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Bridging climate science, citizens, and policy


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On False Equivalence

The Guardian recently ran a couple of really bad climate pieces.  The first has a headline guaranteed to draw eyes, “Miami, the great world city, is drowning while the powers that be look away“.  Who would possibly allow a “great world city” drown?  The monsters!  Know that the author is billed as a “science editor”, which I take to mean he understands basic scientific concepts such as uncertainty, time scale, and accuracy.  What does Robin McKie have to say?

The effect is calamitous. Shops and houses are inundated; city life is paralysed; cars are ruined by the corrosive seawater that immerses them. [...] Only those on higher floors can hope to protect their cars from surging sea waters that corrode and rot the innards of their vehicles. [...] Miami and its surroundings are facing a calamity worthy of the Old Testament.

Really?  Old Testament calamity? Inundated. Paralysed. Ruined. Corrode and rot.

That’s fairly flowery language for a science editor.  How much of it is based in reality?  There are definitely localized effects of sea level rise in Miami.  Seawater is corrosive.  But I missed the news reports of Miami calamities, inundations, being a paralyzed city.  Those are serious effects he describes that aren’t quite as extensive or horrific as his article portrays.

Or, as Time writer Michael Grunwald writes, “I’m sorry to spoil the climate porn, but while the periodic puddles in my Whole Foods parking lot are harbingers of a potentially catastrophic future, they are not currently catastrophic. They are annoying. And so is this kind of yellow climate journalism.”

I agree with Michael on this one.  This type of journalism works against taking the very action that Miami actually is doing right now to adapt to a changing reality.  This quote says it perfectly:

What’s happening in the Middle East right now is calamitous. A blocked entrance is inconvenient.

Thank you, Michael, for some overdue perspective.  He adds,

But let’s get real. The Pacific island of Kiribati is drowning; Miami Beach is not yet drowning, and the Guardian’s persistent adjective inflation (“calamitous,” “astonishing,” “devastating”) can’t change that.

This encouraged a number of climate porn addicts to take to the Twitter and denounce Grunwald’s lack of enthusiasm for not wanting to be a part of their tribe.  Tweets displayed peoples’ camps:

Here is what folks were trying to say: person A has a gun held to their head right now; person B will die sometime in the future, but we don’t know exactly when.  And since the same characteristic will eventually apply to both persons, they both share existential threats.  Ask Kiribatians how much of their daily life is affected by sea level rise and I’d bet dollars to doughnuts you’ll get a very different answer than a Miamians’.  And contrary to most climate activists, that’s not because Miamians are climate uneducated.  It’s because their daily lives aren’t affected by climate change today to the same degree than a Kiribatian is.  Saying they are doesn’t make it so.

I also agree with Mike that this fact doesn’t alter the need to mitigate and adapt.  I agree with TheCostofEnergy that Miami and island nations face different timing and resource issues.  That is precisely why island nations face an existential threat today and Miami doesn’t.  Island nation people have nowhere to move to.  Their islands will disappear and they will be forced to move.  That presents an enormous culture disruption.  Miami has much more adaptive capacity than do island nations.  Miami will have to adapt, there is no doubt about that.  But that’s not an existential threat except in some absurdly narrow use of the term.

Disaster porn language usage has to stop.  It’s not accurate.  It dissuades instead of incentivizes action.  It breaks down instead of builds trust.


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N.C.’s Sea Level Rise Reaction

Many people involved in climate activism have probably heard of North Carolina’s reaction to sea level projections.  The reaction has been exaggerated by some of those same activists.  I read this article and had the following thoughts.

By the end of the century, state officials said, the ocean would be 39 inches higher.

There was no talk of salvation, no plan to hold back the tide. The 39-inch forecast was “a death sentence,” Willo Kelly said, “for ever trying to sell your house.”

Coastal residents joined forces with climate skeptics to attack the science of global warming and persuade North Carolina’s Republican-controlled legislature to deep-six the 39-inch projection, which had been advanced under the outgoing Democratic governor. Now, the state is working on a new forecast that will look only 30 years out and therefore show the seas rising by no more than eight inches.

Up to this point, readers probably have one of two reactions.  They either agree with quoted environmentalists and think N.C. tried to “legislate away sea level rise.”  Or they agree with Kelly’s reactions and the legislature’s boundaries on projection scope.

