Advancing the Science of Climate Change (2010) / Chapter Skim
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6 Changes in the Climate System
6 Changes in the Climate System
Pages 183-234
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From page 183... ...
• Will these changes be steady and gradual, or abrupt? • Will seasonal and interannual climate variations, like El Niño events, continue the same way or will they be different?
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From page 184... ...
Certain other gases, however, absorb and reemit the infrared radiation emitted by the surface, effectively trapping heat in the lower atmosphere and keeping the Earth's surface much warmer -- roughly 59°F (33°C) warmer -- than it would be if greenhouse gases were not present.1 This is called the greenhouse effect, and the gases that cause it -- including water vapor, carbon dioxide (CO2)
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From page 185... ...
However, unlike water vapor molecules, which typically remain in the lower atmosphere for only a few days before they are returned to the surface in the form of precipitation, CO2 molecules are only exchanged slowly with the surface. The excess CO2 emitted by fossil fuel burning and other human activities will thus remain in the atmosphere for many centuries before it can be removed by natural processes (Solomon et al., 2009)
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From page 186... ...
These estimates indicate that the oceans absorbed on average 8.4 ± 1.5 Gt CO2 annually over the last decade (or 26 percent of human emissions) , while the land surface took up 11.0 ± 3.3 Gt per year (29 percent)
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From page 187... ...
. To determine how CO2 levels varied prior to direct atmospheric measurements, scientists have studied the composition of air bubbles trapped in ice cores extracted from the Greenland and Antarctic ice sheets.
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From page 188... ...
. As discussed in further detail in the next section, data from even longer ice cores extracted from the hearts of the Greenland and Antarctic ice sheets -- the bottoms of which contain ice that was formed hundreds of thousands of years ago -- indicate that the current CO2 levels are higher than they have been for at least 800,000 years.
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From page 189... ...
Many current applications [including the radiative forcing values discussed in this chapter] use an ‘adjusted' radiative forcing in which the stratosphere is allowed to relax to thermal steady state, thus focusing on the energy imbalance in the Earth and troposphere system, which is most relevant to surface temperature change."
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From page 190... ...
. Methane is produced from a wide range of human activities, including natural gas management, fossil fuel and biomass burning, animal husbandry, rice cultivation, and waste management (Houweling et al., 2006)
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From page 191... ...
. Unlike CO2, which is only removed slowly from the atmosphere by processes at the land surface, the atmospheric concentration of CH4 is limited mainly by a chemical reaction in the atmosphere that yields CO2 and water vapor.
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From page 192... ...
radiative forcing of a GHG over a particular time scale (usually taken as 100 years) , compared to carbon dioxide, is sometimes expressed as the global warming potential of the gas.
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From page 193... ...
. It has been estimated that the forcing associated with halogenated gases would be 0.2 W/m2 higher than it is today if emissions reductions due to the Montreal Protocol had not taken place (Velders et al., 2007; see also Chapter 17)
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From page 194... ...
. All of these substances have natural sources, but their concentrations have increased as a result of human activities.
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From page 195... ...
. Thus, in total, the changes in atmospheric ozone are responsible for a positive forcing that is on par with the halogenated gases and possibly as large as or slightly larger than the forcing associated with CH4.
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From page 196... ...
Fossil fuel burning, industrial activities, land use change, and other human activities have generally increased the number of aerosol particles in the atmosphere, especially over and downwind of industrialized counties. The net climate forcing associated with aerosols is estimated to be −1.2 W/m2 (Forster et al., 2007; see also Murphy et al., 2009)
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From page 197... ...
Finally, aerosol emissions represent an important dilemma facing policy makers trying to limit the magnitude of future climate change: If aerosol emissions are reduced for health reasons, or as a result of actions taken to reduce GHG emissions, the net negative climate forcing associated with aerosols would decline much more rapidly than the positive forcing associated with GHGs due to the much shorter atmospheric lifetime of aerosols, and this could potentially lead to a rapid acceleration of global warming (see, e.g., Arneth et al., 2009)
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From page 198... ...
. Thus, the total climate forcing associated with modifications to the land surface due to human activities since 1750 could potentially be positive or negative, but the balance of evidence seems to suggest a slight cooling effect.
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From page 199... ...
The estimated total climate forcing associated with changes in solar irradiance since 1750 is 0.3 W/m2. (As noted in the caption for Figure 6.9, the climate forcing associated with solar irradiance changes must be scaled to account for Earth's geometry and the reflection of some of the incident solar energy back to space.)
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From page 200... ...
, energy balance models, and climate models of varying complexity (e.g., Annan et al., 2005; Hegerl et al., 2006; Knutti et al., 2006; Murphy et al., 2004; Wigley et al., 2005)
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From page 201... ...
