Climate Stabilization Targets Emissions, Concentrations, and Impacts over Decades to Millennia (2011) / Chapter Skim
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Pages 105-158
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From page 105... ...
The pattern is derived from experiments with fully coupled Global Climate Models (GCMs) , with validation from efforts to isolate the well-mixed greenhouse gas signal from the historical temperature record.
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From page 106... ...
it has been clear that the pattern of temperature response evolves as the slow component of the warming, associated with equilibration of the deep oceans on multi-century time scales, equilibrates. In particular, on these long time scales the warming of high latitudes in the Southern Hemisphere is much larger relative to the global mean warming than in the earlier periods.
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From page 107... ...
The executive summary of Chapter 10 reports that "[g] eographical patterns of projected SAT warming show greatest temperature increases over land (roughly twice the global average temperature increase)
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From page 108... ...
The JJA patterns' main characteristics are an enhanced warming of the interior of the continents and the Mediterranean Basin, with a gradient that is generally equator to poles rather than north-south. The largest source of variation resides in the inter-model spread rather than the inter-scenario spread, and it is mainly localized over and at the edge of the ice sheets of the Arctic and Antarctica.
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From page 109... ...
by global average annual warming over the same period, using temperature at the surface from the 18 CMIP3 models whose output is available for SRES scenarios A2, A1B, and B1.
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From page 110... ...
110 CLIMATE STABILIZATION TARGETS FIGURE 4.2 Scatterplots comparing regional average warming versus global average warming (both averaged over the calendar year) for western North America (WNA)
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From page 111... ...
White indicates regions where data are not available. The bottom panels show results for the Climate Modelling Intercomparison Project (CMIP3)
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From page 112... ...
112 Annual Pattern, Range by Mod. DJF Pattern, Range by Mod.
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From page 113... ...
DJF Pattern, Range by Scen. JJA Pattern, Range by Scen.
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From page 114... ...
4.2 PRECIPITATION RESPONSE As described in Section 4.1, it is reasonable to assume that the local temperature response to an increase in a well-mixed greenhouse gas is proportional to the global mean temperature response, with a well-defined spatial and seasonal pattern. There are also good reasons to assume that the local precipitation response scales with the global mean surface tem perature response, although the uncertainties are greater, both with regard to the spatial and seasonal structure of the pattern and with regard to the limitations of this pattern scaling assumption.
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From page 115... ...
by global average annual * warming*
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From page 116... ...
The atmosphere is converging water into the regions of excess and diverging water from the regions of deficit. As temperature increases and the amount of water vapor increases, the divergence and convergence of the water vapor flux increase proportionally, increasing the magnitudes of these excesses and deficits This simple picture also explains the magnitude of the increases and decreases in precipitation expected: because water vapor increases by roughly 7% per 1°C, the atmospheric fluxes increase by the same percentage, so that the pattern of precipitation minus evaporation is also amplified by about the same factor.
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From page 117... ...
. In the tropics the circulation is primarily driven by the heat released where water vapor condenses, so one cannot assume that the tropical winds will remain the same as water vapor increases.
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From page 118... ...
For example, the changes in precipitation in the western United States in wintertime are sensitive to the El Niño phenomenon in the equatorial Pacific, the response of which is itself uncertain. And especially over semi-arid land surfaces, arguments based on the assumption that relative humidities do not change can break down.
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From page 119... ...
. A consensus picture emerges in which storms are expected to become more intense on average, roughly by 1-4% per degree global warming, as measured by maximum wind speeds (Knutson et al., 2010)
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From page 120... ...
A poleward shift in the jets and stormtracks due to global warming is a robust feature of the climate models (Miller et al., 2006; Hegerl et al., 2007; Meehl et al., 2007) , with a zonal mean poleward shift of about 100 km per 3°C global temperature increase.
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From page 121... ...
The slowdown in the MOC will have several impacts including a somewhat muted change in surface temperature in the North Atlantic and in maritime western Europe compared to other oceanic regions and modifications in the gradients in tropical sea surface temperature, which will further influence hurricane activity in the tropical Atlantic. The poleward movement of the jet in the Southern Hemisphere will cause the Antarctic Circumpolar Current to narrow and shift southward, impacting the sea-ice extent, the temperature of the water in contact with the floating ice sheets, and the exchange of carbon between the atmosphere and ocean.
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From page 122... ...
. We adapt BN09's analysis to our report's focus on different stabilization targets and make it consistent with its reli ance on pattern scaling in order to infer geographically detailed projections of temperature changes as a linear function of changes in global average temperature (see Methods, Section 4.5)
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From page 123... ...
