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New Study Finds Clouds are Amplifying Human Warming

The mysterious clouds.

For decades, science has been unable to nail down how clouds might change with human warming of the climate. Sure, we knew that added water vapor through a heating-increased amplification of the rate of evaporation and precipitation would likely impact cloud formation. But how would those physical alterations impact climate? Would an added darkening of the Earth through increased cloud cover provide a cooling effect and slow down the rate of human-caused warming (also called a negative feedback)? Or would the added water vapor aloft, itself a powerful greenhouse gas, provide an extra boost to the human heating engine (also called an amplifying feedback)?

The mainstream climate models thus assumed a zero to slightly positive heat feedback from clouds and relied on decadal verification runs to help test for accuracy. A kind of backwards checking that excluded values from clouds due to a lack of needed information.

Pyrocumulonimbus

(From the global climate change perspective, some clouds are worse than others. The above image shows a thunderstorm set off by massive wildfires blazing through the permafrost zone near Great Slave Lake on August 5 of 2014. A pyrocumulonimbus cloud or, colloquially, a fire thunderstorm. Image source: NASA.)

Confusionists Take Advantage of Cloud Uncertainty

It was an uncertainty hanging in the very air above us. An uncertainty many climate confusionists used to sow doubt over a broad range of issues involving how sensitive the Earth is to the human heat forcing. They often argued, through this scientific dim spot, that climate sensitivity was, indeed, quite low and that we had very little to be concerned about regarding an immense dumping of heat trapping gasses into the atmosphere that is now at least 6 times faster than at any time in the deep history of life on Earth.

The shady clouds, in other words, would save us from ourselves.

Not so fast, said Dr. Andrew Dressler who in this paper and this paper recently defended consensus climate science from the cloudy claims of confusionists. Dressler, like mainstream climate science, assumed at least a small degree of positive feedback from changes to clouds and atmospheric water vapor loading. And his observational findings were consistent with an equilibrium climate sensitivity (ECS), or a one century rate of warming, in the range of 2.0 to 4.5 degrees Celsius for each doubling of CO2 (consistent with a multi-century warming [ESS] in the range of 4 to 9 C for each doubling of CO2 — or about a 6 C average).

New Study Finds Changes to Clouds are an Amplifying Feedback

But now, a new study has found that the picture is not quite so rosy as some claimed. The study, led by Dr. Kevin Trenberth, found that net changes to clouds and related additions of water vapor to the upper atmosphere is a positive or amplifying feedback to human caused warming. In other words, the way human heat alters clouds and the related hydrological cycle results in yet more heat being trapped by the Earth System.

This confirms Dr. Dressler’s work and raises a rather unpleasant question — if we have an added heat feedback from clouds under a regime of Earth Systems warming, then how strong is it?

Trenberth notes in an interview published today in The Guardian:

What we do find is that if one looks at tropospheric average temperature rather than surface temperature, then there is a much stronger relationship with energy flow at the top of the Earth’s atmosphere. We are able to find a water vapor signal that is clearly a positive feedback.

Climate Sensitivity Needle May Tilt Toward Upper Range of Estimates

This is somewhat unhappy news.

What it means is that the Earth System is at least as sensitive as climate models suggest. But, even worse, there is a chance that the Earth System may be closer to the upper range of climate sensitivity estimates. It means that accumulation of heat in the atmosphere, in glaciers and in the ocean may happen somewhat faster than consensus models predict and that geophysical changes may, consequently, be greater and more catastrophic.

Whether model simulation of climate sensitivity will need to be altered has not, as yet, been determined. The study is now very new and it will take some time for the more recent data to wash out in the model projections.

But what can be plainly stated is that fossil fuel industry funded voices of false comfort have again been proven dreadfully wrong and that there is some risk that the situation may be even more dangerous than current science anticipates. As such, there is absolutely no reason for further delays in policy action and a very rapid draw-down to zero human carbon emissions.

