Wednesday, December 27, 2017

The Paradox of Sea Level Rise

Sea level is rising.

There is no doubt about it.

But why?

How much of the recent rise is due to greenhouse gas warming?  How much of it is natural? 

The situation is a bit more nuanced than described by some of the media and others (like Zillow in a recent prediction of large numbers of coastal homes flooding in the NW).


Well, let's get our feet wet in this topic by examining sea level records provided by NOAA for several cities.  In each of them, NOAA has also put a "best fit" line for reference.

First Seattle, which has a sea level record going back over a century (1900).  Over the entire period, there has been an average of 2.03 mm increase in sea level per year (.67 feet per century).  The interesting thing is that the upward trend has been going on for a long time, well before the impacts of human emissions of greenhouse gases were significant.  (The radiative impacts of increasingly CO2 became large in the 1970s and later).  And rate of rise has been quite steady, with no hint of a recent acceleration.  In fact, there has been minimal rise during the past 20 years.



San Diego?   A very similar evolution, at a slightly greater rate of rise (2.15 versus 2.03 mm per year)


How about Key West?   A little more:  about 2.40 mm a year (.78 feet per century).  Again, no hint of acceleration of sea level rise during the past decades as human-emitted greenhouse gases have increased rapidly. 


From NOAA's National Climatic Data Center's website I secured this graphic of global sea level rise over the past 140 years.  Pretty steady rise since roughly 1920 and even rising before that.

Sea level can be measured with measurements at the sea surface or from satellites.  The solid line above is a satellite retrieval.  Here is a plot of satellite measurements....but only goes back to the 90s.
Pretty steady rise.



It is important to note that coastal sea level rise is not uniform around the world, with one reason being that the land is not staying at the same elevation!   In some places, the ground is sinking, due to pulling removal of subsurface water or oil, or some other natural process.  For example, the land is rising today in locations that were covered by ice-age glaciers that pushed the land surface down.  After they melted roughly 14,000 years ago, the land rebounded.  That is happening here in the Northwest, particularly for the Olympic Peninsula.

To illustrate all this, here is a a sea level trend map from NOAA.  Some places like the Olympic Peninsula has sea level going down.  Same in Alaska.  But there are large rises where the land is subsiding, such as New Orleans.



So this sea level rise business is pretty nuanced.

Sea level rise is not accelerating appreciably, even thought greenhouse gas concentrations are rapidly rise.  And the rise of sea level began more than a century ago, well before humans could have been a significant cause.    In fact, there was something called the Little Ice Age that occurred during the 1500s to middle 1800s, with subsequent warming that was mainly natural.  The current sea level rise period appears to have its origin in the demise of the Little Ice Age and the warming that followed.

So claims that all or most of the rise in sea level is due to human-emitted greenhouse gases appear problematic because it started before humans could be the main cause.  The casual link is further weakened by the lack of acceleration of sea level rise during the past few decades.

On the other hand, our climate models suggest an accelerated rise of sea level rise due to greenhouse gas warming during this century.  Will our models be correct or are they too sensitive to greenhouse gas impacts?  Time will tell.


So, what sea level rise should we expect in Seattle during the remainder of the century?

Extrapolating the current, steady upward trend implies about a .6 ft rise.  If we include the impacts of greenhouse gas warming, there would be more.  A National Academy of Sciences report did such an analysis suggesting a 4-56 inch increase by 2100, with a mean change of 30 inches (2.5 ft).  But whether such model-driven estimates are reliable is uncertain:  I suspect it will be on the high side considering the slow rise of the past few decades.









39 comments:

Eric Blair said...

One factor that's missing from a lot of the MSM discussion on this topic excludes the recently discovered volcanoes underneath Antarctica, and how their activity may be affecting sea levels:

http://blogs.discovermagazine.com/d-brief/2017/08/14/volcanoes-under-antarctica/

Dan said...

Excellent article, Cliff.

Many people whose only education is talking points think that anthropogenic climate change started a couple of decades ago, when the real answer is that it began with the Industrial Revolution in the 18th century. This process has accelerated not only with further industrialization, but rapid increases in global population.

Humans numbered 1 billion in 1804 and 2 billion in 1927. Today we are fast closing in on 8 billion.

So many policymakers want to legislate how humans choose to use fuel, but the moment that someone talks about discouraging further population increases by revoking child tax credits or closing our doors to most immigration, dissenters start spewing bile.

