The only similar example of rapid warming at this magnitude in the Earth’s recent history is the Paleo-Eocene Thermal Maximum 55m years ago, when global temperatures increased by 5-8C and drove widespread extinction of species on both the oceans and land.
However, scientists not involved in the research caution that the results are still speculative and that other complicating factors could influence if or when a tipping point is reached. The threshold identified by the researchers – a 1,200ppm concentration of atmospheric CO2 – is three times current CO2 concentrations.
If fossil fuel use continues to rapidly expand over the remainder of the century, it is possible levels could get that high. The Representative Concentration Pathways 8.5 scenario (RCP8.5), a very high emissions scenario examined by climate scientists, has the Earth’s atmosphere reaching around 1,100ppm by the year 2100. But this would require the world to continue to expand coal use and eschew climate mitigation, as has been occurring in some areas.
Dissipating ‘climate-cooling’ clouds
Stratocumulus clouds are widespread low-lying clouds, typically present within 2,000 metres of the Earth’s surface. They form large cloud “decks” that typically cover around 20% of the Earth’s tropical ocean regions. They cool the Earth by shading its surface from incoming sunlight, reflecting much of it back to space before it reaches the surface.
Clouds have long been one of the main areas of uncertainty in global climate models. Clouds form and dissipate over scales that are smaller than can be resolved in current global climate models, which makes it difficult to predict how they will respond to future changes driven by increasing greenhouse gas concentrations.
The new study overcomes this hurdle by using a state-of-the-art, high-resolution “large-eddy simulation” model that is capable of resolving the physical processes that govern clouds. The researchers use this model to estimate how cloud properties might change as the world warms.
They found a striking result: in their simulations, stratocumulus cloud decks become unstable and break up into scattered clouds when CO2 levels rise above 1,200ppm. When these clouds break up they no longer shade the surface, triggering global warming of 8C – and as much as 10C in subtropical regions. This is in addition to the 5C or so of global warming above pre-industrial levels associated with 1,200ppm CO2.
Very high levels of CO2 affect stratocumulus clouds by influencing how they absorb and re-emit the heat given off from the Earth’s surface. An atmosphere with lots of CO2 in it is more “opaque” and this causes the re-emission of heat to start at lower levels of the atmosphere. In short, this warms the tops of the stratocumulus clouds – which are typically sustained by cooling at their tops. This also reduces the moisture transported up from the Earth’s surface through convection. Together, these changes make stratocumulus cloud decks more susceptible to breaking up.
The figure below, from the paper, shows this process in action.
It shows three cases: a present-day 400ppm CO2 world (left portion); a 1,200ppm CO2 world (middle); and 1,300ppm world (right). In present-day conditions, stratocumulus clouds reflect 30-60% of the sunlight that hits them back to space. As the Earth warms over time, these clouds gradually sink – and, once a critical threshold is passed after 1,200ppm, they break apart.
The authors find that once the stratocumulus decks have broken up, they only re-form once CO2 concentrations drop substantially, to below 300ppm. They suggest that this would make warming associated with this climate “tipping point” much more difficult to reverse, as CO2 concentrations would have to be drawn down to levels last seen a century ago.
Big climate ‘tipping point’
One of the most concerning aspects of climate change are potential “tipping points” – critical thresholds beyond which rapid climate changes occur that are difficult to reverse. Despite a lot of public attention on tipping points, scientists have found limited evidence of them in climate models, at least over time-frames relevant to humans.
The finding in this paper is important, say scientists, because it represents one of the first firm climate tipping points to come out of modeling exercises. As Prof Andrew Dessler at Texas A&M University, who was not involved in the study, tells Carbon Brief:
“Historically, the models have been frustratingly linear and it’s been hard to get them to ‘tip’. But now people have really been hammering the models and they’ve started discovering weird non-linear behaviour.”
The stratocumulus breakup identified in the study also may help to explain some enduring mysteries about temperatures in the distant past, which current climate models have trouble simulating.
For example, the Arctic was ice free about 50m years ago in the early Eocene. Current climate models suggest that it would require atmospheric concentrations of around 4,000ppm CO2 to trigger these conditions, but records suggest that concentrations were a much lower 2,000ppm during the early Eocene.
“The most interesting thing to me [in the paper] is the proposed link to climates of the past. As they note, it’s kind of hard to get models warm enough during the early Eocene. If this is because models don’t account for stratocumulus breakup, then this could explain why the Eocene was very warm despite CO2 being around 2,000 ppm (a little less than twice RCP8.5).”
‘Don’t freak out’
The paper emphasises that large uncertainties remain and the results they find are very much preliminary. Because they are using a high-resolution large-eddy simulation their model lacks many other factors contained in global climate models that operate over larger geographic scales.