In reality, the strength of climate forcings and feedbacks are continuously varying, and it takes the climate system -- especially the oceans -- a long time to warm up in response to a positive climate forcing. In addition, many estimates of climate sensitivity do not include climate feedbacks associated with processes that operate on decadal to centennial time scales, such as the disappearance of glaciers, changes in vegetation distribution, or changes to the carbon cycle on land and in the oceans; several recent studies that consider some of these processes have suggested that Earth's climate sensitivity may be substantially higher than the aforementioned "best estimate" (Hansen et al., 2008; Sokolov et al., 2009)
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From page 202... ...
. Data from Pinatubo and other volcanic eruptions have been used to estimate the strength of climate feedbacks that operate on relatively short time scales, such as the feedback associated with the correlation between temperature and water vapor in the atmosphere, and for calibrating and validating climate model results (e.g., Soden et al., 2002)
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From page 203... ...
. Perhaps the most dramatic example of natural climate variability is the Ice Age cycle (Figure 6.11)
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From page 204... ...
Human-Induced Climate Change Surface Temperature Measurements Widespread thermometer measurements of sufficient accuracy to reliably estimate large-scale changes in near-surface air temperature over land areas did not become available until the mid-19th century, and routine measurements of ocean temperatures did not become available until the late 19th century. In addition to missing data and individual measurement errors, there are a variety of artificial biases present in long-term temperature records that must be removed to yield records of sufficient accuracy to evaluate climate trends.
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From page 205... ...
. For the GISS data, the uncertainties associated with corrections to the raw data and with the underlying raw data themselves are estimated to yield a total uncertainty in global-average surface temperature estimates of about 0.09°F (0.05°C)
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From page 206... ...
Surface Temperature Changes Global surface temperature records indicate that the Earth has warmed substantially over the past century (Figure 6.12)
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From page 207... ...
There is also a rich tableau of ongoing regional, seasonal, diurnal, and local temperature changes associated with these large-scale, long-term, annual-mean surface warming trends: • Recent analyses of temperature trends over the Midwest and northern Great Plains have revealed that winter temperatures in that region have increased by 7°F (4°C) over the past 30 years (USGCRP, 2009a)
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From page 208... ...
Individual years, or even individual decades, can deviate from the long-term trend due to natural climate variability. Thus, it is not appropriate to look at only a short period of the overall record (such as changes over just the last 5 or 10 years)
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From page 209... ...
-- that naturally tend to reflect longer-term changes in radiative forcing, and that short-term (e.g., decadal-scale) trends are important for identifying and studying the potential for "abrupt" climate changes, which are discussed later in the chapter.
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From page 210... ...
Rather than directly sampling atmospheric conditions, satellites measure the upwelling radiation from the Earth at specific wavelengths, and this information can be used to infer the average temperature of different layers in the atmosphere underneath. As with surface temperature records, the raw satellite data are analyzed by several different research teams, each using its own techniques and assumptions, to produce estimates of inferred temperature changes (Christy et al., 2000, 2003; Mears and Wentz, 2005)
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From page 211... ...
. For comparison, surface temperatures increased +0.29°F (+0.16°C)
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From page 212... ...
. Current research on temperature trends focuses on, among other issues, regional, seasonal, and day-night differences in temperature trends, especially in the tropics, where climate models predict a stronger warming in the upper troposphere than has been observed to date (e.g., Fu and Johanson, 2005)
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From page 213... ...
. Proxy-based temperature and forcing estimates for the past millennium, and for longer time periods such as the Ice Age cycles described above, illustrate the natural variability of the climate system on a wide range of time scales.
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From page 214... ...
0.0 0.0 -0.2 -0.2 -0.4 -0.4 -0.6 -0.6 -0.8 -0.8 -1.0 -1.0 -1.2 -1.2 900 1100 1300 1500 1700 1900 Year FIGURE 6.16 Estimates of surface temperature variations for the last 1,100 years derived from different combinations of proxy evidence (colored lines)
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From page 215... ...
. • Detailed numerical model simulations of the climate system (see the following section for a discussion of climate models)
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From page 216... ...
. FUTURE CLIMATE CHANGE Climate Forcing Scenarios In order to project future changes in the climate system, scientists must first estimate how GHG emissions and other climate forcings will evolve over time.
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From page 217... ...
These representative concentration pathways will be used to conduct new climate model experiments and produce new climate change scenarios. In parallel, new socioeconomic and emissions scenarios will be developed to explore detailed scenarios of socioeconomic drivers, adaptation, mitigation, and other issues such as feedbacks.
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From page 218... ...
