Abrupt Impacts of Climate Change Anticipating Surprises (2013) / Chapter Skim
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2 Abrupt Changes of Primary Concern
2 Abrupt Changes of Primary Concern
Pages 39-126
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From page 39... ...
As described in the Introduction, this section examines both abrupt climate changes in the physical climate system itself and abrupt climate impacts in physical, biological, or human systems that are triggered by a steadily changing climate. Abrupt Changes in the Ocean The Atlantic Meridional Overturning Circulation The Atlantic Meridional Overturning Circulation (AMOC)
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From page 40... ...
Following on this examination, questions have arisen as to the possible likelihood of an abrupt change in the future. The Stability of the Atlantic Meridional Overturning Circulation Climate and Earth system models are used to understand potential changes in the AMOC, including potential feedbacks in the system, although the representation of unresolved physics (such as the parameterization of ocean mixing)
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From page 41... ...
. Rapid melting of the Greenland ice sheet causes increases in freshwater runoff, potentially weakening the AMOC.
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From page 42... ...
initiation of convection and subsequent spin-up of North Atlantic Deep Water (NADW) formation.
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From page 43... ...
This would explain why abrupt changes of the AMOC appear to be pervasive features of the paleoclimate record when vast reservoirs of freshwater were available in the form of ice and proglacial lakes on land. A question that needs to be further addressed is the extent to which projected changes in Greenland ice sheet melting could affect the amount and location of 43
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From page 44... ...
Observations of the Atlantic Meridional Overturning Circulation Recent observational studies have focused on ascertaining two questions of relevance to the AMOC response to climate change: What is the impact of variable North Atlantic Deep Water production on the ocean's meridional overturning? And, what is the current state of the AMOC and its variability?
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From page 45... ...
. The RAPID/MOCHA program (Rapid Climate Change/Meridional Overturning Circulation and Heatflux Array)
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From page 46... ...
is computed from the sum of the Gulf Stream transport through the Florida Straits, directly measured via electromagnetic cables; the Ekman transport, estimated from QuikSCAT winds; and the midocean geostrophic transport, estimated from the moored array instruments. Importantly, this time series demonstrates the significant interannual transport variability.
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From page 47... ...
Summary and the Way Forward Although models do not indicate that AMOC is likely to change abruptly in the coming decades, it is important to monitor the North Atlantic to confirm the understanding of how AMOC responds to a changing climate. Observational studies over the past decade or so reveal a meridional overturning circulation with a tenuous link to the production of deep water masses via local overturning at high latitudes in the North Atlantic.
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From page 48... ...
abrupt impacts of climate chang E Figure 2.4 Existing and proposed monitoring locations for the Atlantic Ocean. Source: Adapted from Schiermeier, 2013.
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From page 49... ...
Although sea-level rise typically is slow compared to many environmental changes, even this type of gradual sea-level rise may force other systems to cross thresholds and trigger abrupt impacts for natural or human systems unless adaptive measures are taken. For example, rising sea level increases the likelihood that a storm surge will overtop a levee or damage other coastal infrastructure, such as coastal roads, sewage treatment plants, or gas lines -- all with potentially large, expensive, and immediate consequences (Nordhaus, 2010)
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From page 50... ...
. In many cases, such areas would be difficult to defend by dikes and dams, and such a large sea level rise would require responses ranging from potentially large and expensive engineering projects to partial or nearcomplete abandonment of now-valuable areas as critical infrastructure such as sewer systems, gas lines, and roads are disrupted, perhaps crossing tipping points for adaptation (Kwadijk et al., 2010)
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From page 51... ...
The low elevation of many parts of the city and surroundings, combined with a water-permeable sand and coral base, make it particularly vulnerable to sea-level rise. Areas at risk from a 1-meter rise in sea level are shown, where 1 meter is within the range of many recently published estimates for sea-level rise by the end of this century.
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From page 52... ...
This may approximate the sea level during the Pliocene period (3–5 million years ago) , the last time that CO2 levels are thought to have been 400 ppm.
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From page 53... ...
. Rise of the global average sea level over the time periods of most interest to human economies is controlled primarily by the mass or density of ocean water.
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From page 54... ...
CO2 minima were reached approximately when the sea level was at a minimum, hence, the extent of the continental ice sheets were at a maximum, and the highest CO2 levels were found during interglacials during the high stands of the sea level. The evolution of the local temperature (as deduced from δD)
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From page 55... ...
