/figure>\n\n\n\n\n\n\n\n

First, there is the IPCC 1.5 \u00b0C report (SR15)<\/a> prepared for the Paris Climate Agreement and published in September\n2018. It doesn\u2019t say all that much about the AMOC, given that it is not a full\nIPCC assessment, but it does say this:<\/p>\n\n\n\n

It is more likely than not that the Atlantic Meridional Overturning Circulation (AMOC) has been weakening  in recent decades, given the detection of the cooling of surface waters in the north Atlantic and evidence that  the Gulf Stream has slowed by 30% since the late 1950s (Srokosz and Bryden, 2015; Caesar et al., 2018).  There is only limited evidence linking the current anomalously weak state of AMOC to anthropogenic warming (Caesar et al., 2018). It is very likely that the AMOC will weaken over the 21 st century. [\u2026]<\/em><\/p><\/blockquote>\n\n\n\n

Weakening of the Atlantic Meridional Overturning Circulation (AMOC) is projected to be highly disruptive to natural and human systems as the delivery of heat to higher latitudes via this current system is reduced.<\/em><\/p><\/blockquote>\n\n\n\n

[Note: those “30% since the late 1950s” are probably in error; they are not supported by either of the two references provided.]<\/p>\n\n\n\n

Then, in November, the\n4th<\/sup> US National Climate\nAssessment<\/a> was\npublished that had been two years in the making. It says:<\/p>\n\n\n\n

The primary concern related to ocean circu\u00adlation is the potential slowing of the Atlantic Ocean Meridional Overturning Circulation (AMOC). An AMOC slowdown would affect poleward heat transport, regional climate, sea level rise along the East Coast of the United States, and the overall response of the Earth\u2019s climate system to human-induced change. [\u2026]<\/em><\/p><\/blockquote>\n\n\n\n

As the atmosphere warms, surface waters entering the North Atlantic may release less heat and become diluted by increased freshwater melt from Greenland and Northern Hemisphere glaciers. Both of these factors would slow the rate of sinking and weaken the entire AMOC. <\/em><\/p><\/blockquote>\n\n\n\n

Though observational data have been insuffi\u00adcient to determine if a long-term slowdown in the AMOC began during the 20th century, one recent study quantifies a 15% weakening since the mid-20th century and another, a weakening over the last 150 years. Over the next few decades, however, it is very likely that the AMOC will weaken.<\/em><\/p><\/blockquote>\n\n\n\n

Finally, Future Earth<\/em> (a global Earth science\nresearch programme) and the Earth League<\/em>\n(a grouping of leading institutions and individuals in the Earth sciences) have\nissued a climate science update for the Katowice climate summit in early\nDecember, called 10 New Insights in Climate Science 2018<\/a>. It says:<\/p>\n\n\n\n

 A weakening of the Atlantic overturning circulation, often referred to as the Gulf Stream system, has been expected from model simulations. Recent studies confirm that it has slowed down by 15% since the middle of the 20th century and is at its weakest in over a thousand years. This is already having observed effects, such as extreme weather in Europe, and further weakening is expected to strongly affect European weather as well as exacerbating sea-level rise at the east coast of North America.<\/em><\/p><\/blockquote>\n\n\n\n

In December also a new\nstudy \u2013 too late to be included in the assessments \u2013 was published by Thibodeau\net al in Geophysical Research Letters<\/a>, which further supports an\nunprecedented AMOC weakening during the past decades. The authors write: <\/p>\n\n\n\n

In this study, we used geochemical evidence to highlight a slowdown in the North Atlantic Ocean circulation over the last century. This change appears to be unique over the last 1,500 years and could be related to global warming and freshwater input from ice sheet melt.<\/em><\/p><\/blockquote>\n\n\n\n

My view<\/strong><\/p>\n\n\n\n

Our regular readers\nknow that one of my topics of interest is the stability of the Gulf Stream\nSystem \u2013 I\u2019ve worked on this on and off for over 25 years, ever since finishing\nmy PhD in physical oceanography. So let me add my own comments on the findings\ncited above.<\/p>\n\n\n\n

First of all, while we don\u2019t have regular direct measurements of the AMOC going back throughout the 20th<\/sup> Century, indirect evidence for an AMOC slowdown is not new. Dima and Lohmann<\/a> already concluded in 2010 that \u201cthe conveyor has been slowing down over the last seven decades<\/em>\u201d (where \u2018conveyor\u2019 refers to the AMOC).<\/p>\n\n\n\n

Strangely, this\nfinding was not discussed at all in the fifth IPCC report published in 2013.\nTherefore, the IPCC now finding that an ongoing slowdown is \u201cmore likely than\nnot\u201d is progress, yet still a very cautious statement. Likewise the statement\nabout the \u201climited evidence\u201d for the slowdown being human-caused is also very\ncautious. Why do I\nfind this overly cautious<\/a>? <\/p>\n\n\n\n

