Introduction to Technical Chapters
The Earth’s temperature varies on a wide range of timescales and for a variety of reasons. The variability on scales of 10,000–100,000 years is paced by cyclic changes in the Earth’s orbit, but strongly depends on the internal operation of the climate system and its connection with other environmental variables. The colder glacial times are marked by decreased concentrations of atmospheric greenhouse gases, which serve to amplify the cooling at the Earth’s surface, resulting in temperature swings on the order of 5°C between glacial times and the warmer interglacial periods, such as the current one (Hansen 2004). Over the last 2,000 years, the changes in the Earth’s orbit have been small (Lean 2005a). Variations in atmospheric concentrations of greenhouse gases were also very small during this period prior to the advent of human impacts in the 19th century (Joos 2005).
The question then of how global mean surface temperature varied over the last 2,000 years is of great interest. When analyzed in conjunction with reconstructions of solar variability, volcanic activity, and other influences on climate during this period, surface temperature reconstructions can be of use in efforts to reduce the level of uncertainty in projections of human-induced greenhouse warming. Such reconstructions provide a measure of the natural variability of the climate system, against which projections of human-induced global warming can be compared. This chapter describes how large-scale surface temperature reconstructions contribute to our understanding of the sensitivity of global mean temperature to natural and human-induced perturbations of the Earth’s energy balance. It also offers a perspective on the importance of surface temperature reconstructions, as compared with other kinds of evidence, in assessing the extent to which the warming of the late 20th century is attributable to human activities.
CONCEPTS AND DEFINITIONS
This report focuses on surface temperature reconstructions over large geographic scales, in particular global mean and hemispheric mean surface temperatures. Global mean surface temperature is a particularly good indicator of the state of the climate system because it is
closely related to the balance between incoming and outgoing energy at the top of the atmosphere.
Global mean surface temperature varies in response to events outside the climate system that affect the global energy balance (NRC 2005). The external forcings considered to be of greatest importance for climate over the last 2,000 years are changes in atmospheric concentrations of carbon dioxide and other greenhouse gases, aerosol concentrations, volcanic activity, and solar radiation. Changes in land use (clearing of forests, increasing the coverage of cultivated land, and desertification) may also contribute to climate variability, but their influence is difficult to quantify (Ruddiman 2003). Human activities have caused increases in the atmospheric concentrations of greenhouse gases and aerosols, which first became appreciable in the 19th century.
The climate system also exhibits internal variability that would occur even in the absence of external forcing. A familiar example of internal climate variability on a year-to-year scale is El Niño, which is a consequence of interactions between the tropical Pacific Ocean and the global atmosphere. Interactions among the more massive, slowly varying components of the climate system could give rise to internal variability of the climate system on timescales of decades to centuries that may be largely unrelated to the external forcings on those timescales.
The change in global mean surface air temperature that occurs in response to a persistent external forcing of 1 watt per square meter over the Earth’s surface is defined as the sensitivity of the climate system (NRC 2003a). An alternative unit, used extensively in this report, is the temperature increase (in °C) that would occur in response to a doubling of the preindustrial atmospheric carbon dioxide concentration. Climate sensitivity is determined by the laws of physics and can be estimated using the methods described in the next section. The fluctuations in global mean surface temperature that occurred in response to past natural forcings provide a check on estimates of climate sensitivity. Other things being equal, the higher the sensitivity, the larger the future warming that can be expected in response to future greenhouse forcing. The strength of the various external forcings can be quantified and compared; knowledge gained from understanding the response to one kind of forcing is applicable to predicting the response to other kinds of forcing.
As in other physical systems, high climate sensitivity is indicative of the prevalence of positive climate feedbacks (NRC 2003a). The most important positive feedback in the climate system involves the increase in the concentration of atmospheric water vapor as the Earth warms. Changes in concentrations of water vapor, a greenhouse gas in its own right, amplify the warming or cooling that occurs in response to changes in concentrations of other greenhouse gases. Another positive feedback involves the decrease in the fractional area covered by snow and ice as temperatures warm, which decreases the reflectivity of the Earth as a whole. Other feedbacks involve changes in cloudiness, lapse rate, the atmospheric circulation, and land surface properties as the Earth warms or cools. The combined effect of the various positive and negative feedbacks determines the sensitivity of the climate system and the sensitivity, in turn, determines how much the Earth will warm in response to a prescribed increase in the atmospheric concentration of carbon dioxide or changes in other external forcings.
Estimation of Climate Sensitivity
The sensitivity of the climate system can be estimated in several different ways. The direct response to a doubling of preindustrial atmospheric carbon dioxide concen-
trations that would be observed in the absence of feedbacks is estimated on the basis of radiative transfer calculations to be about 1°C, and the water vapor feedback (calculated under the assumption of constant relative humidity) nearly doubles this response (e.g., Held and Soden 2000). Numerical experiments conducted with a variety of climate models that incorporate the full suite of climate feedbacks yield a range of climate sensitivities. The least sensitive models exhibit sensitivities roughly comparable to what would be obtained if only the water vapor feedback were included (about 2°C for a carbon dioxide doubling), whereas the most sensitive models estimate a sensitivity five times as large as radiative transfer calculations (Goosse et al. 2005, Webb et al. 2006, Winton 2006). The midrange models estimate a climate sensitivity of around 3°C for a doubling of carbon dioxide.
