December 28, 2012

Three Scenarios

CSIS-CNAS: Security Implications of Climate Change

Summaries of the Three Scenarios

Climate Scenario 1: Expected Climate Change

By 2040 average global temperature rises 1.3°C above the 1990 average.
Warming is greater over land masses and increases from low to high latitudes.

Generally, the most damaging local impacts occur at
  • low latitudes because of ecosystem sensitivity to altered climate and high human vulnerability in developing countries, and
  • in the Arctic because of particularly large temperature changes at high northern latitudes.

Global mean sea level increases by 0.23 meters, causing
  • damage to the most vulnerable coastal wetlands with associated negative impacts on local fisheries,
  • seawater intrusion into groundwater supplies in low-lying coastal areas and small islands, and
  • elevated storm surge and tsunami heights, damaging unprotected coastlines.
Many of the affected areas have large, vulnerable populations requiring international assistance to cope with or escape the effects of sea level rise.
Marine fisheries and agricultural zones shift poleward in response to warming, in some cases moving across international boundaries.
The North Atlantic MOC is not affected significantly. …

The largest and most widespread impacts relate to reductions in water availability and increases in the intensity and frequency of extreme weather events.
The Mediterranean region, sub-Saharan Africa, northern Mexico, and the southwestern United States experience more frequent and longer-lasting drought and associated extreme heat events, in addition to forest loss from increased insect damage and wildfires.

[Northern] mid-latitudes see a mix of benefits and damages.
Benefits include
  • reduced cost of winter heating,
  • decreased mortality and injury from cold exposure, and
  • increased agricultural and forest productivity in wetter regions because of longer growing seasons, CO2 fertilization, and fewer freezes.
Negative consequences include
  • higher cost of summer cooling,
  • more heavy rainfall events,
  • more heat-related death and illness, and
  • more intense storms with associated flooding, wind damage, and loss of life, property, and infrastructure.

Climate Scenario 2: Severe Climate Change

Average global surface temperature rises at an unexpectedly rapid rate to 2.6°C above 1990 levels by 2040 …

[The] rate of [polar] ice flow into the sea [accelerates] rapidly, resulting in 0.52 meters of global mean sea level rise.
[There is] high confidence that the Greenland and West Antarctic Ice Sheets have become unstable and that 4 to 6 meters of sea level rise are now inevitable over the next few centuries.

Water availability decreases strongly in the most affected regions at lower latitudes (dry tropics and subtropics), affecting about 2 billion people worldwide. …
Crop yields decline significantly in the fertile river deltas because of sea level rise and damage from increased storm surges.
Agriculture becomes nonviable in the dry subtropics [due to:]
  • low water availability and
  • increased soil salinization resulting from more rapid evaporation of water from irrigated fields.
Arid regions at low latitudes expand, taking previously marginally productive croplands out of production.
(p 42)

North Atlantic fisheries are affected by significant slowing of the North Atlantic MOC.
Globally, there is
  • widespread coral bleaching,
  • ocean acidification,
  • substantial loss of coastal nursery wetlands, and
  • warming and drying of tributaries that serve as breeding grounds for anadromous fish (ie, ocean-dwelling fish that breed in freshwater, eg, salmon).
Because of a dramatic decrease in the extent of Arctic sea ice, the Arctic marine ecosystem is dramatically altered and the Arctic Ocean is navigable for much of the year.

Developing nations at lower latitudes are affected most severely because of climate sensitivity and low adaptive capacity.
Industrialized nations to the north experience clear net harm and must divert greater proportions of their wealth to adapting to climate change at home.

Climate Scenario 3: Catastrophic Climate Change

Between 2040 and 2100 the impacts associated with climate scenario two progress and large-scale singular events of abrupt climate change occur.
The average global temperature rises to 5.6°C above 1990 levels …
[Mean] sea level rises [of] 2 meters [render] low-lying coastal regions uninhabitable, including many large coastal cities.
The large fertile deltas of the world become largely uncultivable [as] inundation and more frequent and higher storm surges … reach further inland.