The main difference in our approach compared to the analysis in BN09 is the choice of shifting the distribution uniformly to the right rather than trying to build a new distribution of average temperature anomalies on the basis of model output. This is a choice dictated by the use of pattern scaling, which lacks the availability of an ensemble of fully coupled model runs for each CO2 target.
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From page 124... ...
. Global average warming is calculated with respect to 1971-2000 averages.
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From page 125... ...
. Global average warming is calculated with respect to 1971-2000 averages.
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From page 126... ...
Additionally, as discussed in Section 4.2, evapotranspiration on average increases more slowly than does the water-holding capacity of the atmosphere; therefore, if precipitation intensity increases, then the frequency of precipitation must decrease to preserve a global balance of precipitation and evapotranspiration. That climate models behave in this fashion, to first approximation, is documented in a number of papers (Emori and Brown, 2005; Pall et al., 2007; Allan and Soden, 2008; Sugiyama et al., 2010)
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From page 127... ...
They find that the models' ability to reproduce current climate precipitation extremes in the extratropics is plausible, but that there are large differences in the simulations in the tropics, which are compounded by observational issues. On a global basis, the average rate of increase in 20-year return period precipitation was slightly less than the Clausius-Clapyron rate of increase in atmospheric water holding capacity (see Figure 4.10)
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From page 128... ...
128 FIGURE 4.10 Relative changes in 20-year return values averaged over the global land area of annual 24-h precipitation maxima (ΔP20) as a function of globally averaged changes in mean surface temperature for B1, A1B, and A2 global emissions scenarios, with results pooled Figure 4-10.eps from 14 GCM runs and for 2046-2065 and 2081-2100 relative to 1981-2000.
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From page 129... ...
. Sea-ice extent, defined here as the area of ocean with at least 15% sea-ice concentration, is negatively correlated to global average surface temperature so that as globally averaged surface temperature increases, sea-ice extent decreases (e.g., Gregory et al., 2002)
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From page 130... ...
130 Figure 4-11.eps FIGURE 4.11 Arctic and Antarctic sea ice extent anomalies, 1979-2009: Although Arctic sea ice extent underwent a strong decline from 1979 bitmap, landscape to 2009, Antarctic sea-ice underwent a slight increase. Source: Image provided by National Snow and Ice Data Center, University of Colorado, Boulder.
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From page 131... ...
Although the whole Arctic has experienced a trend of decreasing sea ice extent, trends in Antarctic sea ice extent have a strong regional signal and have been of opposing sign as demonstrated in Figure 4.12, taken from Liu et al.
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From page 132... ...
Predictions for Arctic Sea Ice over the 21st Century The consistent conclusion from all of the model studies is that Arctic sea-ice extent will continue to decrease during the 21st century if emissions of GHGs remain unchecked. This is clearly illustrated in Figure 4.13, which shows the September results from IPCC models using 21st century forcings based on the SRES A1B scenario, a medium-high emissions scenario.
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From page 133... ...
and 13 IPCC AR4 climate models, together with the multi-model ensemble mean (solid black line) and standard deviation (dotted black line)
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From page 134... ...
The mechanisms involved in reducing sea-ice cover are all positively correlated with temperature increase, giving rise to a linear relationship be tween annual Arctic sea-ice area reduction and global-averaged surface air temperature. According to one set of estimates, if GHG emissions continue to increase, corresponding temperature increases of 1°C, 2°C, 3°C, and 4°C are associated with Arctic sea-ice area reductions of 13%, 25%, 36% and 50% respectively (e.g., Gregory et al., 2002: Figure 4)
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From page 135... ...
The horizontal black line shows the ice extent at 4.6 million km2 value, which is the minimum sea-ice extent reached in September 2007 according to HadISST analysis. All six 135 models show rapid decline in the ice extent and reach ice-free summer (<1 million km2)
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From page 136... ...
. by the end of the 21st century for a global warming of 2-4°C above late 20th century values or 3-5°C above pre-industrial values This linear rela tionship between sea-ice loss and global-averaged surface air temperature has implications for sea-ice recovery.
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From page 137... ...
IPCC models predict a loss in sea-ice cover in summer and winter ranging from 10-50% in winter, and 33% to total loss in summer by the end of 2100. This is associated with a global warming ranging from 1.7-4.4°C above late 20th century values.
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From page 138... ...
(2005) , in an analysis expanded to include all of the western United States, argued that the PDO alone could not explain the trend in SWE.
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From page 139... ...
: (a) at 824 snow course locations in the western United States and Canada for the period 1950-1957, with negative trends shown by red circles and positive by blue circles; (b)
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From page 140... ...
