Atlantic circulation that helps warm Europe is at its weakest for more than 1500 years

Desdemona Despair

12 April 2018 (UCL News) – In the first comprehensive study of ocean-based records, published in Nature, scientists have observed a marked weakening of Atlantic circulation over the past 150 years. This weakening correlates with the end of the Little Ice Age, around 1850 AD, and the onset of the industrial revolution when glaciers and sea ice melted, causing an influx of freshwater.

It is believed that an influx of freshwater is causing significant disturbance to the ocean currents, and could have a dramatic impact on climates across North America and Western Europe.

“Our results suggest that when simulating historical climate events, leading climate change models are either not sensitive enough to changes in the natural environment, such as the influxes of freshwater, or they are not including all the relevant processes.”

“Given that climate models do not fully capture the events that we are reporting, we have to ask: what does this mean for the future, and how does this relate to the changes expected with global warming?” said lead author, Dr David Thornalley (UCL Geography).

The Atlantic circulation, which is scientifically called the Atlantic Meridional Overturning Circulation (AMOC), is a powerful conveyor belt like system that carriers warm water north from the equator and sends cool water back down from the Arctic and Nordic seas. It is responsible for transporting warm water, and with it warm weather, to Western Europe and regulating water patterns important for marine life.

The AMOC is crucial to the world’s climate, and an abrupt slowdown could trigger various disruptions globally. These include a sudden rise in regional sea levels, changes in the position of major rainfall, arid climate zones and freezing winters across Western Europe.

The Atlantic current is also important for the ocean’s absorption of carbon dioxide, and a slowdown in its operation could lead to more carbon dioxide accumulating in the atmosphere, where it causes global warming.

To investigate variations in the AMOC, changes in the size of sediment grains deposited by a major deep-sea current were examined to infer past changes in the strength of circulation. The study also used the fact that the AMOC transports heat, to work out when the current was weak or strong by examining changes in the abundance of types of marine organisms that prefer warm and cold water. 

The results are supported by another new study in the same issue of Nature, led by Levke Ceasar and Stefan Rahmstorf from the Potsdam Institute for Climate Impact Research, Germany. This work looked at climate model data to confirm that sea-surface temperature patterns can be used as an indicator of AMOC strength. Then, using instrumental data of past sea-surface temperatures, they reveal that AMOC has been weakening more rapidly since 1950 in response to recent global warming.

The two new studies together provide complementary evidence to show that the present-day AMOC is exceptionally weak, offering both a longer-term perspective as well as detailed insight into recent decadal changes.

“Determining the future behaviour of the AMOC will depend on understanding just how sensitive the North Atlantic circulation is to external influences such as the influx of freshwater, and how these will vary or increase in the future.” concluded Dr Thornalley.

The research involved Cardiff University and the University of Reading, and was funded by a National Science Foundation grant, the Leverhulme Trust and the European Union’s Horizon 2020 Research and Innovation Framework Programme.

The research also forms part of a large European Union funded project, ATLAS, which is investigating how an altered AMOC may affect deep-sea ecosystems, such as cold water corals that provide important fish habitats and help recycle ocean nutrients.Links

• Dr David Thornalley’s academic profile
• UCL Geography
• Nature paper

Contact

Natasha Dwownes
Tel: +44 (0)20 3108 3844
Email: n.downes [at] ucl.ac.uk

Atlantic circulation that helps warm UK is at its weakest for over 1500 years

11 April 2018 (RealClimate) – Through two new studies in Nature, the weakening of the Gulf Stream System is back in the scientific headlines. But even before that, interesting new papers have been published – high time for an update on this topic.

Let’s start with tomorrow’s issue of Nature, which besides the two new studies (one of which I was involved in) also includes a News&Views commentary. Everything revolves around the question of whether the Gulf Stream System has already weakened. Climate models predict this will be one consequence of global warming – alongside other problems such as rising sea levels and increasing heat waves, droughts and extreme precipitation. But is such a slowdown already underway today? This question is easier asked than answered. The Atlantic Meridional Overturning Circulation (AMOC, also known as Gulf Stream System) is a huge, three-dimensional flow system throughout the Atlantic, which fluctuates on different time scales. It is therefore by no means enough to put a current meter in the water at one or two points.

Since 2004 there has been a major British-American observation project, called RAPID, which tries to measure the total flow at a particularly suitable latitude (26.5° North) with 226 moored measuring instruments. This provides good results and shows a notable slowdown – but only since 2004, and probably the change over such a short period of time is mainly due to natural fluctuations and in itself hardly reveals anything about the possible effects of climate change.

