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2.3 Scientific Uncertainty
and Security Risks of
Climate Change
Dr. Jay Gulledge
Let me begin by saying that it is a pleasure to be here. I think
this is a really important audience and I am particularly gratified
to see so much attention on this topic coming from the Navy and
the extended community that is working with the Navy on climate
energy issues as well as the rest of the Department of Defense
(DoD) and its civilian leadership.
I want to talk about scientific uncertainty. I think that when
most people hear scientists talk about climate change or get a scientific overview, which this is not going to be, they do not get much
focus on uncertainty, but I think it is absolutely critical because
uncertainty is very tightly linked with risk. That is really what this
topic is all about. The security implications of climate change are
about risk and how we are going to manage that risk.
Dr. Jay Gulledge is a Senior Scientist and Director for Science and
Impacts at the Pew Center on Global Climate Change. He is also a
non-resident Senior Fellow at the Center for a New American Security
(CNAS), an Adjunct Associate Professor at the University of Wyoming,
and a certified ecologist. His work informs environmental and national security policymakers, business leaders, the public, and the press
about science and the impacts of climate change. Dr. Gulledge earned
B.S. and M.S. degrees in biology from the University of Texas and a
Ph.D. in biological sciences from the University of Alaska at Fairbanks.
Before joining Pew Center and CNAS, he served on the faculties of
Tulane University and the University of Louisville, where he developed
courses on global environmental change and ecology. His academic
research investigates how environmental change affects the cycling of
greenhouse gases between terrestrial ecosystems in the atmosphere.
He serves on the editorial board of the Ecological Society of America
and currently serves on the Executive Council of Global Environmental
Change Focus Group in the American Geophysical Union.
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Risk perception is the key to figuring out how we are going
to manage this type of risk. Climate change is very complex, nonlinear, long-term, intergenerational. It is about as difficult and
uncertain a problem as you can find, and it is loaded with risk.
So a risk-management framework is really the appropriate way of
approaching the problem of climate change.
Unfortunately, uncertainty seems to tempt people to ignore risk.
However, if you take a rational look at the uncertainties underlying
climate change, it actually should turn you toward paying more
attention to risk. I think this is also true of the scientific community, which, by and large, is not accustomed to thinking about risk.
Many of the methods of science cut against thinking about risk
because they tend to average over long periods of time and over
large areas of space. A lot of things that entail risk get averaged out
when scientists are working on climate.
For a topic like this, what we really need science to be focusing on, in addition to the basic process of improving understanding, is bringing out and exposing and analyzing the risks in climate
change. One reason that risk perception is difficult, and this goes
back to the averaging problem, is that it is hard to grasp the risk
when there is a heavy focus on averages and changes in averages
as opposed to changes in climate extremes, because much of the
risk is loaded into the extremes. We know this from our past experience. You do not have to have climate change to know that it is
the extremes that cause damage.
If we have an idealized distribution of a climate event—intensity of rainfall or distribution of temperatures, for example—we
know that it is the extreme events, the one in 40 years, one in
50 years events—that cause the most damage: the Dust Bowl type of events; once in a century, perhaps. But with climate change, this
distribution is going to move. By focusing on the middle, the center
line, we miss the fact that a one in 40 years event actually now
becomes a one in 6 years event. It is the same intensity of event.
But now instead of happening once in every one or two generations, it happens several times in a generation. But not only that,
our new one in 6 years event is more damaging than the old one
in 40 years event. This is the type of focus that we need to bring
Chapter 2 Climate Imperatives
49
to the scientific assessment of climate change in order to better
understand the risks, but this is just one aspect of it.
I want to provide a quick case study of how risk perception
has played a role in the way scientists think of the risks of climate
change. Ten years ago, the Intergovernmental Panel on Climate
Change (IPCC) developed this little device here to help policymakers think about what might constitute dangerous interference with
the climate system (Figure 1).