I think the reactions were entirely justified from a personal standpoint and easy to predict if anyone had stopped to think things through.  Nearly everybody would have the same reaction if your property was under threat to be considered worthless – regardless of the underlying reason.  Why?  Because you have an emotional attachment to your property that far exceeds the attachment to a 90-year sea level projection.  You’re going to react to the former more strongly than the latter.  The article identifies the underlying process:

“The main problem they have is fear,” said Michael Orbach, a marine policy professor at Duke University who has met with coastal leaders. “They realize this is going to have a huge impact on the coastal economy and coastal development interests. And, at this point, we don’t actually know what we’re going to do about it.”

This is the problem with the vast majority of climate activists’ language: they coldly announce that civilization will collapse and won’t offer actions people can take to avoid such a collapse.  Well, people will respond to that language, just not the way activists want them to.  People will fight activists and identify with climate skeptics’ arguments since they view the announcements as a threat to their way of life.

Where I differ with Kelly and others is this: she and other coastal residents had better look for viable long-term solutions before that 30-year period is over.  If they prevent long-term planning beyond 2040, inland residents of N.C. will be unfairly burdened with the cost of subsidizing Kelly and others for their lifestyle choices.

Kelly’s view is not without merit, to be sure:

Long before that would happen, though, Kelly worries that codifying the 39-inch forecast would crush the local economy, which relies entirely on tourism and the construction, sale and rental of family beach houses. In Dare County alone, the islands’ largest jurisdiction, the state has identified more than 8,500 structures, with an assessed value of nearly $1.4 billion, that would be inundated if the tides were 39 inches higher.

That’s 8,500 structures in just one county – worth $1.4 billion – an average of $165,000 per structure.  I would absolutely fight to keep my $165,000 worth as long as I could.  Nationwide, the estimate is $700 billion; not a trivial sum is it?  The article has this choice quote:

“What is it you would ask us to do differently right now? Tell people to move away?”   “Preaching abandonment is absurd. People would go in the closet and get the guns out.”

The Coastal Resources Commission bungled their attempt to evaluate the science and establish policy.  By the time they announced results with no action plans, rumors fed by misunderstanding and bias confirmation ran rampant.  The result was Kelly’s actions to change the time horizon that planners could use.

So what are the solutions?  The Commission should establish and maintain relationships with stakeholders.  Get to know the mayors and planners and scientists and property owners.  Find out what their interests are and what motivates them to do what they do.  Identify actions they can take in the next 30 years that sets them up for success afterward.  But don’t release information without context.  Because sea level rise is likely to accelerate in the 2nd half of the 21st century.  But most people will focus on potential direct threats to themselves and their livelihoods, not global concerns.  So get into the weeds with folks.


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N.C.’s Sea Level Rise Reaction

Many people involved in climate activism have probably heard of North Carolina’s reaction to sea level projections.  The reaction has been exaggerated by some of those same activists.  I read this article and had the following thoughts.

By the end of the century, state officials said, the ocean would be 39 inches higher.

There was no talk of salvation, no plan to hold back the tide. The 39-inch forecast was “a death sentence,” Willo Kelly said, “for ever trying to sell your house.”

Coastal residents joined forces with climate skeptics to attack the science of global warming and persuade North Carolina’s Republican-controlled legislature to deep-six the 39-inch projection, which had been advanced under the outgoing Democratic governor. Now, the state is working on a new forecast that will look only 30 years out and therefore show the seas rising by no more than eight inches.

Up to this point, readers probably have one of two reactions.  They either agree with quoted environmentalists and think N.C. tried to “legislate away sea level rise.”  Or they agree with Kelly’s reactions and the legislature’s boundaries on projection scope.

I think the reactions were entirely justified from a personal standpoint and easy to predict if anyone had stopped to think things through.  Nearly everybody would have the same reaction if your property was under threat to be considered worthless – regardless of the underlying reason.  Why?  Because you have an emotional attachment to your property that far exceeds the attachment to a 90-year sea level projection.  You’re going to react to the former more strongly than the latter.  The article identifies the underlying process:

“The main problem they have is fear,” said Michael Orbach, a marine policy professor at Duke University who has met with coastal leaders. “They realize this is going to have a huge impact on the coastal economy and coastal development interests. And, at this point, we don’t actually know what we’re going to do about it.”

This is the problem with the vast majority of climate activists’ language: they coldly announce that civilization will collapse and won’t offer actions people can take to avoid such a collapse.  Well, people will respond to that language, just not the way activists want them to.  People will fight activists and identify with climate skeptics’ arguments since they view the announcements as a threat to their way of life.

