Heat

Turn Down the Heat: Why a 4°C Warmer World Must Be Avoided
The climate modeling community can provide projections of global
mean warming and even regional climatic changes up to at least
a 4–5°C warming, albeit with increasing uncertainty. For most
regions, the patterns of climate change projected for 2°C warming
are expected to be roughly similar, but substantially greater for
warming of 4°C. However, lurking in the tails of the probability
distributions are likely to be many unpleasant surprises. The new
projections for unprecedented heat waves and temperature extremes
for 4°C warming are one illustration of this. Many systems and
changes in the extremes have much more impact than changes in
the mean. Researchers expect that many extremes, including heat
waves, droughts, extreme rainfall, flooding events, and tropical
cyclone intensity, are likely to respond nonlinearly to an increase
in global mean warming itself. They are already observing some of
these effects, which are forcing a recalibration of important impact
parameters, such as the responses of crops and the agricultural
system to climate change. Warming to these levels of risks commits
the climate system to very long-term warming (Solomon,
Plattner, Knutti, and Friedlingstein 2009; Hare and Meinshausen
2006) and to impacts, such as very long-term, multimeter sea-level
rise, because of the response of the ice sheets over thousands of
years (Huybrechts et al. 2011)
The scale and rapidity of climate change will not be occurring
in a vacuum. It will occur in the context of economic growth
and population increases that will place increasing stresses and
demands on a planetary ecosystem already approaching, or
even exceeding, important limits and boundaries (Barnosky et
al. 2012; Rockström et al. 2009). The resilience of many natural
and managed ecosystems is likely to be adversely affected by
both development and growth, as well as the consequences of
climate change.
Although systems interact, sometimes strongly, present tools
for projecting impacts of climate change are not yet equipped to
take into account strong interactions associated with the interconnected
systems impacted by climate change and other planetary
stresses, such as habitat fragmentation, pollution, and invasive
species (Warren 2011). Scientific findings are starting to indicate
that some of these interactions could be quite profound, rather
than second-order effects. Impacts projected for ecosystems, agriculture,
and water supply in the 21st century could lead to largescale
displacement of populations, with manifold consequences
for human security, health, and economic and trade systems.
Little is understood regarding the full human and economic
consequences of a collapse of coral reef ecosystems, combined
with the likely concomitant loss of marine production because of
rising ocean temperatures and increasing acidification, and the
large-scale impacts on human settlements and infrastructure in
low-lying fringe coastal zones of a 1 m sea-level rise within this
century. While each of these sectors have been examined, as yet
researchers do not fully understand the consequences for society
of such wide ranging and concomitant impacts, many of which
are likely before or close to 4°C warming.
An aspect of the risks arising from climate change that requires
further research to better understand the consequences for society
is how nonlinear behavior in the Earth and human systems will
alter and intensify impacts across different levels of warming. This
is discussed in the following sections.
Risks of Nonlinear and Cascading
Impacts
In the outline of impacts presented in this report, an implicit
assumption in nearly all of the modeling and assessment exercises
is that the climate system and affected sectors will respond in a
relatively linear manner to increases in global mean temperature.
Large-scale and disruptive changes in the climate system, or its
operation, are generally not included in modeling exercises, and
not often in impact assessments. However, given the increasing
likelihood of threshold crossing and tipping points being reached or
breached, such risks need to be examined in a full risk assessment
exercise looking at the consequences of 4°C warming, especially
considering that even further warming and sea-level rise would
be expected to follow in the centuries ahead. What follows is a
sketch of potential mechanisms that point to a nonlinearly evolving
cascade of risks associated with rising global mean temperature.
The list does not claim to be exhaustive; for a more extensive
discussion, see, for example, Warren (2011).
Nonlinear Responses of the Earth
System
With global warming exceeding 2°C, the risk of crossing activation
thresholds for nonlinear tipping elements in the Earth System
and irreversible climate change impacts increases (Lenton et al.
2008), as does the likelihood of transitions to unprecedented climate
regimes. A few examples demonstrate the need for further
examination of plausible world futures.
Amazon Rain Forest Die-back
There is a significant risk that the rain forest covering large areas
of the Amazon basin will be lost as a result of an abrupt transition
in climate toward much drier conditions and a related change in
the vegetation system. Once the collapse occurs, conditions would
likely prevent rain forest from re-establishing. The tipping point
for this simulation is estimated to be near 3–5°C global warming
(Lenton et al. 2008; Malhi et al. 2009; Salazar and Nobre 2010).
A collapse would have devastating consequences for biodiversity,
the livelihoods of indigenous people, Amazon basin hydrology
and water security, nutrient cycling, and other ecosystem services.
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