Taken across models, snow amount and snow coverage decrease in the Northern Hemisphere. Although a number of assessments have been per formed of the implications of changes in snow cover on run-off, especially in the western United States (e.g., Hamlet and Lettenmaier, 1999; Christensen et al., 2004; Payne et al., 2004; Christensen and Lettenmaier, 2007)
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From page 141... ...
When it is deep, thawing increases soil moisture storage. Permafrost degradation occurs when temperatures increase in the active layer, and as a result the depth of thaw increases in successive summers and becomes greater than the depth of refreezing.
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From page 142... ...
By 2010, 20 out of 30 mega-cities are on the coast with many low-lying locations threatened by sea level rise. With coastal development continuing at a rapid pace, society is becoming in
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From page 143... ...
Low-lying islands are also vulnerable to sea level rise. An increase in global temperature will cause sea level rise and will change the amount and pattern of precipitation, which is important for the stability of ice sheets.
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From page 144... ...
Sea level rise is an inevitable consequence of global warming for two main reasons: ocean water expands as it heats up, and additional water flows into the oceans from the ice that melts on land.
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From page 145... ...
4-20.eps bitmap Polar Precipitation Each year, about 8 mm of sea level equivalent accumulates as snow on Greenland and Antarctica. Small changes in precipitation in polar regions will have major affects on the mass balance of the Greenland and Antarctic ice sheets.
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From page 146... ...
Some models project stronger net precipitation increases in the first half of the 21st century, while other models project stronger increases after the middle of the century. Subsidence Subsidence is the motion of a surface as it shifts downward relative to a datum such as sea level.
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From page 147... ...
Over the 21st century, the IPCC AR4 projected that thermal expansion will lead to sea level rise of about 0.23±0.09 m for the A1B scenario. Glaciers Terrestrial glaciers and the Greenland and Antarctic ice sheets have the potential to raise global sea level many meters.
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From page 148... ...
NRC rigorous estimates of one standard deviation error for upper-ocean thermal expansion FIGURE 4.22 (a) Total observed sea level rise and its components.
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From page 149... ...
shows ice loss from glaciers and ice caps slightly higher than those reported in IPCC AR4, contributing now about 1.2±0.2 mm y–1 to global sea level rise. Glaciers and ice caps are not in balance with the present climate; glaciers need to decrease in volume by 27% on average, and ice caps need to decrease by 26% to attain equilibrium
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From page 150... ...
As a rough approximation, we can assume that the AAR of every glacier decreases by the same percentage, giving an estimate of the fractional volume change for each glacier. In that case, the minimal sea level rise from glaciers and ice caps will be 0.373±0.021 m over the next 100 years (Bahr et al., 2009)
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From page 151... ...
The colors represent the different methods that were used: black is satellite radar altimetry, orange is aircraft laser altimetry, purple is aircraft/ satellite laser altimetry, red is the flux component method, and blue is satellite gravity. of the mass balance of the Greenland ice sheet that have been made since the early 1990s.
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From page 152... ...
Currently there is no dynamic ice sheet model that can predict the response of the Greenland ice sheet for a warmer climate. We can constrain a possible upper bound of SLR contribution from Greenland assuming a doubling in ice discharge and a continued increase in surface melt using the AR4 A1B scenario (Pfeffer et al., 2008)
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From page 153... ...
The estimated range in sea level rise in 2100 is therefore from about 0.5 to 1 m. The dynamic response of ice sheets to global warming is the largest unknown in the projections of sea level rise over the next century.
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From page 154... ...
. Ocean acidification is in fact a series of interlinked and well known changes in acid-base chemistry and carbonate chemistry due to the net flux of CO2 into surface waters (Figure 4.26)
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From page 155... ...
. Over decadal to century time scales, ocean carbon chemistry is modulated by net CO2 uptake from the atmosphere and trends in ocean circulation and biological productivity, which tend to redistribute dissolved inorganic carbon and alkalinity within the ocean water-column.
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From page 156... ...
The lower panel displays in-situ surface pH based on direct measurements (green symbols) or as calculated from dissolved inorganic carbon and total alkalinity (orange symbols)
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From page 157... ...
. The patterns of ocean acidification in subsurface waters depend on ocean circulation patterns; thermocline waters in subtropical convergence regions and deep-waters in polar regions where cold surface waters sink into the interior ocean are affected more than other parts of the subsurface.
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From page 158... ...
. For most of the surface ocean, climate change feedbacks are weak, and warming and altered ocean circulation have a limited effect on changing pH and W that are determined primarily by atmospheric CO2.
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