Where I differ with Kelly and others is this: she and other coastal residents had better look for viable long-term solutions before that 30-year period is over.  If they prevent long-term planning beyond 2040, inland residents of N.C. will be unfairly burdened with the cost of subsidizing Kelly and others for their lifestyle choices.

Kelly’s view is not without merit, to be sure:

Long before that would happen, though, Kelly worries that codifying the 39-inch forecast would crush the local economy, which relies entirely on tourism and the construction, sale and rental of family beach houses. In Dare County alone, the islands’ largest jurisdiction, the state has identified more than 8,500 structures, with an assessed value of nearly $1.4 billion, that would be inundated if the tides were 39 inches higher.

That’s 8,500 structures in just one county – worth $1.4 billion – an average of $165,000 per structure.  I would absolutely fight to keep my $165,000 worth as long as I could.  Nationwide, the estimate is $700 billion; not a trivial sum is it?  The article has this choice quote:

“What is it you would ask us to do differently right now? Tell people to move away?”   “Preaching abandonment is absurd. People would go in the closet and get the guns out.”

The Coastal Resources Commission bungled their attempt to evaluate the science and establish policy.  By the time they announced results with no action plans, rumors fed by misunderstanding and bias confirmation ran rampant.  The result was Kelly’s actions to change the time horizon that planners could use.

So what are the solutions?  The Commission should establish and maintain relationships with stakeholders.  Get to know the mayors and planners and scientists and property owners.  Find out what their interests are and what motivates them to do what they do.  Identify actions they can take in the next 30 years that sets them up for success afterward.  But don’t release information without context.  Because sea level rise is likely to accelerate in the 2nd half of the 21st century.  But most people will focus on potential direct threats to themselves and their livelihoods, not global concerns.  So get into the weeds with folks.


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Research: Timing Sea-Level Rise Projections

A recent surge (see my last post) of published papers on sea-level rise generated the following paper: “Ice sheet collapse following a prolonged period of stable sea level during the last interglacial” (subs. req’d) by O’Leary et al. in Nature Geoscience.  The authors embarked on this work to provide information on potential events in our future as sea levels rise in response to increasing greenhouse gas concentrations.  From their abstract: “During the last interglacial period, 127–116 kyr ago, global mean sea level reached a peak of 5–9  m above present-day sea level.”  An “interglacial” is a time period in between periods with extensive polar ice sheets.  The last interglacial is also known as the Eemian.  During the Eemian, sea levels were 16-30 feet higher than today’s level.  Scientists estimate that 4 feet of additional sea level rise is already locked-in due to past GHG emissions.  Our current emissions pathway through 2100 locks in 23 feet of future sea level rise (over the course of the next few hundred years).

Most analysis of future sea-level rise suggests that the rise is likely to occur over the course of decades to centuries.  The results are dramatic – more than 1,400 coastal US municipalities would be mostly underwater with an additional 23 feet of sea level – regardless of the timescale in which the seas rise.  Society can respond more easily if the time is century-scale than if it is decadal-scale.  The costs are high regardless, but societal strife might be less if the timescale is longer.  Imagine telling one million people they have to abandon their homes in the next decade due to rising seas while their governments are forced to move infrastructure such as roads and airports (billions of dollars’ worth) and businesses have to move inland as well.  I don’t think the result would be pretty.

O’Leary et al. focused on an even more striking event within the Eemian (which was as warm as projections of likely late 21st century temperatures): a rapid, not gradual, increase in sea levels.  “We show that between 127 and 119 kyr ago, eustatic sea level remained relatively stable at about 3–4 m above present sea level. [...] We suggest that in the last few thousand years of the interglacial, a critical ice sheet stability threshold was crossed, resulting in the catastrophic collapse of polar ice sheets and substantial sea-level rise.”

Their results (along with others’) show that sea levels were stable a few meters above the current level.  Then, something caused polar ice sheets to melt “rapidly” and sea levels to rise quickly from 3-4 m above today’s to 9 m above today’s. So sea levels rose from 10-12 feet above today’s to nearly 30 feet above today’s in less than one thousand years.  If such an event happens again in our future, one thousand years is a relatively long time to respond to the primary change.  Secondary changes might challenge our responses, but that isn’t my focus today.

