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Scientists Predict Looming Climate Shift: Will Ocean Heat Come Back To Haunt Us Once Again?

By Climate Guest Contributor on July 16, 2013 at 11:15 am

"Scientists Predict Looming Climate Shift: Will Ocean Heat Come Back To Haunt Us Once Again?"

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By Rob Painting via Skeptical Science. Reprinted with permission.

Key Points:

  • Despite a large increase in heat being absorbed by the Earth’s climate system (oceans, land & ice), the first decade of the 21st century saw a slowdown in the rate of global surface warming (surface air temperatures).
  • A climate model-based study, Meehl (2011), predicted that this was largely due to anomalous heat removed from the surface ocean and instead transported down into the deep ocean. This anomalous deep ocean warming was later confirmed by observations.
  • This deep ocean warming in the model occurred during negative phases of the Interdecadal Pacific Oscillation (IPO), an index of the mean state of the north and south Pacific Ocean, and was most likely in response to intensification of the wind-driven ocean circulation.
  • Meehl (2013) is an update to their previous work, and the authors show that accelerated warming decades are associated with the positive phase of the IPO. This is a result of a weaker wind-driven ocean circulation, when a large decrease in heat transported to the deep ocean allows the surface ocean to warm quickly, and this in turn raises global surface temperatures.
  • This modelling work, combined with current understanding of the wind-driven ocean circulation, implies that global surface temperaures will rise quickly when the IPO switches from the current negative phase to a positive phase.

Average sea surface temperature trends from the climate model simulations for a) 'hiatus' decades, i.e. decades with no warming of global mean surface temperatures, and b) 'accelerated' decades, i.e. decades with greater-than-average rises in global surface temperatures. The subtropical ocean gyres (green ellipses) are key players in the downward transport of heat. The stippling indicates areas where this trend is statistically significant. From Meehl (2013).

Even with Global Dimming, Still Lots of Warming Down Below

The way that global warming has progressed in the 21st century has probably been a great surprise to many people, no doubt a few climate scientists among them. Despite a strong increase in planet-warming greenhouse gas emissions, warming of global surface temperatures has been rather muted. Surface temperatures have warmed, but at a slower rate than the last two decades of the 20th century.

Of course, the 2000′s doesn’t strictly qualify as a hiatus decade as defined in Meehl (2013) — where hiatus decades are described as negative global surface temperature trends — but it is, nevertheless, a suitable analogue.

Increases in sunlight-blocking sulfate pollution (from industrial activity) during the early part of the 21st century have probably played a role (Hatzianasstissiou [2011]), as have sulfate emissions from tropical volcanoes (Solomon [2011], Vernier [2011]), but the most significant contributor to this muted surface warming trend are natural changes in the way heat is stored in the ocean.When large amounts of heat are efficiently transported to the deep ocean (as in the 2000′s — see Levitus [2012], Nuccitelli [2012] & Balmaseda [2013]), there is less heat available at the ocean surface with which to raise global surface temperature.

This can be better understood by considering the effects of the El Niño-Southern Oscillation (ENSO) on the vertical distribution of heat in the surface layers of the ocean. During El Niño heat builds up in the surface layers where it is able to interact with the atmosphere, and therefore raises global surfaces temperatures. During La Niña much more heat is transported to deeper ocean layers and, with the surface layers cooler-than-normal, global surface temperatures are cooler-than-average. This can be seen in the figure below — using observations from the global system of ARGO floats.

The above is a time-series of globally-averaged (60° South to 60° North) ocean temperature anomaly from the monthly mean, versus depth in metres (pressure in dbar). (B) is a time-series of globally-averaged sea surface temperature anomaly (black, °C), ocean temperature at 160 metre depth (blue), and the Nino3.4 regression estimate for SST (red). From Roemmich & Gilson (2011).

The Interdecadal Pacific Oscillation (Power [1999]) is an index for the mean state of the north and south Pacific Oceans. During the positive phase, El Niño is the dominant global weather pattern, and during the negative phase, La Niña is dominant. During the late 1990′s the positive IPO phase weakened considerably and has been in the negative phase since the year 2000. In other words, La Niña been the dominant pattern of late. It is therefore not surprising that global surface temperatures during the 2000′s have warmed less than previous decades (1977-2000) — when the IPO was in a positive (El Niño-dominant) state.

