fantastical; net greenhouse emissions would need to be cut by 50% this decade and
100% by around 2050. This would require a transformation of the global energy
system at a nearly unimaginable pace and scale. While a 1.5
C increase itself looks
dire, losing, for example, 70–90% of coral reefs, effects get significantly worse
across all metrics with increases of 2.0
C and beyond (IPCC 2014, 2018). The
present “base case scenario” (RCP 2.6), where temperatures are potentially kept
below 2.0
C, also requires large near-term cuts and net-zero by 2075. The “business
as usual” (RCP8.5) scenario leads to a 5
C increase by 2100, but continued marketbased transitions away from more polluting fuels are predicted to result in “only” a
4
C increase without other major efforts. The most likely scenario, involving
modest mitigation, leaves the planet warming to somewhere between 2.5 and
3.0
C by 2100 (IEA 2019). However, these estimates do not include amplifying
feedback mechanisms, including the increasing probability of forest fires, or
breaching irreversible “tipping points” such as the thawing of permafrost (Lenton
et al. 2019). The Arctic is warming twice as fast as the global mean increasing odds
of the latter (NOAA 2020). Even if CO 2 emissions were abruptly and completely
halted, the global mean temperature will increase for decades, sea levels will rise for
centuries, and the planet would not return to pre-industrial temperatures for thousands of years (Zickfeld et al. 2013; Mauritsen and Pincus 2017).
Whatever the final amount of warming, effects will be dramatic, punctuated, and
unevenly distributed across the planet. Many human populations face dire near-term
consequences, including those inhabiting low-lying islands and coastal regions
exposed to flooding, those dependent on vanishing meltwater, or those where the
combination of heat and humidity will soon make outdoor work, then existence
untenable. New models suggest that in the next 50 years, under the best-case
scenario (RCP 2.6), 1.2 billion people will no longer live within the “human comfort
niche” where humans thrived for the past 6000 years (Xu et al. 2020). Along with
increasing probabilities of extreme weather events (Meehl and Tebaldi 2004) and
threats to food supplies (Ukkola et al. 2020; Dahlke et al. 2020; Costanza et al.
2014), these changes are certain to result in massive population displacements.
Yet, it is not just humans who will be on the move, GCC will impose a “universal
redistribution of life on Earth” (Urban 2015; Pecl et al. 2017). The extent of
relocation will depend on the timing and extent of human responses to GCC and
continued rates and distributions of habitat loss along with other anthropogenic
stresses. Of course, some organisms have flourished under human expansion.
These include Coffea arabica, which has vastly increased in range and abundance
since human discovery, although GCC is beginning to reverse this trend (Imbach
et al. 2017). Also included are many disturbance-adapted organisms, and species
exploiting the global commercial infrastructure as a dispersal mechanism. Organisms without such traits, especially those with small endemic ranges, or that flourish
only under a narrow range of environmental conditions, have limited options, and
are expected to fare less well. For example, some coral reef systems, where major
losses are expected no matter what actions are taken, show high levels of endemism
in supported communities (Roberts et al. 2002). In terrestrial systems, many plant
and arthropod species are being extinguished before being known to science. Not
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K. M. Oliver and C. H. V. Higashi
100% by around 2050. This would require a transformation of the global energy
system at a nearly unimaginable pace and scale. While a 1.5
C increase itself looks
dire, losing, for example, 70–90% of coral reefs, effects get significantly worse
across all metrics with increases of 2.0
C and beyond (IPCC 2014, 2018). The
present “base case scenario” (RCP 2.6), where temperatures are potentially kept
below 2.0
C, also requires large near-term cuts and net-zero by 2075. The “business
as usual” (RCP8.5) scenario leads to a 5
C increase by 2100, but continued marketbased transitions away from more polluting fuels are predicted to result in “only” a
4
C increase without other major efforts. The most likely scenario, involving
modest mitigation, leaves the planet warming to somewhere between 2.5 and
3.0
C by 2100 (IEA 2019). However, these estimates do not include amplifying
feedback mechanisms, including the increasing probability of forest fires, or
breaching irreversible “tipping points” such as the thawing of permafrost (Lenton
et al. 2019). The Arctic is warming twice as fast as the global mean increasing odds
of the latter (NOAA 2020). Even if CO 2 emissions were abruptly and completely
halted, the global mean temperature will increase for decades, sea levels will rise for
centuries, and the planet would not return to pre-industrial temperatures for thousands of years (Zickfeld et al. 2013; Mauritsen and Pincus 2017).
Whatever the final amount of warming, effects will be dramatic, punctuated, and
unevenly distributed across the planet. Many human populations face dire near-term
consequences, including those inhabiting low-lying islands and coastal regions
exposed to flooding, those dependent on vanishing meltwater, or those where the
combination of heat and humidity will soon make outdoor work, then existence
untenable. New models suggest that in the next 50 years, under the best-case
scenario (RCP 2.6), 1.2 billion people will no longer live within the “human comfort
niche” where humans thrived for the past 6000 years (Xu et al. 2020). Along with
increasing probabilities of extreme weather events (Meehl and Tebaldi 2004) and
threats to food supplies (Ukkola et al. 2020; Dahlke et al. 2020; Costanza et al.
2014), these changes are certain to result in massive population displacements.
Yet, it is not just humans who will be on the move, GCC will impose a “universal
redistribution of life on Earth” (Urban 2015; Pecl et al. 2017). The extent of
relocation will depend on the timing and extent of human responses to GCC and
continued rates and distributions of habitat loss along with other anthropogenic
stresses. Of course, some organisms have flourished under human expansion.
These include Coffea arabica, which has vastly increased in range and abundance
since human discovery, although GCC is beginning to reverse this trend (Imbach
et al. 2017). Also included are many disturbance-adapted organisms, and species
exploiting the global commercial infrastructure as a dispersal mechanism. Organisms without such traits, especially those with small endemic ranges, or that flourish
only under a narrow range of environmental conditions, have limited options, and
are expected to fare less well. For example, some coral reef systems, where major
losses are expected no matter what actions are taken, show high levels of endemism
in supported communities (Roberts et al. 2002). In terrestrial systems, many plant
and arthropod species are being extinguished before being known to science. Not
266
K. M. Oliver and C. H. V. Higashi