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The water south of Greenland has been cooling, so what causes that?

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Sea surface temperature trend 1993 – 2018, from European Atlas of the Seas

Let’s compare two possibilities by a back-of-envelope calculation.

(1) Is it due to a reduced heat transport of the Atlantic Meridional Overturning Circulation (AMOC)?

(2) Or is it simply due to the influx of cold meltwater as the Greenland Ice Sheet is losing ice?

The latter is often suggested. The meltwater also contributes indirectly to slowing the AMOC, but not because it is cold but because it is freshwater (not saline), which contributes to the first option (i.e. AMOC decline).

AMOC heat transport

For that we take the AMOC flow rate times the temperature difference of 15 °C between the northward upper branch and southward deep return flow to obtain the heat transport.

17,000,000 m3/s x 15 K x 1025 kg/m3 x 4 kJ/kgK = 1 PW (1)

(Here, 1 PW = 1015 Watt and 4 kJ/kgK is the heat capacity of water.)

An AMOC weakening by 15 % thus cools the region at a rate of 0.15 PW = 1.5 x 1014 W and according to model simulations can fully explain the observed cooling trend (2). Of course, this slowdown is not only due to Greenland meltwater – other factors like increasing precipitation probably play a larger role, but the impact of Greenland melting is not negligible, as we argue in (3).

Greenland ice melt

Here we start by taking the Greenland mass loss rate into the ocean, times the temperature difference between the meltwater and the water it replaces. Note we are interested in the longer-term temperature trend over decades over the region with the meltwater properly mixed in, not at some temporary patches of meltwater floating locally at the surface.

Total Greenland mass loss has been on average 270 Gt/year for the last two decades (4).

Most of that evaporates though, and what ends up in the ocean of this, according to a recent study by Jason Box (5), is around 100 Gt/year, about 30% of which in form of ice and 70% in form of meltwater.

100 Gt/year = 3000 tons/second – that sounds a lot but the AMOC flow is more than 5000 times larger.

Assuming the ice and meltwater runoff occurs at 0 °C and replaces water that is 10 °C (a very high assumption corresponding to summer conditions and not the long-term average), the cooling rate is:

3,000,000 kg/s x 10 K x 4 kJ/kgK = 1.2 x 1011 W

So in comparison, the cooling effect of a 15 % AMOC slowdown is over 1,000 times larger than the direct cooling effect of the Greenland meltwater.

For the part entering the ocean as ice, we must also consider that to melt ice requires energy. The heat of fusion of water is 334 kJ/kg so that adds 900 tons/s x 334 kJ/kg = 3 x 1011 W.

So it turns out that those suggesting that ‘cold’ meltwater might cause the cold blob in the northern Atlantic are doubly wrong: if we talk about the direct impact of stuff coming off Greenland, than ice is the dominant factor and the energy that’s required to melt the ice. But both the direct effect of meltwater and of icebergs entering the ocean are completely dwarfed by the weakening of the AMOC (regardless of whether we take the numbers of Box et al. or other estimates). And Greenland’s contribution to that is not because the meltwater is ‘cold’, but because it is fresh – it contains no salt and dilutes the saltiness of the ocean water, thereby reducing its density.

As an additional observation: the cooling patch shown above often vanishes in summer, covered up by a warm surface layer – just when the Greenland melt season is on – only to resurface when deeper mixing starts in autumn. Which again supports the idea that it is not due to a direct effect of cold meltwater influx. Also compare the temperature change directly at the Greenland coast, where the meltwater enters, in the image above.

Finally, some have suggested that the cold blob south of Greenland has been caused by increased heat loss to the atmosphere. That of course is relevant for short-term weather variability – if a cold wind blows over the ocean it will of course cool the surface – but I do not think it can explain the long-term trend, as we discussed earlier here at Realclimate.

References

1.            Trenberth, K. E. & Fasullo, J. T. (2017) Atlantic meridional heat transports computed from balancing Earth’s energy locally, Geophys. Res. Let. 44: 1919-1927.

2.            Caesar, L., Rahmstorf, S., Robinson, A., Feulner, G., & Saba, V. (2018) Observed fingerprint of a weakening Atlantic Ocean overturning circulation, Nature 556: 191-196.

3.            Rahmstorf, S., J.E. Box, G. Feulner, M.E. Mann, A. Robinson, S. Rutherford, and E.J. Schaffernicht, 2015: Exceptional twentieth-century slowdown in Atlantic Ocean overturning circulation. Nature Climate Change, 5, 475–480, doi:10.1038/nclimate2554.