If you want to look further back in time, you have to look for other sources of evidence. In my view, it is the ocean temperatures that are most likely to solve the mystery – because firstly, there is a lot of good data and, secondly, the AMOC has a dominant influence on sea temperatures in large parts of the North Atlantic. In our study – together with colleagues from Princeton and the University of Madrid – we therefore compare all available measurement data sets since the late 19th century with a simulation of a climate model in which the ocean currents are computed in very high resolution. […]

What changes in the AMOC do the data show? The time series from the two new and some earlier studies are shown in Fig. 2. Each of the six curves is based on a different data type and methodology, but they show a largely consistent picture. The green curve shows changes in water mass based on deep sea coral data, the blue curve shows the grain sizes mentioned and the yellow curve shows the RAPID measurements discussed above. The three remaining curves are based on temperature changes – but also on three different methods. The curve from Rahmstorf, et al., 2015 was based on a network of land-based proxy data such as tree rings and ice cores, while the new red curve from Thornalley et al. was based on sediment data. And the new curve from our study (dark blue) uses measured sea surface temperatures, as shown in Fig. 1.

The curves all show a long-term slowdown that is accelerating. The red curve is so smooth because these particular sediment data have too low a time resolution to show shorter fluctuations. The blue curve shows an early decrease already in the 19th century, which Thornalley and colleagues attribute to an earlier warming at the end of the so-called “Little Ice Age”, when the inflow of meltwater could have slowed the formation of deep water in the Labrador Sea. This is not necessarily a contradiction to the other data series, because the two sediment cores used are located in the area of the deep outflow of Labrador Sea Water – but this is only one of two deep currents that together make up the southward part of the overturning circulation of the Atlantic, and thus the heat transport to the north. Therefore, the time evolution of ocean temperatures does not always have to coincide with that of the Labrador Sea Water.

In our study we conclude that the AMOC has weakened by about 15 percent since the middle of the 20th century. [more]

Stronger evidence for a weaker Atlantic overturning circulation

ABSTRACT: The Atlantic meridional overturning circulation (AMOC) is a system of ocean currents that has an essential role in Earth’s climate, redistributing heat and influencing the carbon cycle^{1, 2}. The AMOC has been shown to be weakening in recent years¹; this decline may reflect decadal-scale variability in convection in the Labrador Sea, but short observational datasets preclude a longer-term perspective on the modern state and variability of Labrador Sea convection and the AMOC^{1, 3,4,5}. Here we provide several lines of palaeo-oceanographic evidence that Labrador Sea deep convection and the AMOC have been anomalously weak over the past 150 years or so (since the end of the Little Ice Age, LIA, approximately AD 1850) compared with the preceding 1,500 years. Our palaeoclimate reconstructions indicate that the transition occurred either as a predominantly abrupt shift towards the end of the LIA, or as a more gradual, continued decline over the past 150 years; this ambiguity probably arises from non-AMOC influences on the various proxies or from the different sensitivities of these proxies to individual components of the AMOC. We suggest that enhanced freshwater fluxes from the Arctic and Nordic seas towards the end of the LIA—sourced from melting glaciers and thickened sea ice that developed earlier in the LIA—weakened Labrador Sea convection and the AMOC. The lack of a subsequent recovery may have resulted from hysteresis or from twentieth-century melting of the Greenland Ice Sheet⁶. Our results suggest that recent decadal variability in Labrador Sea convection and the AMOC has occurred during an atypical, weak background state. Future work should aim to constrain the roles of internal climate variability and early anthropogenic forcing in the AMOC weakening described here.

Anomalously weak Labrador Sea convection and Atlantic overturning during the past 150 years

ABSTRACT: The Atlantic meridional overturning circulation (AMOC)—a system of ocean currents in the North Atlantic—has a major impact on climate, yet its evolution during the industrial era is poorly known owing to a lack of direct current measurements. Here we provide evidence for a weakening of the AMOC by about 3 ± 1 sverdrups (around 15 per cent) since the mid-twentieth century. This weakening is revealed by a characteristic spatial and seasonal sea-surface temperature ‘fingerprint’—consisting of a pattern of cooling in the subpolar Atlantic Ocean and warming in the Gulf Stream region—and is calibrated through an ensemble of model simulations from the CMIP5 project. We find this fingerprint both in a high-resolution climate model in response to increasing atmospheric carbon dioxide concentrations, and in the temperature trends observed since the late nineteenth century. The pattern can be explained by a slowdown in the AMOC and reduced northward heat transport, as well as an associated northward shift of the Gulf Stream. Comparisons with recent direct measurements from the RAPID project and several other studies provide a consistent depiction of record-low AMOC values in recent years.

Observed fingerprint of a weakening Atlantic Ocean overturning circulation