The great bulks of the Greenland and Antarctic ice sheets actually have pulled ocean water toward them, so that their coastal sea levels are notably higher than they would be without that gravitational attraction. If the ice melts, adding water to the ocean, it is useful then to think of a two-step process (see Figure A)
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From page 56... ...
Melting of ice raises the global average sea level, and reduces the gravitational attraction from the ice, which allows the sea level near the ice to fall while sea level far from the ice rises more than the global average.
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From page 57... ...
Greenland, as well as (c) melting of the mounting glaciers and ice sheets tabulated by Meier.
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From page 58... ...
Potential Abrupt Changes to Polar Ice Sheets Ice-sheet volume is controlled by the balance between mass input and mass loss; mass input is almost entirely due to snowfall, and mass loss is from iceberg calving supplied by flow of the ice sheet, or runoff of melt water. As summarized in, for example, Meehl et al.
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From page 59... ...
Increased ice-sheet flow can raise sea level by shifting non-floating ice into icebergs or into floating-but-still-attached ice shelves, which can melt both from beneath and on the surface. Rapid sea-level rise from these processes is limited to those regions where the bed of the ice sheet is well below sea level and thus capable of feeding ice shelves or directly calving icebergs rapidly, but this still represents notable potential contributions to sea-level rise, including the deep fjords in Greenland (roughly 0.5 m; Bindschadler et al., 2013)
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From page 60... ...
. The rate of discharge into deep water across a grounding line in general increases 60
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From page 61... ...
Much process-based research coupling field work, remote sensing, and modeling is required to advance assessment of the likelihood of a threshold-crossing leading to abrupt sea-level rise from the ice sheets, as well as to improve projections of moregradual sea-level rise that could lead to threshold-crossing events in other systems. Great progress has been made recently in assessing the current rate of mass loss from the ice sheets (Shepherd et al., 2012)
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From page 62... ...
Because air carries much less heat than an equivalent volume of water, physical understanding indicates that the most rapid melting of ice leading to abrupt sea-level rise is restricted to ice sheets flowing rapidly into deeper water capable of melting ice rapidly and carrying away large volumes of icebergs. In Greenland, such deep water in contact with ice is restricted to narrow bedrock troughs where friction between ice and fjord walls limits discharge.
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From page 63... ...
However, the current rate of increase of atmospheric CO2 exceeds the rate at which natural processes can buffer these pH changes. Although ocean acidification is not an abrupt climate change, i.e., the change occurs at the same rate as the forcing, the impacts of ocean acidification on ocean biology have the potential to cause rapid (over multiple decades)
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From page 64... ...
. Scientists are particularly concerned with the ability of reef-building corals to persist in the face of ocean acidification combined with the other stressors such as temperature increase, sea level rise, and changes in storm intensity all also associated with climate change.
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From page 65... ...
Warming ocean temperatures lead to lower oxygen solubility. A warming surface ocean is also likely to increase the density stratification of the water column (i.e., Steinacher et al., 2010)
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From page 66... ...
However, the lifetime of nitrate in the global ocean is thousands of years, so any change in the global nitrate inventory would also take place on this long time scale. Likelihood of Abrupt Changes Changes in global ocean oxygen concentrations have the potential to be abrupt because of the threshold to anoxic conditions, under which the region becomes uninhabitable for aerobic organisms including fish and benthic organisms.
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From page 67... ...
OMZs are not well represented in global climate models due to limited understanding of the physical and biological processes that affect them. In particular, the processes that lead deep water to be exchanged with the surface water remain poorly understood; for example, how rapidly a given parcel of ocean water is ventilated needs to be better resolved.
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From page 68... ...
Abrupt climate change due to variations in the atmospheric circulation and its attendant patterns of climate variability can arise through two principal mechanisms: (1) through abrupt changes in the time-dependent behavior of the circulation; or (2)
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From page 69... ...
Abrupt Changes of Primary Concern FIGURE 2.8 In the above example, the largest changes in rainfall due to the shift in the circulation are found on the flanks of the original precipitation regions. A slowly evolving change in the circulation may thus lead to seemingly abrupt changes in precipitation in regions where the existing spatial gradients in rainfall are largest.
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From page 70... ...
) , the atmospheric circulation exhibits large and abrupt changes, including a sudden poleward jump in the middle latitude jetstream of roughly 10 degrees latitude.