Links:

Climate Variability and Relationships Between Top of Atmosphere Radiation and Temperatures on Earth

Changes in Water Vapor and Clouds are Amplifying Global Warming

A Determination of the Cloud Feedback from Climate Variations Over the Past Decade

Clouds and The Earth’s Energy Budget

NASA

Hat Tip to TodaysGuestIs

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Nature: Human-Destabilized Antarctica Capable of Glacial Outbursts Contributing to Sea Level Rise of 14+ Feet Per Century

“Our new results suggest that the Antarctic Ice Sheet is more unstable than previously considered…” — Peter Clark, Paleoclimatologist at Oregon State University

*    *    *    *    *

Massive glacial destabilization and irreversible collapse caused by human warming. That’s what a flurry of recent studies issued by NASA’s Jet Propulsion Laboratory found is now happening to vast sections of Antarctica’s towering glaciers.

The NASA climate scientists found that rapid human warming of the deep ocean was resulting in hotter than usual water upwelling beneath the ocean-fronting glaciers of Antarctica. The warmer waters ate away at the great glaciers from underneath, making them less stable and propelling them with ever-higher velocity toward the world’s oceans. The studies, led by Eric Rignot, concluded that large sections of Antarctica were now destabilized and that six key glaciers representing what could well be termed the entire flank of West Antarctica were now locked in an unstoppable plunge into the ocean.

These glaciers in irreversible collapse combined with a growing number of destabilized and destabilizing glaciers all around Antarctica and northward to Greenland to represent an extreme risk for a much more rapid than expected sea level rise this century.

Now a new study published this week in the prestigious science journal Nature adds to what has become an extraordinary torrent of evidence for heightening risks to speeding human-spurred sea level rise.

Antarctica glacial velocity map

(Antarctica glacial velocity map composed by Eric Rignot in 2011. Blue = fast. Brown = slow. Note the numerous high-speed glacial flows plunging deep into Antarctica. By the 2010s, Antarctica was losing between 1,300 and 2,000 gigatons of ice each year. Image credit: Antarcticaglaciers.org.)

A Glimpse into Earth’s Past Provides Stark Evidence For a Human-Warmed Future

The study, entitled Millennial-Scale Variability in Antarctic Ice Sheet Discharge During the Last Deglaciation, and published in Nature, took a closer look at Antarctic ice sheet instability and its contribution to sea level rise during the end of the last ice age by taking sediment cores from iceberg rafted debris (IBRD) fields around the frozen continent. These fields contained minerals left by melting glaciers discharged into the Southern Ocean during the major glacial melt events at the end of the last ice age and provide a good proxy for the rate of glacial discharge from Antarctica.

Analysis of these sediment cores resulted in the finding that Antarctica experienced 8 separate large glacial outburst events during the end of the last ice age. These events began about 20,000 years ago and ended 9,000 years ago. This is directly counter to conventional thinking that had assumed Antarctic melt started late and ended early during the last glacial melt period. It also hints that the Antarctic ice sheet is far less stable that previously assumed, making it much more vulnerable to current, human-caused heat forcings.

Each large outburst event contributed significantly to global sea level rise. The largest and most violent event was found to have occurred around 14,900 years ago. Lasting 350 years, this single episode, known as meltwater pulse 1A, resulted in a 50 foot sea level rise, pushing global oceans higher by 14.2 feet per century. It was previously unknown that Antarctica contributed in any way to this rapid sea rise event. But large deposits of iceberg sediment during this time of surging oceans provides strong evidence that Antarctica was a major contributor.

Post-Glacial_Sea_Level

(Pace of post glacial sea level rise since the end of the last ice age. Note the steep rise in sea levels occurring in conjunction with meltwater pulse 1A, a pulse that scientists now know included a major contribution from Antarctic melt. Image source: Commons.)

Professor Clark of Oregon State’s College of Earth, Ocean and Atmospheric Sciences who contributed to the paper noted:

“During that time, the sea level on a global basis rose about 50 feet in just 350 years – or about 20 times faster than sea level rise over the last century. We don’t yet know what triggered these eight episodes or pulses, but it appears that once the melting of the ice sheet began it was amplified by physical processes.”

Implications for Current-Day Human Warming

At first blush, the findings of this study may seem innocuous. But upon reflection, one quickly comes to the conclusion that they are rather stark.

