How Likely is a 4°C World?
[If the] emission pledges made at … Copenhagen and Cancun [were] fully met [they would] place the world on a trajectory for a global mean warming of well over 3°C [— with] about a 20% chance of exceeding 4°C in 2100.
If these pledges are not met:
- [On a medium business-as-usual reference pathway] there is a … more than 40% [likelihood] of warming exceeding 4°C by 2100 [— with] a 10% [risk] of this occurring [by the 2070s.]
- On a higher fossil fuel intensive business-as-usual pathway {(SRES A1FI) warming reaches 4°C by the 2080s [— with a] probability of 10% of exceeding this level by the 2060s.}
There are technically and economically feasible emission pathways that could still limit warming to 2°C or below in the 21st century.
(p 23)
Changes in Extreme Temperatures
[Toward] the end of the century [using SRES A2 assumptions] about every second European summer could be as warm as or warmer than the summer of 2003 [— a 5 sigma event with a natural return time of several million years].
[Under] unmitigated emission scenarios,
- the European summer of 2003 would be classed as an anomalously cold summer relative to the new climate by the end of the century …
- days exceeding the present-day 99th percentile occur more than 20 times as frequently [and]
- the intensity, duration, and frequency of summer heat waves are expected to be substantially greater over all continents, with the largest increases over Europe, North and South America, and East Asia.
[These are the expected consequences of a 3+°C rise in global average temperatures.
The effects of a 4+°C degree rise would almost certainly be more severe.]
(p 37)
Frequency of Significantly Warmer Months
[In 4+°C world the] warmest July month in the Sahara and the Middle East will see temperatures as high as 45°C, or 6–7°C above the warmest July simulated for the present day.
In the Mediterranean and central United States, the warmest July in the period 2080–2100 will see temperatures close to 35°C, or up to 9°C above the warmest July for the present day.
[In] the Southern Hemisphere, [monthly average temperatures] will be as warm as 40°C in Australia, or about 5°C warmer than the most extreme present-day January.
[These are monthly average temperatures —] which include night-time temperatures.
Daytime [daily maximum] temperatures can be expected to significantly exceed the monthly average.
Monthly heat extremes exceeding 3 standard deviations or more that occur during summer months are associated with the most prolonged, and therefore high-impact, heat waves.
[The] number of such prolonged heat waves will increase dramatically … over essentially all continental regions, with the tropics and the [Northern Hemisphere] subtropics and mid-latitudes most severely impacted.
The Impacts of More Frequent Heat Waves
Prolonged heat waves are generally the most destructive as mortality and morbidity rates are strongly linked to heat wave duration, with excess deaths increasing each additional hot day.
[In a 4+°C world] a completely new class of heat waves, with magnitudes never experienced before in the 20th century, would occur regularly. …
[The] agricultural sector would be strongly impacted as extreme heat can cause severe yield losses.
Ecosystems in tropical and sub-tropical regions would be particularly vulnerable … as they are not adapted to extremes never experienced before.
[The] increase in absolute temperatures relative to the past variability is largest in these regions and thus the impacts on ecosystems would become extreme here.
(p 40, emphasis added)
Sea Level Rise
[During the last interglacial, 120,000 years ago, when) global mean temperature was … likely 1-2°C above current values [—] sea level was 6.6–9.4 m above the present level …
[This indicates that] ice sheets may have been very sensitive to changes in climate conditions and [have collapsed] in the past.
(p 30-31)
Risks of Sea-level Rise
[In] the Caribbean [total] cumulative capital GDP loss [is] estimated at US$68.2 billion [(8.3% of GDP) by] 2080, including
- present value of permanently lost land [and]
- relocation and reconstruction costs.
Annual GDP costs [are estimated] at $13.5 billion (1.6% of GDP) [—] mainly in the tourism and agricultural sectors.
{The tourism industry —] a major source of economic growth in these regions [— is predicted] to be very sensitive to sea-level rise.}
These estimates do not include
- water supply costs,
- increased health care costs,
- nonmarket damages, and
- increased tropical cyclone damages.
