Ocean Conveyor Belt Confounds Climate Science
The Atlantic Meridional Overturning Circulation (AMOC), also known as the “Ocean Conveyor Belt,” has been the subject of much study since its discovery. The AMOC is primarily responsible for Europe's temperate climate and some scientists have warned that global warming could cause the ocean's flow to slow down or even stop. This rather counter intuitive result of a warming climate would result in a much colder Europe—perhaps even a new mini-ice age. A new analysis of data from satellites and drifting sensors finds no evidence that the conveyor belt is slowing. In fact, a NASA backed study indicates that the conveyor is far less susceptible to throttling by climate change than some climate change alarmists feared.
The heat transported by the AMOC makes a substantial contribution to the moderate climate of maritime and continental Europe. Scientists have long feared that any significant slowdown in the overturning circulation would have profound implications for climate change. In a 2005 Nature paper, “Slowing of the Atlantic meridional overturning circulation at 25°N,” Harry L. Bryden, Hannah R. Longworth and Stuart A. Cunningham analyzed temperature and salinity measurements made during five brief ship surveys between 1957 and 2004. These data seemed to indicate that the northward flow of the Atlantic conveyor suffered a 30% decline in volume around the turn of the century. These findings led to warnings that a scenario similar to the disaster film The Day After Tomorrow could be just around the corner.
“'Mini Ice Age' May Be Coming Soon, Sea Study Warns,” was the headline on the National Geographic News site. “Chilling new evidence from the Atlantic Ocean is raising fears that western Europe could soon be gripped by a mini ice age,” reported their 2005 news article. “The study supports computer model predictions suggesting that global warming will switch off the North Atlantic current in the next 50 to 100 years.” But even in 2005, it was noted that the magnitude of these findings did not seem to match up with observed climate variation.
The changes reported were so big that they should have already cut oceanic heating of Europe by about one-fifth—enough to cool the British Isles by 1°C and Scandinavia by 2°C. Richard Wood, chief oceanographer at the UK Met Office's Hadley Centre, said at the time that Bryden et al.'s findings left a lot unexplained. More recently, continuous data measurements from cable-moored instrument arrays identified large yearly fluctuations in conveyor flow even larger than those found by the old ship surveys.
The AMOC, bringing warmth to Europe and Scandinavia since the start of the Holocene.
Now, a new study by physical oceanographer Joshua Willis, working at NASA's Jet Propulsion Laboratory in Pasadena, California, has further reduced the likelihood of a faltering conveyor belt. Centimeter-accuracy satellite measurements combined with observations from free-floating subsurface Argo drifters indicate that the flow has not diminished in the past seven years. If anything, the satellite data indicate an increase in flow. Here is how Dr. Willis described the work in the abstract from “Can in situ floats and satellite altimeters detect long-term changes in Atlantic Ocean overturning?”:
Global warming has been predicted to slow the Atlantic Meridional Overturning Circulation (AMOC), resulting in significant regional climate impacts across the North Atlantic and beyond. Here, satellite observations of sea surface height (SSH) along with temperature, salinity and velocity from profiling floats are used to estimate changes in the northward-flowing, upper limb of the AMOC at latitudes around 41°N. The 2004 through 2006 mean overturning is found to be 15.5 ± 2.4 Sv (106 m3/s) with somewhat smaller seasonal and interannual variability than at lower latitudes. There is no significant trend in overturning strength between 2002 and 2009. Altimeter data, however, suggest an increase of 2.6 Sv since 1993, consistent with North Atlantic warming during this same period. Despite significant seasonal to interannual fluctuations, these observations demonstrate that substantial slowing of the AMOC did not occur during the past 7 years and is unlikely to have occurred in the past 2 decades.
For the non-oceanographer the increase of 2.6 Sv may not seem that significant but is is, in fact, a lot of water flow. The “Sv” stands for “Sverdrup,” the largest quantity commonly used to express the volume of flowing water. It is named in honor of Norwegian oceanographer Harald Sverdrup who defined it: 1 Sv represents 106 cubic meters per second (~264 million US gallons per second). This is a flow equivalent to of all the rivers in the world that empty into the ocean. So, not only is the increase of 2.6 Sv over the past decade sizable, the seasonal and year to year variation of ±2.4 Sv is also impressively large. Simply put, the conveyor belt is not behaving the way scientists thought it did.
The globe spanning ocean conveyor belt, also known as the thermohaline circulation (THC), is a large-scale ocean circulation that is driven by differences in water density that occur in different parts of the ocean. Thermohaline is a combination of thermo, referring to temperature, and haline referring to salt content, both factors that help determine the density of sea water. Density differences are created by surface heat and freshwater flowing from rivers and melting ice.