For the good of the species, we as a global society need to quit accepting economic migration from rapidly expanding countries to the stabler countries of the West. We can't stop Pakistan, the Philippines, and Sub-Saharan Africa from breeding themselves into oblivion, but we can refuse to accept their migrants. Only by forcing their children to stay in the country of their birth can we make the point that uncontrolled breeding must come to an end.

Literacy and enfranchisement for women are important components of this effort as well, while the biggest forces standing in the way of pushing down birthrates are Catholicism, Islam, and poverty.

Immigration proponents in the West need to answer for their policies that encourage further population expansion, and by extension, further increased carbon emissions.

But from these interests we hear nothing.

Andrew Lincicome said...

Solar Cycle activity levels seems to cover these trends like a glove.

David Riggs said...

It seems strange that melting glaciers and warming oceans isn't already having a demonstrative effect on sea level rise. How could this be?

Richard Harrington said...

Thanks for another fact-filled post.

You did miss a highly relevant factor - erosion.

Most of the islands that have disappeared in recent years were coral-based atols, which rarely get much above sea level, and frequently get wiped out by severe weather events.

Steve Allwine said...

It seems that this analysis is missing some key components. Ocean levels are also dictated by the currents and wind and as shown in the US map, sea level rise is not uniform.

The US, especially along the Gulf Coast, is likely to see notable level of rise over the century, however when you look at a world-wide map, you'll see that sea level rise + current motion + wind forcing, places like the South Western Pacific, between Australia and Indonesia, are going to get absolutely clobbered. They're already experiencing 10+ mm/year of rise.

I'd like to see Cliff Mass's data set expand beyond that of the continental US, as it's not showing the whole picture. NOAA's satellite altimetry from the TOPEX, J1 and J2 satellites tells a much more nuanced story concerning measured sea level trends.

GaryP. said...

Cliff, you miss that the ice sheets in Antarctic are breaking and sliding into the ocean. Past data isn't going to show that effect and yet it cause seas to rise by 6 to 11 ft.

Quit yer whinin'! said...

Excellent job, Cliff! Crushing the fever dreams of the AGW alarmists!

Elston Hill said...

I am confused. I have seen articles which say that human influence on climate goes back thousands of years, not just a century. That humans started impacting climate as they burned forests to grow crops. That was initially at a much slower rate. And I understood from these articles that that there is a correlation to human activity that goes back thousands of year.

Bruce Kay said...

I was a bit puzzled by your use of "paradox" in your title, not being sure how you intended it. It seems that most any uncertainty can seem paradoxical so I guess it makes sense but what i did find was a good article on the "Climate change paradox" which in practical terms is the paralysis of action despite any awareness of the facts. That is the paradox that is of vital interest, in terms of risk.

https://www.theguardian.com/commentisfree/2017/jun/07/caring-about-climate-change-its-time-to-build-a-bridge-between-data-and-emotion

snapdragon said...

One thing I wonder about is why people blame melting sea ice for the sea level rise. Doesn't ice take up more space than water? Isn't that what happens in my ice cube trays in the freezer? If the sea ice were melting, wouldn't the sea level fall?
I can see that glacial melt would cause sea levels to rise, but not the melting of sea ice or polar ice... right? Or is that just too simplistic?

Unknown said...

Your blog post here appears to contradict the most recent conclusions of the Fourth National Climate Assessment which asserts that "human-caused climate change has made a substantial contribution to GMSL rise since 1900 (high confidence)." The same Assessment reports a doubling of the rate of eustatic sea level rise from the period 1901 - 1990 (1.2 mm/yr) to the period 1993 - 2015 (3.4 mm/yr). Is it your contention that the National Assessment is wrong?

Earthwater said...

From the reports I've read, most of the sea level rise to date is from the ocean is warming and expanding. In our region, as you noted, the Olympics are rising, so Neah Bay is rising or breaker even. The Puget sound trough is sinking a bit.
The big unknown for the future is the rate of melting for the continental glaciers, especially Greenland and West Antarctica. The last IPCC report provided very conservative estimates of sea level rise based on uncertainty of modeling glacial melt. It should be used as a lower estimate. The wild cards appear to be: in Greenland, honeycombing, and base melting; in Antarctica, acceleration of glacial advance from ice sheet advance. The future can no longer be predicted from the past, so we be doing some risk assessment and planning now.

Organic Farmer said...

Good post Cliff.

Questioning sea level rise rates, caused by human contributed greenhouse gasses, is good science! Not denial!