Large areas of important wetlands would be lost, affecting fisheries and water supply for many communities: …
- 22% in Jamaica,
- 17% in Belize, and
- 15% in the Bahamas …
(p 34, emphasis added)
What are Emissions Scenarios?
The Special Report on Emissions Scenarios (SRES), published by the IPCC in 2000, has provided the climate projections for the Fourth Assessment Report (AR4) of the Intergovernmental Panel on Climate Change (IPCC).
They do not include mitigation assumptions.
Since then, a new set of four scenarios (the representative concentration pathways or RCPs) has been designed, which includes mitigation pathways.
The Fifth Assessment Report (AR5) will be based on these.
SRES Scenarios
The SRES study includes consideration of 40 different scenarios, each making different assumptions about the driving forces determining future greenhouse gas emissions.
These emissions scenarios are organized into families:
[The] A1 family [subsets are] based on their technological emphasis:
- A1FI: An emphasis on fossil-fuels …
- A1B: A balanced emphasis on all energy sources.
- A1T: Emphasis on non-fossil energy sources.
Observed emissions rates
Between the years 2000-2009, growth in CO2 emissions from fossil fuel burning was, on average, 3% per year, which exceeds the growth estimated by [at least 34] of the 40 SRES scenarios …
Human-caused greenhouse gas emissions set a record in 2010, a 6% jump on 2009 emissions, exceeding even the "worst case" scenario cited in the IPCC Fourth Assessment Report.
(Adapted from Wikipedia, 14 March 2013)
Representative Concentration Pathways
… RCPs are consistent sets of projections of only the components of radiative forcing (the change in the balance between incoming and outgoing radiation to the atmosphere caused primarily by changes in atmospheric composition) that are meant to serve as input for climate modeling.
These radiative forcing trajectories are not associated with unique socioeconomic or emissions scenarios, and instead can result from different combinations of economic, technological, demographic, policy, and institutional futures. …
Four RCPs were selected, defined, and named according to their total radiative forcing in 2100:
- RCP 8.5:
Rising radiative forcing pathway leading to 8.5 W/m² in 2100 - RCP 6:
Stabilization without overshoot pathway to 6 W/m² at stabilization after 2100 - RCP 4.5:
Stabilization without overshoot pathway to 4.5 W/m² at stabilization after 2100 - RCP 3-PD:
Peak in radiative forcing at ~ 3 W/m² before 2100 and decline
(p 22)
(Turn Down the Heat, 2012)
Contents
21st Century Projections
How Likely is a 4°C World?
Ocean Acidification
Droughts and Precipitation
Tropical Cyclones
Sea Level Rise Projections
Changes in Extreme Temperatures
World Bank
- Turn Down the Heat, Potsdam Institute for Climate Impact Research and Climate Analytics, International Bank for Reconstruction and Development / The World Bank, November 2012.
21st Century Projections
The nonmitigation IPCC Special Report on Emissions Scenarios (SRES, 2000) gave a warming range for 2100 of 1.6–6.9°C ["best guess" 2.3–4.5°C] above preindustrial temperatures.
[About] half the uncertainty range [is attributable] to the uncertainties in the climate system response to [rising greenhouse gases. …]
[The] remaining uncertainty being due to different assumptions about how the world population, economy, and technology will develop during the 21st century. …
[A] range of new scenarios was developed for the IPCC AR5, three of which are derived from mitigation scenarios.
Three of [these so-called Representative Concentration Pathways (RCPs)] are derived from mitigation scenarios produced by Integrated Assessment Models (IAMs) that are constructed to simulate the international energy-economic system …
Figure 20
Probabilistic temperature estimates for old (SRES) and new (RCP) IPCC scenarios.
Depending on which global emissions path is followed, the 4°C temperature threshold could be exceeded before the end of the century.
… RCP 8.5, is the only nonmitigation pathway within this AR5 scenario group and is comparable to the highest AR4 SRES scenario (SRES A1FI).
It projects warming by 2100 of close to 5°C [and still steeply rising.]
RCP 6 [assumes] a limited degree of climate policy intervention, [and] projects warming exceeding 4°C by 2100 with a probability of more than 15%.