Distinct boundaries exist between masses of water which form at the surface, and subsequently maintain their own identity within the ocean. These masses of water can be arranged one on top of the other, according to density. When driven by gravity, wind and Earth's rotation they form a complicated system that circulates water around the world's oceans in a complicated pattern. The term MOC, short for meridional overturning circulation, is often used for the overall circulation system since the currents are not driven by just water density. The AMOC is the Atlantic portion of the larger worldwide MOC system.
The conveyor belt on a continuous ocean map. Image by Avsa.
Scientists earliest vision of the MOC was of a large, orderly circulation that moved heat energy around the globe. The major currents were thought to be fairly steady, but could increase or decrease with changes in global temperature. Only a weakened MOC would flow during an ice age, and anything that could interrupt its smooth functioning could cause abrupt climate change (e.g. the Bølling-Allerød Warming (BA) and the subsequent Little Dryas cooling). Disruption of the conveyor belt's flow is often cited as a mechanism for climate change “tipping points.”
Now, as it turns out, the flow is neither steady nor does it respond to the projected effects of climate change as scientists supposed. A number of climate scientists have hypothesized that global warming would weaken the MOC. This prediction eventually made its way into the IPCC's 2007 AR4 Working Group I report (see “10.3.4 Changes in the Atlantic Meridional Overturning Circulation”). Here is part of what the the IPCC said about global warming and the MOC:
The reduction in MOC strength associated with increasing greenhouse gases represents a negative feedback for the warming in and around the North Atlantic. That is, through reducing the transport of heat from low to high latitudes, SSTs are cooler than they would otherwise be if the MOC was unchanged. As such, warming is reduced over and downstream of the North Atlantic. It is important to note that in models where the MOC weakens, warming still occurs downstream over Europe due to the overall dominant role of the radiative forcing associated with increasing greenhouse gases.
The IPCC based its predictions on simulations from 19 coupled models integrated from 1850 to 2100 under SRES A1B atmospheric CO2 and aerosol scenarios up to year 2100. Some of the models continue the integration to year 2200 with the forcing held constant at the values of year 2100. The results from these simulations are shown in figure 10.15 from the report, reproduced below. Estimates of late-20th century MOC based on direct observation are shown as vertical bars on the left.
Three of the simulations show a steady or rapid slow down of the MOC that is unrelated to global warming. A few others have late-20th century simulated values that are inconsistent with observational estimates. Of the model simulations consistent late-20th century observations, no simulation predicted an increase in the MOC during the 21st century. The predicted flow reductions range from “indistinguishable within the simulated natural variability to over 50% relative to the 1960 to 1990 mean.” None of the models projects an abrupt transition to an off state of the MOC. In other words, no tipping points are predicted.
That seems all well and good, except the MOC is not following the IPCC script. As the paper by Dr. Willis shows, there has been no slowdown over the past 7 years and probably none over the past 20 years, years during which global temperatures are purported to have risen significantly. This is not the first time that the MOC has thrown scientists a curve. As was reported in “Conveyor Belt Model Broken,” when floating measurement buoys became available scientists discovered that the conveyor belt did not flow in large, smoothly predicable masses. In correspondence with Dr. Susan Lozier, lead author of the paper that article was based on, she claimed that their findings did not change the theoretical behavior of the MOC.
“The North Atlantic waters are indeed overturning, flowing equatorward at depth and returning poleward at the surface, but we are now suggesting that the flow at depth is not confined to a narrow boundary current, or 'conveyor belt',” she stated. “The climate models care first and foremost about the return of the surface waters and our research has no bearing in the slightest on those waters.” That this finding does not potentially impact the climate system as a whole is simply false modesty—any change in the behavior of ocean currents affects how they interact. The conveyor belt doesn't work as scientists thought and that has implications for global heat transfer, and hence climate, over time. Now it would appear that discrepancy was only the beginning and climate science has once again gotten the conveyor belt currents wrong.
Did someone mention an ice age?
Climate models are called GCM, general circulation models, because the two most important parts of the models are atmospheric and ocean circulation simulations. Of the two, the ocean holds the most heat energy and has the strongest impact on climate change. It is clear that climate science does not understand precisely how ocean circulation works, and that implies that all of their model simulations do not reflect reality. The upshot of this new information is that Europe's threatened ice age no longer seems imminent, at least not from a conveyor belt slowdown caused by global warming.
This shows the weakness of the science behind climate change. The predictions of future climate change are based on current understanding of how climate works—the theory. And the theory is based on observations of climate behavior in the past—the data. Except that the data regarding fluctuations in the MOC were spotty and incomplete. Now, with better data it looks like the theory is wrong. This in turn, means that all existing models are based on incorrect assumptions and may also have been calibrated using erroneous historical data. Yet predictions of future disaster generated by these models form the heart of the climate change alarmists' case for radical socioeconomic change. And those of us skeptical of climate science's prognostications are considered the foolish ones.
Be safe, enjoy the interglacial and stay skeptical.