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.
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.
- 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.
(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).
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)