In total, about 200 feet of potential sea level rise is currently locked in all of Antarctica’s ice. This compares to Greenland which, if it completely melted, would contribute about 24 feet to sea level rise. Until recently, it was assumed that this large store of ice in Antarctica was relatively stable and responded only slowly to climate perturbations.

The first challenge to the notion of Antarctic ice sheet stability came in 1968 when glaciologists began issuing warnings that the West Antarctic Ice Sheet was unstable. But, until recent years, this instability lacked an observed physical mechanism to explain what appeared to be an ongoing and increasingly rapid glacial rush toward the sea. By the mid 2000s, scientific reports began to emerge showing that warm water upwelling along the coasts of Antarctica was eating away larger and larger chunks of the great glaciers’ bases and speeding their flow to the sea. This year, a flurry of reports sounded a death knell for entire sections of West Antarctic ice, locking in many feet of sea level rise with more likely to come.

Temperature increase over last 22,000 years

(Temperature and CO2 increase from 22,000 YBP through 6,000 YBP. Note that major glacial outburst events in Antarctica began after only .2 C of atmospheric warming and 10 ppm of CO2 increase around 20,000 years ago. Such a response shows a very high degree of glacial and Earth Systems climate sensitivity. Also note that total warming over a 12,000 year timeframe was 3.7 C. Warming of 4, 6 and even 9 C is possible by the end of this Century under BAU human greenhouse gas forcing. Image source: Nature via Skeptical Science.)

But until this recent Nature paper, such a rapid destabilization of Antarctica’s massive glaciers was without a paleoclimate context. For few studies had challenged an assumed but sparsely supported perception of past Antarctic ice sheet stability. To the contrary, the paper found an Antarctic ice sheet that was very sensitive to warming. The ice sheet issued its first major glacial outburst 20,000 years ago as the world showed its first hint of thaw from a great ice age. The giant glaciers continued their lashing out in 8 major events until warming stopped with the advent of the Holocene around 9,000 years ago. At that point, Earth’s climate had begun to settle into a new equilibrium and Antarctica quieted its grumbling.

Now that humans are warming the atmosphere and oceans at a pace at least 30 times that of the last ice age, we are discovering that Antarctica in the deep past showed a major destabilization response to even the slightest hint of warming.

A Warming Ocean is Likely Antarctica’s Soft Underbelly

Rapidly accumulating evidence that ocean warming is providing a powerful blow to the world’s glaciers combines with the recent study to provide a stark implication — any addition to atmospheric heat is rapidly transferred to the world’s oceans which, in turn, goes to work melting ocean-contacting glaciers. Furthermore, glacial destabilization is likely to remain in play so long as the global energy imbalance toward continued warming remains in force and destabilized glaciers haven’t yet hit the ocean. As such, we should consider that start time for glacial destabilization and increasing rates of sea level rise to be now and not in some distant, far off, future.

Glacial systems may well present considerable atmospheric inertia. But when presented with warming waters, the glaciers must yield. This makes sense as water’s heat capacity is four times greater than that of air. So the melting force of water just one degree C above freezing is about four times that of the same volume of air at the same temperature. Glaciers in constant contact with the potent heat capacity of warming oceans essentially don’t stand a chance. And with many of Antarctica’s glaciers directly in the firing line of waters warmed by human greenhouse gas forcing rising from the deep ocean, it would appear, at this time, that the great frozen continent may well have a very soft underbelly.

Antarctic Waterfall

(Waterfall spilling from the heart of a melting Antarctic glacier and into the Southern Ocean. Image source: Antarctica.gov.au)

Inertia Not So Strong as the Forcing; Stark Implications For Sea Level Rise

Since so much has happened over the past year, it is useful to put the current state of science into an overall context. In doing so we should consider these scientific findings:

  1. Large portions of Antarctica are already undergoing destabilization or irreversible collapse.
  2. In the deep past, Antarctica responded very rapidly to even a small global temperature change and remained unstable so long as warming continued.
  3. This rapid glacial response and destabilization was likely due to a broad basal exposure to warming oceans.
  4. The current pace of human warming is now 30 times faster than at the end of the last ice age.
  5. The deep ocean is accumulating heat faster than any other Earth System.
  6. At peak, the most violent glacial outburst flood events included glacial discharges from Antarctica and pushed sea levels higher by 14.2 feet each century during the end of the last ice age.
  7. Current human CO2 heat forcing at 400 ppm + is enough to raise sea levels by 75 feet according to paleoclimate estimates.
  8. Current human CO2e heat forcing at 481 ppm + is enough to raise sea levels by 120 feet according to paleoclimate estimates.

