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New Study on sea level rise find ‘no compelling evidence that recent rates of sea level rise are abnormal’

Special Report on Sea Level Rise

by Judith Curry

I have now completed my assessment of sea level rise and climate change.

The complete report can be downloaded here [ Special Report- Sea Level Rise].

My preliminary compilation of information was provided in the 7 part Climate Etc. series Sea level rise acceleration (or not).

This report reflects 18 months of work on this topic. Why have I devoted so much time to the sea level rise issue? First, I regard sea level rise to be the most consequential potential impact of predicted global warming. Second, there is a great deal of public confusion about the issue, including decision makers. Third, a number of CFAN’s clients have queried me about a range of specific concerns that they have regarding sea level rise (and I have been doing consulting on this topic).

Why do I think an independent assessment of the sea level rise issue by yours truly is needed, given the plethora of international and national assessment reports? My clients are concerned about the alarmist predictions they have encountered. I have seen various ‘experts’ make public statements projecting 21stcentury sea level to be as high as 9 m [30 feet]. My clients are looking for someone that they trust to provide an objective assessment that focuses on their issues of concern.

I am not a published expert on sea level rise, although I have published some relevant papers in oceanography and the climate dynamics of the polar regions. What I bring to this assessment is a broader perspective on the issues of climate dynamics, climate modeling and uncertainty than most of the community working on the sea level rise issue. In any event, it is arguably useful for a knowledgeable person outside of the publishing sea level community to provide an independent assessment.

The process that I used in preparing this Report was to:

  • read in detail the relevant international and national assessment reports,
  • conduct an extensive literature survey of recent publications (twitter has been invaluable for this),
  • post my initial chapters on the blog for comments and references that I might have missed, and
  • elicit comments on a draft of the report from several different clients regarding readability and needed clarifications.

My report provides a different way of framing the sea level rise issue and of interpreting the evidence, in a way that I don’t think is inconsistent with the evidence in the mainstream sea level rise assessments.

While on the plane last week headed for Thanksgiving travel, I received an email from Roger Caizza, informing me that a draft was available for review of the IPCC Special Report on Ocean and Cryosphere in a Changing Climate. Since I am a registered reviewer with U.S. Global Change Research Program, I signed up to review this Report. I did a quick read of chapters 3 and 4 over the weekend, and I saw nothing in this Report that made me feel uncomfortable about my own Report. Later next week, I will do a more careful read and submit comments. I will obviously not make any public comments on the IPCC draft, other than to say I was fairly impressed with what I read.

A few comments are in order regarding my impressions of the sea level and cryosphere communities. I know very few of these individuals personally (and then in only the most casual way). I have to say that I am quite impressed overall with this community. They have made a massive amount of progress since the IPCC AR5, particularly in understanding the ice sheets. They ‘get’ the importance of natural internal variability, deep uncertainty and worst-case scenarios. That said, the way all this gets portrayed in the media does not provide a balanced perspective or one that is useful for decision makers.

Report summary

The overall report is 80 pages long, with 7 pages of that being the reference list. The focus of the report is on the 21st century, with one Chapter specific to U.S. coastal sea level rise.

Here is the text of the introduction, minus the references and footnotes.

The alarm over sea level rise

The public discourse on the threat of sea level rise is typified by these dire statements from climate scientists:

“That’s the big thing – sea-level rise – the planet could become ungovernable.” – Dr. James Hansen, former Director, NASA GISS

“We’re talking about literally giving up on our coastal cities of the world and moving inland.” – Dr. Michael Mann, Penn State 

 The alarm over sea level rise is not so much about the 7-8 inches or so that global sea level has risen since 1900. Rather, it is about projections of 21stcentury sea level rise from human-caused global warming.

This Report refers extensively to the Assessment Reports prepared by the Intergovernmental Panel on Climate Change (IPCC), since these Reports are used to guide policies developed by the UN Framework Convention on Climate Change, including the 2015 Paris Agreement.