Indeed land masses rise and fall each having profound effects on sea level rise. Melt ice off Greenland and it rebounds. Remember ice shelf's and sea ice are already part of the sea level, wether liquid or frozen.

My personal "hunch" is regional ice ages are just as much an effect of global "warming" as anything else. (Please let's ditch global warming, and refer human induced increase in atmospheric greenhouse gasses as "climate change"

I find the recent reports on the intensification of the Aleutian low causing snow rates on the higher peaks of BC and AK to double!

An example of warmer more humid air causing more snow.

Hello glacial growth.

Mark Anderson said...

A good illustration of how tricky this science is and how easily we can fool ourselves.

homero flores said...

Cliff,
You raise an interesting argument (pun intended): perhaps greenhouse emissions are not causing the sea level rise we're experiencing, since sea level rise has been happening for over a hundred years.

There are a few things that your argument is missing:

First, you're right about the little ice age, but you're leaving out the fact that CO2 emissions have been increasing due to human activity since the industrial revolution, at least since 1850 (see figure 5 in this document https://www.esrl.noaa.gov/gmd/aggi/aggi.html), thus their effects started ramping up since then and could be added to the warming of the planet after the little ice age. In fact, we could still be warming up after the little ice age, I don't know, but the greenhouse gasses have been increasing, and thus the solar forcing on the planet.

Second, the oceans have a huge thermal inertia, and thus even though they are absorbing most of the extra solar forcing, the changes in temperature - and thus expansion - are small now, but will keep happening and accelerating.

Finally, as a suggestion, it would be great if you made a reference to a source that is a counter argument to yours, such as NOAA's: https://oceanservice.noaa.gov/facts/sealevel.html. A paragraph on it states the following "The two major causes of global sea level rise are thermal expansion caused by warming of the ocean (since water expands as it warms) and increased melting of land-based ice, such as glaciers and ice sheets. The oceans are absorbing more than 90 percent of the increased atmospheric heat associated with emissions from human activity".

Homero

Deek Deek said...

Thanks Cliff for another informative article. I am not surprised that there is little indication of ocean level rise due to greenhouse gas warming. I expect that there is a significant time delay due to the large thermal inertia of the ice and the ocean. It takes alot of energy to melt ice which adds water to the ocean and heat the oceans which results in thermal expansion of the water.

Doc Wellness said...

There are studies showing that sea level rise is accelerating, attributed to melting in Greenland and Antarctica.

https://www.washingtonpost.com/news/energy-environment/wp/2017/06/26/sea-level-rise-isnt-just-happening-its-getting-faster/?utm_term=.277fbf88711a

scott springer said...

Sea level along the west coast is a complicated combination of global changes and regional changes, including GIA, the gravitational effect of mass loss in glaciers in the coast ranges, and PDO and/or El Nino. There are a number of studies that have looked at these factors in some detail, such as

http://onlinelibrary.wiley.com/doi/10.1002/2016JC011815/full

Evidence for global sea level acceleration is beginning to appear, e.g.

https://www.nature.com/articles/nclimate3325

BMFH said...

Thankyou

Craig said...

You have left out a very important component of sea level rise along our coast and that is uplift caused by the subduction zone off shore which is slowing pushing up the land. This is canceling out much of the rise we should be seeing here and in some places like southern Oregon it is actually outpacing it so the sea level there is going down. Unfortunately when we experience the next subduction zone earthquake we will get all our sea level rise at once which could be as much as 2 meters. Of course most of that is from the land sinking because of the earthquake, but the accumulated sea level rise will add to the total.

Also the temporary rise caused by the thermal expansion of water during El Nino years could be a factor if climate change effects the frequency and strength of those. The changes in sea level during El Ninos can be 10s of centimeters along our coast.

jayemarr said...

Maybe Zillow could move away from sea level hysteria and try to discourage people from building homes on top of eroding cliffs?

Jennifer Gervais said...

Sea level rise will be a function of at least two variables you haven't discussed in your post. The first is the increased volume that a body of water will take on as it heats up. This is operating now. The second is the melting of glaciers around the world, which will inject enormous quantities of water into the oceans. We have not seen much of the latter yet, and models disagree about how fast that glacial ice melt will contribute to rising sea levels. However, in the last several years, projections have begun suggesting that much more severe sea level rise may occur prior to the end of this century than was previously thought. Note that ice already on or in the oceans will not add to sea level rise, any more than ice melting in a cold drink will overflow the container. It's the ice balanced on the rim that has the potential to spill the liquid over the top. Thank you for your thoughtful posts.