The main points there are that an AMOC slowdown leads to a particular fingerprint pattern in sea surface temperature change \u2013 which is basically what Dima and Lohmann already identified, and this pattern is predicted by high-resolution climate models in response to rising greenhouse gases, and it is also found in the observations. There is no known alternative explanation for what might cause this fingerprint. That fingerprint is not subtle: it is so strong that the subpolar Atlantic is the world\u2019s only region which has resisted global warming over the past hundred years and even has cooled down, reaching record low temperatures in 2015 when the globe as a whole was record-hot.<\/p>\n\n\n\n

\"\"

Although the AMOC slowdown fingerprint is most clearly seen in long-term sea surface temperature trends, it is also apparent in the 2018 temperature anomaly, despite a single year including a lot of short-term variability noise. No place on Earth had a larger cold anomaly than the subpolar Atlantic. Image: Berkeley Earth<\/a> project.<\/em> <\/figcaption><\/figure>\n\n\n\n

In fact, the strength of this pattern and the conclusion that it corresponds to a 15% AMOC slowdown just matches the median slowdown found in the historic climate runs of the CMIP5 climate models \u2013 in other words, it is exactly what the models predict as a response to human-caused climate change. The physical mechanism is understood – how warming and ice melt weakens the AMOC (and that these factors are human-caused), and how an AMOC weakening causes the observed surface temperature fingerprint. In addition, there are several independent data sets that show this slowdown to be unprecedented for at least a millennium. <\/p>\n\n\n\n

In IPCC jargon, personally I would therefore give the statement that the AMOC has slowed down since the early-mid 20th<\/sup> , and that this is at least partly human-caused, a \u201cvery likely\u201d rating.<\/p>\n\n\n\n

Links<\/strong><\/p>\n\n\n\n

If you doubt that the AMOC has weakened, read this<\/a><\/p>\n\n\n\n

Stronger evidence for a weaker Atlantic overturning circulation<\/a><\/p>\n\n\n\n

The underestimated danger of a breakdown of the Gulf Stream System<\/a><\/p>\n\n\n\n

AMOC slowdown: Connecting the dots<\/a><\/p>\n\n\n\n

What\u2019s going on in the North Atlantic?<\/a><\/p>\n\n<\/div> ","protected":false},"excerpt":{"rendered":"

Last year, twenty thousand peer reviewed studies on \u2018climate change\u2019 were published. No single person can keep track of all those \u2013 you\u2019d have to read 55 papers every single day. (And, by the way, that huge mass of publications is why climate deniers will always find something to cherry-pick that suits their agenda.) That […]<\/p>\n","protected":false},"author":10,"featured_media":0,"comment_status":"open","ping_status":"closed","sticky":false,"template":"","format":"standard","meta":{"_exactmetrics_skip_tracking":false,"_exactmetrics_sitenote_active":false,"_exactmetrics_sitenote_note":"","_exactmetrics_sitenote_category":0,"_genesis_hide_title":false,"_genesis_hide_breadcrumbs":false,"_genesis_hide_singular_image":false,"_genesis_hide_footer_widgets":false,"_genesis_custom_body_class":"","_genesis_custom_post_class":"","_genesis_layout":"","footnotes":""},"categories":[1],"tags":[60,61,62],"class_list":{"0":"post-22250","1":"post","2":"type-post","3":"status-publish","4":"format-standard","6":"category-climate-science","7":"tag-amoc","8":"tag-gulf-stream","9":"tag-ipcc","10":"entry"},"aioseo_notices":[],"post_mailing_queue_ids":[],"_links":{"self":[{"href":"https:\/\/www.realclimate.org\/index.php\/wp-json\/wp\/v2\/posts\/22250","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/www.realclimate.org\/index.php\/wp-json\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/www.realclimate.org\/index.php\/wp-json\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/www.realclimate.org\/index.php\/wp-json\/wp\/v2\/users\/10"}],"replies":[{"embeddable":true,"href":"https:\/\/www.realclimate.org\/index.php\/wp-json\/wp\/v2\/comments?post=22250"}],"version-history":[{"count":6,"href":"https:\/\/www.realclimate.org\/index.php\/wp-json\/wp\/v2\/posts\/22250\/revisions"}],"predecessor-version":[{"id":22261,"href":"https:\/\/www.realclimate.org\/index.php\/wp-json\/wp\/v2\/posts\/22250\/revisions\/22261"}],"wp:attachment":[{"href":"https:\/\/www.realclimate.org\/index.php\/wp-json\/wp\/v2\/media?parent=22250"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/www.realclimate.org\/index.php\/wp-json\/wp\/v2\/categories?post=22250"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/www.realclimate.org\/index.php\/wp-json\/wp\/v2\/tags?post=22250"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}