According to the IPCC, the projected 21st century sea level rise depends on the amount of greenhouse gas emissions. The likely range of projected sea level rise by the end of the 21st century is from 0.26 to 0.82 m [10 to 32 inches], depending on the emissions scenario.

The primary concern over future sea level rise is related to the potential collapse of the West Antarctic Ice Sheet, which could cause global mean sea level to rise substantially above the IPCC’s likely range in the 21st century. The IPCC AR5 has medium confidence that this additional contribution from the West Antarctic ice sheet would not exceed several tenths of a meter [less than a foot] of sea level rise during the 21st century.

Subsequent to the 2013 IPCC AR5, there has been a focus on the possible worst-case scenario for global sea level rise. Estimates of the maximum possible global sea level rise by the end of the 21st century range from 1.6 to 3 meters [5-10 feet], and even higher. These extreme values of possible sea level rise are regarded as extremely unlikely or so unlikely that we cannot even assign a probability. Nevertheless, these extreme, barely possible values of sea level rise are now becoming anchored as outcomes that are driving local adaptation plans [link].

Is the alarm over sea level rise a ‘false alarm,’ or not? The following four issues frame this report:

  1. Whether recent global sea level rise is unusual in context of the historical and geological record.
  2. The extent to which recent global sea level rise is caused by human-caused global warming, relative to natural causes of global sea level rise.
  3. The extent to which local sea level rise is influenced by the global sea level rise, relative to local vertical land motion and land use practices.
  4. Projections of sea level rise (global and local) for the 21st century, from all causes.

This Report critically evaluates the assessment and conclusions from the IPCC and other recent assessment reports regarding sea level rise, and includes an assessment of recent research and the knowledge frontiers. The IPCC and other assessment reports have been framed around assessing support for the hypothesis of human-caused climate change. As a result, natural processes of climate variability have been relatively neglected in these assessments. Arguments are presented here supporting the important and even dominant role that natural processes play in global and regional sea level variations and change.

Understanding and predicting sea level rise is a vibrant and active area of research. The challenges and uncertainties are well recognized by international scientific community, as formulated by the World Climate Research Programme (WCRP) Grand Challenge on Regional Sea Level Change and Coastal Impacts.


Here are my conclusions:

Mean global sea level has risen at a slow creep for more than 150 years; since 1900, global mean sea level has risen about 7-8 inches. The implications of the highest values of projected sea-level rise under future climate change scenarios are profound, with far reaching socioeconomic and environmental implications. However, these projections are regarded as deeply uncertain and the highest of these projections strain credulity.

The IPCC and other assessment reports are framed around providing support for the hypothesis of human-caused climate change. As a result, natural processes of climate variability have been relatively neglected in these assessments. Arguments have been presented here supporting the important and even dominant role that natural processes play in global and regional sea level variations and change.

With regards to the four issues raised in the Introduction:

  1. Is the recent sea level rise (since 1993) of magnitude 3 mm/year unusual?

No, although this conclusion is conditional on the quality of the global sea level data. The available evidence shows the following:

  • Sea level was apparently higher than present at the time of the Holocene Climate Optimum (~ 5000 years ago), at least in some regions.
  • Tide gauges show that sea levels began to rise during the 19th century, after several centuries associated with cooling and sea level decline. Tide gauges also show that rates of global mean sea level rise between 1920 and 1950 were comparable to recent rates.
  • Recent research has concluded that there is no consistent or compelling evidence that recent rates of sea level rise are abnormal in the context of the historical records back to the 19th century that are available across Europe.
  1. Has recent global sea level rise been caused by human-caused global warming?

 Identifying a potential human fingerprint on recent sea level rise is confounded by the large magnitude of natural internal variability associated with ocean circulation patterns. There is not yet convincing evidence of a fingerprint on sea level rise associated with human-caused global warming:

  • The slow emergence of fossil fuel emissions prior to 1950 did not contribute significantly to sea level rise observed in the 19th and early 20th centuries.
  • The recent acceleration in mean global sea level rise (since 1995) is caused by mass loss from Greenland that appears to have been larger during the 1930’s, with both periods associated with the warm phase of the Atlantic Multidecadal Oscillation.
  1. To what extent is local sea level rise influenced by global sea level rise?

