Most analysts would agree that the … chances of keeping below 2°C [are] extremely slim, with 3°C much more likely, and [4°C a real possibility.]
— Four degrees and beyond, Philosophical Transactions of the Royal Society A, 13 January 2011, p 15.
Contents
Four Degree World
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Royal Society
- Four degrees and beyond: the potential for a global temperature increase of four degrees and its implications, Philosophical Transactions of the Royal Society A: Physical, Mathematical and Engineering Sciences, 369:6-19, 13 January 2011.
Mark New, Diana Liverman, Heike Schroeder & Kevin Anderson.
Introduction
The idea of a 2°C temperature target derives partly from a convergence of two themes in the IPCC assessments.- First, an accumulation of potential impacts, with increasing certainty and severity, when moving from a 2°C warming to 3°C and 4°C, suggested that many of the more serious impacts could be avoided by keeping below 2°C …
- Second, a sequence of the IPCC mid-range emission scenarios projected global temperature increases of 2°C by the end of the twenty-first century. …
It was … a pragmatic choice, being both potentially achievable … and a catchy number.
Feasibility of 2°C
[Bowerman et al.] show that a total of 1 trillion tonnes of carbon (TtC) from 1750 to 2500 will produce a ‘most likely’ peak warming of 2°C.
[The] uncertainties in carbon cycling and climate system response to atmospheric CO2 mean that there is considerable likelihood of exceeding … 2°C.
[The] relationship between cumulative emissions and peak temperature is largely insensitive to the emissions pathway; therefore, a continued steep rise of emissions after 2010, with a high peak and steep post-peak decline, can produce the same peak temperature as a flatter emissions profile, provided they both keep to a 1 TtC cumulative total.
[For] less aggressive emissions reduction policies: 2 and 3 TtC result in most likely peak temperatures of 3°C and 4°C over preindustrial.
(p 8)
[Anderson & Bows] explore how the approximately 0.5 TtC that can still be emitted while remaining within the 1 TtC total could be apportioned between Annex 1 and non-Annex 1 nations.
Any reasonable assumptions about when non-Annex 1 emissions might peak and how steeply they will be able to decline chew up most of the remaining budget, requiring Annex 1 nations to immediately and radically reduce emissions at rates steeper than have been contemplated by most previous studies.
[They] conclude that keeping below 2°C is virtually impossible …
While the shape of any emissions profile leading to a particular cumulative budget is not a determinant of peak temperature, it does affect the rate of warming …
These different rates of warming have important implications …
Implications of policy failure
Betts et al. use a series of global climate model simulations … to explore the timings of climate change under a high-end, roughly business-as-usual scenario, IPCC SRES A1FI, where emissions have reached 30 Gt of CO2 (8 GtC) per year by 2100.
All but two of the models reach 4°C before the end of the twenty-first century, with the most sensitive model reaching 4°C by 2061, a warming rate of 0.5°C per decade.
All the models warm by 2°C between 2045 and 2060.
(p 9)
What might a 4°C world look like?
Land areas warm more than the oceans, so for almost all areas of human habitation, temperature increases will exceed, frequently by more than one-and-a-half times, the global average.
Temperature changes at high latitudes are projected to be especially amplified, largely owing to snow and ice albedo feedbacks …
[Boreal] summer temperatures are at least twice the global average warming, and Arctic Ocean winter temperatures warm three times faster than average.
While global average precipitation is projected to increase, most areas that are currently arid and semi-arid are projected to dry, while the moist tropics and mid-latitudes are projected to become wetter, a signal that appears to be emerging in recent precipitation trends. …
Sanderson et al. [compared] global climate models that warm by at least 4°C by 2100 with those that warm less rapidly under the IPCC SRES A2 emissions scenario. …
In areas where precipitation decreases, temperature increases tend to be amplified, probably owing to reduced evaporative cooling of the land surface.
The broadly constant ratio of local climate change to global temperature change implies that these local changes are amplified in a 4°C world; for example, a local change of 3°C in a +2°C world [becomes 7.5°C in a +4°C world. …]
Nicholls et al. … propose that in a world that warms by 4°C by 2100, global sea level will increase between 0.5 and 2m by the end of the century …
A warming of 4°C will also commit the world to larger SLRs beyond 2100, as the ocean equilibrates thermally to atmospheric warming …
[These] post-2100 increases could be large should irreversible melting of the Greenland ice sheet be triggered and some level of break-up of the West Antarctic ice sheet occur.