If O’Leary’s estimate of one thousand years is too long by one order of magnitude, serious challenges would exist.  If sea levels rise by 17 feet in less than one hundred years, how would we respond?  How much infrastructure would we leave to the rising seas because moving it is just too costly?  In addition to continued sampling of former underwater sites in Australia and elsewhere to gauge total sea level change, scientists need to refine methods to pin down the timing of past changes.  How frequently did past rapid changes occur?  What were their magnitude?  Then, decision makers need to establish two-tiered response plans.  The first should address the most common type of change: gradual rise/fall over hundreds to thousands of years.  The second therefore should address the more serious challenge of sub-centennial rapid sea level change.  We can’t implement plans that aren’t formulated beforehand.

It is worth noting that Mercer first wrote about polar ice sheet collapse 35 years ago.  This phenomenon is not new but is being better informed by subsequent research.  Hat tip New York Times.


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Research: Updated Projections of Future Sea Level Rise

An international team of climate scientists led by Anders Levermann wrote a paper than appeared in the Proceedings of the National Academy of Sciences (PNAS) of the US that described long-term (2,000y) sea level changes in response to different stabilized temperature thresholds.  You can find a short Reuters summary of the paper here.  I will provide more detail and share some observations of this paper.  The paper garnered a good amount of attention in climate activist circles since publication.

First, a little historical perspective.  Global mean sea levels rose in the 20th and 21st centuries: about 0.2m.  Prior to research conducted in the past five years, projections of additional 21st century sea-level rise ranged from another 0.2m to 2.0m.  These projections did not, in general, consider feedbacks; the parent simulations did not consider cryosphere processes (i.e., melting glaciers and the land-based Greenland and Antarctic ice sheets).  More recent research included more feedbacks and cryosphere processes, but their treatment remains immature.  Additionally, recent research started to examine projections based on realistic emissions scenarios, after researchers began to accept the fact that policymakers are unlikely to enact meaningful climate policy any time soon.  As such, sea-level projection ranges increased.  Which is where this latest paper comes in.

Levermann et al. reported a 2.3 m/°C sea-level rise projection in the next two thousand years.  Benjamin Strauss’s PNAS paper put this into context:

[W]e have already committed to a long-term future sea level >1.3 or 1.9m higher than today and are adding about 0.32 m/decade to the total:10 times the rate of observed contemporary sea-level rise

Thus, if global temperatures rise only 1°C and stabilize there (an extremely unlikely scenario), sea-levels two thousand years from now could be 2.3 m higher.  This might not sound like much, but an additional 7.5 feet of sea level rise would inundate 1.5 million U.S. peoples’ homes at high tide.  With 2°C, sea levels could rise 4.6m.  On our current emissions pathway, global mean temperatures would rise 4°C, which would result in an additional 9.2 m of sea level rise.  That’s 30 feet higher than today!  Levermann notes that these higher sea level projections are supported by sea level heights that occurred in the distant past (paleoclimate), even with their associated uncertainties.

According to Strauss’s analysis, such a rise would threaten more than 1,400 municipalities – and those are just U.S. municipalities that exist today, not tomorrow.  Globally, billions of people would be adversely affected.  What social stresses would billions of people moving inland exert?  The U.S. experienced its own small glimpse into this future post-Hurricane Katrina as a few thousand people permanently abandoned their below-sea level neighborhoods.

And now a couple of important points.  Some of the processes Levermann utilized involved linear change.  In complex systems like the Earth’s climate, very few changes are linear.  Many more are exponential.  When I discuss linear and exponential change with my students, I include a number of different examples because our species doesn’t easily understand exponential change.  We typically severely underestimate the final value of something that changes exponentially.  If changes within the climate system occur exponentially, the Levermann projections probably won’t be valid.  But their estimate will likely be an underestimate.

A USA Today article on the Levermann paper has this quote:

Looking at such a distant tomorrow “could scare people about something that might not happen for centuries,” says Jayantha Obeysekera of the South Florida Water Management District, a regional government agency. He says such long-term projections may not be helpful to U.S. planners who tend to focus on the next few decades.

Should people be scared that their communities will be underwater in 2000 years?  I don’t think they will be, number one.  Few people pay attention to trends that will affect them in 2 or 20 years.  200 or 2000 are well beyond anybody’s individual concern.  But I think society as a whole should examine this updated projection.  Do we want to condemn one-third of Florida to the bottom of the Atlantic Ocean?  Do we want to condemn thousands of towns and cities to that fate, even if the time horizon may be well beyond our lifetime?

