Effects of Sea Level Decadal Variability on Acceleration and Trend Difference*

James Houston1 and Robert Dean2

¹ Army Engineer Research and Development Center, Corps of Engineers

2 Department of Civil and Coastal Engineering, University of Florida

* Based on Houston and Dean, 2012, Journal of Coastal Research (Accepted)

Definition of Acceleration

Time (Years)

Sea L

evel

Car going a steady 120 miles/hr

is moving fast, but not accelerating

Velocity = Trend = Rate of change of sea level

(mm/yr)

Acceleration = Rate of change of trend (mm/yr2)

Sea Level Rise not Constant in Time - Has Decadal

Variability Likely Caused by Weather Variability

Holgate (2007)

1.7 mm/yr

About 20-yr

oscillation

Sea Level Rise not Constant in Time - Has Decadal

Variability Likely Caused by Weather Variability

Holgate (2007)

Calculated Acceleration

Depends on Record Length

40 years

X

X

Sea Level Rise not Constant in Time - Has Decadal

Variability Likely Caused by Weather Variability

Holgate (2007)

Calculated Acceleration

Depends on Record Length

40 years

X

X

Sea Level Rise is Not Spatially Uniform Like a

Bathtub Filling with Water

Sea Level Change Is Not Spatially Uniform

Satellite altimeter measurements over 19 + years

3-4 times world

average

Sea level falling

Douglas (1992)

All tide gauge records in the

world with length ≥ 10 years

Sea level rise in 100

yrs if acceleration

persists

+ 5 m

- 5 m

+ 1 m

- 1 m

+10m

- 10m

+5m

- 5m

All tide gages in world (1123)

with lengths ≥ 10 yr

Houston and Dean (2012)

75 yrs

Douglas (1992) used only

records ≥ 75 years

Sea level rise in 100

yrs if acceleration

persists

Violating the Prime Directive

• PRIME DIRECTIVE - Use record lengths > ~ 75 yrs to

determine sea level acceleration and don’t trust results

where this criterion is violated

• Sallenger et al (2012) consider

40 - 60 yr records, leading

to invalid conclusions

• 900 world media outlets picked

up the Sallenger story of a “hot

spot” of sea level rise along the

east coast

• Rahmstorf and Vermeer (2011) use record lengths as short

as 30 yrs to erroneously claim their model and projections

superior to those of the Intergovernmental Panel on Climate

Change

Sallenger et al Use Trend Difference = T2-T1

20-Year Record

Trend Difference ~ Acceleration * Record Length

20-Year Record

Trend Difference Versus Record Length

Significant scatter even for 100-yr records

All tide gages in world (1123)

with lengths ≥ 10 yr

Sallenger et al (2012) Trend Difference for 40-Year Period Centered in 1990

New York “Hotspot”

Sallenger et al (2012)

Sallenger et al (2012) obtained

trend difference of 3 mm/yr for

40-yr period centered in 1990

They

projected

This trend

difference

to 2100

They

projected

This trend

difference

to 2100

Sallenger et al (2012) Trend Difference for 40-Year Period Centered in 1990

New York

Fernandinaa

Neutral

Sallenger et al (2012)

Galveston

“Cold”

Sallenger et al (2012)

San Francisco

“Cold”

Sallenger et al (2012)

Bromirski et al

(2011)

Sallenger et al (2012)

• Violate “prime directive” that requires record lengths > ~ 75

yrs to determine the underlying acceleration

• By taking short records, they just see

decadal variations, not underlying

trends

• It is not valid to project trend difference

based on one 40-yr period because

trend difference varies every year

• They say, “Trend differences for

windows longer than 72 yr were not

significantly different than zero.”

• OF COURSE! When they analyze records of proper length,

the east coast is not even a “lukewarm” spot of sea level

acceleration

Based on Church and

White (2006) worldwide data

Rahmstorf and Vermeer (2011)

Based on Rahmstorf

and Vermeer model

They calculate acceleration from beginning year to 2001 and

plot value at beginning year

~ 60 to 30-yr record lengths

>75 yr record lengths

Dots are worldwide data of

Church and White (2006)

1. Two Linear Trends

2. 20-yr decadal oscillation with 5-mm amplitude

3. 50-yr decadal oscillations with 4-mm amplitude

Trend 0.8 mm/yr

Trend 1.9 mm/yr

Synthetic Record

Trend change

~ 1926

Synthetic Record

Data of

Church

and White

Synthetic Record Versus Data

Trend Change

Decadal Variations

Data of

Church

and White

Acceleration due to

trend change

Decadal variations set to zero Dip from ~ 1895-1930 due primarily

to trend change in 1926

Rahmstorf and Vermeer claim the dip from ~ 1895 to 1930 is

due to temperature changing little from “1940 to 1980”

Acceleration Due to Trend Change

Zero after ~ 1926

Acceleration from

1870 – 2001 matches data

Data of Church

and White

Acceleration from 1940 due to

decadal variations

Trend change set to zero

Negative contribution

1917 - 1933

Acceleration Due to Decadal Variations

Acceleration from decadal variations

Decadal oscillations

and linear trends

Data of

Church

and White

Synthetic Record Versus Data

Linear trends and decadal oscillations explain dip

from ~ 1895-1930 and acceleration after 1940

Rahmstorf and Vermeer, 2011

• Violate the “prime directive” by considering records as

short as 30 – 60 yrs, which are dominated by decadal

variations

• The trend change + decadal oscillations from 1917 to 1933

produce the dip in acceleration from

~ 1895 to 1930

• Decadal variations cause the

acceleration beginning in ~ 1940

• Their model does not include the trend

change or decadal variations that

determine the curve they claim shows

their model to be superior to that of the

Intergovernmental Panel on Climate Change

Conclusions

• Tide records > ~ 75 yrs must be used to determine

acceleration because decadal variations

dominate shorter records and obscure

the underlying acceleration

• It is not valid to project sea level rise

based on short records

• Journal papers are not always correct

- Just require 2 of 3 reviewers’ approval

- The Intergovernmental Panel on

Climate Change (IPCC) has > 40

eminent authors who agree on sea

level projections and several hundred

reviewers conduct two detailed reviews

- Use sea level projections of IPCC (Sept, 2013)