4.            NASA Vital Signs, https://climate.nasa.gov/vital-signs/ice-sheets/

5.            Box, J. E., et al. (2022), Greenland ice sheet climate disequilibrium and committed sea-level rise, Nature Clim. Change, 12(9), 808-813, doi: 10.1038/s41558-022-01

Sea level in the IPCC 6th assessment report (AR6)

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My top 3 impressions up-front:

  • The sea level projections for the year 2100 have been adjusted upwards again.
  • The IPCC has introduced a new high-end risk scenario, stating that a global rise “approaching 2 m by 2100 and 5 m by 2150 under a very high greenhouse gas emissions scenario cannot be ruled out due to deep uncertainty in ice sheet processes.”
  • The IPCC gives more consideration to the large long-term sea-level rise beyond the year 2100.

And here is the key sea-level graphic from the Summary for Policy Makers:

Source: IPCC AR6, Figure SPM.8

This is a pretty clear illustration of how sea level starts to rise slowly; but in the long run, sea-level rise caused by fossil-fuel burning and deforestation in our generation could literally go off the chart and inundate many coastal cities and wipe entire island nations off the map. But first things first.

Observed Past Rise

Let’s dive a little deeper into the full report and start with the observed sea level change. Since 1901 sea level has risen by 20 cm, a rise unprecedented in at least 3,000 years (disclosure: I co-authored some of the research behind the latter conclusion).

Source: IPCC AR6 Fig. 2.28b

Since 1900 the rise has greatly accelerated. During the most recent period analyzed, 2006-2018, it’s been rising at a rate of 3.7 mm/year – nearly three times as fast as during 1901-1971 (1.3 mm/year). The IPCC calls this a “robust acceleration (high confidence) of global mean sea level rise over the 20th century”, as did the SROCC in 2019.

The finding of sea-level acceleration is not new. The AR4 already concluded in 2007: “There is high confidence that the rate of sea level rise has increased between the mid-19th and the mid-20th centuries.” And the AR5 found in 2013 that “there is high confidence that the rate of sea level rise has increased during the last two centuries, and it is likely that global mean sea level has accelerated since the early 1900’s.” (Which has not stopped “climate skeptics” from repeatedly claiming a lack of acceleration.)

Source: IPCC AR6 Fig. 2.28c

The reason for earlier hedged wording by the IPCC was the possibility of natural decadal variability affecting the trend estimates, but the AR6 now concludes “that the main driver of the observed global mean sea-level rise since at least 1970 is very likely anthropogenic forcing”. That is the result of so-called “attribution studies” – attempts to differentiate with the help of a combination of data, models, pattern detection and statistics between all possible human-caused and natural factors in the observed changes. However, on the level of basic physical reasoning, it is of course a no-brainer that warming will cause land-ice to melt (and melt faster as it gets hotter) and ocean waters to expand, so sea-level rise is the inevitable result.

And there is this:

New observational evidence leads to an assessed sea level rise over the period 1901 to 2018 that is consistent with the sum of individual components contributing to sea level rise, including expansion due to ocean warming and melting of glaciers and ice sheets (high confidence).

IPCC AR6

That’s an important consistency check; the independent data add up to the overall observed rise.

The Future Until 2100

It is virtually certain that global mean sea level will continue to rise over the 21st century in response to continued warming of the climate system.

IPCC AR6

By how much? That depends on our emissions and is shown in the following figure. The take-away message is: for high emissions we’d likely get close to a meter, sticking to the Paris agreement would cut that down to half a meter.

Source: IPCC AR6, Figure SPM.8

And how does that compare to the recent previous reports? Here is the comparison the IPCC shows:

Projections of global mean sea level for 2050 (left) and 2100 (right). The different colours and boxes represent three emissions scenarios: RCP8.5/SSP5-8.5 (red), RCP4.5/SSP2-4.5 (light blue/yellow) and RCP2.6/SSP1-2.6 (dark blue). Projections are given for AR6, SROCC, AR5, a survey of 106 experts (Survey), structured expert judgment (SEJ), models including marine ice cliff instability (MICI) and projections including only medium-confidence processes (MED). Source: IPCC (2021) Figure 9.25.

If you look at the 2100 projections for the last three reports (AR5, SROCC, AR6) you can see that the numbers have increased each time – and remember that the AR5 numbers had already increased by ~60% compared to the AR4. This illustrates the fact that IPCC has been too “cautious” in the past (which is not a virtue in risk assessment), having to correct itself upward again and again (all the while “climate skeptics” try to paint the IPCC as “alarmist”, for want of any better arguments to play down the climate crisis).