Considered together, these points would seem to indicate that glacial inertia to heat forcing is not so great as previously hoped. More likely, the energy balance of the Earth System more rapidly responds to total heat content, energy imbalance, and pace of heat accumulation than previous sensitivity estimates assumed. If this meta-analysis is true, and it seems to be based on a growing pile of evidence, sea level rise for the current century is likely to be far greater than previously anticipated by scientific assessments. The top range of a 3 foot sea level rise for this century under IPCC modeling is likely, given current realities, to instead be a low estimate. A more realistic range, given a greatly reduced true glacial inertia, is probably 3-9 feet through 2100 with higher outside potentials during large glacial outburst flood events.

Given changing ocean and atmospheric conditions together with the rising potential of large rainfalls over certain glacial zones during summer as the 21rst century progresses, climate analysts should consider such large glacial outburst floods to be a potential high risk event under current extreme human warming. It is also worth noting that these glacial systems have probably never experienced a set of forces so powerful or rapid as they are likely to face as the 21rst century progresses. Recent scientific assessments are essentially playing catch-up to these new and emerging realities.

Links:

Grim News From NASA: West Antarctic’s Entire Flank is Collapsing into the Southern Ocean

Millennial-Scale Variability in Antarctic Ice Sheet Discharge During the Last Deglaciation

Commons

Nature

Skeptical Science

Antarctica.gov.au

Antarctic Ice Sheet Unstable at End of Last Ice Age

Antarcticaglaciers.org

A Faustian Bargain on the Short Road to Hell: Living in a World at 480 CO2e

On the highway to a smokestack hell, Faust met a devil who said to him:

“Give me all your tomorrows, all your children and all your children’s children, and I will make this one day, for you, a paradise.”

*    *    *    *    *

Understanding how much warming may be in store from all the CO2, methane, N2O and other greenhouse gasses humans have pumped into the atmosphere can be a bit problematic. First, definitions have tended to be confused due to the fact that equilibrium climate sensitivity measures (Charney) used to project warming for this century by the IPCC only take into account about half of long-term (slow feedback) warming should CO2 and other greenhouse gas levels remain high.

For example, equilibrium climate sensitivity measures show an effective rate of warming by about 3 degrees Celsius (C) for every doubling of CO2 from 1880 onward. By this measure, we get about 3 C worth of warming over this century once we hit 550 ppm CO2 and about 6 C worth of warming at levels around 1100 ppm. It is important to stress that these short term warming projections do not take into account long-term ‘slow’ feedbacks to a given rise of CO2 that are strong enough to double the ultimate temperature increase. This larger Earth Systems Sensitivity (ESS) measure is both observable in paleoclimate and in the various model runs that project a given level of atmospheric CO2 out through the centuries.

Fast Feedback vs Slow Feedback Climate Sensitivity

(Fast feedback equilibrium climate sensitivity over one century vs long term sensitivity over multiple centuries to a given greenhouse gas forcing. Note that approximately double the amount of warming occurs after ‘slow feedbacks’ like ice sheet response and environmental ghg emissions are taken into account. Image source: Leeds.)

So both paleoclimate and most model runs end up with a long term warming of about 6 C at 550 ppm CO2 and of about 12 C at 1100 ppm CO2.

It is here that we run into an additional difficulty. We don’t ultimately know how long, long-term will really be. We hope, and our climate models seem to support this hope, that such ‘long term’ warming from the so-called slow feedbacks like ice sheet albedo response and natural carbon emissions won’t appear in force this century. But given the stunning pace of human greenhouse gas build-up combined with a number of observed ‘slow feedback’ responses going on now, we don’t really know for certain. And there is some reason to believe that the ‘slow feedbacks’ might not be so slow after all.