Some Background Context

Present warming of the global oceans is a result of increased atmospheric concentrations of greenhouse gases warming the thin cool-skin layer at the sea surface. This reduces the typical rate of heat loss from the ocean to the atmosphere above, and therefore causes the oceans to grow warmer over time. So much like greenhouse gases trap heat in the atmosphere, by reducing the rate of heat loss to space, they also reduce the rate of heat loss from the ocean too. While changes in the ocean circulation brought on by warming can, and probably do, contribute to overall uptake of heat by the ocean, this greenhouse gas-induced warming of the surface ocean is the dominant player over the long-term.

Though not strictly true, it’s easier to think of the ocean circulation as two distinct but interconnected processes — the wind-driven ocean circulation which warms and ventilates (mixes air into) the surface-to-deep ocean (down to about 2000 metres), and the thermohaline circulation which warms and ventilates the deep-to-abyssal ocean (2000 mtrs to ocean floor), and which is mainly driven by changes in the bouyancy and density of seawater. The wind-driven ocean circulation can be described as a pool of warm salty surface ocean sitting on top of a cold fresh (less salty) abyssal ocean.

The transport of heat down into the surface to deep ocean occurs via the subtropical ocean gyres (green ellipses in Figure 1). These are large rotating masses of water, in each ocean basin, where ocean currents converge at their centre and are forced downwards, taking warm surface water with them. Future SkS posts will deal with the ocean gyres and how they operate, but note that changes in the winds which power the ocean gyres affect the transport of heat down into the ocean, and also the transport of heat from the tropics to the polar regions. Intensified tropical easterly trades winds, for instance, spin up the ocean gyres leading to greater downward, and poleward, heat transport (see Roemmich [2007]).

Accelerated Warming and Hiatus Decades

Meehl (2013) once again uses the NCAR CCSM4 climate model to evaluate decadal periods of either accelerated warming, or slight cooling (hiatus decades). To accomplish this the authors use a moderate future (fossil fuel) emissions scenario for the 21st century combined with historical simulations of the 20th century. This future emissions scenario is chosen because the climate forcing is not so strong that hiatus decades disappear entirely in the latter part of the 21st century simulations.

From five climate model runs of the 21st century the authors derive 500 years worth of simulations. The hiatus decades were chosen based on a slight cooling trend in global surface temperatures of less than -0.08°C per decade. Eight decades match this criteria and the composite of their surface temperature trends are shown in Figure 1(a). If a zero decade-long temperature trend were chosen as well, then there are many more decades that fall under that criteria, including three instances where no warming lasts for 14 years, and one where this persists for 15 years.

For the accelerated warming decades the authors choose decades where the global surface warming is at least 0.41°C per decade (around twice the observed warming over the last few decades based on GISTEMP). Like the hiatus decades, these large values for the accelerated warming decades were chosen so that the trends were obvious. The composite of the trends for these accelerated decades are shown in Figure 1(b).

It is apparent in Figure 1, that the hiatus and accelerated warming decades are virtually the mirror image of each other. The warm sea surface temperatures in the gyres, during hiatus decades, indicate convergence of near-surface currents and strong downwelling of heat. With accelerated decades the vertical, and poleward, transport of heat by the gyres seems to shutdown, enabling strong sea surface warming in the tropics – where most solar radiation enters the ocean. The strong warming in the polar regions is related to changes in the thermohaline circulation.

A comparison of the accelerated warming and hiatus with regard to all the other decades in the simulations is shown below. Obvious is the tendency for warming of the deeper ocean layers during hiatus decades, and warming of the surface ocean layer (0-300m) during accelerated warming decades.

(A) Average linear trends for hiatus decades versus trends for all other decades, broken down into 3 ocean layers. The top-of-the-atmosphere radiation balance (left-hand columns) is strongly positive, indicating a strong global warming trend. The error bars denote 5 percent statistical significance. (B) Same as above except for accelerated warming decades. From Meehl (2013).

Although I have not delved into it here (to keep things less complicated), changes to the thermohaline circulation, specifically the Atlantic Meridional Overturning Circulation (AMOC) and Antarctic Bottom Water (AABW) formation, also play a part in the hiatus and accelerated warming decades in the climate model. The thermohaline changes associated with the IPO suggest a common mechanism at work — most likely the changes in wind forcing. All three processes (IPO, AMOC & AABW) are contributors in each hiatus and accelerated decades, but the authors analysis reveals the IPO to play the dominant role.