Related to that are notable changes in grappling with uncertainty and risk. The IPCC is now showing very likely (5-95 percentile) as well as likely (17-83 percentile) ranges. In the AR5, it had made the rather ad-hoc argument that “global mean sea level rise is likely (medium confidence) to be in the 5 to 95% range of projections from proces-based models”. So their likely range was actually the modelled very likely range.

The IPCC now splits the uncertainty into two types, hence the two different shadings in the uncertainty bars, in an attempt to also cover uncertainty in processes which we still cannot confidently model. They write:

Importantly, likely range projections do not include those ice-sheet-related processes whose quantification is highly uncertain or that are characterized by deep uncertainty. Higher amounts of global mean sea level rise before 2100 could be caused by earlier-than-projected disintegration of marine ice shelves, the abrupt, widespread onset of Marine Ice Sheet Instability (MISI) and Marine Ice Cliff Instability (MICI) around Antarctica, and faster-than-projected changes in the surface mass balance and dynamical ice loss from Greenland. In a low-likelihood, high-impact storyline and a high CO2 emissions scenario, such processes could in combination contribute more than one additional meter of sea level rise by 2100.

Note that this uncertainty goes to one side: up. For estimating this uncertainty they use an expert survey as well as a smaller but more detailed structured expert judgement. I co-authored the survey (see also 7-minute video about it) with Ben Horton and others, as well as a predecessor survey published in 2014, and I am happy to see that the IPCC now includes this type of expert judgement to assess risks that can’t yet be modelled reliably, but cannot be just ignored either. In dealing with the climate crisis, it simply is not enough to consider what is likely to happen – it is even more important to understand what the risks are.

Think about it: If someone builds a nuclear facility near to your house, would you be satisfied with knowing that it is “likely” to work well (say, 83% certain)? Or would you like to know about a few percent chance that it could blow up like Chernobyl in your lifetime?

With the high-end risk scenarios, the IPCC is catching up with other assessments such as the US National Climate Assessment of 2017, which already showed a “high” scenario of 2 meters and an “extreme” scenario of 2.5 meters of rise by 2100.

The Long Term Future

One of the headline statements of the AR6 is:

Many changes due to past and future greenhouse gas emissions are irreversible for centuries to millennia, especially changes in the ocean, ice sheets and global sea level.

IPCC AR6

That’s because huge ice sheets take a long time to melt in a warmer climate, and the ocean waters take a long time to warm up as you go further down, away from the surface. So by what we are doing now in the next couple of decades we determine the rate and amount of sea-level rise for millennia to come, condemning many generations to continually changing coastlines and forcing them to abandon many coastal cities, large and small. That we cannot turn this back is the reason why the precautionary principle should be applied to the climate crisis.

Just look at the ranges expected by the year 2300, in the right-hand panel of the first image above. Even in the blue mitigation scenario, which limits warming to well below 2 °C, our descendants may well have to deal with 2-3 meters of sea-level rise, which would be catastrophic for the people living at the world’s coastlines. Not only would it be extremely hard and costly – if possible at all – to defend cities like New York during a storm surge with a so much higher sea level. We would see massive coastal erosion happening all around. And remember that “nuisance flooding” is already causing real problems after just 20 cm of sea-level rise, for example along the eastern seaboard of the US!

At least with this Paris scenario and a good portion of sheer luck, we may get away with less than a meter rise. But with further unmitigated increase in emissions, a desastrous 2 meter rise is about as likely as an utterly devastating 7 meter rise. What would our descendants think we were doing?

Two graphs show the path to 1.5 degrees

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In the Paris Agreement, just about all of the world’s nations pledged to “pursue efforts to limit the temperature increase to 1.5 degrees Celsius above pre-industrial levels”. On Saturday, the top climate diplomats from the U.S. and China, John Kerry and Xie Zhenhua, reiterated in a joint statement that they want to step up their climate mitigation efforts to keep that goal “within reach”.

But is that still possible? Here are two graphs.

Global temperature trend (relative to mean 1880-1910, NASA data). The colored curve shows the moving average over 12 months, the black line the linear trend over the last 50 years. Transient warmth following two strong El Niño events in the tropical Pacific is indicated by arrows. If everything continued like this, the 1.5 degree limit would be exceeded around 2040.

