Dr James Hansen presentation summary by Cris Vernon
- Dr James Hansen presentation summary by posted by Cris Vernon
Nov 22, 2006
Can We Still Avoid Dangerous Human-Made Climate Change?
Figures are at link (above) to Cris Vernon's article,
some of the figures are also at:
[Copied text only summary by Cris Vernon, below]
Dr James Hansen: Can We Still Avoid Dangerous Human-Made Climate
Posted by Chris Vernon November 22, 2006
Dr James Hansen, physicist, adjunct professor: Earth and Environmental
Sciences, Columbia University, director: NASA's Goddard Institute for
Space Science and their lead climate scientist spoke to a packed
lecture theatre at Bristol University on Friday (17Nov06). Outside the
scientific community Hansen is probably best known for accusing the
Bush administration of trying to silence him after he gave a lecture
December 2005 calling for prompt reductions in emissions of greenhouse
gases linked to global warming.
Hansen was speaking on climate change but I did have the opportunity
to ask him about peak oil.
Dr James Hansen (image source NYT)
Hansen believes there is a huge gap between what is understood about
global warming by the science community and what is known about it by
the people who need to know, that is the public and policy makers.
This belief has driven Hansen to communicate the science directly to
the public as frequently and as clearly as he is able. Why is this
important? Because we have just a decade to embark on a fundamentally
different path regarding our use of fossil fuels if we are to avoid
dangerous human-made climate change.
The world has experienced 0.8�C global average temperature rise over
the last century with 0.6�C of that occurring over the last 30 years.
Figure 1 illustrates the temperature anomalies of the 1st half decade
of this century over the 1951-�80 average.
It is exactly what one would expect from forced climate change, the
increase is larger over the land due to the thermal inertia of the
oceans and it is larger at higher latitudes than low latitudes due to
Forcings and the paleoclimate
Hansen�s expertise lies partially in radiative transfer in planetary
atmospheres, a large part of his lecture addressed climatic forcing.
Looking back at paleoclimatic data enables an understanding of
climatic forcing to be developed. Figure 2 shows the familiar Vostok
ice core temperature data, illustrating the variation between the last
ice age and the start of the Holocene, two periods when the planet was
in radiation balance.
The reason for the temperature difference during these two periods?
The climate forcing, mainly of increased ice sheets and reduced
greenhouse gas concentrations. Figure 3 calculates the global
equilibrium climatic sensitivity to climate forcing producing a result
of ~3/4�C per W/m2.
This result is easily checked back through time as we have pretty
accurate (�12m on a variance of 100m) historic sea level data from
which ice sheet area can be calculated and the ice cores provide
historic greenhouse gas concentrations. He didn�t mention it but I
gather it is trivial to work out the forcing (W/m2) from ice sheet
area and greenhouse gas concentrations. Given these forcings and the
previous results of 3/4�C per W/m2, global temperatures can be
calculated. Comparing this calculation with the observed temperatures
from ice cores shows how just these two forcings account remarkably
well for the temperature changes.
Hansen made another point about the temperature changes and greenhouse
gas concentration changes, specifically the temporal relationship
between them. Data on both can be obtained from the same ice core with
high relative temporal accuracy, with only a need to correct for the
time it takes between fluffy snow falling and solid ice forming
impervious bubbles (the air is younger than the ice, the temperature
data comes from the ice and the gas data comes from the trapped air).
Comparing the temperature change and gas concentrations Hansen said:
The correlation is maximum when there is a 700 year lag, the
temperature leads the greenhouse gas change. So the greenhouse gas
changes are a feedback of the climate change.
That was news to me.
The conclusions from this part of the lecture were that greenhouse
gases and ice area are the chief mechanisms for paleoclimatic changes,
however they were �merely� feedbacks from the instigators of climate
change which Hansen describes as orbital variations, other small
forcings and chaos, stressing the fact that long term climate is very
sensitive to very small forcings.
The system is different now though as humans have taken control of one
of the main mechanisms through our emissions of greenhouse gases.
At this point Hansen criticised Al Gore�s presentation in An
Inconvenient Truth of current greenhouse gas concentrations in
comparison to paleo concentrations. Saying Gore was wrong to suggest
the temperature change we are likely to experience from current
concentrations is proportional to that seen in the paleoclimate. Those
paleoclimate changes were predominately driven by ice cover, which as
long as Greenland and Antarctica stay roughly the same size isn�t a
major factor now and also aerosols which had scope to reduce their
cooling contribution as climate changed from cold/dry=dusty to warm
Dangerous Climate Change
So what is dangerous climate change? Surprisingly there doesn�t seem
to be very much research on this. Hansen suggested the following
metrics to characterise climate change:
* Extermination of animal and plant species, specifically polar
and alpine species and those suffering unsustainable migration rates.
* Ice sheet disintegration, leading to long term deviations from
paleoclimate data and sea level rise.
* Regional changes including droughts and floods.
For the last 30 years temperatures have risen by 0.2�C per decade.
Over northern hemisphere land areas a given isotherm is moving forward
at a rate of 50km per decade. A study of 1,700 species found that in
the last half of the 20th century the average migration rate forward
was about 6km per decade, much slower than the rate the isotherms were
Hansen suggested a 3�C warming from where we are now would result in a
likely species extinction rate of 50%. 3�C was described as the
business as usual scenario. The �Alternative� scenario with falling
CO2 emissions and only 1�C temperature increase would result in a
likely species extinction rate of 10%.
Greenland received particular attention. Three points:
* The area of Greenland experiencing summer melting is increasing
at around 0.7% per year with 2005 setting the record melt. This melt
water serves as lubrication speeding up the transport of ice to the
* The GRACE satellite can measure the mass of Greenland ice sheet
and is showing a reduction of 162�22km3/yr over recent years.
* The number of ice-quakes, similar to earthquakes and caused by
ice sheets suddenly surging forward, has increased markedly. The
seismic magnitudes are in the range 4.5 to 5.1 and they are located at
the mouths of ice flows discharging ice into the ocean. See figure 4.
Hansen believes the IPCC are conservative with their estimations for
sea level rise as they only consider thermal expansion and alpine
glacier melt, believing the major ice sheets to be in approximate mass
equilibrium and taking millennia to respond. He suggested ice sheet
disintegration may start slowly but multiple positive feedbacks can
lead to rapid non-linear collapse, noting the equilibrium sea level
rise for 3�C warming is 25�10m.