A Looming Climate Shift

During the late 1970′s global surface temperatures changed so quickly (Graham [1995]) that some researchers labelled this a ‘climate shift’ (Miller [1994], Wainwright [2008]). It was later shown that this climate shift was simply the human-caused global warming trend combining with the positive (warm surface ocean) phase of the Interdecadal Pacific Oscillation (Meehl [2009]). With both driving forces moving in same (warming) direction, they combined to nudge global surface temperatures sharply upwards.

The late 1970′s climate shift is an appropriate analogue for the near-future because, based on the modeling in Meehl (2013) and other peer-reviewed scientific papers not discussed here, the current negative IPO (cool surface ocean-warm deep ocean) trend is likely to come to an end sometime soon. The NCAR climate model suggests that these phases can last up to 15 years, but are generally around a decade in length. This climate model-based IPO cycle length is shorter than the IPO cycle length observed during the 20th century, however the reasons for this disparity are not yet clear.

Whenever this phase reversal does kick in there are likely to be significant changes in global rainfall patterns (Dai [2012]), drought, mass coral bleaching and fish catches (caused by change in the wind-driven upwelling of nutrients in key areas of the ocean). Just how significant this is will depend on how much heat remains in the surface ocean during the next positive phase of the IPO.

So to answer the question posed in the title – will ocean heat come back to come to haunt us? Yes, but perhaps not in the way some might think. Heat buried in the deep layers of the ocean will not re-surface any time soon. Instead, when the subtropical ocean gyres spin down, they will no longer be efficiently removing heat from the tropical surface ocean. The transport of ocean heat to depths, and to the poles, will drastically slow down, and this will allow the surface of the tropical oceans to warm rapidly. That heat is very likely to haunt us.

Rob Painting via Skeptical Science. Reprinted with permission.

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24 Responses to Scientists Predict Looming Climate Shift: Will Ocean Heat Come Back To Haunt Us Once Again?

  1. Paul Klinkman says:

    I’ve taken my share of (accurate, it turns out) potshots at the overly-conservative climate change forecasting community over the years. I need to acknowledge that they seem to be straightening out and flying right these days, or at least this article looks good. Congrats!

  2. Superman1 says:

    Putting air temperature effects aside for a moment, what are the adverse effects of deep ocean heating alone?

    • prokaryotes says:

      … variations in seafloor depth and heat flow with age that provide the primary constraints on the thermal structure and evolution of the oceanic lithosphere http://www.earth.northwestern.edu/people/seth/research/thermal.html

      It probably means: more heat in the deep ocean = more seismic activity. The added energy adds to seismic uptake from crust rebound from ice sheet thaw, seabed slides (i.e. methane destabilization) and added stress from overall thermal expansion stress.

      • prokaryotes says:

        Amongst other advantages, this allows a more faithful determination of the so-called dynamic ocean topography, i.e. the deviation of the ocean surface from the equilibrium with the force of gravity. This ocean topography is essentially determined by ocean currents. Therefore, the gravity field models calculated with GOCE measurements are of great interest for oceanography and climate research.

        Besides GOCE, long-term measurement data from the twin-satellite mission GRACE (Gravity Recovery and Climate Experiment) of the GFZ were included in the new EIGEN-6C. GRACE allows the determination of large-scale temporal changes in the gravitational field caused for example by climate-induced mass displacements on the Earth’s surface. These include the melting of large glaciers in the Polar Regions and the seasonal variation of water stored in large river systems. Temporal gravity changes determined with GRACE are included in the EIGEN-6C model. Therefore, the new Potsdamer potato is for the first time no longer a solid body, but a surface that varies over time. Particularly in order to record these climate-related processes for the long term, a follow-on mission for the GRACE mission that ends in 2015 is urgently needed. http://tinyurl.com/q6t96dn

    • Paul Klinkman says:

      Most of the deep ocean species will be extinct before humanity has even seen them for the first time. Perhaps we’ll see fossils of them someday.