The North Atlantic MOC stops at mid-century, generating large-scale collapse of North Atlantic marine ecosystems and associated fisheries.
Northwestern Europe experiences colder winters, shorter growing seasons, and reduced crop yields …
[Globally, the] MOC collapse increases average temperatures in most regions and reorganizes precipitation patterns in unpredictable ways, hampering water resource planning around the world and drying out existing grain-exporting regions.
Southern Europe and the Mediterranean region … continue to experience hotter, drier summers with more heat waves, more frequent and larger wildfires, and lower crop yields.

Agriculture in the traditional breadbaskets is severely compromised by alternating persistent drought and extreme storm events that bring irregular severe flooding.
Crops are physiologically stressed by temperatures and grow more slowly …
Even in … regions with increased precipitation, summertime soil moisture is reduced by increased evaporation.
Breadbasket-like climates shift strongly northward into formerly sub-arctic regions with … little infrastructure …
[Extreme] year-to-year climate variability … makes sustainable [agriculture] difficult on the scale needed to feed the world population.

Mountain glaciers are virtually gone and annual snow pack dramatically reduced in regions where large human populations [have] relied on glaciers and annual snowfall for water supply and storage, including Central Asia, the Andes, Europe, and western North America.
[The] area requiring remote water sources for habitability [increases] dramatically [as] such remote sources [become] less available.
{Half of the world’s human population experiences persistent water scarcity.}

Arid regions expand rapidly, overtaking regions [previously able] to support dense populations.
The dry subtropics, including the Mediterranean region, much of Central Asia, northern Mexico, much of South America, and the southwestern United States are no longer [habitable.]
(p 43)

[Tropical] and mid-latitude storm activity and associated wind and flood damage becomes much more intense and occurs annually, leading to frequent losses of life, property, and infrastructure in many countries every year.

[Water] availability and loss of food security disproportionately affect poor countries at lower latitudes …
[However,] extreme weather events are more or less evenly distributed, with perhaps greater frequency at mid-latitudes because of stronger extratropical storm systems, including severe winter storms.
(p 44)



Scenario-based Approach

Underlying Assumptions

General Patterns of Projected Climate Change


Washington DC.

  • The Age of Consequences: The Foreign Policy and National Security Implications of Global Climate Change, 5 November, 2007.
    Kurt M Campbell, Jay Gulledge, JR McNeill, John Podesta, Peter Ogden, Leon Fuerth, R James Woolsey, Alexander TJ Lennon, Julianne Smith, Richard Weitz and Derek Mix.


    Jay Gulledge: Senior Scientist and Program Manager, Science and Impacts, Pew Center on Global Climate Change.

    The scenarios outlined in this section are not predictions of future conditions and should not be read or cited as such.


    As the purpose of this project is to assess potential security risks of future climate change, the primary criterion for the climate impacts scenarios outlined here is plausibility rather than probability.
    Rather than asking, What is the most likely climate-driven outcome?, we ask, What potential climate-driven outcomes are plausible, given current scientific understanding? Recent observations indicate that projections from climate models have been too conservative …
    [The] effects of climate change are unfolding faster and more dramatically than expected.
    Given the uncertainty in calculating climate change, and the fact that existing estimates may be biased low at this time, plausibility is an important measure of future impacts.
    [For this reason] potential changes that the IPCC or other assessments may characterize as improbable are considered plausible here if significant uncertainty persists regarding their probability …
    [For example, the] collapse of the North Atlantic overturning circulation …
    (p 35, italics added)

    Scenario-based Approach

    [A scenario is a] coherent, internally consistent and plausible description of a possible future state of the world.
    Scenarios are not predictions or forecasts but are alternative images without ascribed likelihoods of how the future might unfold.
    In this volume we develop a group of three impacts scenarios:
    • expected,
    • severe, and
    • catastrophic.
    Although guided in general by the IPCC AR4 and other authoritative sources, these impacts scenarios are unique to this study and were created specifically for its purposes.

    The IPCC uses independent scenarios of man made greenhouse gas emissions called SRES [IPCC Special Report on Emission Scenarios, 2000] scenarios in its assessment process.
    The SRES scenarios make assumptions about future population growth, economic and infrastructure development, and energy policy that result in plausible, alternative pathways of future greenhouse gas emissions.
    [Alternative] SRES emission scenarios are used to drive climate models, which in turn produce alternative projections of future climate conditions.
    [The] SRES A1B emission scenario is used in our study solely to derive levels of temperature change for each of our three impacts scenarios. …
    [In] some cases we present a range of estimates provided in the published literature based on a variety of emission scenarios for a given temperature change.
    From the perspective of risk assessment, the upper ends of such ranges are most relevant.