[The] range of changes in temperature for the RCP scenarios is wider than for the AR4 SRES scenarios.
[This is because] the RCPs span a greater range of plausible emissions scenarios, including both scenarios assuming no mitigation efforts (RCP 8.5) and scenarios that assume … ambitious mitigation efforts (RCP 3-PD).
Figure 21
Probabilistic temperature estimates for new (RCP) IPCC scenarios, based on the synthesized carbon-cycle and climate system understanding of the IPCC AR4.
Figure 22
(p 21, emphasis added)
Median estimates (lines) from probabilistic temperature projections for two nonmitigation emission scenarios (SRES A1FI and a reference scenario close to SRES A1B), both of which come close to, or exceed by a substantial margin, 4°C warming by 2100.
The results for these emissions are compared to scenarios in which current pledges are met and to mitigation scenarios holding warming below 2°C with a 50% chance or more …
How Likely is a 4°C World?
RCP 8.5, the highest of the new IPCC AR5 RCP scenarios, can be used to explore the regional implications of a 4°C or warmer world. …
[Pronounced] warming (between 4°C and 10°C) is likely to occur over land. …
The subtropical region consisting of the Mediterranean, northern Africa and the Middle East and the contiguous United States is likely to see a monthly summer temperature rise of more than 6°C.
(p 24)
Figure 23
The correlation between regional warming and precipitation changes in the form of joint distributions of mean regional temperature and precipitation changes in 2100 is shown for the RCP 3-PD (blue) and RCP 8.5 (orange) scenarios.
Figure 24
(p 25)
Simulated historic and 21st century global mean temperature anomalies, relative to the preindustrial period (1880–1900), for 24 CMIP5 models based on the RCP 8.5 scenario.
CO2 Concentration and Ocean Acidification
The increase of carbon dioxide concentration to the present-day value of 390 ppm has caused the pH to drop by 0.1 since preindustrial conditions [—] equivalent to a 30% increase in ocean acidity …
The scenarios of 4°C warming or more by 2100 correspond to … a 150% [increase in acidity.]
[This] is likely to have very severe consequences for coral reefs, various species of marine calcifying organisms, and ocean ecosystems generally. …
{Reduced growth, coral skeleton weakening, and increased temperature dependence would start to affect coral reefs [at] below 450 ppm.}
[At 450 ppm and above,] coral reef growth around the world is expected to slow … considerably …
[At] 550 ppm reefs are expected to start to dissolve.
(p 24)
[If] multiple stressors [—] such as high ocean surface-water temperature events, sea-level rise, and deterioration in water quality [—] are included [it is estimated that a CO2 level of below 350 ppm is required to ensure the long-term survival of coral reefs. …]
If mitigation measures are not implemented soon to reduce carbon dioxide emissions … ocean acidification can be expected to extend into the deep ocean [and] slowing and reversing [it will become] much more difficult.
(p 25)
Droughts and Precipitation
The most robust large-scale feature[s of the] climate model projections [are]- an increase in precipitation in the tropics …
- a decrease in the subtropics [and]
- an increase in mid to high latitudes.
On the regional scale, observational evidence suggests soil-moisture feedbacks might [provide] a negative feedback that [offsets the] increased dryness trend …
[However, it is] unclear if and how [such] small-scale feedbacks translate to longer time scales and larger subcontinental spatial scales.
[Total precipitation is] generally projected to increase by roughly 10%.
[Extreme] precipitation events [expressed as total annual precipitation during the five wettest days in the year] is projected to increase by 20% in RCP 8.5 (4+°C) [—] indicating an [increased] risk of flooding.
[These] projected increases [are] concentrated in … December, January, and February … over- the Amazon Basin [and] southern South America,
- western [and central] North America …
- northern Europe, and
- Central Asia. …
[Significant] soil moisture decreases are projected to occur over much of the Americas, as well as the Mediterranean, southern Africa, and Australia.
(p 26)
The most extreme droughts … are projected over- the Amazon,
- western United States,
- the Mediterranean,
- southern Africa, and
- southern Australia
Implications for Economic Growth and Human Development
According to [integrated assessment models — which] bring together the biophysical impacts of climate change and economic indicators [—] food prices can be expected to rise sharply, regardless of the exact amount of warming.