Figure 1. IPCC Five “Reasons for Concern”
Developed to Characterize Risk1
They came up with five categories of climate impacts or risks
that they thought might be of interest to the policymakers. These
included risk to unique and threatened systems like the Great
Dark is more dangerous; light is less so. Included in the IPCC Third Assessment
Report (TAR) in 2001 but not in the Fourth Assessment Report (AR4) in
2007. [1, 2] Updated after AR4 by Smith et al. in 2009. [3]
1
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Barrier Reef or the city of New Orleans as well as risks of extreme
weather events. They also identified distribution of impacts as
a factor. Is it going to hurt poor countries more than rich countries, poor people within a country? Yet another was distribution
through space and time. You could even think of it across generations. Another was aggregate impact; what is the total number of
people exposed to water stress or exposed to damages? And then
risk of large-scale discontinuities like a massive sea-level rise or
shutdown of the Atlantic thermohaline circulation.
The dark color shown on the vertical axis reflects warming
beyond the year 1990 temperature. Lighter shading indicates some
risk, while darker implies significant risk. This is just a qualitative
assessment by a panel of experts as to how much warming constitutes risk for these categories. This was done about 10 years ago
based on the information available for the IPCC TAR.
Figure 2 shows a recent update after about 10 more years of
information, where we had much more climate data analyzed. We
had events like Hurricane Katrina. We had a very deadly heat wave
in Europe in 2004 that killed tens of thousands of people in rich
countries. Based on a decade of additional information, the same
group of experts redrew the chart like so. And, of course, what you
see immediately is that the colors come down farther on the chart
saying that risks kick in at lower temperatures than these experts
thought before. Now, this is just based on their change of perception, based on the information that became available. How we
look at the type of information that is available is really key to this
perception.
Now, I am going to go quickly through the kinds of things that
we really have a good handle on in climate science as a foundation for saying we know we have risk and we have got to manage
this problem. In 2007 (in the AR4), the IPCC concluded that the
warming of the climate system is unequivocal. The evidence is
very strong. We have thermometer measurements over 150 years.
We have a synchronous retreat of mountain glaciers around the
world. We have decline of ice and snow pack all over the world.
We have acceleration of global sea-level rise. None of those things
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Figure 2. Proposed AR4 “Reasons for Concern” [3]
can happen without a net increase in the temperature of the Earth’s
surface.
The IPCC also concluded that it is very likely, which they define
as greater than a 90% chance, that since the mid-20th century, that
warming of the climate system is mostly caused by accumulation
of manmade greenhouse gases in the atmosphere. We also have
good confidence about a number of other things. Changes are
already underway. The impacts are occurring. A report of the U.S.
Global Change Research Program from last summer said that that
is true in the United States. The temperature change of the 21st
century is very likely to be larger than what we have seen in the
20th century.
After a degree or two of warming globally, the net effects are
likely to be negative around the world, even if there are some benefits. So it is possible that adaptive capacity could be overwhelmed
by unmitigated warming. Both developed and developing countries are vulnerable. That is a relatively recent finding. We had
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this ongoing assumption that rich countries were really not very
vulnerable.
But that is a lot of information. It does tell us that we have this
risk and it needs to be managed. But it does not really tell us the
specifics of what that means. What does it entail to manage these
risks? There are a lot of uncertainties that make this ambiguous.
How much the climate will warm for a given amount of greenhouse
gases in the atmosphere is still uncertain. I will talk more about
that. We do not know how much greenhouse gases will be emitted in the future. That also applies to other human activities that
change the climate—the timing and magnitude of future change.
Even if we knew the quantity of greenhouse gases we would emit
in the future, the specifics of where, when, and how the climate
will change are very uncertain, including the regional details.
We also do not have a handle on the potential amplifying
effects or positive feedbacks such as thawing of the frozen soils of
the Arctic that might release a lot of methane into the atmosphere
and cause additional warming. And then there are tipping points
and thresholds and irreversibility of certain types of changes that
we cannot model or predict. So we have a lot of uncertainty that
is risk-intensive.