    [It] is essential to distinguish between a prediction and a projection.
    • A projection describes an outcome that is deemed plausible, often subjectively, in the context of current uncertainties …
    • [A] prediction describes the statistically most probable outcome based on the best current knowledge.
    Michael MacCracken:
    [A] projection specifically allows for significant changes in the set of [determinants] that might influence the [future climate], creating ‘if this, then that’ types of statements.
    The greater the degree of uncertainty surrounding determinants of future climate conditions, such as future man made greenhouse gas emissions, the less certain a prediction can be and the more important projections become for risk assessment.
    (p 36)

    The running 10-year average of annual frequency shows a dramatic and abrupt increase above the previous maximum observed in the mid-1950s, previously considered extreme.

    Box 1: Two Myths About Climate Change

    Myth 1: Future climate change will be smooth and gradual

    The history of climate reveals that climate change occurs in fits and starts, with abrupt and sometimes dramatic changes rather than gradually over time.
    This … implies that surprising changes are likely in the future even if average climate change is projected accurately.
    Hypothetically, a projection of 1 meter of sea level rise over one century could prove correct, but it could occur in several quick pulses with relatively static periods in between.
    This type of change is more difficult to prepare for than gradual change, as large-scale public works projects intended to adapt to such a change are likely to require several decades to complete.
    Surprises from abrupt climate change may therefore increase the burden of climate impacts beyond what is expected, with unforeseen security implications.

    Myth 2: Impacts will be moderate in industrialized nations.

    … By virtue of its large size and varied geography, the United States already experiences a wide range of severe climate-related impacts, including
    • droughts,
    • heat waves,
    • flash floods, and
    • hurricanes,
    all of which are likely to be exacerbated by climate change.
    [The] western United States, southern Europe, and southern Australia [can expect to] experience progressively more severe and persistent drought, heat waves, and wildfires in future decades as a result of climate change.
    The United States is also one of the most susceptible countries to future sea level rise, with the largest number of coastal cities and two agricultural river deltas near or below sea level.
    The United States and coastal countries of the European Union are likely to experience some of the greatest losses of coastal wetlands.

    The misconception that climate change impacts will spare the industrialized world may stem from confusion between the concepts of impacts and vulnerability.
    Vulnerability measures the ability of a population to withstand impacts, but low vulnerability does not imply low impacts.
    [The] United States may be more capable of devoting resources to preparing for, adapting to, and recovering from climate change impacts than developing countries [but because] it will be severely impacted, [it] will need to divert great financial and material resources toward coping with climate change.
    Severe climate change impacts in wealthy nations portend diversion of foreign aid to domestic projects, generating greater potential for environmental refugees to migrate to wealthy countries.
    (p 37, italics added)

    Underlying Assumptions in the Three Climate Impacts Scenarios

    [The expected climate change scenario is based on SRES A1B] s a medium-range emission scenario that considers continued growth of man made greenhouse gas emissions under rapid economic growth, technological development, and ongoing efficiency improvements, but with significant continued reliance on fossil fuels.
    Atmospheric carbon dioxide (CO2) rises to a concentration of about 700 parts per million (ppm) … by the end of the 21st century … associated with a global surface temperature increase of 1.7 to 4.4°C, with a best estimate of 2.8°C.
    [Assuming] that society takes no [effective] actions to limit climate change …
    [The main source of uncertainty] in the temperature outcome [is the] physical climate sensitivity to greenhouse gas forcing …
    [2-4.5°C, "best estimate" 3°C ie a 3 fold amplification of the 1°C baseline heat trapping effect of a doubling of C02 from 280 to 560 ppm by positive climate system feedbacks such as increased water vapour and reduced seaice.]
    (p 38)

    Table 2
    Projections of Global Average Surface Warming and Sea Level Rise Relative to 1990.
    *Projections for scenarios 2 and 3 are unique to this study and are meaningful only the context of this study.
    (p 41)

    Climate Scenario 1: Expected Climate Change

    [This 2040 scenario] accepts the temperature change projected in the AR4 for emission scenario A1B.