Moving beyond the inevitable question regarding fear, what do these results mean?  We need to include results like these in planning processes.  If nothing else, planners and policymakers have more realistic estimates of likely future sea level in hand.  Those estimates will continue to change (hopefully for the better) with additional research.  But decision-making shouldn’t stop with the expectation that some future projection might be perfect because it won’t be.  The decisions we make today will have profound effects on the eventual level of the sea in the distant future.  There will also be countless effects on the climate, societies, and ecosystems until we reach that level.   That is what today’s decision-making needs to address.

Climate Central has a useful map to investigate how potential thresholds implicate sea level rise with respect to US states at different points in the future.


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How the IPCC Underestimated Climate Change – Scientific American article

Scientific American published an article summarizing what I’ve written about for a couple of years: the IPCC’s projections aren’t 100% correct.  Gasp – the horror!  But, contrary to what skeptics think, the direction the IPCC’s reports were wrong are opposite of what they claim.  The projections time and again underestimated future changes.  I think a valid complaint, and one I’ve made many times myself, is that the IPCC process is too conservative – it takes too long to get the kind of consensus they’re looking for.  Rapidly changing conditions are not well handled by the IPCC process.  When there is conflicting evidence of something, the IPCC has tended to say nothing in an effort not to upset anybody.  The good news is there are indications this is changing.  The list:

1. Emissions

This is the biggest one.  Too many studies focused on moderate emission pathways, when yearly updates showed our actual emissions were on the high range of those considered by the IPCC.  I actually posted on this two days ago: CO2 Emissions Continue to Track At Top of IPCC Range.  This has implications for every other process that follows.

2. Temperature

More accurately, energy in the climate system is the variable of interest.  It is easy to point out that temperatures since 2000 haven’t increased as much as projected.  It is also easy to compare observed trends since 1980 and claim AR4 models over-predicted temperature rise.  This conflates a couple of issues: the AR4 wasn’t used to project since 1980.  More importantly, the difference between observed trends since 1980 and projected temperatures from half of the AR4 models was less than 0.04°C (0.072°F).  That’s pretty darned small.  With respect to the trend since 2000, the real issue is energy gain.  The vast majority of energy has accumulated in the oceans:

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More specifically, if the heat is transported quickly to the deep ocean (>2000ft), the sea surface temperature doesn’t increase rapidly.  Nor does atmosphere or land temperatures change.  This is true at least in the short-term.  When the ocean transports this heat from the deep back to the surface, we should be able to more easily measure that heat.  Put simply, the temporary hiatus of temperature rise is just that: temporary.  Are we prepared for when that hiatus ends?

The relatively small increase in near-surface air and land temperatures is thus explained.  The IPCC never claimed the 4.3° to 11.5°F temperature rise (AR4 projection) would happen by 2020 – it is likely to happen by 2100.  Expect more synergy between projected temperatures and observed temperatures in the coming years.  Also remember that climate is made up of long-term weather observations.

Additionally, aerosols emitted by developing nations have been observed to reflect some of the incoming solar radiation back to space.  Once these aerosols precipitate out of the atmosphere or are not emitted at some point in the future, the absorption of longwave radiation by the remaining greenhouse gases will be more prominent.  The higher the concentration of gases, the more radiation will be absorbed and the faster the future temperature rise is likely to be.  These aerosols are thus masking the signal that would otherwise be measured if they weren’t present.

3. Arctic Meltdown

This is the big story of 2012.  The Arctic sea ice melted in summer 2012 to a new record low: an area the size of the United States melted this year!  Even as late as 2007 (prior to the previous record-low melt), the IPCC projected that Arctic ice wouldn’t decrease much until at least 2050.  Instead, we’re decades ahead of this projection – despite only a relatively small global temperature increase in the past 25 years (0.15°C or so).  What will happen when temperatures increase by multiple degrees Centigrade?

4. Ice sheets

These are the land-based sheets, which are melting up to 100 years faster than the IPCC’s first three reports.  2007′s report was the first to identify more rapid ice sheet melt.  The problem is complex cryospheric dynamics.  Understandably, the most remote and inhospitable regions on Earth are the least studied.  Duh.  That’s changing, with efforts like the fourth International Polar Year, the results of which are still being studied and published.  Needless to say, modern instrumentation and larger field campaigns have resulted in advances in polar knowledge.