      • prokaryotes says:

        Deoxygenation in warming oceans—Looking back to the future

        Earth’s climate has varied dramatically over its long history, fromsnowball glaciations to greenhouse extremes. Ancient warming is one rea-son we study the past—to get the fullest possible picture of what mightlie ahead. No episode of past warming was more dramatic than the Paleo-cene-Eocene Thermal Maximum (PETM) ~55 m.y. ago: a rapid temperature increase of ~5–8 °C unfolded over a mere tens of thousands of years,driving severe perturbations to the marine and terrestrial carbon cycles and concomitant impacts on ecologies in both settings. As we anticipate the impacts possible via modern climate trends, with similar levels of projected carbon release and temperature rise but over only centuries, thePETM has become a conservative poster child for why we look back in time to inform our understanding of the future.Among the growing list of concerns in the face of the current warm-ing trend are widespread decreases in the dissolved oxygen content of seawater, or ‘ocean deoxygenation’ (Keeling et al., 2010), in part becauseO2is less soluble in warmer water. In this regard, however, the PETM’s potential as an ancient window to view modern climate has been only partly realized. Oxygen loss has been implicated in perturbations to organ-isms living on the deep sea floor, but little is known about the intensityand global extent of those O2-challenged conditions. Dickson et al. (2012,p. 639 in this issue of Geology) set their sights on filling this gap through the use of molybdenum isotope measurements applied to an exceptional set of drill-core samples collected in the Arctic Ocean. Their data point convincingly to low-O2conditions during the PETM that were more expansive than today’s. http://geology.gsapubs.org/content/40/7/671.full.pdf+html

      • Mulga Mumblebrain says:

        More likely is their seeing fossils of us.

    • Joan Savage says:

      What do we mean by deep?

      There’s a stack of ocean currents one atop another. The report only tackles the top 2000 meters of the water column monitored by the ARGO system, and yet there’s water deeper than that, such as the Antarctic bottom water, which could be doing something significant, but how to find out.

    • Mulga Mumblebrain says:

      It will wake the Kraken.

  3. Superman1 says:

    The excess energy trapped by greenhouse gases is allocated among atmosphere, ocean, land, and myriad endothermic processes, like melting ice. They are all inter-connected in this one biosphere, and will all come back to haunt us. There may be focus on atmospheric effects for reasons beyond science, but impacts in all the above areas should be serious.

  4. DRT says:

    So, to point out the obvious, the spate of droughts, heat waves, fires, floods, etc. we have seen so far are just the warm up act. Stick around for the main act and things will start to get interesting.

    • colinc says:

      Yes, and moments after the “main act” begins a couple billion will leave the theater in a “hell-a” hurry and another couple billion will be stacked up against the exits then depart nearly as quick as the first contingent. Most of the rest will probably be wanting a refund, which is when the show will become spectacular.

  5. Jeff Huggins says:

    Follow The Logic

    The facts that the dynamics of temperature increase and weather instability are complex and non-linear, and that short-term fluctuations tend to be more immediately apparent than long-term trends to most people, mean that it will be darn hard, and perhaps impossible (in practical terms, given the way our politics and media work), to convince a clear majority of the people to make addressing climate change a *top* priority for them, short of waiting for huge catastrophes that force the issue.

    In turn, this suggests that we will have to find more intelligent and direct ways to pick leaders who will actually lead the way (not merely follow the public’s appetite) to address climate change with the forthrightness and verve and speed that the task requires.

    In turn, this means that it will be crucial for us to pick a nominee (for president) who already “gets it” and has the backbone, temperament, wherewithal, will, and genuine leadership ability to get the job done. And, someone who will place (as a priority) tackling the climate challenge above his or her own political ambitions.

    In turn, this means that leading (and self-proclaimed) progressive and Democratic organizations who care about climate change should be leading the way to facilitate this — that is, the nomination and subsequent election of a person who has this understanding, this commitment, this skill set, and this will to lead the country to face and responsibly address climate change.

    In turn, this means that we (these organizations leading the way, and ourselves) should be vetting the would-be candidates carefully, looking to understand their specific positions, ideas, and commitments regarding climate change, in concrete and specific terms.

    Can we follow this logic? It’s not all that hard.

    I ask, and have been asking, CP and CAP to lead such a charge. Along with 350.org, the Sierra Club, and all other organizations concerned about climate change. We should all realize how crucial it is that our next president should be someone who will lead, vigorously and effectively. We should all realize how crucial it is that we understand a person clearly and confidently (his or her positions, concrete ideas, commitments, willingness to lead, willingness to place climate change as a priority above his/her own political ambitions, clarity and forthrightness, and so forth) BEFORE we choose to nominate and elect that person, or someone else, as president.

    Cheers and Be Well,

    Jeff

    • Mulga Mumblebrain says:

      Will El Nino and La Ninas become things of the past, phenomena of the relative climate stability of the last few millennia?

  6. Joan Savage says:

    I keep coming back to this article, and going off looking for more references and graphs to help me connect the dots better. Both the article and the comment/questions have been very stimulating. Thanks.