    Climate Scenario 2: Severe Climate Change

    [This 2040 scenario] assumes that the AR4 projections of both warming and attendant impacts are systematically biased low.
    Multiple lines of evidence support this assumption …
    (p 38)

    [Firstly, the models] either omit or do not account for uncertainty in potentially important positive feedbacks that could amplify warming …
    • release of greenhouse gases from thawing permafrost,
    • reduced ocean and terrestrial CO2 removal from the atmosphere …
    [There] is some evidence that such feedbacks may already be occurring in response to the present warming trend. …

    [There is also observational evidence that suggests that the] IPCC models underestimate the responsiveness of some aspects of the climate system system to a given amount of warming [eg:]
    • changes in global ice cover,
    • sea level rise,
    • tropical storm activity …
    [This] scenario assumes that omitted positive feedbacks occur quickly and amplify warming strongly, and that the climate system components respond more strongly to warming than predicted.
    As a result, impacts accrue at twice the rate projected for [scenario 1.]

    Based on current understanding of physical inertia in the climate system, a doubling of the rate of warming seems highly unlikely on the 30-year time scale.
    [However, given] that the IPCC projections show only average change with a smooth evolution over time and have tended to underestimate climate system response to warming … a combination of underestimated change and abrupt episodes could plausibly result in an unexpectedly large and rapid warming in a matter of a few decades …

    [A] recent study … found greater than a one-in-twenty chance that warming could exceed 2°C relative to 1990 by 2040 for the highest SRES emission scenario.
    This level of warming is not greatly different from projected in scenario two.

    Climate Scenario 3: Catastrophic Climate Change

    … Based on current scientific understanding of climate change, we assume that abrupt, large-scale climate events cannot plausibly occur in the next three decades, but could plausibly do so over the course of this century.
    [This 2100 scenario] extends the rapid warming and attendant accelerated impacts associated with scenario two to the end of the 21st century, leading to …
    • rapid loss of polar land ice,
    • abrupt 2 meter sea level rise, and
    • collapse of the Atlantic meridional overturning circulation (MOC).
    [This assumes a doubling of] the best estimate of modeled surface warming under emission scenario A1B for the year 2100.
    [This] result (5.6°C warming by 2095 relative to 1990) compares well with the upper-end projection of a group of models that incorporated carbon cycle feedbacks and therefore simulated higher atmospheric CO2 growth rates than … the IPCC models.
    These models still did not incorporate all possible positive feedbacks, such as increased greenhouse gas emissions from thawing permafrost, so our most extreme warming scenario could potentially prove conservative.
    [However,] there is little utility in assuming higher projected temperatures, as impacts have generally not been assessed for 21st century warming greater than 5°C.
    (p 39)

    Sea Level Rise

    … 10% of the world’s population currently lives in low-lying coastal zones and that this proportion is growing …

    [It is likely that the omission of] ice sheet dynamics and melt point drainage … systematically biases the IPCC projections low.
    For the IPCC, this omission was perhaps unavoidable because current knowledge of ice sheet dynamics simply does not permit the process to be modeled.
    For our purposes, such an omission is unacceptable as it would lead to an unrealistically low upper limit. …

    The IPCC’s model projections for sea level rise from the 2001 Third Assessment Report (TAR) were higher than the latest projections of the AR4.
    [Observed] sea level rise for the period 1990 to 2006 tracks the upper uncertainty bound of the TAR projections, and therefore exceeds all AR4 model projections for sea level rise during the same period.
    [Adopting] the upper bound of projected sea level rise in the TAR … yields a sea level rise for scenario one of 23 cm in the year 2040 relative to 1990. …

    [The] largest uncertainty with regard to sea level rise rests on which of two mechanisms … will dominate future sea level rise …
    • thermal ocean water expansion or
    • freshwater contributions from land-based ice sheets …

    [Because of the] fundamental uncertainties [of current] model-based estimates … nine leading climatologists with relevant expertise [were surveyed.]
    All of the experts agreed that at least 1 meter of sea level rise by the end of the 21st century was plausible, and at least three felt that 2 meters were plausible. …