In many of the most vulnerable coastal locations, the dominant causes of local sea level rise are natural oceanic and geologic processes and land use practices. Land use and engineering in the major coastal cities have brought on many of the worst problems, notably landfilling in coastal wetland areas and groundwater extraction.

  1. How much will sea level rise in the 21st century?

Local sea level in many regions will continue to rise in the 21st century – independent of global climate change.

Emissions scenario choice exerts a great deal of influence on predicted sea level rise after 2050. If RCP8.5 is rejected as an extremely unlikely or impossible scenario, then the appropriate range of sea level rise scenarios to consider for 2100 is 0.2–1.6 m. Values exceeding 2 feet are increasingly weakly justified. Values exceeding 1.6 m require a cascade of extremely unlikely to impossible events, the joint likelihood of which is arguably impossible.

Further, these values of sea level rise are contingent on the climate models predicting the correct amount of temperature increase. There are numerous reasons to think that the climate models are predicting too much warming for the 21st century, and hence the more extreme values of sea level rise (above 1 m) are arguably too high.

Kopp et al. (2017) state:

“The breadth of published projections, as well as of remaining structural uncertainties, highlight the fact that future sea-level rise remains an arena of deep uncertainty.”

Climate-related decisions involve incomplete information from a fast-moving and irreducibly uncertain science. The challenges of understanding the causes of sea level rise and projecting future climate change and sea level rise are well-recognized by the international community of climate and sea level researchers, as summarized in the World Climate Research Programme (WCRP) Grand Challenges. The WCRP Grand Challenge on Sea Level Rise states:

“Despite considerable progress during the last decade, major gaps remain in our understanding of past and contemporary sea level change and their causes. These uncertainties arise from limitations in our current conceptual understanding of relevant physical processes, deficiencies in our observing and monitoring systems, and inaccuracies in statistical and numerical modelling approaches to simulate or forecast sea level.”

“A significant part of this large uncertainty arises from inappropriate (or sometimes missing) model representations of some physical processes that affect sea level and needs to be reduced for more accurate sea level projections.”

“Improved sea level predictions/projections, particularly over the next decades, are critically dependent on understanding observed natural variability, accurately reproducing it in models, and mapping its future behavior under climate change.”


I’m publishing this through my company Climate Forecast Applications Network(CFAN) as a Special Report. I regard the currently posted version to be final, although I will correct any errors that are spotted.

I considered publishing this in some other venue, but it wasn’t obvious as to where/how I might publish this, and I didn’t want to wrestle with copyright issues. Plus I wanted to make sure this is publicly available, and hope that it will reach a wide audience.

I encourage you to read the entire report. I look forward to your comments, and would like to thank you again for comments on the original Sea level rise (acceleration or not) blog posts.

Conflicts of interest statement

I imagine that the first reaction to this Report from the activist wing will be to question my ‘motives.’ Well my motives are the same that they have always been: sound, evidence-based science and helping decision makers make effective use of that science.

From CE’s About page:

Funding disclosure:  Funding sources for my research  have included NSF, NASA, NOAA, DOD and DOE. Recent government contracts for CFAN include a DOE contract to develop extended range regional wind power forecasts, a DOD contract to predict extreme events associated with climate variability/change having implications for regional stability, and a NOAA contract to improve sub seasonal forecasting. CFAN contracts with private sector and other non-governmental organizations include energy and power companies, reinsurance companies, financial companies, other weather service providers, NGOs, development banks and government agencies.

Specifically with regards to the issue of sea level rise, CFAN’s clients are in the insurance, energy and public sectors.  As a matter of company policy, CFAN does not name its clients.

If anyone thinks that I can be bought or otherwise influenced, they should think harder. Many have tried, none have succeeded.