[The] SRES A2 scenario (with warming of about 4.1°C above preindustrial temperatures) indicates a significant risk of increased climate-induced poverty.
The largest increase in poverty … is likely to occur in Africa …
[However,] Bangladesh and Mexico [are] also projected to see substantial climate-induced poverty increases.
Tropical Cyclones
[Average] maximum cyclone intensity (defined by maximum speed) is likely to increase in the future (SREX).
[Uncertainty] remains as to whether the global frequency of tropical cyclones will decrease or remain essentially the same.
[In the absence of additional protection measures, increasing] exposure through economic growth and development is likely to lead to higher economic losses in the future …
In the East Asia and Pacific and South Asian regions as a whole, gross domestic product (GDP) has outpaced increased losses …
[In] all other regions … the risk of loss of wealth because of tropical cyclone disasters appears to be increasing faster than wealth.
[Mortality] risk from tropical cyclones depends on such factors as- tropical cyclone intensity,
- exposure,
- levels of poverty, and
- governance structures.
[Warming] reaching roughly 4°C by 2100 is likely to double the present economic damage … with most damages concentrated in- North America,
- East Asia, and
- the Caribbean and Central American region.
Sea-Level Rise Projections
[Process based projections of sea level rise are use] numeric models that represent the physical processes at play [—] such as air, temperature, and precipitation.
[However, in] the case of Greenland and Antarctic ice sheets … uncertainties in the scientific understanding about the response to global warming lead to less confidence in [projecting] sea-level rise … for the current century.
[Alternative] semi-empirical approaches [— which use] the observed relation between past sea level rise and global mean temperature [over the last 2,000 years] to project future sea level rise [—] have their own limitations and challenges. …
Using a semi-empirical model indicates that scenarios that approach 4°C warming by 2100 (2090–2099) lead to median estimates of sea-level rise of [96 cm] above 1980–1999 levels …
(p 29, italics added)
Components are- thermal expansion,
- mountain glaciers, and ice caps (MGIC),
- Greenland Ice Sheet (GIS), and
- Antarctic Ice Sheet (AIS).
Table 2 (Adapted): Global Mean Sea-Level Projections between Present-Day (1980–99) and the 2090–99 Period
Process Based Semi-empirical Scenario Thermal (cm) MGIC (cm) GIS (cm) AIS (cm) Total (cm) 2°C 19 13 23 23 79 4°C 27 16 26 26 96
(p 31)
[There is also] an unquantifiable risk of nonlinear responses from the West Antarctic Ice Sheet and … other components of Greenland and Antarctica. …
The West Antarctic Ice Sheet is grounded mainly below sea level, with the deepest points far inland, and has the potential to raise eustatic global sea level by about 3.3 m.
This estimate takes into account that the reverse bedslope could trigger instability of the ice sheet, leading to an unhalted retreat. …
[It is] unclear how likely such a collapse is and at what rate it would contribute to sea-level rise. …
Process-based modeling considerations at the very high end of physically plausible ice-sheet melt … suggest that sea-level rise of as much as 2 m by 2100 might be possible at maximum.
(p 29)
(p 30)Predictability of Future Sea-level Changes
Global mean thermal expansion is relatively well simulated by climate models …
Projected melt in mountain glaciers and ice caps is also considered reliable …
The Greenland and Antarctic ice sheets … potential contributions to future global mean sea-level rise is very large, namely 7 m and 57 m, respectively, for complete melting. …
[The] critical threshold for complete disintegration of the Greenland ice sheet might be [a sustained increase of] 1.6°C …
The time frame for such a complete disintegration [is not precisely known but] is of the order of at least several centuries …
[The] physics of the large ice sheets is poorly understood [consequently] model simulations [have] so far [been unable] to reproduce their presently observed contribution to current sea-level rise.
Regional variations of future sea-level also have uncertainties … concerning ocean dynamics …
[Nonetheless,] they remain within reach of the current generation of coupled ocean-atmosphere models …
Regional Sea-level Rise Risks
Sea level is not “flat” nor uniformly distributed over the Earth.