Dan McShane said...

The negative relative sea level (RSL) change on the Olympic Peninsula is primarily from seismic strain on the Cascadia Fault as well as crustal under plating along the convergent margin. Nearly all the local glacial loading rebound took place shortly after the Puget and Juan de Fuca ice lobes retreated. Our local glacial isostatic adjustment (GIA) rebound was very rapid compared to other locations such as Hudson Bay where the coast is still rising or the lower Red River Valley on the US Canada border where land levels are still rising and are implicated on flooding on the Red River. GIA is a complicated factor that needs to be incorporated into sea-level rise evaluation both at a local level and global level. It is by no means a simple problem and is a subject of inquiry that is critical to global sea-level rise.

Another factor that needs to be considered in evaluating sea level rise related to climate change is water stored in reservoirs. This volume has had a considerable dampening effect on the observed sea level increase during the era of large dam building. This is particularly true post 1950. Accounting for this factor will reduce sea level by 1.3 to 1.8 mm/year. If this correction is made there does appear to be an increase in rate of sea level rise starting in about 1950 over the previous rate increase that may be related to GIA and ice loss from the "little Ice Age" (See Figure 6 from Kemp and others, 2011)( http://www.pnas.org/content/108/27/11017.full).

Sea level change is complicated stuff and reviewing sea level change should be done with caution.

JeffB said...

The keyword is adaptation. If it’s half a foot or a foot in this century that’s not going to rise to the level of alarm or urgency but instead warning and adaptation. And we can’t assume that every potential future danger constitutes a need for alarm today because we have limited resources. Good luck getting most of the Puget Sound to worry about the few with waterfront property who might experience some flooding in the next 100 years.

ran380 said...

Hi Dr. Mass

Your main thesis argument is that the rate of sea level rise is not accelerating.

However, it's apparent from the global sea level change graph in your post that the rate of change is increasing. However, I wanted to do some basic data analysis.

I downloaded data from the EPA website on global sea level change:
https://www.epa.gov/climate-indicators/climate-change-indicators-sea-level

I divided the data into three four-decade periods (1980-1920, 1920-1960, 1973-2013) and did a linear fit on the sea level change. I found slopes of .0511, .0703 and .0947 inches/year. This simplistic analysis suggests that the rate of sea level rise has almost double in the past 100 years.

http://i68.tinypic.com/eu3eas.png
http://i63.tinypic.com/2h71mrs.png
http://i65.tinypic.com/ot45ma.png

I did a quick literature search and I found that most recent work supports the acceleration of sea level rise using much more sophisticated analysis to decouple natural variations:
https://www.nature.com/articles/nature14093
http://onlinelibrary.wiley.com/doi/10.1002/2017GL072845/full
http://onlinelibrary.wiley.com/doi/10.1002/grl.50731/full

At the very minimum, this topic requires a much more involved analysis in order to make such a strong claim that seems to be at odds with the consensus in the field.

Kind regards,

Randolph Lopez

nutso fasst said...

Clearly, the cause of rising relative sea level at many tide gauges is in part anthropogenic.

The rapid rise at islands south of New Orleans is likely due to both loss of sedimentation from the re-routed Mississippi River and subsidence from oil extraction from beneath the Gulf (https://coastal.er.usgs.gov/gc-subsidence/induced-subsidence.html).

The Texas shoreline south of Houston to Galveston is subsiding due to groundwater extraction (https://txpub.usgs.gov/houston_subsidence/home/), as are areas around greater Miami and the eastern seaboard.

Rincon Island off the Ventura, CA coast was constructed for oil extraction. Its tide gauge record shows a choppy mean overall rise of 3.22 mm/year from 1962-1990. Up the coast at relatively pristine Port San Luis, however, mean rise is a non-accelerating 0.84 mm/year since 1945. And south at Los Angeles, mean rise has been a non-accelerating 0.96 mm/year since 1923. Those rates are close to that at Brest, France—a non-accelerating 1.05 mm/year since 1807. But when Gov. Brown warned that Los Angeles Int'l Airport, 38,100 mm above sea level, could be under water within 200 years, the LA Times dutifully reported it. If not for the outcry of 'deniers', Brown may have created a new talking point for credulous California Climatastrophists.

Average global sea level has risen and may be rising still, but the clear lack of rate increase in areas not affected by human-caused subsidence casts reasonable doubt that atmospheric CO2 increase is having any effect.

Sharon Steinbis said...

