National Seminar on Socio-Economic Impact of Extreme Weather and Climate Change
21-22 June 2007, Putrajaya
Climate Change and Extreme Weather: IPCC Findings
Yap Kok Seng
Malaysian Meteorological Department
E-mail: yks@met.gov.my
1.
Introduction
Climate change refers to a statistically significant variation in either the state of the
Earth’s climate or its variability, persisting for an extended period (typically decades
or longer). Climate change may be due to natural internal processes, external forcing, or to persistent anthropogenic changes in composition of the atmosphere or in
land use. Climate variability refers to variations in the mean state and other statistics
such as standard deviations, the occurrence of extremes etc. of climate on all temporal and spatial scales beyond that of individual weather events. Variability may be
due to natural internal processes within the climate system (internal variability), or to
variations in natural or anthropogenic external forcing (external forcing). Hence the
differences between climate change and variability are only in temporal and spatial
scale of climate they deal with, rather than their causes or drivers.
The Intergovernmental Panel on Climate Change (IPCC) uses climate change to
refer to any change in climate over time, whether due to natural variability or as a
result of human activity. The Framework Convention on Climate Change (UNFCCC)
defines climate change as a change of climate which is attributed directly or indirectly to human activity that alters the composition of the global atmosphere and that is
in addition to natural climate variability observed over comparable time periods. That
is, the UNFCCC makes a distinction between “climate change” attributable to human
activity altering the atmospheric compositions and “climate variability” attributable to
natural causes.
In recent years, attention on climate change has been targeted at global warming
effects of atmospheric concentration of the greenhouse gases (CO2, CH4, O3, N2O,
halocarbons) which have increased markedly as a result of human activities since
1750 and which now far exceed pre-industrial values as determined from ice cores
spanning many thousands of years. The global atmospheric concentration of CO 2
has increased from a pre-industrial value of about 280 ppm to 379 ppm over the last
650,000 years (180-300 ppm).
2. Observed Climate Change and Extreme Weather Events
Based on the Summary of Policymakers of Working Group I of the IPCC Fourth Assessment Report, formally approved in February, 2007, the findings can be summarized as folllows:
Global surface temperature over the past 100 years (1906-2005) has risen by
0.74oC, larger than the corresponding temperature rise for 1901-2000 given in the
Third Assessment Report (TAR), 2001 of 0.6oC. The warming over the last 50 years
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National Seminar on Socio-Economic Impact of Extreme Weather and Climate Change
21-22 June 2007, Putrajaya
is nearly twice that for the last 100 years. Eleven of the last twelve years (19952006) rank among the 12 warmest years in the instrumental record of global surface
temperature (since 1850).
Most of the observed increase in globally averaged temperature since the mid-20th
century is very likely (>90% probability) due to the observed increase in anthropogenic greenhouse gas concentrations, with a higher confidence level than the TAR’S
conclusion that “most of observed warming over the last 50 years likely (>66% probability) have been due to the increase in greenhouse gas concentration”.
At continental, regional, and ocean basin scales, numerous long-term changes in
climate have been observed, including changes in Arctic temperatures and ice
sheets, widespread changes in precipitation amounts, ocean salinity, wind patterns
and aspects of extreme weathers.
In the context of global warming, extreme weather and climate events including
droughts, heavy precipitation, heat waves and the intensity of tropical cyclones have
assumed significant changes in intensity, areas and frequency of occurrence (Table
1). More intense and longer droughts have been observed over wider areas since
the 1970’s, particularly in the tropics and subtropics. The frequency of heavy precipitation events has increased over most land areas. Widespread changes in extreme
temperature have been observed over the last 50 years. Cold days, cold nights and
Table 1: recent trends, an assessment of the human influence on the trend, and projections of
extreme weather events (after IPCC, 2007).
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National Seminar on Socio-Economic Impact of Extreme Weather and Climate Change
21-22 June 2007, Putrajaya
Frost have become less frequent, while hot day, hot nights, and heat waves have
become more frequent. There is observational evidence for an increase of intense
tropical cyclone (hurricanes in the North Atlantic and typhoons in the North Pacific)
since about 1970, correlated with increase of tropical sea surface temperature.
There is no clear trend in the annual numbers of tropical cyclones.