There are a range of other potential thresholds in the climate system and large ecosystems that might be crossed as the world warms from 2°C to 4°C and beyond.
These include- permanent absence of summer sea ice in the Arctic,
- loss of the large proportion of reef-building tropical corals,
- melting of permafrost at rates that result in positive feedbacks to greenhouse gas warming through CH4 and CO2 releases and
- die-back of the Amazon forest.
While the locations of these thresholds are not precisely defined, it is clear that the risk of these transitions occurring is much larger at 4°C …
[The] nature of the changes in climate we experience may well start shifting from incremental to transformative.
(p 10)
Impacts and adaptation
Zelazowski et al. examine the changes in potential climatic niche for humid tropical forests under 2°C and 4°C global warming scenarios.
In South America, African and Asia, large fractions of current environmental niches are lost, but there are also gains, especially on the western margins of the Congo basin …
However … much of the area that might become suitable is already under agriculture, so the chances of successful migration of forests into these areas are either slim or, at best, rather slow. …
Nicholls et al. evaluate the range of impacts from SLR should the world warm by 4°C by 2100. …
[For] a 2m SLR: the annual cost of enhancing and maintaining sea defences that have kept up with rising sea levels through to 2100 is $270 billion.
Many of the nations most at risk from SLR — such as Bangladesh and Vietnam — will find it difficult to meet the costs of full protection without contributions from richer nations …
[The] risk of continued SLR after global temperatures have peaked means that many more people could be at risk, particularly if areas that are protected up to 2100 are then forced to be abandoned.
(p 11)
[In] some river basins, at 4°C, climate change starts to outweigh population growth as the primary driver of water stress.
[Even] small increases in global temperature are projected to reduce crop yields owing to combined increases in heat and water stress.
Much of sub-Saharan Africa (SSA) is thought to be particularly vulnerable, owing to a combination of climate change and limited adaptive capacity of small farmers.
A warming of 4°C or higher will exacerbate these stresses, but also raises the question of whether particular types of agriculture become unsustainable [due to] intolerable frequencies of crop failure or complete loss of suitable growing conditions.
[The] potential impacts of a 5°C global temperature increase on SSA agriculture [include] drying over most of the region … resulting in large decreases in growing season length, especially (greater than 20%) in the Sahel and over most of southern Africa. …
[In] much of southern Africa [rain-fed crop] failures are projected to occur once every 2 years. …
Within any sector — water, agriculture, coastal flooding or ecological function — impacts are clearly amplified in a +4°C world …
[Furthermore,] interactions between these sectors … may result in societal impacts that are greater than the sum of individual sectoral impacts …
[For] example, shifts to biofuel production … and programmes to prevent forest conversion to agriculture may place an additional stress on food and water security. …
The larger impacts on society associated with a 4°C world clearly present greater challenges for adaptation …- [The] continued failure of the parties to the UNFCCC to agree on emissions reductions means that those planning adaptation responses have to consider a wider range of possible futures, with a poorly defined upper bound.
- [Responses] that might be most appropriate for a 2°C world may be maladaptive in a +4°C world …
For example, a reservoir built to help communities adapt to moderate temperature increases may become dry if they continue to increase, or coastal protection designed for 2°C may be overcome at 4°C.
- [For] some of the more vulnerable regions, a +4°C world may require a complete transformation in many aspects of society …
[For] example, high crop failure frequency in southern Africa may require shifts to entirely new crops and farming methods, or SLR may require the relocation of cities. …
An interesting dynamic emerges between the potential impacts of climate change and the rate at which climate change occurs …- If climate warms rapidly — as might occur with a steep rise in emissions, with a high peak emissions rate [—] a temperature of anywhere between 2°C and 4°C might be reached by the 2050s or 2060s, precisely at the time when vulnerability as a result of population demands for food and water is highest.
A slower rise in temperature … would mean that maximum climate impacts would occur after demand for food and water begins to decline in line with a shrinking population. - [Early] and rapid warming reduces the time available for adaptation [and] will require a transformative rather than incrementalist adaptive response.
Faster and more serious impacts require more resources [and may lead to] the least well-resourced communities [being] ‘left behind’.