In this context, the current level of CO2, at around 400 ppm, results in a warming this century of around 1 to 1.5 degrees Celsius (if the slow feedbacks are as slow as expected) and a long-term warming of about 2-3 degrees Celsius. And it is at this point where an already complex dynamic begins to break down, taking on a number of, yet more complex, factors.

A Host of Extra Gasses No-One Really Talks About

At issue is the fact that humans have emitted a massive volume of additional greenhouse gas into the atmosphere. These gasses have grown in proportion and heating effect alongside the, admittedly larger and more significant, CO2 emission. And each has made their own additional contribution to human warming.

Some of these gasses, like methane, have been a typical part of natural atmospheres for millions of years. At times, methane concentrations are observed to have spiked to levels even higher than those seen today. But the periods during which such levels were apparent were also times of global crisis — the hothouse mass extinction events.

Methane Since 1984 MLO

(Atmospheric methane concentrations since 1984 as observed at the Mauna Loa Observatory. Image source: NOAA ESRL.)

But the other gasses: nitrous oxide, CFCs, HFCs, nitrogen triflouride, and a host of nearly 50 other industrial chemicals that contribute to warming were either never in the atmosphere before or were present at much lower levels than what is seen today. The result of this added pollution is yet more potential warming, in addition to a number of other difficult to deal with impacts. A pollution impact that is outside the context of past global crises and that puts current day greenhouse gas forcing at a critical and unstable level.

Methane levels alone have more than doubled since the start of the Industrial Revolution, rising from about 750 parts per billion to about 1835 parts per billion today. This value, depending on how it’s calculated over time, is equivalent to an additional CO2 forcing of between 22 and 110 parts per million. And though methane is the strongest non-CO2 warming agent, adding them all together can result in a value that is quite a bit higher than the base CO2 level would indicate.

Nitrous Oxide MLO

(Atmospheric nitrous oxide levels since 1997 as observed at the Mauna Loa Observatory. Image source: NOAA ESRL.)

In addition, on the negative side of the ledger, human fossil fuel burning (primarily coal) burning emits sulfur dioxide, other sulfates and various aerosols which, overall, create strong negative feedbacks in the climate system by reflecting incoming sunlight. The net result is a temporary suppression of a portion of human-caused warming. The reason this suppression is temporary is due to the fact that the sulfur dioxide and related sulfates rapidly wash out of the atmosphere. So if coal burning ceases, the reflective particles rapidly fall away and we readily come to witness the full strength of the human greenhouse gas emission.

Which brings us to the question — what is the full strength of the current human emission and how long will it last? There’s a term for this number: CO2e. In other words — the equivalent CO2 forcing of all greenhouse gasses added together.

Fortunately for our exploration, there’s been a bit of work done on just this subject. Last year, MIT’s Advanced Global Atmospheric Gasses Experiment issued a report describing model data that determined the current CO2 equivalent forcing from all of the more than 50 greenhouse contributing trace gasses in the atmosphere. And the results were somewhat disconcerting. As of June of 2013, that amount was equal to 478 parts per million CO2. Or a CO2e of 478 parts per million when all the other greenhouse gasses were added to the already high and rapidly rising levels of CO2. Adding in the current rate of CO2 rise, we end up with about 480 parts per million of CO2e from all greenhouse gasses by this year. So if we’re talking about the total burden of all greenhouse gasses and the one that will be with us through the long term, 480 is, unfortunately, the number we should be dealing with and not 400.