Sea level has risen at an average rate of 1.8 (1.3 to 2.3) mm per year over 1961 to
2003. The rate was faster over 1993 to 2003, at about 3.1 (2.4 to 3.8) mm per year.
3. Climate Scenarios
To obtain a realistic estimate of the possible climate evolution in future, one needs
not only to know the internal dynamics of the climate system, but also the future development of the greenhouse gas concentration. To estimate the human induced
effect, so called scenarios have been developed, which are based on different plausible pathways for the future development of population growths, income, standard
of living, energy consumption and energy sources. These scenarios correspond to
different emissions of greenhouse gases (CO2, CH4, O3, N2O, halocarbons) and their
concentration in the atmosphere.
These scenarios have been compiled by IPCC which published 35 emissionsscenarios based on different story lines, which are more commonly known as the
“SRES-Scenarios“. These scenarios can be categorized into four groups and are
represented by four “marker” scenarios.
The A1 scenarios describe a world with a large economic growth. World
population will have peaked by the middle of this century and gradually declined
thereafter. New and efficient technologies will be introduced world wide, regional differences in income and living standard will even out. This group can be differentiated
into three sub-groups according which technology is used to provide the energy:
A1FI places the main emphasis on fossil fuel, A1T on non-fossil fuel, and A1B on a
mixture of different energy sources.
The A2-scenarios describe a heterogeneous world. The main assumption is
regional independence and the continuation of regional differences. World population
increases steadily during the next 100 years. The economical development, the living standard and the incomes are regionally quite different, and technological progress is proceeding only slowly.
The B1-scenarios assume, similar to the A1-scenarios, a world population,
which stabilizes during the next century. The economical development proceeds into
the direction of a service oriented society with a reduced usage of natural resources
and the introduction of clean and efficient technologies. The main emphasis lies on
global and sustainable solutions.
The B2-scenarios anticipate a development, in which regionally different sustainable solutions of economical, ecological and social problems are found. World’s
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National Seminar on Socio-Economic Impact of Extreme Weather and Climate Change
21-22 June 2007, Putrajaya
population rises uncurbed, but slower than in the A2 scenario. The economical development is slower than in the other scenarios. As economical issues become less
important, more diverse technological development emerges. The main emphasis is
placed on environmental conservation and social justice, but on a regional scale.
The IPCC 2007 study focussed its attention on three cases: a low case (B1), and
medium case (A1B) and a high case (A2).
4. Climate Projections
Likely range for globally average surface air warming for six SRES emissions is 1.16.4oC and the best estimate for the high scenario is 4.0 oC. For the low B1 scenario
the mean near-surface temperature will rise between 1.1 to 2.9oC by the year 2100
with a best estimate of 1.8oC, for the medium A1B , the rise is between 1.7 to 4.4 oC
with a best estimate of 2.8oC and for the high A2 scenario the rise is between 2.0 to
5.4oC with a best estimate of 3.4oC (Fig. 1). Although these six SRES projections
are broadly consistent with the span in the TAR (1.4 to 5.8 oC) the new assessment
of the likely ranges now has been derived based on a large number of climate models of increasing complexity and realism, as well as inclusion of feedbacks from the
carbon cycle.
Fig. 1: Multi-model means of surface warming relative to 1980-1999 for the scenarios A2, A1B
and B1. Numbers indicate the number of models which have been run for a given scenario.
The gray bars at right indicate the best estimate (solid line within each bar) and the likely
range assessed for the SRES marker scenarios. (IPCC, 2007)
The warming does not display large differences between the different scenarios during the first three decades of the present century, while in the final two decades the
amplitude of the warming pattern is directly related to the scenarios. There is now
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National Seminar on Socio-Economic Impact of Extreme Weather and Climate Change
21-22 June 2007, Putrajaya
higher confidence in projected patterns of warming and other regional-scale features,
including changes in wind patterns, precipitation, and some aspects of extremes and
of ice.
The pattern of warming (Fig. 2) shows the highest temperature rises over land and in
most high northern latitudes, and a reduced warming in part of the North Atlantic and
the Southern Ocean.
Fig. 2: Projected global average temperature changes for the early and late 21 st century relative to the period 1980 – 1999, as calculated by multi-model averages for a low (B1), a medium
(A1B) and a high SRES scenario for the decades 2020-2039 (left) and 2090-2099 (right). (IPCC,
2007)
Temperature extremes
It is very likely that heat waves will be more intense, more frequent and longer lasting
in a future warmer climate. Cold episodes are projected to decrease significantly in a
future warmer climate. Almost everywhere, daily minimum temperatures are projected to increase faster than daily maximum temperatures leading to a decrease in diurnal range.