Mitigation options outside of the UN Framework Convention on Climate Change
The lack of agreement [among nation states has] led to a renewed focus on the role and potential of non-nation-state actors (NNSAs) — regional, city and local government, the private sector, nonprofit organizations and individuals …
(p 13)
[Cities] produce somewhere between 30 and 75% of global greenhouse gas emissions …
[London has] committed to emission reductions of 60% below 1990 baseline levels by 2025 and [Los Angeles] to 35% [by 2030. …]
[Networks] such as ‘Cities for Climate Protection’ and the C40 group of global cities … share best [practice] strategies and advocate for strong climate policies [in] support [of] local action.
California [has committed] to an 80% cut in emissions by 2050 and adopting standards for the carbon content of products that [has sent] ripples across the USA.
[The establishment of regional] carbon markets such as the Regional Greenhouse Gas Initiative (RGGI) or Western Climate Initiative (WCI) [mean that] more than half the [U.S.] population [live] within a jurisdiction with a greenhouse gas emissions reduction commitment …
[New] automobile standards and renewable obligations are also controlling emissions …
[Major corporations] from the energy, manufacturing, mining, cement and retail sectors [are taking steps to reduce their emissions …]
[Some] to comply with government policies or to take advantage of carbon trading opportunities …
[Others to] reflect pledges on corporate social responsibility or the realization that there may be market savings in low-carbon pathways and market gains from being seen as a ‘green brand’.
[In aggregate these actions] are likely to be insufficient alone to avoid a 2°C or 3°C climate change, but they may help to reduce the rates of global emissions growth [and thus provide more time for a global agreement to be reached.]
(p 14)
Geoengineering: the silver bullet?
[The 2009] report on geoengineering by the Royal Society [concluded] that although geoengineering is technically possible, there are major uncertainties, and that geoengineering provides no justification to diminish efforts in mitigation and adaptation.- [Stratospheric] aerosols are effective, affordable and timely [but] potentially less safe than afforestation or carbon removal and storage.
- Afforestation is considered less effective and timely …
- [Ocean] fertilization is generally seen as less effective, affordable, timely and safe than other technologies.
- [Carbon] dioxide removal may be preferable, partly because it deals with the ocean acidification problem …
- [Solar] radiation management may be necessary to respond to rapid climate change or tipping points because it [could] be implemented faster.
[However,] there are risks that any interruption could result in a sudden rise in temperature and thus a rapid return to higher temperatures associated with high greenhouse gas concentrations.
Some of the less risky approaches are those with long implementation times — during which temperatures may continue to increase …
[Whereas] a failure to sustain the shorter term technologies of solar radiation management could bring a swift rise in the temperature.
[Whether] the risks of temperatures as high as 4°C are so great that geoengineering research and implementation should be accelerated [remains a subject of debate.]
Research agenda for a 4°C world
- [The] broadly linear response seen in many components of the climate system — ENSO, monsoon rainfall, ocean circulation — under more moderate warming may break down in a +4°C world.
[Climate] model simulations that persist for many decades at 4°C or higher [are needed] to explore the potential behaviour and stability of these key climate process phenomena. - [Furthering understanding] of biogeochemical feedbacks, such as Arctic methane release, ocean CO2 drawdown, ocean floor methane hydrates and forest carbon cycles [is] critical to putting constraints on the upper end of climate change.
- [Research is needed into] the existence and location of thresholds such as permanent crop failure … to enable the sort of new thinking about adaptation that is required and to estimate the costs and needed development investments to support such adaptation.
- Research into flexible, staged approaches to adaptation that are robust to significant uncertainties is needed [in order to design an] institutional framework that [integrates] both incremental and transformative [adaptive strategies] across sectors and … geographical scales.
- [Modelling] of future emissions pathways needs to work within a cumulative budget framework, rather than stabilization of concentrations in the atmosphere.
This allows for- incorporation of recent and concurrent emissions data, providing a realistic launch point for emissions profiles that fit the required budgets for a particular temperature target [and]
- transparent discussion of how remaining emissions can be partitioned between nations.
- Further research [into] geoengineering, with careful consideration of the relative risks, costs and benefits of alternative technologies as compared with the costs and benefits of mitigation and adaptation. …
- [Different] emissions pathways leading to the same peak temperature can result in quite different warming rates, and so further exploration of the sensitivity of systems to different rates of warming is required.
- A clearer understanding of the aggregate contributions of NNSAs towards mitigation and adaptation efforts is required, and how they can be promoted and fostered alongside international and domestic efforts. …
(p 16) - Climate change: a summary of the science, September, 2010.
