If sea levels rose to where they were during the Last Interglacial Period, large parts of the Gulf of Mexico region would be under water (red areas), including half of Florida and several Caribbean islands. Image: Jeremy Weiss |
Melting
ice sheets contributed much more to rising sea levels than thermal
expansion of warming ocean waters during the Last Interglacial Period, a
UA-led team of researchers has found. The results further suggest that
ocean levels continue to rise long after warming of the atmosphere
levels off.
Thermal
expansion of seawater contributed only slightly to rising sea levels
compared to melting ice sheets during the Last Interglacial Period, a
University of Arizona-led team of researchers has found.
The
study combined paleoclimate records with computer simulations of
atmosphere-ocean interactions and the team’s co-authored paper is
accepted for publication in Geophysical Research Letters.
As
the world’s climate becomes warmer due to increased greenhouse gases in
the atmosphere, sea levels are expected to rise by up to three feet by
the end of this century.
But
the question remains: How much of that will be due to ice sheets
melting as opposed to the oceans’ 332 million cubic miles of water
increasing in volume as they warm up?
For
the study, UA team members analyzed paleoceanic records of global
distribution of sea surface temperatures of the warmest 5,000-year
period during the Last Interglacial, a warm period that lasted from
130,000 to 120,000 years ago.
The
researchers then compared the data to results of computer-based climate
models simulating ocean temperatures during a 200-year snapshot as if
taken 125,000 years ago and calculating the contributions from thermal
expansion of sea water.
The
team found that thermal expansion could have contributed no more than
40 cm – less than 1.5 feet – to the rising sea levels during
that time, which exceeded today’s level up to eight meters or 26 feet.
At
the same time, the paleoclimate data revealed average ocean
temperatures that were only about 0.7 C, or 1.3 F, above those of today.
“This
means that even small amounts of warming may have committed us to more
ice sheet melting than we previously thought. The temperature during
that time of high sea levels wasn’t that much warmer than it is today,”
says Nicholas McKay, a doctoral student at the UA’s department of
geosciences and the paper’s lead author.
McKay
pointed out that even if ocean levels rose to similar heights as during
the Last Interglacial, they would do so at a rate of up to three feet
per century.
“Even
though the oceans are absorbing a good deal of the total global
warming, the atmosphere is warming faster than the oceans,” McKay adds.
“Moreover, ocean warming is lagging behind the warming of the
atmosphere. The melting of large polar ice sheets lags even farther
behind.”
“As
a result, even if we stopped greenhouse gas emissions right now, the
Earth would keep warming, the oceans would keep warming, the ice sheets
would keep shrinking, and sea levels would keep rising for a long time,”
he explains.
“They
are absorbing most of that heat, but they lag behind. Especially the
large ice sheets are not in equilibrium with global climate,” McKay
adds.
Jonathan
Overpeck, co-director of the UA’s Institute of the Environment and a
professor with joint appointments in the department of geosciences and
atmospheric sciences, says: “This study marks the strongest case yet
made that humans – by warming the atmosphere and oceans – are pushing
the Earth’s climate toward the threshold where we will likely be
committed to four to six or even more meters of sea level rise in coming
centuries.”
Overpeck,
who is McKay’s doctoral advisor and a coauthor of the study, adds:
“Unless we dramatically curb global warming, we are in for centuries of
sea level rise at a rate of up to three feet per century, with the bulk
of the water coming from the melting of the great polar ice sheets –
both the Greenland and Antarctic Ice Sheets.”
According
to the authors, the new results imply that 4.1 to 5.8 meters, or 13.5
to 19 feet, of sea level rise during the Last Interglacial period was
derived from the Antarctic Ice Sheet, “reemphasizing the concern that
both the Antarctic and Greenland Ice Sheets may be more sensitive to
warming temperatures than widely thought.”
“The
central question we asked was, ‘What are the warmest 5,000 years we can
find for all these records, and what was the corresponding sea level
rise during that time?'” McKay says.
Evidence
for elevated sea levels is scattered all around the globe, he added. On
Barbados and the Bahamas, for example, notches cut by waves into the
rock six or more meters above the present shoreline have been dated to
being 125,000 years old.
“Based
on previous studies, we know that the sea level during the Last
Interglacial was up to 8.5 meters higher than today,” McKay explains.
“We
already knew that the vast majority came from the melting of the large
ice sheets in Greenland and Antarctica, but how much could the expansion
of seawater have added to that?”
Given
that sea surface temperatures were about 0.7 degrees warmer than today,
the team calculated that even if the warmer temperatures reached all
the way down to 2,000 meters depth – more than 6,500 feet, which is
highly unlikely – expansion would have accounted for no more than 40
centimeters, less than a foot and a half.
“That
means almost all of the substantial sea level rise in the Last
Interglacial must have come from the large ice sheets, with only a small
contribution from melted mountain glaciers and small ice caps,” McKay
says.
According
to coauthor Bette Otto-Bliesner, senior scientist at the National
Center for Atmospheric Research (NCAR) in Boulder, Colo., getting the
same estimate of the role ocean expansion played on sea level rise
increases confidence in the data and the climate models.
“The
models allow us to attribute changes we observe in the paleoclimate
record to the physical mechanisms that caused those changes,”
Otto-Bliesner says. “This helps tremendously in being able to
distinguish mere correlations from cause-and-effect relationships.”
The
authors cautioned that past evidence is not a prediction of the future,
mostly because global temperatures during the Last Interglacial were
driven by changes in the Earth’s orbit around the sun.
However, current
global warming is driven by increasing greenhouse gas concentrations.
The
seasonal differences between the northern and the southern hemispheres
were more pronounced during the Last Interglacial than they will be in
the future.
“We
expect something quite different for the future because we’re not
changing things seasonally, we’re warming the globe in all seasons,”
McKay says.
“The
question is, when we think about warming on a global scale and
contemplate letting the climate system change to a new warmer state,
what would we expect for the ice sheets and sea levels based on the
paleoclimate record? The Last Interglacial is the most recent time when
sea levels were much higher and it’s a time for which we have lots of
data,” McKay adds.
“The
message is that the last time glaciers and ice sheets melted, sea
levels rose by more than eight meters. Much of the world’s population
lives relatively close to sea level. This is going to have huge impacts,
especially on poor countries,” he adds.
“If
you live a meter above sea level, it’s irrelevant what causes the rise.
Whether sea levels are rising for natural reasons or for anthropogenic
reasons, you’re still going to be under water sooner or later.”