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From page 71... ...
2. Numerical evidence of a poleward shift in the Southern Hemisphere and North Atlantic middle latitude jetstreams in response to increasing greenhouse gases (e.g., Fyfe et al., 1999; Kushner et al., 2001; Cai et al., 2003; Yin, 2005; Miller et al., 2006; Meehl et al., 2007b; Barnes and Polvani, 2013)
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From page 72... ...
It is also difficult to detect statistically robust abrupt changes in the circulation. Detection of an abrupt climate change requires demonstrating that the system was stationary before and after the change occurred.
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From page 73... ...
. BOX 2.3 COUPLING OF ATMOSPHERIC AND LAND SURFACE AS A CURRENT RESEARCH FRONTIER The coupling between land surface vegetation and atmosphere could also potentially cause abrupt changes of atmospheric circulation at regional scales.
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From page 74... ...
Maintaining and enhancing the current observational network of remotely sensed and in-situ measurements that can be used to infer changes in the atmospheric circulation is essential. The likelihood of abrupt changes in the atmospheric circulation remains unclear, as does the potential for inducing abrupt climate change in regions of large gradients in surface weather.
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From page 75... ...
by total cost. Climate Change Is Affecting Extremes Climate change is expected to shift frequency statistics for weather and climate events, as illustrated in Figure 2.10, in ways that affect the likelihood of extreme events on the tails of the distribution, either the high side ("extremely hot" for example)
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From page 76... ...
While total precipitation in the United States increased by about 7 percent over the past century, the heaviest 1 percent of rain events increased by nearly 20 percent (Bull et al., 2007)
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From page 77... ...
Abrupt Changes of Primary Concern FIGURE 2.10 Potential effects of changes in temperature distribution on extremes: a) effects of a simple shift of the entire distribution toward a warmer climate; b)
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From page 78... ...
Source Hansen et al., 2012. Climate change may also be affecting other weather and climate extremes, with impacts and trends that vary regionally.
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From page 79... ...
Links Between Extreme Events and Abrupt Change While extreme events per se are not abrupt climate changes as defined in this report, changes in extreme events could lead to abrupt changes in two ways: (1) an abrupt change in a weather or climate extremes regime, for example a sudden shift to persistent drought conditions; or (2)
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From page 80... ...
. Summary and the Way Forward The connection between extreme climate and related abrupt climate change is poorly understood, given the relatively poor understanding of both extreme climate events and abrupt changes.
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From page 81... ...
This is central to the ability to improve the quantitative understanding of the thresholds that can trigger abrupt changes and the probability distribution changes of the extreme climate events with the slow varying climate states and forcings that can be monitored. Abrupt Changes at High Latitudes Potential Climate Surprises Due to High-Latitude Methane and Carbon Cycles Interest in high-latitude methane and carbon cycles is motivated by the existence of very large stores of carbon (C)
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From page 82... ...
is particularly worrisome as it is many times more potent as a greenhouse gas than carbon dioxide (CO2) over short time scales.
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From page 83... ...
. Under business-as-usual climate forcing scenarios, much of the upper permafrost is projected to thaw within a time scale of about a century (Camill, 2005, Lawrence and Slater, 2005)
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From page 84... ...
, so the methane fraction of the net carbon emissions to the atmosphere can be, and usually is, much lower than this. Projecting the future water balance and moisture state of Arctic soils -- and thus the ratio of CO2 to CH4 production -- contributes the largest uncertainty in forecasting methane emissions from Arctic land surfaces.
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From page 85... ...
Throughout most of the world ocean, a water depth of about 700 m is required for hydrate stability. In the Arctic, due to colder-than-average water temperatures, only about 200 m of water depth is required, which increases the vulnerability of those methane hydrates to a warming Arctic Ocean.
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From page 86... ...
Potential response to a warming climate Climate change has the potential to impact ocean methane hydrate deposits through changes in ocean water temperature near the sea bed, or variations in pressure associated with changing sea level. Of the two, temperature changes are thought to be most important, both during the last deglaciation (Mienert et al., 2005)
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From page 87... ...
. Over time scales of centuries and millennia, the ocean hydrate pool has the potential to be a significant amplifier of the anthropogenic fossil fuel carbon release.
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From page 88... ...