    During warming at the end of the last ice age sea level rise was dominated by the retreat of land-based ice sheets and occurred at an average rate of 1 to 2 meters per century for several thousand years.
    (p 40, italics added)

    The warmest point of the last interglacial period, around 125,000 years ago, was about 1°C warmer than the present global average temperature for only a few centuries, yet saw an average sea level 4 to 6 meters higher than at present.
    [Given that] the modern warming trend has already been under way for nearly a century … 2 meters is a plausible upper bound for the increase in sea level during the during the 21st century under a scenario of rapid warming and ice sheet-dominated sea level rise, as assumed in scenario three. …

    To obtain a projection of sea level rise for scenario two, we use the projection of the 2001 IPCC report as a scaling function.
    The upper end of the projection is about 0.23 meters in 2040 and 0.88 meters in 2100, giving a ratio of 0.26.
    Multiplying this ratio by the posited rise of 2 meters per century yields a sea level rise projection of 0.52 meters for the year 2040 relative to 1990 in scenario two. …

    [Many] more centuries will pass before sea level equilibrates with the change in temperature.
    Sustained warming of about 3°C would eventually eliminate the Greenland Ice Sheet in future centuries, ultimately raising sea level by 6 meters …
    [Contributions] from Antarctica would increase the total even more.
    (p 41)

    General Patterns of Projected Climate Change

    This section reviews general patterns of climate change as projected by the IPCC Fourth Assessment Report (AR4).
    The purpose is to provide a general template of regional patterns of climate impacts at subcontinental scales, over which to lay the generalities described [in] the three scenarios …
    Unless otherwise indicated the results described in this section are extracted from chapters 10 and 11 of the Contribution of Working Group I to the AR4, which present projections of future climate change based on modeling experiments using mostly aggregated results of up to 21 different global circulation models. …


    All models in the AR4 show global surface warming in proportion to the amount of man made greenhouse gases released to the atmosphere.
    For the A1B emission scenario, average global surface warming relative to 1990 is about 1.3°C in 2040 and 2.8°C in 2100.
    [However,] changes are far from uniform globally.
    • Temperature over land, particularly in continental interiors, warms about twice as much [quickly as temperature over ocean.]
    • High northern latitudes … warm about twice as fast as the global average {[leading] to faster thawing of permafrost, with consequent
      • infrastructure damage (eg, collapsed roads and buildings, coastal erosion) and
      • feedbacks that amplify climate change (eg, CH4 and CO2 release from thawed organic soils) …}
    • [The] average change in any given location [does not increase smoothly] over time [but is intead] associated with larger extremes …
      [Overall, high temperature extreme events and impacts (eg, heat waves, droughts, and wildfires) predominate over low temperature events and impacts (eg, freezes).]
    • [Seasonal] differences, with winter temperatures rising more rapidly than summer temperatures, especially at higher latitudes.
      Wintertime warming in the Arctic over the 21st century is projected to be three to four times greater than the global wintertime average warming, resulting in much faster loss of ice cover and associated impacts (eg, faster sea level rise).


    Under the A1B scenario, global average precipitation increases by 2% in 2040 and 5.5% in 2100.
    [A] global change of a few percent translates into [regional changes exceeding] 20% [in some] areas.
    [Extreme drought and] rainfall events are … expected to become more frequent [due to] this intensification of the global water cycle.
    • Increased precipitation generally prevails in the tropics and at high latitudes …
      {[Monsoonal] rainfall is projected to increase in South and Southeast Asia …
      [However, this may not be beneficial] as rain is already plentiful at this time of year [and] the added rainfall will likely increase damage from flooding.}
    • Decreased precipitation prevails in the subtropics and mid-latitudes …
    (p 44)

    Central America experiences the largest decline in summer precipitation.
    The main areas projected to experience greater drought are
    • the Mediterranean region,
    • Central America,
    • Australia and New Zealand, and
    • southwestern North America.
    Decreases in precipitation and related water resources are projected to affect several important rain-fed agricultural regions, particularly in South and East Asia, in Australia, and in northern Europe.
    {[A] decrease in summer precipitation is projected for Amazonia, where the world’s largest complex of wet tropical forest depends on high year-round precipitation.}

    Two important correlates of precipitation are [soil moisture and] annual runoff (ie, surface water flow) …
    These parameters are critical to water supply for consumption and [irrigation/crop] production.