The first graph shows the global temperature trend. Warming has progressed essentially linearly for fifty years in response to increasing CO2 emissions. Although the latter accelerate the rise of CO2 in the atmosphere, on the other hand, radiative forcing (which causes warming) increases only with the logarithm of CO2 concentration, and therefore roughly linearly since the 1970s. Any acceleration of warming over the last decade is not a significant trend change. It is linked to two El Niño events in recent years, but that is part of natural variability. Does anyone remember the discussion about the supposed “warming pause” in the early 2000s? It also never was statistically significant, nor did it signify a trend change.

Therefore, if emissions continue to grow, we expect a further roughly linear increase in temperature, which would then exceed 1.5 degrees around 2040. If we lower emissions, the trend will flatten out and become roughly horizontal as we reach zero emissions. Therefore, these observational data do not argue against the possibility to still keep warming below 1.5°C.

Exemplary emission trajectories with CO2 emission budgets that, according to the IPCC, correspond to limiting warming to 1.5 °C with 50% probability (solid) or limiting it to 1.75 °C with 67% probability. The same emissions as in 2019 were assumed as the starting point in 2021, assuming the “corona spike” in 2020 is likely to be temporary.

The second graph shows global CO2 emission trajectories with which we can still limit warming to 1.5 °C, at least with 50:50 probability. This means: given the uncertainties, this could also land us at 1.6 degrees, but with a bit of luck, it could land us a bit below 1.5 degrees. The core conclusions:

  •     It is not yet impossible to keep warming below 1.5 °C.
  •     This requires roughly a halving of global CO2 emissions by 2030 (as already stated in the IPCC 1.5 degree report).
  •     If the world dithers for another ten years before emissions fall, it will no longer be possible (red curve).

It should be noted that I have not assumed net-negative emissions here. Many scenarios assume that we first emit too much and that our children then have to pull CO2 out of the atmosphere after mid-century – I think this is not very realistic and also ethically questionable. I think we will probably not be able to achieve more than reducing global emissions to net zero. Even that would require CO2 sinks to compensate for unavoidable residual emissions, e.g. from agriculture.

Conclusion: The limitation to 1.5 degrees is still possible and from my point of view also urgently advised to avert catastrophic risks, but it requires immediate decisive measures. I am curious to see what the climate summit scheduled by US President Joe Biden will bring in the coming days!

Link

Fact check by Climate Analytics to the claim that we can no longer limit warming to 1.5°C.

This article originally appeared in German at KlimaLounge.

New studies confirm weakening of the Gulf Stream circulation (AMOC)

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Many of the earlier predictions of climate research have now become reality. The world is getting warmer, sea levels are rising faster and faster, and more frequent heat waves, extreme rainfall, devastating wildfires and more severe tropical storms are affecting many millions of people. Now there is growing evidence that another climate forecast is already coming true: the Gulf Stream system in the Atlantic is apparently weakening, with consequences for Europe too.

[Read more…] about New studies confirm weakening of the Gulf Stream circulation (AMOC)

How much CO2 your country can still emit, in three simple steps

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Everyone is talking about emissions budgets – what are they and what do they mean for your country?

Our CO2 emissions are causing global heating. If we want to stop global warming at a given temperature level, we can emit only a limited amount of CO2. That’s our emissions budget. I explained it here at RealClimate a couple of years ago:

First of all – what the heck is an “emissions budget” for CO2? Behind this concept is the fact that the amount of global warming that is reached before temperatures stabilise depends (to good approximation) on the cumulative emissions of CO2, i.e. the grand total that humanity has emitted. That is because any additional amount of CO2 in the atmosphere will remain there for a very long time (to the extent that our emissions this century will like prevent the next Ice Age due to begin 50 000 years from now). That is quite different from many atmospheric pollutants that we are used to, for example smog. When you put filters on dirty power stations, the smog will disappear. When you do this ten years later, you just have to stand the smog for a further ten years before it goes away. Not so with CO2 and global warming. If you keep emitting CO2 for another ten years, CO2 levels in the atmosphere will increase further for another ten years, and then stay higher for centuries to come. Limiting global warming to a given level (like 1.5 °C) will require more and more rapid (and thus costly) emissions reductions with every year of delay, and simply become unattainable at some point.

In her recent speech at the French National Assembly, Greta Thunberg rightly made the emissions budget her central issue.

So let’s look at how the emissions budget concept can be used to guide policy on future emissions trajectories for countries.

[Read more…] about How much CO2 your country can still emit, in three simple steps

Can planting trees save our climate?

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In recent weeks, a new study by researchers at ETH Zurich has hit the headlines worldwide (Bastin et al. 2019). It is about trees. The researchers asked themselves the question: how much carbon could we store if we planted trees everywhere in the world where the land is not already used for agriculture or cities? Since the leaves of trees extract carbon in the form of carbon dioxide – CO2 – from the air and then release the oxygen – O2 – again, this is a great climate protection measure. The researchers estimated 200 billion tons of carbon could be stored in this way – provided we plant over a trillion trees.