Aerosols and the Faustian Bargain

Unfortunately, to determine the current forcing one has to also take into account those pesky aerosols we mentioned above. And, luckily, we also have a reliable measure that provides the negative forcing or relative cooling effect of sulfur dioxide in the current atmosphere. As of 2013, the IPCC had found that sulfates and other effects due to aerosols provided a net negative forcing of about .8 Watts per meter squared or about 1/2 the positive forcing of CO2 which was, then, at around 390 ppmv (2011), about 1.68 Watts per meter squared. This approximate 1/2 value, when divided by the then observed rise in CO2 since 1880 gives us a rough equivalent negative forcing value of minus 55 parts per million CO2e.

ipcc_rad_forc_ar5

(IPCC AR5 Radiative Forcing Assessment. Image source: IPCC)

So subtracting out the net effect of sulfates and other aerosols brings us to a total net forcing from all factors related to human changes to the atmosphere of about 425 ppm CO2e. A rather disturbing final number both due to its departure over the current 400 ppm CO2 value and due to the fact that though most greenhouse gasses have atmospheric residence times of decades to centuries, the cooling sulfates would likely last for 1-2 years before falling out entirely. This means that once fossil emissions stop, we may as well just add +55 ppm CO2e to the current total.

This warmth masking factor of human coal emissions was described by James Hansen as a kind of Faustian bargain in which current burning of the dirty fuel provides temporary respite to warming at the cost of even more rapid future temperature increases. And it is just this devil’s deal in which we are now entangled.

425 CO2e: A Dangerous Interim

So it is likely that current atmospheric forcing, including all greenhouse gasses and all human sulfates, is probably at around 425 ppm CO2e. And since the residence times of these gasses are decades to millennium, while Earth Systems feedbacks appear to be enough to maintain high methane levels indefinitely, we should probably view this as an interim figure when considering how much short and long-term warming is likely locked in.

In the short term, using equilibrium climate sensitivity measures, we are likely to end up with between 1.2 and 1.8 C warming over the course of this century even if all greenhouse gas levels, along with sulfate levels, were to remain stable and if the slow feedbacks move along at the expected pace. Meanwhile, long-term warming of between 2.4 and 3.6 C would be expected if all atmospheric gas levels were to stabilize.

But unless an ongoing regime of sulfate aerosol spraying of the stratosphere were put into place, the sulfates would, predictably, fall out once human emissions stopped. And that rapidly brings us back to the 480 ppm CO2e number.

480 CO2e: What is Probably Locked in Long-Term

Looking at the more permanent 480 CO2e value, the fact begins to sink in that we are already well on the way to extreme climate difficulties. For 480 CO2e, without the reflective aerosols, means that the world probably ends up warming by between 1.8 and 2.3 C before the slow feedbacks kick in and between 3.5 and 4.5 C long-term. At these levels, major ice sheet destabilization and melt is eventually likely to result in between 50 and 140 feet of sea level rise with the only remaining glaciers in the end confined to central and eastern Antarctica.

The only saving grace to a cold turkey cessation of emissions now is that most of the worst amplifying feedbacks are likely to be kept in check and thus prevent rapidly accelerated warming and climate destabilization. The extra 1.7 to 2.2 C worth of long-term warming likely comes from a combination of albedo loss, permafrost thaw and related ghg release keeping currently high levels high long-term, and, perhaps, a methane belch in the 1-50 gigaton range that spikes atmospheric levels.

I say likely to be kept in check… but we have to also consider that there is a low, but not out of the question, risk of setting off a kind of mini-runaway that generates warming far beyond the expected range and pushes climates to a hothouse state not seen since the PETM or Permian extinction events. There is little evidence for such an event in response to current climate forcings in the models at this time, but we have a number of scientists, including Peter Wadhams, Natalia Shakhova, and Igor Simeletov, who have raised the possibility, based on their observations of Arctic sea ice and carbon stores, that just such an event could be in the offing. Unfortunately, without more in-depth research into the potential pace of release of current carbon stores (permafrost, forest, clathrate, ocean) we don’t have a scientific oracle that provides a comfortable certainty on this key issue.

It’s worth noting that this best possible future, where the risk of a mini-runaway in warming to PETM or Permian levels remains low, probably won’t happen as business as usual fossil fuel emissions continue unabated with no sign of being rationally held in check. Under the current regime, a CO2e of about 550 ppm, enough to warm the Earth between 5-6 C long term, is locked in within 25-30 years. A climate state that pushes the risk of a mini-runaway to moderate. Meanwhile, levels that would almost certainly set off a Permian or PETM type, anoxic ocean, extinction event, at around 800 ppm CO2e, become possible under BAU by 2060-2080.