5. Sea Level Rise

It’s nice being relevant.  I just posted something new on this yesterday: NOAA Sea-Level Rise Report Issued – Dec 2012.  The 3.3mm of sea-level rise per year is higher than the 2001 report’s projection of 2mm per year.  Integrated over 100 years, that 1mm difference results in 4″ more SLR.  But again, with emission and energy underestimates, the 3.3mm rate of SLR is expected to increase in future decades, according to the latest research.  Again, another mm per year results in another 4″ 100 years from now.  Factors affecting SLR that the IPCC didn’t address in 2007 includes global ocean warming (warmer water takes up more volume), faster ice sheet melt, and faster glacial melt.  Additionally, feedback mechanisms are still poorly understood and therefore not well represented in today’s state-of-the-art models.

6. Ocean Acidification

The first 3 IPCC reports didn’t even mention this effect.  In the past 250 years, ocean acidity has increased by 30% – not a trivial amount!  As the article points out, research on this didn’t even start until after 2000.

7. Thawing Tundra

Another area that is not well-studied and therefore not well understood.  The mechanics and processes need to be observed so they can be modeled more effectively.  1.5 trillion tons of carbon are locked away in the currently frozen tundra.  If these regions thaw, as is likely since the Arctic has observed the most warming to date, methane could be released to the atmosphere.  Since methane acts as a more efficient GHG over short time frames, this could accelerate short-term warming much more quickly than projected (See temperatures above).  The SciAm article points out the AR5, to be released next year, will once again not include projections on this topic.

8. Tipping Points

This is probably the most controversial aspect of this list.  Put simply, no one knows where potential tipping points exist, if they do at all.  The only way we’re likely to find out about tipping points is by looking in the past some day in the future.  By then, of course, moving back to other side of the tipping point will be all but impossible on any time-frame relevant to people alive then.

Summary

There are plenty of problems with the UNFCCC’s IPCC process.  Underestimation of critical variables is but one problem plaguing it.  Blame it on scientists who, by training, are very conservative in their projections and language.  They also didn’t think policymakers would fail to curtail greenhouse gas emissions.  Do policymakers relying on the IPCC projections know of and/or understand this nuance?  If not, how robust will their decisions be?  The IPCC process needs to be more transparent, including allowing more viewpoints to be expressed, say in an Appendix compendium.  The risks associated with underestimating future change are higher than the opposite.


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Research: Sea-Level Rise in Response to Warming Climate

From the top, I want to include important context for the research results I am presenting.  This research is based on peak warming of only either 1.5°C or 2°C.  It is my educated opinion that such goals are unrealistic.  Prevention of warming past 2°C is no longer a viable option based on the globe’s history of burning carbon-intensive fossil fuels as well as the medium- to long-term future, which doesn’t promise much of a difference.  Furthermore, as I have stated numerous times in the past year, policy discussion would be better served if scientists would conduct research on developments that are much likelier to occur and not the world they want to see (i.e., higher vs. lower emissions scenarios).  That said, this research fulfills an important role in the overall discussion because I think some of the results can be used as a “floor” – conditions are likely to reach higher magnitudes than those found in this and similar papers.

Michiel Schaeffer, William Hare, Stefan Rahmstorf & Martin Vermeer’s Nature paper was published on June 24, 2012.  They examined sea-level rise in response to warming scenarios using a semi-empirical model.  By 2100, global sea-level rise would be ~60cm above the 2000 level if global GHG emissions were zeroed by 2016.  This is an obvious fantasy world, but it provides a useful benchmark for other scenarios the scientists examined.  The reason sea-level rise would continue through the 21st century even if we haled emissions completely in the next 3-4 years is the response of the climate system to the anthropogenic forcing imparted on it through the 20th and early 21st centuries.  If 1.5°C or 2°C warming is not exceeded, global sea-level rise would be 75-80cm above the 2000 level.  The authors also report that unmitigated emissions could result in 100cm rise above 2000 levels.  It is important to note that 20th century sea level rise has been estimated to be ~20cm.  It doesn’t require much thought to realize that the rate of sea-level rise has increased throughout the 20th century and continues to do so in the 21st.  Moreover, it is clear that since we will most likely warm beyond 2°C, the 75-100cm projection can be viewed as a reasonable estimate for a “floor”: actual sea-level rise could be greater than this.

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