Specifically, climate models suggest that large-scale subsidence in the atmosphere – colder air becoming denser and moving towards the ground – weakens as the world warms. This has the effect of lifting up and cooling cloud tops, which counteracts possible stratocumulus breakup. While the paper tries to account for this, the weakening of subsidence that occurs is uncertain and varies across climate models.
If subsidence weakens on the faster end of the range found in climate models, it would mean that stratocumulus breakup would not occur until CO2 levels reach a much more improbable 2,200ppm – and reform if levels fall beneath 1,900ppm.
Carbon Brief asked Marvel if there is any reason to worry that a cloud tipping point could occur at lower CO2 concentrations. She says:
“I can’t really imagine a complicating feedback that would trigger stratocumulus breakup at a lower CO2 level; in fact, as they point out, weakening the large-scale subsidence in the troposphere would counteract this instability.”
Marvel cautions that while the tipping point found in the new paper is interesting, it “doesn’t merit freaking out”. Existing projections are enough of a concern, she adds: “We already have more than enough reasons to avoid hurling ourselves to an Eocene climate. Let’s try to not get to 1,200ppm.”
Dessler similarly cautions that the results are still quite uncertain, telling Carbon Brief that he is “not worried yet”. He suggests that the study’s conclusions should be viewed “as ‘low confidence’ until more work is done on this and other groups/models can reproduce it.”
The tipping point identified in this new paper should be easy to avoid with any sort of concerted efforts to mitigate climate change, even if they fall far short of Paris Agreement current goals of limiting temperature rise to 1.5C or 2C above pre-industrial levels.
But the potential presence of massive tipping points that could usher in potentially catastrophic warming should provide a sobering example of the risks of climate inaction in the face of large “unknown unknowns” in the climate system.
The research, published in Nature, says that a recent weakening of the “Atlantic Meridional Overturning Circulation” (AMOC) is coming to an end, but will stay at a “prolonged minimum” for the next two decades.
This would see relatively low levels of heat uptake in the Atlantic Ocean, thus boosting rising temperatures at the Earth’s surface.
The indirect “proxy” data used to analyse the AMOC suggest that its weakening is part of a natural cycle, rather than being caused by human-caused warming – as had been proposed by two studies published earlier this year.
However, researchers not involved in the study warn that it is “debatable whether such strong conclusions” about the AMOC can be drawn using proxy data, rather than direct observations.
The AMOC is a system of currents that brings warm, salty water in the upper layers of the ocean up from the Gulf of Mexico into the North Atlantic. It then sends cold, more dense water back again in the deep ocean on a constant conveyor belt.
This conveyor plays a crucial role in western Europe’s climate as the incoming warm water releases heat into the atmosphere. Without it, for example, UK winters would be around 5C colder.
Recent research has suggested that the AMOC has weakened by around 15% since the middle of the 20th century. This could lead to considerable changes in climate and rainfall patterns throughout the northern hemisphere.
Palaeoclimate studies of Earth’s distant past link weakening – or even complete shutdowns – of the AMOC to abrupt periods of cold in the northern hemisphere. This suggests that a weakening AMOC in modern times could see northern hemisphere temperatures fall.
But, the new study notes, such past changes happened before humans started pumping billions of tonnes of CO2 into the atmosphere. For example, the researchers suggest the AMOC was in a “weak” phase between 1975 and 1998 – yet global surface temperatures rose rapidly during that period.
As the AMOC has only been monitored directly and continuously since 2004, the study uses a series of “proxies” to infer changes in AMOC strength back to 1945.
These proxies include measurements of ocean temperature and salinity, as well as observations of sea surface height from satellites and tide gauges. Taken together, these proxies “paint a consistent picture” of changes in the AMOC, says co-author Prof Ka-Kit Tung, professor of applied mathematics and adjunct professor in atmospheric science at the University of Washington.
The researchers theorise that human-caused warming has essentially changed the principal role of the AMOC from shifting heat northwards to storing heat in the deep Atlantic.
When the AMOC is strong, there is more warm, salty water in the North Atlantic and the subsequent sinking transports more heat to the deep ocean. This lessens human-caused warming at the Earth’s surface, the researchers say. During periods of weak AMOC, less heat is being shifted into the deep ocean, and so more stays at the surface and temperatures rise rapidly.
This is illustrated in the charts below. The coloured lines in the upper chart show the different proxies for the strength of the AMOC since 1945 – and the inset shows the direct measurements from the RAPID project since 2004. The lower chart shows the variations in global surface temperature (black line) from 1850.
Since 1945, the AMOC has gone through several phases, the researchers say: a strong AMOC up until the mid-1970s, then a weak phase up to the late 1990s, and another strong phase until the mid-2010s. These three phases coincide with periods of slower and faster global surface warming, the study says – showing how the AMOC can suppress or enhance human-caused warming.