    Soil moisture generally corresponds with precipitation …
    [But] declines in some areas [where] greater evaporation [from warmer temperatures predominates over increased precipitation.]
    The biggest changes in soil moisture include
    • a strong increase in a narrow band of equatorial Africa …
    • a moderate increase in a band extending from northern and eastern Europe and into Central Asia [and]
    • decreases by 10% or greater over much of
      • the United States, Mexico and Central America,
      • southern Europe and the Mediterranean basin … including parts of the Middle East …
      • southern Africa,
      • the Tibetan Plateau, and … much of northern Asia.

    Runoff follows a pattern very similar to precipitation, with
    • increases in high northern latitudes and parts of the tropics, including Central, South, and Southeast Asia, tropical eastern Africa, the northern Andes and the east-central region of South America around Uruguay, and extreme southern Brazil [and]
    • decreases … in the southwestern United States, Central America, the Mediterranean region (including southern Europe, northern Africa, and the Middle East), southern Africa, and northeastern South America, including Amazonia.
    (p 45)

    Box 2: Summary of IPCC Findings for Regional Climate Projections

    The following summaries, excerpted from the Executive Summary of Chapter 11 of the Contribution of Working Group I to the Fourth Assessment Report …
    [The following] changes are assessed as … very likely (greater than 90% likelihood) …


    Warming is very likely to be larger than the global annual mean warming throughout the continent and in all seasons, with drier subtropical regions warming more than the moister tropics. …

    Mediterranean and Europe

    Annual precipitation is very likely to
    • increase in most of northern Europe and
    • decrease in most of the Mediterranean area. …
    Extremes of daily precipitation are very likely to increase in northern Europe.
    The annual number of precipitation days is very likely to decrease in the Mediterranean area.
    The duration of the snow season is very likely to shorten … in most of Europe.


    Precipitation in boreal winter is very likely to increase in northern Asia and the Tibetan Plateau …
    It is very likely that heat waves/hot spells in summer will be of longer duration, more intense, and more frequent in East Asia.
    Fewer very cold days are very likely in East Asia and South Asia.
    There is very likely to be an increase in the frequency of intense precipitation events in parts of South Asia, and in East Asia.

    North America

    Annual mean precipitation is very likely to increase in Canada and the northeast United States …
    Snow season length and snow depth are very likely to decrease in most of North America except in the northernmost part of Canada …
    (p 46, italics added)

    Australia and New Zealand

    Precipitation is very likely to decrease in southwestern Australia in winter. …
    Increased frequency of extreme high daily temperatures in Australia and New Zealand, and a decrease in the frequency of cold extremes is very likely.
    Extremes of daily precipitation are very likely to increase, except possibly in areas of significant decrease in mean rainfall (southern Australia in winter and spring).

    Polar Regions

    The Arctic is very likely to warm during this century more than the global mean.
    Warming is projected to be largest in winter and smallest in summer.
    Annual arctic precipitation is very likely to increase.
    It is very likely that the relative precipitation increase will be largest in winter and smallest in summer.
    Arctic sea ice is very likely to decrease in its extent and thickness.

    Small Islands

    All Caribbean, Indian Ocean, and North and South Pacific islands are very likely to warm during this century.
    (p 47)

    Regional Sensitivity to Climate Change

    A given change in climate such as a degree of warming or a 10% change in precipitation does not affect all regions the same way. …

    Some regions experience a very stable climate, and natural and human systems have developed around this stability …
    [In] such regions even a small change may generate significant impacts. For instance, in wet tropical systems moderate decreases in precipitation may lead to the collapse of productive rainforests.
    Alternatively, settlements and infrastructure in wet tropical regions may be damaged by increased flooding from small increases in precipitation during the rainy season.
    Semi-arid regions that are already marginal for supporting natural and human systems may be rendered uninhabitable by small decreases in precipitation or runoff.

