[Vision2020] Pubs.GISS: Hansen & Sato July 2011, in press: Paleoclimate implications for human-made climate change

[Ted Moffett]

Goddard Institute for Space Studies Publication Abstracts
Hansen and Sato 2011, in press

http://pubs.giss.nasa.gov/abs/ha05510d.html
---------------------
Science Briefs

7-20-2011

Earth's Climate History: Implications for Tomorrow

http://www.giss.nasa.gov/research/briefs/hansen_15/

By James E. Hansen and Makiko Sato — July 2011

The past is the key to the future. Contrary to popular belief, climate
models are not the principal basis for assessing human-made climate
effects. Our most precise knowledge comes from Earth's paleoclimate,
its ancient climate, and how it responded to past changes of climate
forcings, including atmospheric composition. Our second essential
source of information is provided by global observations today,
especially satellite observations. which reveal how the climate system
is responding to rapid human-made changes of atmospheric composition,
especially atmospheric carbon dioxide (CO2). Models help us interpret
past and present climate changes, and, in so far as they succeed in
simulating past changes, they provide a tool to help evaluate the
impacts of alternative policies that affect climate.

Paleoclimate data yield our best assessment of climate sensitivity,
which is the eventual global temperature change in response to a
specified climate forcing. A climate forcing is an imposed change of
Earth's energy balance, as may be caused, for example, by a change of
the sun's brightness or a human-made change of atmospheric CO2. For
convenience scientists often consider a standard forcing, doubled
atmospheric CO2, because that is a level of forcing that humans will
impose this century if fossil fuel use continues unabated.

We show from paleoclimate data that the eventual global warming due to
doubled CO2 will be about 3°C (5.4°F) when only so-called fast
feedbacks have responded to the forcing. Fast feedbacks are changes of
quantities such as atmospheric water vapor and clouds, which change as
climate changes, thus amplifying or diminishing climate change. Fast
feedbacks come into play as global temperature changes, so their full
effect is delayed several centuries by the thermal inertia of the
ocean, which slows full climate response. However, about half of the
fast-feedback climate response is expected to occur within a few
decades. Climate response time is one of the important 'details' that
climate models help to elucidate.

We also show that slow feedbacks amplify the global response to a
climate forcing. The principal slow feedback is the area of Earth
covered by ice sheets. It is easy to see why this feedback amplifies
the climate change, because reduction of ice sheet size due to warming
exposes a darker surface, which absorbs more sunlight, thus causing
more warming. However, it is difficult for us to say how long it will
take ice sheets to respond to human-made climate forcing because there
are no documented past changes of atmospheric CO2 nearly as rapid as
the current human-made change.

Humans lived in a rather different world during the last ice age,
which peaked 20,000 years ago. An ice sheet covered Canada and parts
of the United States, including Seattle, Minneapolis and New York
City. The ice sheet, more than a mile thick on average, would have
towered over today's tallest buildings. Glacial-interglacial climate
oscillations were driven by climate forcings much smaller than the
human-made forcing due to increasing atmospheric CO2 — but those weak
natural forcings had a long time to operate, which allowed slow
climate feedbacks such as melting or growing ice sheets to come into
play

Ice sheet response to climate change is a problem where satellite
observations may help. Also ice sheets models, as they become more
realistic and are tested against observed ice sheet changes, may aid
our understanding. But first let us obtain broad guidance from climate
changes in the 'recent' past: the Pliocene and Pleistocene, the past
5.3 million years.

Figure 1 shows global surface temperature for the past 5.3 million
years as inferred from cores of ocean sediments taken all around the
global ocean. The last 800,000 years are expanded in the lower half of
the figure. Assumptions are required to estimate global surface
temperature change from deep ocean changes, but we argue and present
evidence that the ocean core record yields a better measure of global
mean change than that provided by polar ice cores.

Civilization developed during the Holocene, the interglacial period of
the past 10,000 years during which global temperature and sea level
have been unusually stable. Figure 1 shows two prior interglacial
periods that were warmer than the Holocene: the Eemian (about 130,000
years ago) and the Holsteinian (about 400,000 years ago). In both
periods sea level reached heights at least 4-6 meters (13-20 feet)
greater than today. In the early Pliocene global temperature was no
more than 1-2°C warmer than today, yet sea level was 15-25 meters
(50-80 feet) higher.

The paleoclimate record makes it clear that a target to keep human
made global warming less than 2°C, as proposed in some international
discussions, is not sufficient — it is a prescription for disaster.
Assessment of the dangerous level of CO2, and the dangerous level of
warming, is made difficult by the inertia of the climate system. The
inertia, especially of the ocean and ice sheets, allows us to
introduce powerful climate forcings such as atmospheric CO2 with only
moderate initial response. But that inertia is not our friend — it
means that we are building in changes for future generations that will
be difficult, if not impossible, to avoid.

One big uncertainty is how fast ice sheets can respond to warming. Our
best assessment will probably be from precise measurements of changes
of the mass of the Greenland and Antarctic ice sheets, which can be
monitored via measurements of Earth's gravitational field by
satellites.

Figure 2 shows that both Greenland and Antarctic ice sheets are now
losing mass at significant rates, as much as a few hundred cubic
kilometers per year. We suggest that mass loss from disintegrating ice
sheets probably can be approximated better by exponential mass loss
than by linear mass loss. If either ice sheet were to lose mass at a
rate with doubling time of 10 years or less, multi-meter sea level
rise would occur this century.

The available record (Fig. 2) is too brief to provide an indication of
the shape of future ice mass loss, but the data will become extremely
useful as the record lengthens. Continuation of these satellite
measurements should have high priority.

References

Hansen, J.E., and Mki. Sato, 2011: Paleoclimate implications for
human-made climate change. In Climate Change at the Eve of the Second
Decade of the Century: Inferences from Paleoclimate and Regional
Aspects: Proceedings of the Milutin Milankovitch 130th Anniversary
Symposium. A. Berger, F. Mesinger, and D. Šijači, Eds. Springer,
submitted.

Velicogna, I., 2009: Increasing rates of ice mass loss from the
Greenland and Antarctic ice sheets revealed by GRACE. Geophys. Res.
Lett., 36, L19503, doi:10.1029/2009GL040222.

Contact
Please address all inquiries about this research to Dr. James Hansen.

------------------------------------------
Vision2020 Post: Ted Moffett