I will give you an example. One of the fundamental key uncertainties about climate change is the climate sensitivity, which climate scientists define as the equilibrium warming of the Earth’s
surface for a doubling of CO2 in the atmosphere. Right now, we
are on pace for a doubling at about the middle of this century.
Figure 3 shows several probability distributions for the climate sensitivity. The climate sensitivity is shown on the horizontal axis, and
the probability appears on the vertical axis. You can see that they
are all shaped similarly. They have this long tail off to the right that
means it is difficult to limit the upper bound for warming.
The best estimate is around 3°C. That is sort of the weighted
mean of these distributions. But there is a 5–15% chance that it
could be as high as 5°C. We have what statisticians and economists call a “fat tail.” As a result, there is an uncomfortable chance
that the temperature change will be greater than 10°C. It is very
hard to cut that tail off.
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Figure 3. Risks Associated with Doubled CO2
So just to give you a sense of what that means, whether the
sensitivity is 3°C, 5°C, or 10°C, let’s look at Figure 3, which was
adapted from the IPCC AR4. It shows several different categories
of impact. You can see, for example, that mountain glaciers start to
disappear with 1°C of warming. If this climate sensitivity, the best
estimate, is correct, the first darker vertical line shows where that
would be for a doubling of CO2 in the atmosphere. We are really
far into this risk chart. We are getting significantly into this bottom
risk of the rising risk of dangerous positive feedbacks and these
singularities, these global singularities, like massive sea-level rise.
But there is a 10% chance that the climate sensitivity is out as
far as the line labeled ~10% and a 2–3% chance that it could be
beyond that, where we have not even assessed what the impacts
might look like out beyond that amount of warming. So another
aspect of this problem is that currently our models and our judgments of climate change are underestimating current observed
change. It is only recently that we have the data necessary to make
this comparison. That is the comparison of projections to observe
change.
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Next, let’s look at sea-level rise (Figure 4). The squiggly line
is based on our satellite observations available since 1993. The
shaded plume is the range of IPCC model projections. The dashed
line is the average of those models. As you can see, the models are
not keeping pace. The actual change is almost three-fold the rate
of the average of the models.
Figure 4. Sea-Level Rise
There are other things that are being underestimated right now.
The polar ice sheets, in both the Arctic and the Antarctic, are now
losing mass. In 2001, the IPCC said that this would not happen
during the 21st century. It is already happening. The reason is we
have observations now that we did not have for the 2001 TAR. The
small glaciers and ice caps are also losing ice faster than projected.
Global precipitation was projected to increase, but it is increasing
more rapidly. While climate models had forecast that the tropics
would widen, they are widening faster than projected.
So what we have is a situation where our view of uncertainty is
being modified by the observations. Ideally, we would try to set up
our analysis so that we have an equal chance of either overestimating or underestimating change. This is a normal distribution. And I
would point out that the 2008 National Defense Authorization Act
Chapter 2 Climate Imperatives
55
mandated that the military, the DoD, and the Quadrennial Defense
Review (QDR) use the mid-projection of the IPCC AR4 as a basis
for risk assessment. [4, 5]
What we are actually seeing now is a distribution of the probability of severity along the lines of climate change that might look
more like Figure 5. It is similar to that climate sensitivity distribution. But it could also just be related to how much change we will
have in sea level, for example, and other aspects of climate. So the
risks are loaded to the more severe side. It is more likely that we are
underestimating change than overestimating change at this point.
Let’s look at another quick case study. The IPCC projected a
range of sea-level rise for the 21st century between 20 and 80 cm.
The Defense Authorization Act then would require the QDR to
examine the mid-range of this, so perhaps about half a meter would
be a reasonable assessment. But since the IPCC AR4 was released,
several other studies have been released that have projected sealevel rise between half a meter and 2 meters. The full uncertainty
range is actually like this. And, in fact, we are missing by this criterion, we are missing most of the risk when we do our assessment.