Mean Precipitation, Precipitation extremes and droughts
Models project that precipitation generally increases in the areas of regional tropical
precipitation maxima (such as the monsoon regimes) heavy precipitation will continue to become more frequent. The storm tracks of the mid-latitudes are suggested to
move further poleward. This is connected with a shift in the main precipitation areas
(Fig. 3). It leads to a decrease of precipitation in the subtropics, particularly during
the months of winter in the northern hemisphere, and to an increase of precipitation
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National Seminar on Socio-Economic Impact of Extreme Weather and Climate Change
21-22 June 2007, Putrajaya
in the mid to high latitudes. In summer in the northern hemisphere the subtropics
exhibit a decrease of precipitation as well with the exception of the monsoon region
of East Asia, where it increases. Areas affected by droughts will likely increase.
Snow and Ice
The snow and ice-coverage will decrease in both hemispheres. Glaciers and ice
caps lose mass owing to a dominance of summer melting over winter precipitation
increases. This contributes to sea level rise. There is a projected reduction of sea ice
in the 21st century both in the Artic and Antarctic where the projected reduction is
accelerated in the Arctic.
Mean tropical Pacific climate change and El Niňo
Multi model averages show a weak shift towards average background conditions
which may be described as “El Niňo” like, with sea surface temperature in the central
and eastern Pacific warming more than those in the west, with weakened tropical
circulations and an eastward shift in mean precipitation.
The models project continued inter-annual variability of ENSO and there is no consistent indication at this time of discernable changes in projected ENSO amplitude or
frequency in the 21st century.
Tropical cyclones (typhoons and hurricanes)
In the future climate, tropical cyclones are projected to become more intense, with
larger peak wind speeds and more heavy precipitation associated with ongoing increases of tropical SST. There is less confidence in projections of a global decrease
in numbers of tropical cyclones (see Table 1).
Fig. 3: Projected global average precipitation changes for the late 21 st century (2090-2099) relative to the period 1980–1999. The estimates are based on multi-model average projections for
the medium (A1B) scenario for December to February (left) and June to August (right). White
areas are where less than 66% of the models agree in the sign of the change and stippled areas are where more than 90% of the models agree in the sign of the change. (IPCC, 2007)
Sea level
Sea level will continue to rise in the warming climate. The 6 SRES scenarios gives a
range of 0.18-0.59 m rise in sea level by 2090-2099 relative to 1980-1999 levels,
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National Seminar on Socio-Economic Impact of Extreme Weather and Climate Change
21-22 June 2007, Putrajaya
with B1 scenario projecting a rise of 0.18-0.38m, A1T scenario projecting a rise of
0.20-0.45m, B2 scenario projecting a rise of 0.20-0.45m, A1B scenario projecting a
rise of 0.21-0.48m, A2 scenario a rise of 0.23-0.51m and A1Fscenario a rise of 0.260.59m.
Ocean acidification
Increasing atmospheric CO2 concentrations leads directly to increasing acidification
of the surface ocean. Multi model projections based on SRES scenarios gives reduction in pH of between 0.14 to 0.35 units in the 21 st century, adding to the present
decrease of 0.1 units from pre industrial times.
Atlantic Ocean Meridional Overturning Circulation
Model simulations project the Atlantic Ocean meridional overturning circulation to
slow down during the course of the 21st century, with a multi-model ensemble average of about 25% reduction. Models show that there is still warming of surface temperatures around the Northern Atlantic Ocean and Europe due to the larger radiative
effects of the increase in greenhouse gases.
5. Concluding Remarks
The climate will be warmer in future independent of the scenario assumed. However,
the degree of this warming, particularly during the latter half of the century, is determined by the strength of the scenario pathway followed.
IPCC scientists have interpreted the increase in climate variability and extreme
weather events as signals of the impacts of climate change due to global warming.
Whether these increases are due to global warming as a result of anthropogenic activities of mankind, or due to long term natural variability of the climate itself, it is
clear we need to take further steps to better prepare ourselves against the impacts of
such changes.
Literature:
IPCC 2007: Climate Change 2007 - The physical science basis. Summary for Policymakers. Online version available under: www.ipcc.ch
IPCC 2007: Climate Change 2007 – The physical basis, Chp. 10
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