Most modern climate models also include representations of the oceans, cryosphere, and land surface, as well as the exchanges of energy, moisture, and materials among these components. Earth system models additionally simulate a wide range of biophysical processes including atmospheric chemistry and the biogeochemistry of ecosystems on land and in the oceans (Figure 6.17)
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From page 219... ...
. Our confidence in the ability of climate models to reliably project certain aspects of future climate stems from the extensive development and testing processes used to design models and evaluate their performance -- including simulations of 20th-century climate when the climate forcing and response are both reasonably well known (up to the limits of observations and recordkeeping)
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From page 220... ...
One widely used approach is statistical downscaling, wherein empirical relationships between past observations of local- and regional-scale climate variations are used to translate large-scale projections from global climate models to smaller space scales and shorter time scales. Alternatively, finer-scale regional models can be "nested" within coarserresolution global models to simulate regional climate changes (e.g., Hay et al., 2002; Leung et al., 2003; UCAR, 2007)
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From page 221... ...
, the CMIP3 climate models project increases in mean surface temperatures over the 21st century ranging from 2.0°F to 11.5°F (1.1°C to 6.4°C) , relative to the 1980-1999 average, by the end of the century.
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From page 222... ...
. The similarity between the three panels on the left-hand side of Figure 6.20 also illustrates how temperature increases over the next few decades reflect past emissions as well as somewhat similar GHG emissions over the next few decades for the three selected SRES scenarios (none of which include explicit policy interventions)
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From page 223... ...
. Projections of future climate also indicate that snow cover and sea ice extent will continue to decrease (Meehl et al., 2007a; USGCRP, 2009a; Zhang, 2010)
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From page 224... ...
Later chapters also explore how changes in temperature, precipitation, and other aspects of the physical climate system are likely to affect ecosystems (see Chapter 9) , agriculture (Chapter 10)
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From page 225... ...
For example, there is high confidence that global temperatures will continue to rise, that the rate and magnitude of future temperature change depends strongly on current and future rates of GHG emissions, and that climate change -- in interaction with other global and regional environmental changes -- poses significant risks for a number of human and natural systems. Global climate models and, increasingly, regional techniques are also starting to provide useful information about future climate and climate-related changes on local to regional scales.
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From page 226... ...
The mean (average) of the 21 model experiments is also shown in 6.22.pdf the bottom right panel.
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From page 227... ...
Analyses of the differences among models -- such as CMIP3 and previous model intercomparison projects -- are also a key tool for model development. The other main type of "known" uncertainty in model-based projections of future climate change is associated with processes that are either not resolved or not very well simulated in the current generation of global climate models.
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From page 228... ...
Other surprises that may be associated with future climate change include so-called "low-probability, high-impact" events, such as an unprecedented heat wave or drought, or when multiple climate changes interact with each other or with other environmental stresses to yield an unexpectedly severe impact on a human or environmental system. Some of these potential -- or in some cases already observed -- surprises are discussed in later chapters.
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From page 229... ...
For example, the ability to realistically simulate natural climate variations, such as the El Niño-Southern Oscillation, is a critical test for climate models. Improved understanding of regional variability modes is also critical for improving regional climate projections, as discussed below.
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From page 230... ...
, to further develop methods for downscaling climate scenarios and providing regional climate information, and to develop data and information systems for pairing socioeconomic and climate scenarios for use in impacts research and to support the needs of particular decision makers. Improve understanding of climate system forcing, feedbacks, and sensitivity.
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From page 231... ...
Intensive regional measurement campaigns (ground-based, airborne, satellite) should be con ducted that are designed from the start with guidance from global aerosol models so that the improved knowledge of the processes can be directly applied in the predictive models that are used to assess future climate change scenarios.
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From page 232... ...
While this research should not be expected to eliminate uncertainties, especially given the inherent uncertainty in projections of future climate forcing, efforts to expand and improve model simulations of future climate changes can be expected to yield more, more robust, and more relevant information for decision making, including the effectiveness of various actions that can be taken to respond to climate change. It should also be noted that improvements in modeling go hand-in-hand with improvements in understanding and observation.
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From page 233... ...
Advance understanding of thresholds, abrupt changes, and other climate "surprises." Some of the largest potential risks associated with future climate change come not from the relatively smooth changes in average climate conditions that are reasonably well understood and resolved in current climate models, but from extreme events, abrupt changes, and surprises that might occur when thresholds in the climate system (or related human or environmental systems) are crossed.
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From page 234... ...
Sustained observations will be critical for identifying the signs of possible thresholds and for supporting the development of improved representations of extreme events and nonlinear processes in climate models. Expanded historical and paleoclimatic records would also be valuable for understanding the impacts associated with abrupt changes in the past.
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