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Extreme Events
Sonia Barbey & Xiaoyu Liu
“Extreme events” may be climatic. In fact, in climatology, extreme events can be seen as
being the highest or the lowest value of a climatic element observed, in a particular
location, during a specific time frame or during specific period.
However one of the most pragmatic yet tolerant explanations has come across so far:
“There are a number of ways extreme climate events can be defined, such as extreme
daily temperatures, extreme daily rainfall amounts, large areas experiencing unusually
warm monthly temperatures, or even storm events such as hurricanes. Extreme events can
also be defined by the impact an event has on society. That may involve excessive loss of
life, excessive economic or monetary losses or both.” (Easterling et al 2000)
If this value was the greatest of all the recorded extremes it would be considered the
absolute extreme.
In this overview of Extreme events we will be consulting literature composed of
scientific articles as well as lay magazine articles. We will than assess what information
these articles give us about extreme events. Finally, we will outline the 3 main points we
find most crucial to our topic, “Extreme Events”.
Observed variability and trends in extreme climate events: A brief review
DR Easterling et al
This article is a great way into extreme events. It explains some of the main events and
some of the difficulties scientists are having with them. It explains that in order to create
better models we need more international cooperation in order to get better data.
However, it outlines that this is getting increasingly difficult. It shows on a global scale
how extreme events are very present in some areas and not in others.
“Heavy precipitation and high streamflow in the contiguous United States: Trends in the
twentieth century”
Groisman et al.
This article is an extensive study of some of the repercussions of the changes in
precipitation in the United States. It suggests that these extreme precipitation events
(which they have found to be increasing at a statistically significant level 95%) may have
repercussions on streamflow. However due to the multiple contributors of streamflow and
the fact that “precipitation is not routinely monitored within the U.S. streamflow
network” they cannot generalize their results to the entire US. Rather they split the results
in the eastern US where their data strongly supports a relationship between maximum
streamflow and heavy precipitation events. For the western US snowmelt has been
occurring earlier, and is thus a major contributor to maximum streamflow and thus it is
difficult to extricate precipitation from streamflow in these parts. In Central US the data
is too sparse to make any claims. This is an interesting article because it looks not only at
the extreme precipitation events but at what is MEANS to have increased precipitation in
certain areas. One may derive other repercussions from these increased extreme events;
this paper is an illustration of how one may do this, with streamflow.
Quantifying the risk of extreme seasonal precipitation events in a changing climate
Palmer et al.
This article looks at the chance of having more extreme events in England and
Bangladesh. Once they examine this (In parts of the UK, precipitation will increase by
factor of 5 over the next 50-100 years and similar results were found for Bangladesh)
they try to give an economic value to their probabilistic projections. They finally discuss
how these extreme events would benefit from being included into more general climate
models although the scales of these models are quite different. They claim that inclusion
of these extreme climate events would help us determine and define “future
computational requirements for the climate change problem”
Increasing risk of great floods in a changing climate
Milly et al.
This paper examines increased flood risk due to extreme precipitation events due to
anthropogenic sources. The paper looks at the flood increase in the 20th century. They do
find that “The 100 year flood was exceeded 21 times in our observational record of
20166 station-years. Flood events were concentrated in the latter half of the record: half
the observations were made after 1953 and 16 of the flood events occurred after 1953”. In
effect due to increased anthropogenic activities and emissions one could assume that
these events are linked, at least enough to say that flood risk is associated with radiatively
forced climate change. The significance of their results is “tentative”. They end up
suggesting more stream gouging programs and more tropical hydroclimate simulations.
Climate change scenarios for the US National Assessment.
MacCracken et al.
This article has more of an agenda. It looks at and assesses climate projections in order to
establish whether they can be taken into account by the USNA to provide “the nation”
with what?. It explains what it looks for in models and how much significance is needed
for them to be significant. It also extracts what they call “key findings”. These 10 points
summarize what has been looked at, what can be expected and what requires vigilant
preparation due to climate change. In terms of extreme events, this article is useful
because it underlines that we still need to include extreme events such as “10 year
drought in California”. Overall it explains why and how models are used and underline
their strengths and weaknesses, supplementing their predictions with other factors such as
“historical trends, physical and biological relationships […]”
Global Hydrological Cycles and World Water Resources” Taikan Oki and Shinjiro Kanae
Science in www.sciencemag.org August 2006
This article states that climate change due to anthropogenic sources will affect
hydrological cycles. However, it stresses that the extent to which this effect will take
place is highly uncertain due to the high variability of extreme events, however, some of
the ways in which hydrological cycles will be affected have been assessed: temperatures
will affect snowmelt, water rise will salinate aquifers, and precipitation will increase,
amongst many other changes.
“Water Supply Chapter 6” David Major and Richard Goldberg
This is more of a policy related article. But it outlines some of the difficulties we
encounter when having to prepare for extreme events in terms of water resources and
changing precipitation patterns.
The five year forecast November 27, 2006 issue of New York Magazine
Clive Thompson
This article is an interesting lay article. It outlines how climate events are heavily
interlinked, with each other and with other disciplines. For example we thank El Nino
events for “extremely mild” winters and fewer hurricanes. It is also interesting that
climate events are linked to many other issues such as economy, making it a “meaty”
article for many people, not just people interested in climate.
Extreme Events Examples:
Heat Waves:
Definition:
Hot sustained temperatures have been known to produce notable impacts on human
mortality, regional economies and ecosystems.