The largest uncertainty is the concentration of methane hydrate, especially in the shallow sediment column near the sediment water interface. Coupled atmosphere-ocean climate models can be used to simulate the thermal response of the ocean water column to climate change with a moderate degree of uncertainty and the subsequent penetration of heat into the sediment column.
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From page 89... ...
Measurements from aircraft, manned and unmanned, are the third potential monitoring approach, providing vertical resolution of the concentrations, which gives much tighter constraint on local-source fluxes. Summary and the Way Forward Arctic carbon stores are poised to play a significant amplifying role in the centurytimescale buildup of CO2 and methane in the atmosphere, but are unlikely to do so abruptly, on a time scale of one or a few decades.
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From page 90... ...
An instantaneous release, for example, would cause the atmospheric methane concentration to spike immediately, then decay back toward the unperturbed value on a time scale of approximately one decade. The climatic impact of a spike of methane would be shaped by the long time scale of the Earth's temperature response to radiative (greenhouse gas)
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From page 91... ...
Abrupt Changes of Primary Concern BOX 2.4 Continued TABLE Summary of methane release scenarios compared with present-day methane fluxes and the radiative impact of business-as-usual CO2 rise. Arctic Arctic CH4 Increase flux/ Wetland CH4 emission factor relative flux, per m2 CH4 Conc.
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From page 92... ...
Coverage is especially sparse in the continental interiors of Canada and Russia, and most notably so in the vast James Bay and West Siberian lowlands, as they contain very large stocks of frozen soil carbon in the form of peatland soils that have accumulated since the last glacial maximum. A second key component for permafrost monitoring is measurements of active-layer depth (the thickness of seasonally thawed soil, measured downward from the soil surface)
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From page 93... ...
, and given the definitions used in this report, the changes already experienced qualify as an abrupt climate change. Projections from climate models suggest that ice loss will continue in the future, with a possibility of September ice-free conditions later this century (e.g., Stroeve et al., 2012b; Massonnet et al., 2012)
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From page 94... ...
. This bears resemblance to the so-called Rapid Ice Loss Events simulated in a number of climate models (Holland et al., 2006)
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From page 95... ...
Abrupt Changes of Primary Concern FIGURE 2.14 Extent of Arctic sea ice in September 1979, 2000, 2007, and 2013. The magenta line shows the 1981 to 2010 median extent for September.
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From page 96... ...
. And still others suggest a bifurcation in the transition to a year-round ice-free state (e.g., North, 1984, 1990; Ridley FIGURE 2.15 Climate Feedback Loop: The melting of Arctic sea ice is an example of a positive feedback loop.
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From page 97... ...
discusses how differences in the strength of various climate feedbacks can lead to differences in the likelihood of hysteresis in the Arctic sea ice system. The numerical evidence for irreversible change to a year-round ice-free state was first discussed in studies with simple diffusive climate models (e.g., North, 1984, 1990)
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From page 98... ...
Climate models suggest that Antarctic sea ice will decline through the 21st century (e.g., Arzel et al., 2006)
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From page 99... ...
(2006) related simulated rapid ice loss events to anomalous ocean heat transport into the Arctic from the North Atlantic.
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From page 100... ...
More work is needed to determine why different models exhibit different behavior in this regard. Finally, very little work has been done on the Antarctic sea ice system in terms of possible abrupt change.
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From page 101... ...
. The ubiquity of biological response to climate indicates that climate changes underway will cause existing ecosystems to change noticeably.
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From page 102... ...
. Such rainfall regimes cover nearly half of the global land, where either a gradual climate change across the ecosystem thresholds or a strong perturbation due to either extreme climate events, land use, or diseases could trigger abrupt ecosystem changes.
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From page 103... ...
. Climate change represents yet another source of stress on an already distressed system.
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From page 104... ...
. The length of the dry season and rainfall variability are also important, however.
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From page 105... ...
These processes can potentially lead to abrupt changes in Amazon forest structure and extent.
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From page 106... ...
Initially the forest was unaffected, but after the third year the largest trees began to die. Detailed simulations support Nepstad's hypothesis that the key factor was incomplete recharge of deep soil moisture, which supports prodigious rates of photosynthesis and transpiration by the largest trees during the dry season (Figure 2.17; Ivanov et al., 2012)
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From page 107... ...
. In areas close to the biogeographic boundary, increasing variability of rainfall or longer dry seasons can shift forests to savannas, without changes in mean PE or Pc.
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From page 108... ...
