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Initial Publication Date: December 7, 2016
Activate Your Learning: High-Frequency Sea Level Change
What do you see in the data? What trends can you describe? You should be able to identify at least 3 observable trends or patterns present in these data. These form the basis for the questions below.
Figure 4.24 (repeated): This diagram shows sea level position for the last five million years since the end of the Miocene and the beginning of the Pliocene. Sea level is established here using an "albedo proxy" to establish sea levels.
Credit: Root Routledge, created from source information and graphs from James Hansen publications. Copyright Root Routledge, but available for non-commercial distribution
Question 1 - Essay
The first interval or trend that might be recognized lasts from ~5.5 to ~3.5 mya - What is the nature of this interval? How would you describe sea-level variation during this period?
ANSWER:
Over this time period sea-level, although dynamic and with several scales of cycles, shows an overall average, long-term sea-level drawdown by a few meters to perhaps a almost 15 meters of sea-level fall (equal to about 10 to perhaps as much as 30 feet of long-term decline in sea-level). In shorter time scales, there are very short-term rise and fall events active on scales of a few thousand years, as well as cycles with durations of maybe a million years.
Question 2 - Essay
The second interval or trend that might be recognized lasts from ~3.5 to ~1 mya - What is the nature of this interval? How would you describe sea-level variation during this period?
ANSWER:
Over this time period the rate of fall over the full duration became much more pronounced (steeper slope) indicating that sea-levels fell at a faster rate than during the preceeding time interval. Moreover the range over which sea-levels rise and fall becomes much more substantial and drastic. In the short-term, water levels may have oscillated by as much as 75 meters (~150 feet) within a just a few 1000 years. The million year cycles are still evident but are obscured by the extreme volatility in the high-order cycles that operate on the finer time scales. With regard to modern sea-levels, rarely within this time interval were sea-levels higher than they are today. Knowing that we are currently in a deglaciating interval today, it is highly likely that much of this long-term sea-level decline is coupled to growth of ice sheets on land.
Question 3 - Essay
The third interval or trend that might be recognized lasts from ~1 my to ~11,000 kya - What is the nature of this interval? How would you describe sea-level variation during this period?
ANSWER:
For the last million years, data from this interval shows that sea-levels were primarily well-below the sea-levels of today. Extreme rates of rise and fall are noted with oscillations ranging up to 100 meters or more. The anatomy of these rise and fall events, when studied carefully shows lower rates of sea-level fall followed by more rapid sea-level rise events. This pattern will be explored in the coming section.
Question 4 - Essay
Over the last 5 million years (up to the beginning of the Holocene), how would you summarize the overall, integrated trend, into a cohesive statement about sea-level change?
ANSWER:
In a grand sense, the pattern of sea-level change has been one of significant volatility, but with an overall long-term average decline in sea-level. Moreover, for the last few million years sea-level has been, by far and large, much lower than the present sea-level and rarely within this interval has sea-level been much higher than it is today
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