The situation is, therefore, once again worse than expected…

Links:

400 PPM CO2? Add in Other Gasses and It’s 478 CO2e

Earth Systems Sensitivity

Leeds Climate Sensitivity

Jules Charney (bio)

NOAA ESRL

Radiative Forcing Links:

Real Climate: Radiative Forcing

The Advanced Global Atmospheric Gasses Experiment

NOAA: Radiative Forcing of Non-Greenhouse Gasses

IPCC: Initial Radiative Forcing Assessment

Non-CO2 Greenhouse Gasses: Scientific Understanding, Control and Implementation

CDIAC: Recent Greenhouse Gas Concentrations and Analysis

IPCC AR4 Appendix/Glossary

Nitrous Oxide and Climate Change

“Slow Feedbacks,” Paleoclimate Data Show Equilibrium Climate Sensitivity Misses Half of Future Warming

Over the past month or so, there’s been quite a bit of controversy over a scientific measurement called equilibrium climate sensitivity (ECS). Among the media, confusion abounds. In a recent instance, The Economist, taking a number of scientific studies out of context, made the dubious claim that a slower pace of temperature increase during the first decade of the 20th century indicated a lower level of climate sensitivity. Other news outlets continue to remark on new climate sensitivity studies without appearing to understand what equilibrium climate sensitivity really means or, more importantly, understanding the inherent limitations of model-based ECS estimates.

Because there’s been such a high level of interest in and confusion over ECS recently, it’s worthwhile taking a closer look at this measurement’s scope and limitations. In an effort to clear up some of the confusion surrounding ECS, this article will attempt to answer these questions:

  • How is ECS defined?
  • How accurate is ECS?
  • And, lastly, does a slower pace of warming over the first decade of the 21rst century mean climate sensitivity is less than previously expected?

Definition of Equilibrium Climate Sensitivity

In its broadest sense, ECS defines the long-term increase in surface air temperatures that results from a doubling of atmospheric carbon dioxide. The measure is important because it gives a broad indication of how much climate change and related harm to humans and environments results from a given amount of carbon dioxide being pumped into Earth’s atmosphere.

Under these basic principles, ECS provides a good guideline. However, ECS operates under a major handicap. The measurement does not include the effects of slow climate feedbacks like loss of ice sheets or albedo change to Earth’s surface.

How Accurate is Equilibrium Climate Sensitivity?

Because ECS leaves out slow feedbacks, it isn’t a very accurate measure of potential long-term warming. That said, in the early days of global warming modeling, ECS was seen as the most useful measure because models had difficulty handling complex physical forces that resulted from slow feedbacks. The result was that climate science came to rely on a less accurate measure because it was more expedient to use in modeling.

For these reasons, ECS was developed as a simpler way to model atmospheric temperature increase caused by carbon emissions over the long term. And because it was difficult to model slow feedbacks, they were not included in the measurement. So, though ECS is a useful measurement for the purposes of more easily modeling atmospheric temperature increases, ECS dramatically undershoots long-term global temperature increases.

The reason for this is that slow feedbacks such as albedo change and ice sheet melt have powerful impacts. We know this because ECS models tend to present about half the total sensitivity observed in the paleoclimate data for a doubling of CO2. This second and more accurate, but far more difficult to model, measure of climate sensitivity based on all global feedbacks acting in concert is called Earth System Sensitivity (ESS).

In total, the combined Earth System Sensitivity is far greater than the more model friendly Equilibrium Climate Sensitivity. By how much? Based on paleoclimate data, total long term ESS is probably about double that of current ECS estimates.

Muddling Models

Unfortunately, these different definitions can be very confusing to the layman observer. As an example, the IPCC estimates ECS to be between 1.5 to 4.5 degrees Celsius for each doubling of CO2. From an average of these measures, the IPCC gets its estimate ECS of about 3 degrees Celsius. On the other hand, observations of past Earth climates show temperatures averaging at least 6 degrees Celsius warmer when CO2 levels were around 560 parts per million, or about double what the IPCC estimates for ECS.