The media impact of the new study was mainly based on the statement in the ETH press release that planting trees could offset two thirds of the man-made CO2 increase in the atmosphere to date. To be able to largely compensate for the consequences of more than two centuries of industrial development with such a simple and hardly controversial measure – that sounds like a dream! And it was immediately welcomed by those who still dream of climate mitigation that doesn’t hurt anyone.

[Read more…] about Can planting trees save our climate?

First successful model simulation of the past 3 million years of climate change

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Guest post by Matteo Willeit, Potsdam Institute for Climate Impact Research

A new study published in Science Advances shows that the main features of natural climate variability over the last 3 million years can be reproduced with an efficient model of the Earth system.

The Quaternary is the most recent geological Period, covering the past ~2.6 million years. It is defined by the presence of glacial-interglacial cycles associated with the cyclic growth and decay of continental ice sheets in the Northern Hemisphere. Climate variations during the Quaternary are best seen in oxygen isotopes measured in deep-sea sediment cores, which represent variations in global ice volume and ocean temperature. These data show clearly that there has been a general trend towards larger ice sheets and cooler temperatures over the last 3 million years, accompanied by an increase in the amplitude of glacial-interglacial variations and a transition from mostly symmetry cycles with a periodicity of 40,000 years to strongly asymmetric 100,000-year cycles at around 1 million years ago.  However, the ultimate causes of these transitions in glacial cycle dynamics remain debated.

[Read more…] about First successful model simulation of the past 3 million years of climate change

What the 2018 climate assessments say about the Gulf Stream System slowdown

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Last year, twenty thousand peer reviewed studies on ‘climate change’ were published. No single person can keep track of all those – you’d 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 is why climate assessments are so important, where a lot of scientists pool their expertise and discuss and assess and summarize the state of the art.

So let us have a quick look what last year’s 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.

[Read more…] about What the 2018 climate assessments say about the Gulf Stream System slowdown

Does a slow AMOC increase the rate of global warming?

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Established understanding of the AMOC (sometimes popularly called Gulf Stream System) says that a weaker AMOC leads to a slightly cooler global mean surface temperature due to changes in ocean heat storage. But now, a new paper in Nature claims the opposite and even predicts a phase of rapid global warming. What’s the story?

By Stefan Rahmstorf and Michael Mann

In 1751, the captain of an English slave-trading ship made a historic discovery. While sailing at latitude 25°N in the subtropical North Atlantic Ocean, Captain Henry Ellis lowered a “bucket sea-gauge” down through the warm surface waters into the deep. By means of a long rope and a system of valves, water from various depths could be brought up to the deck, where its temperature was read from a built-in thermometer. To his surprise Captain Ellis found that the deep water was icy cold.

These were the first ever recorded temperature measurements of the deep ocean. And they revealed what is now known to be a fundamental feature of all the world oceans: deep water is always cold. The warm waters of the tropics and subtropics are confined to a thin layer at the surface; the heat of the sun does not slowly warm up the depths as might be expected. Ellis wrote:

“This experiment, which seem’d at first but mere food for curiosity, became in the interim very useful to us. By its means we supplied our cold bath, and cooled our wines or water at pleasure; which is vastly agreeable to us in this burning climate.”

[Read more…] about Does a slow AMOC increase the rate of global warming?

Will climate change bring benefits from reduced cold-related mortality? Insights from the latest epidemiological research

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Guest post by Veronika Huber

Climate skeptics sometimes like to claim that although global warming will lead to more deaths from heat, it will overall save lives due to fewer deaths from cold. But is this true? Epidemiological studies suggest the opposite.

Mortality statistics generally show a distinct seasonality. More people die in the colder winter months than in the warmer summer months. In European countries, for example, the difference between the average number of deaths in winter (December – March) and in the remaining months of the year is 10% to 30%. Only a proportion of these winter excess deaths are directly related to low ambient temperatures (rather than other seasonal factors). Yet, it is reasonable to suspect that fewer people will die from cold as winters are getting milder with climate change. On the other hand, excess mortality from heat may also be high, with, for example, up to 70,000 additional deaths attributed to the 2003 summer heat wave in Europe. So, will the expected reduction in cold-related mortality be large enough to compensate for the equally anticipated increase in heat-related mortality under climate change? [Read more…] about Will climate change bring benefits from reduced cold-related mortality? Insights from the latest epidemiological research