The sensitivity estimates derived from the models are checked by comparing observed and simulated responses to various known external forcings. For example, model simulations that consider surface temperature reconstructions for the past 700 years combined with instrumental data estimate climate sensitivity to be between 1.5 and 6.2°C (Hegerl et al. 2006).
ATTRIBUTION OF GLOBAL WARMING TO HUMAN INFLUENCES
The attribution of the large-scale warming of the late 20th century to human influences is supported in part by evidence that the warmth of the most recent one or two decades stands out above the background or natural variability of the last 2,000 years. To place this paleoclimatic evidence in context, it is necessary to consider the other evidence on which the attribution is based.
Based on evidence summarized in Chapter 2, it is known that global mean surface temperature has risen by about 0.6°C during the past century and that most of this warming took place during the period 1920–1940 and during the last 30 years. The troposphere is warming at a rate compatible with the warming of the Earth’s surface (CCSP and SGCR 2006). The spatial pattern of the observed temperature trends resembles the “fingerprint” of greenhouse warming in climate models, with cooling in the stratosphere and an uptake of heat by the oceans (e.g., Meehl et al. 2004, Hansen et al. 2005, Barnett et al. 2005). The warming is also reflected in a host of other indicators: For example, glaciers are retreating, permafrost is melting, snowcover is decreasing, Arctic sea ice is thinning, rivers and lakes are melting earlier and freezing later, bird migration and nesting dates are changing, flowers are blooming earlier, and the ranges of many insect and plant species are spreading to higher latitudes and higher elevations (e.g., ACIA 2004, Parmesan and Yohe 2003, Root et al. 2003, Berteaux et al. 2004, Bradshaw and Holzapfel 2006).
It is also well established that atmospheric concentrations of greenhouse gases have been increasing due to human activities. In recent decades the increases have been documented on the basis of direct measurements at a network of stations. Increases in concentrations of carbon dioxide, methane, and nitrous oxide starting in the 19th century, following many millennia of nearly constant concentrations, are clearly discernible in air bubbles trapped in ice cores recovered from the Greenland and Antarctic ice sheets (Petit et al. 1999, Siegenthaler et al. 2005a, Spahni et al. 2005). The attribution of these increases to human activities rests on both isotopic evidence and the fact that they are consistent with inventories of emissions of these gases from the burning of fossil fuels and other human activities, taking into account the storage in the oceans and the land biosphere. Based on station and ice core measurements, the combined
forcing due to the greenhouse gases injected into the atmosphere by human activities is about 2.5 watts per square meter (IPCC 2001).
Based on a climate sensitivity of 3°C for a carbon dioxide doubling, as estimated in the preceding section, a greenhouse forcing of 2.5 watts per square meter is sufficient to produce a warming of around 2°C. The observed warming during the 20th century of around 0.6°C is less than the estimated response to the greenhouse forcing for two reasons:
it is partially offset by increases in the concentration of sulfate and other aerosols, which tend to produce cooling at the Earth’s surface (e.g., Santer et al. 1995), and
part of the warming has not been realized yet because the oceans and polar ice sheets have not had sufficient time to equilibrate with the forcing (e.g., Hansen et al. 2005).
The observed 0.6°C warming during the 20th century is much larger than the internal variability in climate models. Model simulations that include both externally forced and internal variability, including plausible prescriptions of time-varying sulfate aerosols, yield time series of global mean temperature that resemble the observations (Stott et al. 2000, Ammann et al. 2003). To the extent that the warmth of the most recent one or two decades stands out above the natural variability in mean surface temperature over the last 2,000 years, the surface temperature record serves as supporting evidence that human activities are largely responsible for the recent warming. However, the attribution of the recent global warming to human activities does not rest solely or even principally upon paleoclimate evidence.
The next chapter of this report provides a brief description of the instrumental record and some considerations that apply to estimating large-scale surface temperature variations on the basis of observations at a limited number of sites. Most of what we know about how the temperature of the Earth has varied on the timescale of the last 2,000 years is based on proxy records, including documentary records, archeological evidence, and a variety of natural sources including tree rings, corals, ice cores, ocean and lake sediments, borehole temperatures, and glacier length records. The sources and characteristics of the various proxy datasets are discussed in Chapters 3–8. The statistical procedures and assumptions that are used in reconstructions of surface temperatures from proxy data are discussed in Chapter 9. Paleoclimate models and an expanded discussion of variations in climate forcing over the last two millennia are presented in Chapter 10. Finally, Chapter 11 describes the synthesis of evidence derived from a variety of different proxies to produce large-scale surface temperature reconstructions. These techniques have been a subject of controversy in a number of recent papers in the refereed literature, so Chapter 11 also assesses their strengths, limitations, and prospects for improvement.