    [By] contrast, regions with historically large climate variability require larger changes of future climate to move natural and human systems beyond the bounds of the climate extremes to which they have adapted.
    For instance, in spite of great natural climate variability, the Arctic is expected to be heavily impacted by climate change because the degree of warming is projected to be large …

    The areas most sensitive to a combination of projected temperature and precipitation change relative to natural variability are in tropical Central and South America, tropical and southern Africa, Southeast Asia, and the polar regions. …
    Marginal agricultural lands generally show intermediate to high climate sensitivity, including in the southwestern United States, Central America, sub-Saharan Africa, southern Europe, Central Asia, including the Middle East, and eastern China.
    Most of these regions also bear large human populations.

    Extreme Weather Events

    Droughts, flash floods, heat waves, and wildfires are all projected to occur more frequently and to become more intense in regions where such events are already common.

    Intense tropical and mid-latitude storms with heavier precipitation and higher wind speeds are also projected.
    There is evidence that many of these events already occur more frequently and have become more intense.
    Projections indicate … a decrease in the frequency of low-intensity storms.
    As a consequence, the total number of storms decreases globally even as the number of intense storms increases.

      Precipitation and drought

      [The] IPCC projects that a larger fraction of total precipitation will fall during extreme events, especially in the moist tropics and in mid and high latitudes where increased mean precipitation is projected. …
      Even in areas projected to become drier, the average intensity of precipitation may increase because of longer dry spells and greater accumulation of atmospheric moisture between events.
      (p 48)

      This portends increased incidence and duration of drought, punctuated by extreme precipitation, which may be either rainfall or snowfall, depending on latitude and season.
      In general, the risk of drought is expected to increase during summers in the continental interiors. …

      The Asian, African, and Australian monsoons are projected to bring increased rainfall to certain regions of these continents.
      Because this rain falls during what is already the rainy season, it may cause more flooding without bringing additional benefits.
      In Mexico and Central America, the monsoon is projected to bring less precipitation to the region, contributing to the increased drought generally projected for the region.


      Hotter temperature extremes and more frequent, more intense, and longer-lasting heat waves [will lead to] increased heat-related illness and mortality.
      Growing seasons will also become longer because of earlier spring warming and later fall cooling, but crops will face greater heat stress and associated drought during the growing season.
      Cold spells will become less frequent, causing fewer deaths and economic losses …

      Tropical cyclones and mid-latitude storms

      Intensification of winter mid-latitude storms may bring more frequent severe snow storms, such as those experienced in the northcentral United States in February and March of 2007.
      Near coasts, both tropical and mid-latitude storms will increase wave heights and storm surge heights, increasing the incidence of severe coastal flooding …
      • [In] 2004 Hurricane Catarina became the only hurricane to strike Brazil in recorded history.
      • [In] 2005 the remnants of Hurricane Vince became the first tropical storm on record to make landfall on the Iberian Peninsula.
      • In June 2007 Cyclone Gonu, the first category five hurricane documented in the Arabian Sea, temporarily halted shipping through the Strait of Hormuz, the primary artery for exporting Persian Gulf oil.
      Whether such historical aberrations are related to global warming remains unknown …
      [However, extreme] weather events exceeding historical precedents [are anticipated effects] of climate change
    (p 49)

    Singular, Abrupt Events

      Collapse of the Atlantic meridional overturning circulation

      [Models] project a slowing of the Atlantic MOC of up to 60 percent, but none indicates a complete shutdown during the 21st century.
      As a result, the IPCC places the likelihood of a shutdown of the MOC during the 21st century at not more than 10 percent.
      [A] slowing of the MOC of up to 60% does not produce a cooling of Europe, as the warming effect of increasing atmospheric greenhouse gases outweighs the cooling effect of the slowing MOC.