Yet ECS is, most often, the official, published estimate for how much the Earth will warm. Yet, as shown above, given our current understanding of past climates, the ECS model estimates are short by half.

When looking at the stunning impact of CO2 on global temperatures in the paleoclimate data, one wonders why ECS is used, so often, without this broader qualification? Why, instead, don’t estimates of ECS provide a broader indication that end temperature increases are likely be double those seen in the climate models?

Lower Climate Sensitivity?

For the most part, scientists are trying to determine if slower atmospheric warming since 2000 indicates that climate sensitivity, in this case an already, short by half, equilibrium climate sensitivity, is less than previously expected.

For context, average atmospheric temperatures increased by about .2 degrees Celsius during the 1990s while atmospheric temperatures during the 2000s increased by about .1 degree Celsius. This apparent slowdown in atmospheric warming has caused some to question whether equilibrium climate sensitivity is less than previously expected.

One paper, published by Alexander Otto in Nature indicated a long-term equilibrium climate sensitivity of about 2 degrees Celsius based on new data from recent decades. This new model estimate is still in the range of 1.5-4.5 degrees Celsius provided by most model runs. Furthermore, the study found almost no significant changes to equilibrium climate sensitivity in the long-term trend.

Another Nature study, conducted by Roger Bodman of Victoria University, also found that model estimates for equilibrium climate sensitivity were not lower than previous estimates.

Conclusions

The conclusions to draw from this information are manifold.

The first is that model estimates for equilibrium climate sensitivity are not the best measure of total, long-term climate change. For that we should take a look at past Earth climates. From these measurements, we can find an Earth System Sensitivity of about 6 degrees Celsius for each doubling of CO2. This measure is consistent with Earth climates 50-65 million years ago when CO2 measured about 580 parts per million and temperatures were more than 6 degrees Celsius warmer.

The second conclusion is that long-term change will likely take many centuries to completely unfold. So long-term climate sensitivity measures like ECS or ESS are not good indicators for how fast Earth temperatures will increase within a given decade. Some decades may see relatively slow increases, some little or no increase, and some remarkably rapid increases. However, the overall trend will be for warming, and probably a more rapid pace of warming than that seen in the geological record due to the fact that the human CO2 forcing is currently so powerful.

Furthermore, the .2 degrees Celsius warming during the 1990s and the .1 degrees Celsius atmospheric warming during the 2000s are not entirely indicative of what the long-term trend will look like. In all likelihood, natural variability favored warming more during the 1990s and less so during the 2000s. This is hinted at in the El Nino/La Nina cycle with many powerful El Ninos (which tend to warm the atmosphere) evident during the 1990s while La Ninas (which tend to cool the atmosphere) were more prevalent during the late 2000s.

The third conclusion is that major Earth system feedbacks are beginning to kick in that are likely to make observations of the climates of past decades less relevant. Loss of albedo, ice sheet melt, ocean heat uptake, and environmental carbon release all have a roll to play in future atmospheric warming. Together with a continued and growing human CO2 forcing, these and other factors will determine the pace of warming over the next century. Under business as usual, worldwide CO2 levels hit somewhere between 800 and 1000 parts per million by the end of this century. Such a strong forcing is likely to have very powerful and damaging effects on Earth’s climate system. Even the models that do not take into account slow feedbacks are showing warming of 5-7 degrees Celsius by the end of this century if business as usual emissions continue. And for such a high degree of warming to take place even without the additional contribution from a number of slow feedbacks would be a terrible result.

Fourth, it should be noted that ‘slow feedbacks’ are already beginning to emerge at a more rapid pace than previously estimated. One example, loss of sea ice, is already reducing the northern polar region’s albedo. Another instance, methane hydrate and tundra methane release, are also already adding a positive amplifying feedback to human-caused warming.

Finally, it is worth noting that a more rapid than expected melt of polar ice would temporarily keep temperatures lower at the cost of a more rapid pace of sea level rise combined with much more extreme weather. Such higher than expected paces of melt are entirely possible and, in certain regions, appear to be happening now. In one example northern polar sea ice has experienced an 80% loss of volume since 1980. The result is that northern hemisphere sea ice melt is occurring 80 years ahead of model projections.

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