The strong AMOC phase in the early 2000s, for example, was partly responsible for the much talked-about slowdown in global surface temperature rise, Tung tells Carbon Brief:
“The other partner is the Southern Ocean. Together they account for 70% of the heat that was stored away from the atmosphere. With more heat stored away at depth in the oceans, less is available to heat the surface, so surface warming slows.”
The AMOC has since weakened rapidly – as has been observed in the RAPID data – and global temperature rise has swiftly resumed.
The proxy data suggest that the weakening of the AMOC is coming to an end, but will stay in a “prolonged AMOC minimum” that will last around two decades. Tung explains:
“If past patterns hold, then we are entering into a flatter minimum. AMOC will stop declining for a couple decades. During this time, surprisingly – which is a new finding of our work – surface warming will actually be more rapid.”
However, writing on the website RealClimate, climate scientists Prof Michael Mann, from Penn State, and Prof Stefan Rahmstorf, from the Potsdam Institute for Climate Impact Research, say that a weaker AMOC bringing warming runs counter to existing research:
“Knight et al. (2005) found that decadal variability of the AMOC can cause small variations in global mean surface temperature, with a strong AMOC linked to high global surface temperatures. Liu et al. (2017) found that their climate model warms less under the same greenhouse gas scenario when the AMOC is weakened more.”
They therefore conclude that the idea that a weak AMOC promotes rapid global warming is “not supported by any convincing evidence”:
“We…do not doubt that rapid global warming will continue until we strongly reduce greenhouse gas emissions – but for reasons that have nothing to with the AMOC.”
Trend or cycle?
Two studies on the recent weakening of the AMOC, published in Nature in April, linked the shift to rising global temperatures and melting of the Greenland ice sheet.
The theory goes that meltwater from the Greenland ice sheet – along with melting Arctic sea ice and greater rainfall over the North Atlantic – causes an influx of freshwater into the ocean. This extra freshwater reduces the sinking of the cooling seawater, reducing how much warm water is dragged up from the tropics, thus weakening the circulation.
But the new study proposes that changes in AMOC are actually part of a natural cycle over multiple decades, with peaks and troughs over the past 60-70 years. Tung explains:
“The peaks are sharper. The trough, where AMOC is the weakest, is more flat and lasting longer – about two or three decades – followed by a rapidly increasing AMOC – 12-13 years – and then a rapidly decreasing AMOC – 13 years – from the sharp peak.”
However, as their proxy record only goes back to 1945, there is only one full AMOC cycle to base future projections on, notes Tung:
“We have only one cycle, so some caution is needed in describing it as a natural cycle with a certain period, because we do not even know if it is periodic. However, it does not look like it is an anthropogenically forced downward trend, because there were ups and downs in the AMOC strength.”
Tung and his co-author propose that the cycle is driven by a “self-reinforcing feedback”. A strong AMOC transports more warm water up to the high latitudes of the North Atlantic, where it helps melt glaciers and sea ice and – over decades – adds freshwater to the surface waters. This then slows the sinking of cold water, thus weakening the AMOC. Eventually, this pattern reverses again, explains Tung:
“When AMOC is slower, it transports less of the warm water northward, ice melt decreases and freshwater output decreases. The seawater becomes more saline gradually, and more sinking then resumes.”
However, while proxy data studies “broadly agree” that the AMOC has swung between strong and weak phases over past decades – and some suggest a long-term weakening trend – they “differ in the details”, says Dr Jon Robson, a senior research scientist at the University of Reading, who was not involved in the study. He tells Carbon Brief:
“We are also still largely in the dark about whether [the recent AMOC] changes are all natural variability or whether human activity has had an influence.”
One of the main caveats of the study is that the lack of a long-term record for AMOC, say Dr Gerard McCarthy and Prof Peter Thorne from Maynooth University in Ireland in an accompanying Nature News & Views article:
“By necessity, the authors used proxies for AMOC strength because no direct observations of sufficient length exist.”
This is a common problem, says Prof Meric Srokosz, professor of physical oceanography at the National Oceanography Centre in Southampton and science coordinator of the RAPID project. He tells Carbon Brief:
“It is debatable whether such strong conclusions as the paper draws regarding the behaviour and influence of the AMOC can be drawn based on proxies that are not verified against observations over multidecadal timescales of interest.”
Scientists need observations of the AMOC over several decades to unpick its long-term influence on global climate and ocean heat storage, adds Srokosz. With regard to this and other studies using proxies for the AMOC, he notes that:
“The bottom line is, without long-term observations to verify the proxies over the time scales of interest, no proxy reconstruction of the AMOC can give definitive answers to the questions about past and future AMOC changes.”
OCEANS | JUNE 6. 2015.
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