. A recent study shows that the dry season length over part of the Amazonia has increased much faster than that represented by climate models for both the current and future climate (Fu et al.
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From page 109... ...
Hence it appears that the tools required for monitoring and provision of early warning are at hand. Multi-spectral and active microwave data from satellites, plus an effective network of ecological plots, appear capable of monitoring response to climate change.
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From page 110... ...
. Note that regional proxies, such as the oxygen-isotope temperature reconstructions from the Greenland Ice Core Project that record Dansgaard-Oeschger events, often indicate faster regional rates of climate change than the overall global average for glacial-interglacial transitions, just as today warming is more pronounced in Arctic regions than in equatorial regions (Barnosky et al., 2003; Diffenbaugh and Field, 2013)
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From page 111... ...
Most projections of future climate-driven extinctions rest upon the assumption that potential geographic distribution of each species is ultimately determined by the climatic tolerances of the populations that make up that species. These tolerances define a species "climate envelope" which moves in space as the global climate changes, FIGURE 2.19 Global climatic conditions (here exemplified by temperature rise)
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From page 112... ...
. However, those past climate changes were considerably slower and less intense than what species are expected to experience over the next 30 to 80 years, projections which lead to forecasts of significant future extinctions (Moritz and Agudo, 2013)
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From page 113... ...
The figure shows two methods of calculating the velocity of climate change for different time periods at the end of this century. The top panel shows the velocity in terms of nearest equivalent temperature, i.e., the climate change velocity in the CMIP5 RCP8.5 ensemble, calculated by identifying the closest location (to each grid point)
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From page 114... ...
. It is an open question whether the climatic tolerances of local populations can evolve fast enough to keep up with rapid climate change (Aitken et al., 2008; Hoffmann and Sgro, 2011; Moritz and Agudo, 2013)
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From page 115... ...
. A critical consideration is that the biotic pressures induced by climate change will interact with other well-known anthropogenic drivers of extinction to amplify what are already elevated extinction rates.
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From page 116... ...
. Several authors suggest that the extinction crisis is already so severe, even without climate change included as a driver, that a mass extinction of species is plausible within decades to centuries.
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From page 117... ...
The likelihood of extinction from climate change is low for species that have short generation times, produce prodigious numbers of offspring, and have very large geographic ranges. However, even for such species, the interaction of climate change with habitat fragmentation may cause the extirpation of many populations.
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From page 118... ...
3. Those that are effectively trapped by habitat fragmentation in areas where climate changes detrimentally, even though suitable climatic habitat may exist for them elsewhere in the world.
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From page 119... ...
. These changes will be most prominent in what are today's most important reservoirs of biodiversity (including the Amazon, discussed in more detail in the "Abrupt Changes in Ecosystems" section above)
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From page 120... ...
. Coral reefs, which plausibly as a result of climate change could disappear entirely by 2100 and almost certainly will be reduced much in areal extent within the next few decades (Hoegh-Guldberg, 1999; Mumby et al., 2007; Pandolfi et al., 2011; Ricke et al., 2013)
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From page 121... ...
At best, changes of such magnitude would trigger dramatic re-organization of ecosystems across the globe that would play out over the next few centuries; at worst, extinction rates would elevate considerably for the many species adapted to pre-global warming conditions, via mechanisms described above (inability to disperse or evolve fast enough to keep pace with the extremely rapid rate of climate change, and disruption of ecological interactions within communities as species respond individualistically)
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From page 122... ...
. The main differences today, with respect to extinction potentials, are that anthropogenic climate change is much more rapid and moving global climate outside the bounds living species evolved in, and the global human population, and the pressures people place on other species, are orders of magnitude higher than was the case at the last glacialinterglacial transition (Barnosky et al., 2012)
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From page 123... ...
, and what loss of diversity through extinctions would actually cost humanity. What can be monitored to see abrupt changes coming?
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From page 124... ...
Better understanding of the role of species interactions in affecting resilience to climate change 8. Better understanding of the costs -- in ecosystem services, economics, and aesthetic/emotional value -- of losing species through extinction With improved understanding of these issues, society can make more informed decisions about potential intervention actions (Figure 2.21)
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From page 125... ...
Abrupt Changes of Primary Concern FIGURE 2.21 Improved understanding of adaptive capacity, sensitivity, and exposure to climate change can allow for more informed policy decisions. Potential actions are shown as a function of these variables.
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