So it is important to consider this full range of uncertainty.
Figure 5. Global Sea-Level Rise Projected to 2100 [6–8]
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Just to give you some sense of what it means to have sea-level
rise in this range, take a look at Figure 6. The upper image shows
Vietnam’s Mekong Delta, an important rice-exporting region that
feeds part of Asia. The highlighted land area would be inundated
Figure 6. Sea-Level Rise Scenarios
Chapter 2 Climate Imperatives
57
by 1 meter of sea-level rise. The lower image shows similar data
for Africa’s Niger Delta. It has oil fields, croplands, and, as I
understand it, a rebel movement. This is the type of region where
we might be worried about instability.
These are the great river deltas of the world. They are heavily
populated. They produce a huge proportion of global crops. And
they are all vulnerable to sea-level rise and storm surges. Storm
surges are going to become a problem even earlier than inundation
is going to become a problem. And this is just to show that sealevel rise is not the only problem. It is a good case study because
its features are well studied. But yet, it is very uncertain.
Water resources are going to be a huge problem. Figure 7 shows
a mid-21st century projection for changes in surface water availability. The smaller circles indicate increasingly less water. The largest
circle is more water. I think you can see that there are regions here
that would be of concern in the littorals for social stability. I would
point out that Southern Europe and Western United States are also
in pretty bad shape here. And so we have vulnerability exposure in
both rich countries and the developing world.
Figure 7. Projection for Changes in Surface Water Availability
To bring this full circle to CNA’s Military Advisory Board, they
said that “the U.S. and Europe may experience mounting pressure
to accept large numbers of immigrant and refugee populations as
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drought increases and food production declines in Latin America
and Africa.” [9] Well, I just want to end on that note as a way of
saying this overview was not really an assessment of the risk of the
vulnerabilities and the security implications. But I think this illustrates the problem.
References
1. IPCC, Third Assessment Report: Climate Change 2001, IPCC,
2001.
2. IPCC, Fourth Assessment Report: Climate Change 2007, IPCC,
2007.
3. Joel B. Smith, Stephen H. Schneider, Michael Oppenheimer,
Gary W. Yohee, William Hare, Michael D. Mastrandrea, Anand
Patwardhan, Ian Burton, Jan Corfee-Morlot, Chris H. D. Magadza,
Hans-Martin Füssel, A. Barrie Pittock, Atiq Rahman, Avelino
Suarez, Jean-Pascal van Ypersele, “Assessing dangerous climate
change through an update of the Intergovernmental Panel on
Climate Change (IPCC) “reasons for concern”, Proc. Natl. Acad. Sci.
USA 106(11):4133–4137, 2009, doi: 10.1073/pnas.0812355106.
4. U.S. Congress, House, H.R. 1585: National Defense Authorization
Act for Fiscal Year 2008, 110th Congress, 2007–2008, http://
www.govtrack.us/congress/bill.xpd?bill=h110-1585.
5. Department of Defense, Quadrennial Defense Review Report,
Feb 2010, http://www.defense.gov/qdr/.
6. Stefan Rahmstorf, “A Semi-Empirical Approach to Projecting
Future Sea-Level Rise,” Science 315(5810)368–370, 2007, doi:
10.1126/science.1135456.
7. Martin Vermeer, Stefan Rahmstorf, “Global sea level linked to
global temperature,” Proc. Natl. Acad. Sci. USA 106(51):21527–
21532, 2009, doi: 10.1073/pnas.0907765106.
8. W. T. Pfeffer, J. T. Harper, S. O’Neel, “Kinematic Constraints on
Glacier Contributions to 21st-Century Sea-Level Rise,” Science
321(5894)1340–1343, 2008, doi: 10.1126/science.1159099.
9. The CNA Corporation, National Security and the Threat of
Climate Change, Alexandria, Virginia: CNA, 2007, http://
securityandclimate.cna.org/report/.