Data

Alberto Zaragoza Comendador said...

Honestly, I find the acceleration in SLR (or lack thereof) a red herring.

Natural SLR is not constant; sometimes it's faster, sometimes it's slower, but throughout the last millennia it has rarely if ever been at the current (3mm/year or so) rate. How do we know this? Because if 3mm/year was normal the seas would have risen 3 meters since the year 1000, 6 meters since Jesus Christ, etc. And there is no evidence of that. So the "background" or natural SLR seems to be about 1 or 1.5mm/year, though obviously with fluctuations.

Also, the "no acceleration" meme conflates two different things. One is the lack of acceleration since the pre-industrial era (XIX century), i.e. since before significant man-made warming. This "lack of acceleration" is false - of course SLR has accelerated since then, though this may of course not be obvious looking at independent stations. The other "no acceleration" argument is the one that uses the satellites; as you say SLR measured that way has been quite steady, but it's been "steady" at a much higher level than pre-industrial SLR.

(Imagine if someone showed you China's economic growth since 1980 and said "hey, they've been growing fast all this time, in fact faster at the beginning of then period than as of late... see? No acceleration!" Of course, the acceleration in economic growth is with respect to the previous period).

Is SLR currently accelerating? That's unclear because the satellite record is so short. Has it accelerated since humans started to change the composition of the atmosphere? Yes, by 1.5mm/year or so. Is the acceleration due to humans? For sure at least part is due to humans. Current ocean warming is about 0.7w/m2 and, though estimates of ocean heat before 1950 are almost non-existent, the evidence points pretty strongly to an increase in this rate of heating since then; in other words going back to 1960 you'd find a rate of warming not of 0.7w/m2 but of 0.2w/m2 or so (speaking off the top of my head, it's been a while since I last checked a paper on ocean heat). So, while part of the ocean warming may be natural, and indeed the planet may conceivably have had a small (say 0.1 or 0.2w/m2) positive energy imbalance for centuries or millennia, the majority of the ocean warming is man-made. Since the 0.7w/m2 we currently have are estimated to cause an ocean expansion of about 0.8-1mm/year, this gives you a lower bound on how much of the acceleration is man-made. I say a lower bound because, on top of man-made ocean warming, you'd have to include the (increased) glacier melt and, perhaps, SLR from groundwater depletion (this is more speculative but according to some authors it may have added 0.2 or 0.3mm/year in the last decades).

David Young said...

Interesting post Cliff. It seems to me that given the last century of rising sea levels that there are some obvious actions to take. The Federal government could stop subsidizing flood insurance which would encourage people to stop rebuilding in low lying areas. Building codes in low lying areas have been changing too, but perhaps more needs to be done in this area too.

Bill Wise said...

In the chart depicting "global sea level rise over the past 140 years" there appears a decided inflection point somewhere 1930 to 1940 with a more rapid increase after that point. Am I misreading this chart or are we just seeing the inflection as insignificant?

Jack said...

We live on the cool crust of a molten planet that just won't sit still. Constant Continental Drift, tweaks in the orbit and axis of the Earth's orbit and the variable intensity of the Sun all have a major effect on our planet. Nothing is constant and that includes the ruler you use to measure the water level in one of two lifetimes.

ElenaW said...

Happy New Year! Some of us did miss you and just thought you were taking a well deserved vacation.

jimijr said...

Good summary. Key West is an especially good data point since the platform is neither rising nor falling and the term of record goes back to the 1870's. An inch per decade is what I tell folks. Doesn't sound like much but the 1-inch since Wilma occupies thousands of square miles and represents a huge volume of water which surely made Irma's surge worse when driven into the amplifying bays and inlets.

Colin said...

Isostatic rebound is negligible compared to tectonic uplift (1st paragraph of the 3rd page of the document Cliff posted). Perhaps a semantic point, if it wernt for implications from the potential "big one" earthquake instantly sinking the peninsula by 6ft then adding some 3 ft of sea level rise. A potential 9ft rise on the peninsula combining sea level rise and tectonics is significant.
Cheers!

Jim Steele said...

Steve Allowing says, "you'll see that sea level rise + current motion + wind forcing, places like the South Western Pacific, between Australia and Indonesia, are going to get absolutely clobbered. They're already experiencing 10+ mm/year of rise."

Please provide a link t that supports the 10+/yr claim.

Since 2010 that area has show a decline in sea level.