      If, however, the rate of warming and loss of polar ice has been underestimated, as assumed in scenario three, then the chance of a collapse during this century could be considerably higher.
      Should an abrupt shutdown occur, a cooling of the North Atlantic region, including northwestern Europe, is more likely. …
      NW Arnell [1] [has] simulated a shutdown of the Atlantic MOC in a global circulation model in the year 2055 and followed its subsequent effects on water resources, energy use, human health, agriculture, and settlement and infrastructure.
      [The] effects of a MOC collapse in scenario three [are based] on the results of that study.
      (p 50)

      Arnell forced a global climate model (HadCM3) with greenhouse gas emission scenario SRES A2 and separately forced a shutdown of the MOC by imposing an artificial freshwater pulse in the North Atlantic. …
      The impact of shutting down the MOC was compared to impacts of the A2 scenario without the freshwater pulse to shut down the MOC. …

      Areas of northwestern Europe cooled by as much as 3°C, with broader areas of Europe and northeastern North America cooling by 1 to 2°C.
      Many other parts of the world warmed because of a redistribution of heat from changes in ocean currents. …

      Precipitation changes were more widespread than cooling, with attendant changes in runoff, drought, and flooding.
      • The largest decreases in precipitation occurred in North Africa, the Middle East, Central America, the Caribbean, and northeast South America, including Amazonia. …
        {Several of the world’s major grain-exporting regions, particularly in North America and South Asia, were affected by increased drought …}
      • The largest increase in precipitation was centered on the southwestern United States, providing a net reduction in the number of people in the country under water stress.
        {[Increased] precipitation in East Africa and East and Southeast Asia results in a net of one billion fewer people under water stress with MOC collapse, but adds to flood hazards in these regions. …}

      [While] the effects of accelerated global warming without MOC collapse are larger than the effects of MOC collapse … accelerated climate change is expected to intensify current precipitation patterns, offering some degree of predictability and maintaining current geographic patterns of large-scale food production.
      By reorganizing precipitation patterns, MOC collapse may threaten major crop regions with decreased precipitation, raising the possibility of major disruptions in global food supply.

    1. NW Arnell, Global impacts of abrupt climate change: an initial assessment, Tyndall Centre for Climate Change Research, Working Paper 99, 2006.

    (p 51, italics added)

      Abrupt sea level rise

      The IPCC projects sea level rise in the range of 0.18 to 0.59 meters by the end of the century. [However,] this projection excludes an estimate of accelerated ice loss from the Greenland and Antarctic Ice Sheets …
      [And while] IPCC projections depict a gradual change in sea level over the next century … abrupt and intermittent rises may be more likely …

      [About] one-tenth of the world’s population lives in coastal regions within 10 meters of sea level, and the global population continues to migrate coastward. …
      [Although] it is currently extremely difficult to quantify future damage to humanity from sea level rise … damage from a rise of 2 meters during the current century would clearly be catastrophic for many regions …

      Sea level rise causes or contributes to several distinct types of impacts, including:
      • inundation,
      • increased flooding from coastal storms,
      • coastal erosion,
      • saltwater intrusion into coastal water supplies,
      • rising water tables, and
      • coastal and upstream wetland loss with attendant impacts on fisheries and other ecosystem services. …
      There are dozens of coastal cities worldwide in both industrialized and developing nations that lie at least partly below 1 to 2 meters elevation, but most of them have flood protection.
      Hence, inundation from extreme high tides alone might not [give rise to crises in] most of these cities within the coming century [—] although enhanced defenses will be required to avoid increasing damages. …

      [Coastal] wetlands are probably the most vulnerable of all natural systems to inundation …
      (p 52)

      … 75% of the commercial fish catch and 90% of recreational fish catch in the United States depends on wetlands that serve as nurseries and feeding grounds for fish and shellfish. …
      Chronic saltwater inundation [— which cannot be controlled by levees or other surface-level devices —] is devastating to agricultural production [and] coastal groundwater supplies …

      [In] the near term … more frequent and more severe flooding from coastal storms is likely to be the largest impact of sea level rise along low-lying coastlines.

      Existing flood protection systems built to withstand extreme storm surges will be overcome much more frequently as local sea levels rise. …
      • levees around New Orleans were designed to withstand storm surges associated with category three hurricanes, which historically attained heights of 2.8 to 3.7 meters.
        Such defenses would be reduced effectively to category two-level protection with 1 meter of sea level rise and category one-level protection with 2 meters of sea level rise. …
      • current flood defenses in New York City were designed to protect against the 100-year flood …
        However, 1 meter of sea level rise would lower the return interval of such a flood to as little as five years.
        This estimate does not account for storm intensification, which would raise maximum storm surge and wave heights further …
      The most critical areas of low-lying coastlines are cities and farmed deltas.
    (p 53)

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