The presence of mountains, deep-ocean ridges, and even ice sheets perturb the gravity field of the Earth and give the ocean surface mountains and valleys.
Wind and ocean currents further shape the sea surface, with strong currents featuring a cross-current surface slope (because of Earth rotation).
This effect results in a so-called “dynamic” sea-level pattern, which describes local deviations from the gravity-shaped surface (also called [the] geoid), which the ocean would have if it were at rest.
(p 31)
Climate change perturbs both the geoid and the dynamic topography.
The redistribution of mass because of melting of continental ice (mountain glaciers, ice caps, and ice sheets) changes the gravity field (and therefore the geoid). …
Changes in the wind field and in the ocean currents can also … lead to strong local sea-level changes. …
A clear feature of the regional projections … is the relatively high sea-level rise at low latitudes (in the tropics) and below-average sea-level rise at higher latitudes.
(p 32)
- Close to the main ice-melt sources (Greenland, Arctic Canada, Alaska, Patagonia, and Antarctica), crustal uplift and reduced self-attraction cause a below-average rise, and even a sea-level fall, in the very near-field of a mass source. …
- In the northeastern North American coast {the sea level is rising faster than the global mean, and might continue to do so, if the gravitational depression from the nearby melting Greenland and Canadian glaciers is moderate.}
- Along the eastern Asian coast and in the Indian Ocean, however, which are far from melting glaciers, both gravitational forces and ocean dynamics act to enhance sea-level rise, which can be up to 20% higher than the global mean.
(p 32-33)
Changes in Extreme Temperatures
A Substantial Increase in Heat Extremes
[Our] analysis indicates that monthly heat extremes will increase dramatically in a world with global mean temperature more than 4°C warmer than preindustrial temperatures.
Temperature anomalies that are associated with highly unusual heat extremes today (namely, 3-sigma events occurring only once in [740] years in a stationary climate) will have become the norm over most (greater than 50%) continental areas by the end of the 21st century.
Five-sigma events [— with return times of several millions of years —] will become common, especially in the tropics and in the Northern Hemisphere mid-latitudes during summertime.
(p 37)
Monthly mean temperatures- over oceans will increase between 0°C and 4°C and
- over continents between 4°C and 10°C. …
[In] the near Arctic region [“arctic amplification” would result] in temperature anomalies of over 10°C [during winter. …]
The subtropical region [—] consisting of the Mediterranean, northern Africa, and the Middle East, as well as the contiguous United States [— is] likely to see monthly summer temperatures rise by more than 6°C.
All land areas show a mean warming of at least 1-sigma above the present-day mean and most land areas (greater than 80%) show warming of at least 2-sigma.
Roughly half of the land area will likely experience a mean warming of more than 3-sigma [from December to February] and more than 4-sigma [from June to August].
Shifts in Temperature by Region
[Tropics]
[A] 4°C warmer world will consistently cause temperatures in [South America, Central Africa and the Pacific islands] to shift by more than 6 standard deviations for all months of the year.
[This implies that] the coolest months in 2080–2100 [will be] substantially warmer than the warmest months in the end of the 20th century.
[Southern Hemisphere Mid-Latitudes]
[Monthly] temperatures over the continents by the end of the 21st century lie in the range of 2- to 4-sigma above the present-day mean in both seasons.
[Northern Hemisphere Mid-Latitudes]
Over large regions [the continental warming] is much stronger in summer, reaching 4- to 5-sigma, than in winter.
[In] large regions of North America, southern Europe, and central Asia, including the Tibetan plateau. …- 80% to 100% [of] summer months will be warmer than 3-sigma and …
- about 50% … will be warmer than 5-sigma.
[Due to soil moisture feedbacks] temperatures of the warmest July within the period 2080–2100 in the Mediterranean region … are expected to approach 35°C [or] about 9°C warmer than the warmest July estimated for the present day.
[That is because once] the soil has completely dried out due to strong evaporation during heat waves, no more heat can be converted into latent heat [—] thus further increasing temperatures.
This effect is much more important during summers and has been a characteristic of major heat and drought events in Europe and North America.
(p 39)