Pattern:
A global coupled climate model Parallel Climate Model (PCM) shows that there is a
distinct geographic pattern to future changes in heat waves. Model results for areas of
Europe and North America, associated with the severe heat waves in Chicago in 1995
and Paris in 2003, show that future heat waves in these areas will become more intense,
more frequent, and longer lasting in the second half of the 21st century. Observations and
the model show that present-day heat waves over Europe and North America coincide
with a specific atmospheric circulation pattern that is intensified by ongoing increases in
greenhouse gases, indicating that it will produce more severe heat waves in those regions
in the future.
Physical Mechanism:
Heat waves are generally associated with specific atmospheric circulation patterns
represented by semi-stationary 500-hPa positive height anomalies that dynamically
produce subsidence, clear skies, light winds, warm-air advection, and prolonged hot
conditions at the surface. The 500-hPa height anomalies are most strongly related to
positive warm season precipitation anomalies over the Indian monsoon region and
associated positive convective heating anomalies that drive mid-latitude teleconnection
patterns in response to anomalous tropical convective heating in future climate.
Hydrological Cycle Change
There is evidence that precipitation extremes, particularly heavy rainfall events, are
increasing in the United States and Australia, also suggesting an enhanced hydrologic
cycle as the planet warms.
There is some evidence for an increase in intense rainfall events (United States, tropical
Australia, Japan, and Mexico), but analyses are far from complete and subject to many
discontinuities in the record. The strongest increases in extreme precipitation are
documented in the United States and in Australia. The extreme precipitation amounts are
increasing, in some areas by as much as 12%. However, for many areas this is being
offset by decreases in more moderate events, and in other areas of the country (e.g., the
Northwest, Southeast, and Northeast) both amounts are increasing. Such an increase, at
least in the United States, has been supported by an observed increase in water vapor and
total precipitation. Changes have not been detected in the year-to-year or season-toseason variability of snow extent during the satellite era.
Temperature
Interannual temperature variability has not changed significantly over the past century.
However, on shorter timescales and higher frequencies however, e.g., days to a week,
there is some evidence for a decrease in temperature variability across much of the
Northern Hemisphere. Related to the decrease in high-frequency temperature variability,
there has been a tendency for fewer low-temperature extremes and a reduction in the
number of freezes in disparate locations such as the northeastern United States and
Queensland, Australia.
Summary
What evidence exists from past/current observations?
In many of the articles it is stated that anthropogenic contributions to GHGs have had an
impact on extreme events. This has been outlined in the works concerning water
management, the five year forecast, flood risk as well as streamflow increase. The
twentieth century has wit ????
What do model simulations of past/present indicate?
It is very difficult to model extreme events but the forecasts done by Milly in terms of
predicting floods suggest extreme events will happen more and more frequently.
However most authors are hoping that extreme events will be given a bigger place in
future climate models.
What are the models suggesting for the future, and what are the assumptions in those
projections?
The assumptions are that our emissions of GHGs will remain the same. Thus there could
be some good news in store if we can manage to harness some of them.
The assumptions are that
What are the principal uncertainties in the scientific evidence and in the forecasts?
The emissions are uncertain; hopefully we will be seeing some changes. Also the extreme
variability of extreme events makes it difficult for them to be predicted and difficult to
incorporate in to the climate models.
3 points
1) Extreme events have been closely linked to GHG emissions
2) Extreme events are very variable and are difficult to model
3) Extreme events have many repercussions and impacts. We need to get prepared
even we cannot model it very well.
Bibliography
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Observed variability and trends in extreme climate events: A brief review
DR Easterling, JL Evans, PY Groisman, TR Karl, KE … - Bulletin of the
American Meteorological Society, 2000 - agci.org
Easterling, DR and co-authors, 2000. Observed climate variability and change of
relevance to the biosphere. Journal of Geophysical Research-Atmos., 105 (D15):
20101-20114.
Meehl, GA and C. Tebaldi, 2004. More intense, more frequent, and longer lasting
heat waves in the 21st century. Science, 305 (5686): 994-997.
Stott, PA, DA Stone and MR Allen, 2004. Human contribution to the European
heat wave of 2003. Nature, 432 (7017): 610-614.
Groisman, PY and co-authors, 2004. Contemporary changes of the hydrological
cycle over the contiguous United States: Trends derived from in situ observations.
Journal of Hydrometeorology, 5 (1): 64-85.
Groisman, PY, WE Knight and TR Karl, 2001. Heavy precipitation and high
streamflow in the contiguous United States: Trends in the twentieth century.
Journal of Climate, 82 (2): 219-246.
Palmer, TN and J Raisanen, 2002. Quantifying the risk of extreme seasonal
precipitation events in a changing climate, Nature, 415: 512-514.
Milly, PCD and co-authors, 2002. Increasing risk of great floods in a changing
climate, Nature, 415: 514-517.
MacCracken, MC and co-authors, 2003. Climate change scenarios for the US
National Assessment. Bulletin of the American Meteorological Society, 84 (12):
1711-+.
Major, D. and R. Goldberg. 2001. Water Supply. In: Rosenzweig, C. and W.D.
Solecki (Eds.) 2001. Climate Change and a Global City: The Potential
Consequences of Climate Variability and Change – Metro East Coast. New York:
Columbia Earth Institute. p. 87-101.
Taikan Oki and Shinjiro Kanae Science in www.sciencemag.org August 2006
Global Hydrological Cycles and World Water Resources”