Cairns AU. http://www.psmsl.org/data/obtaining/rlr.monthly.plots/953_high.png

Weipa Au. http://www.psmsl.org/data/obtaining/rlr.monthly.plots/1157_high.png

Turtle Head Island. AU. http://www.psmsl.org/data/obtaining/rlr.monthly.plots/1749_high.png

PSMSL Sea level gauges do not show the most recent decades of change but researchers have reported sea level fall around Papua New Guinea causing coral mortality. https://www.biogeosciences.net/14/817/2017/

Jim Steele said...

The IPCC and sea level experts have bemoaned the lack of closure for sea level rise budgets, acknowledging about 25% can not be accounted for. For example the IPCC’s 4th Assessment estimated a 20th century sea level trend rising by 1.8 mm/year. However their summation of contributing factors, melting ice and thermal expansion, could only account for 1.1 mm/year


The steady rise of about 2 mm/yr obscures decades of sea level acceleration and deceleration. El Nino and La Nino driven circulation patterns can cause more rainfall to land on continents and drop sea level or more rain to fall back to the ocean and raise sea level. In 2010 to 2011 there was a 7mm drop in global sea level due to more rainfall shifting to endorheic basins.

That drop happened despite an increase in solar induced melting on Greenland associated with the North Atlantic Oscillation. Greenland ice sheet was considered to be in equilibrium from the 1960s to the 1990s and thus not a contributor to the 20th century steady rate of rise. Greenlands episodic melts offer better explanations of fluctuating rates ion rise and fall.

Similarly IPCC estimates of contributions from Antarctica ranged from gaining ice to losing ice . The most recent study by NASA's Zwally wrote in 2015 "Mass gains of the Antarctic ice sheet exceed losses" contending Antarctica has been gaining ice over all, with central and eastern Antarctic gains offsetting west Antarctic losses. https://ntrs.nasa.gov/search.jsp?R=20120013495

The biggest omission in sea level rise analyses is the mostly ignored contribution from ground water discharge. Groundwater stored during and since the last ice age has yet to reach an equilibrium with modern sea levels. Gleeson 2015 determined, “total [fresh] groundwater volume in the upper 2 km of continental crust is approximately 22.6 million cubic kilometers, of which 0.1–5.0 million cubic kilometers is less than 50 years old. If all that freshwater stored underground reached the oceans, sea level would rise 62,430 millimeters (204 feet), that’s 8 times any theoretical contribution from Greenland’s ice sheet if it melted completely.

The discharge of rainwater that has infiltrated the subsurface and comprises the upper shallow levels of an aquifer, will be affected by decadal climate changes associated with EL Nino and La Nina. However the discharge of deeper aquifers takes centuries and millennia and is unaffected by recent weather changes. Thus groundwater discharge is best candidate to explain the slow steady rise of sea level. Considering warming has been estimated to account for about 0.3 to 0.4 mm/year, deep groundwater discharge could explain about 1.5 mm/yr sea level rise.

Some researcher suggest a large contribution from melting glaciers but 1) meltwater does not magically end up in the ocean and often takes a slow route via the subsurface that may take centuries or more. SIgnificant portions of the melt water from glaciers of the Little Ice Age are just now reaching the ocean. And 2) most glaciers such as those in Glacier National Park lost ice at a greater rate during by the 1920s and 30s, than measured now. So even if meltwater was routed directly to the oceans, that contributing factor would be decreasing now.

Spencer said...

I think looking at longer time scales is important here. Compare the 2mm/year warming we're currently showing with this graph of sea levels since the last ice age. We notice:

1. Over the last 4000 years there has been more like 0.25mm/year rise, so the rate of increase is definitely increasing.

2. Sea level rise lags considerably behind temperature change. Global temperatures have been stable to within .5C for about 10 thousand years (graph), but the sea level continued rising rapidly until 7 thousand years ago. So the "natural" lag from temperature increases to sea level rise seems to be thousands of years.

I'm also not convinced by Cliff's argument that the trend over even the last 100 years is best fit by a linear line. There should definitely be an inflection point sometime between the 0.25mm/year of 2000BCE and the current 2mm/year, but I haven't seen a graph clearly showing that this inflection point is before the industrial revolution (and thus not anthropogenic).

Spencer said...

Cliff, as tedious as it may seem, I feel like you should start all these climate posts with a big disclaimer like "Climate change is real and is caused by humans!" Even if you disagree with the anthropogenic cause of a particular phenomenon (sea levels, forest fire frequency, etc), those who don't regularly read your blog could easily conclude that you disagree with the whole package.